Re: [Haskell-cafe] Functor and Haskell
Daryoush Mehrtash-2 wrote: I am not sure I follow how the endofunctor gave me the 2nd functor. As I read the transformation there are two catagories C and D and two functors F and G between the same two catagories. My problem is that I only have one functor between the Hask and List catagories. So where does the 2nd functor come into picture that also maps between the same C and D catagories? Consider singleton :: a - [a] singleton x = [x] Here F is the identity functor, and G is the list functor. And yes, C=D= category of (a subset of) Haskell types. -- View this message in context: http://www.nabble.com/Functor-and-Haskell-tp23166441p23170956.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Is 78 characters still a good option? Was: [Haskell-cafe] breaking too long lines
The question of which column width is right is not a revealing one -- there is little technical or scientific basis to prefer 117 to 80. The line length that we prefer is similarly unenlightening. The number of people who, when pushing for column widths greater than 80, choose 132 instead shows us how just how unexploratory and hidebound folks are on this issue. Like you didn't pick 132 because it's the other traditional terminal size. Column width is rather like driving on a certain side of the road. Is one side better than the other? No. Let's change it! Why? The year after you change the ordinance, people will still be crashing into one another when they get up in the morning and switch by mistake -- and people out in the country will drive on the old side for a decade afterward, to Hell with the law. All this and *not for any benefit*. The effect of liberalizing the line length will be anarchy. I'll be resizing my terminal several times a week because some goofball likes 91 characters while another likes 354. Whereas now, we expect code from most sources to fit in 80 columns *and it does*. I've not changed the width of my terminal for code, email or reading in, well, uhm -- well ever since I changed it to 80. 80 columns is a wise limit because it's in wide use. It's not about whether I like it best -- it's about whether I expect most everyone else to be happy with code that I write in it (and I do). In matters where there is no clearcut best answer, social convention creates a clear expectation. Now in the presence of new knowledge, social convention must be changed. Unfortunately, there is no new science on line lengths. There is no reason to prefer any other column width to 80; the number was chosen a long time ago. Let's accept it and move on. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANN: list-tries-0.0 - first release
Felipe Lessa wrote: On Wed, Apr 22, 2009 at 12:29:05AM +0300, Matti Niemenmaa wrote: Good to hear that it works for someone else, too. (I don't have a new enough version of containers installed myself, after upgrading to 6.10.2.) Just bear in mind that some functions won't work. What exactly don't work? What is the problem with containers-0.2? The following three bugs are the problem: (1) http://hackage.haskell.org/trac/ghc/ticket/2644 (2) http://hackage.haskell.org/trac/ghc/ticket/2769 (3) http://hackage.haskell.org/trac/ghc/ticket/2960 (1) means that any intersection function will fail on a trie parametrized with an IntMap. (2) I worked around, it just means that mapAccumDescWithKey on a trie parametrized with a Map as well as mapAccumDesc and mapAccumDescWithKey on a trie parametrized with an IntMap will have worse performance, as they go via lists. Finally, (3) means that using any Traversable function on a trie parametrized with an IntMap will error out. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Is 78 characters still a good option? Was: [Haskell-cafe] breaking too long lines
Really, the whole thing makes me wish we had blasphemy laws. If any person, in speaking or in writing, shall indicate a preference for column widths other than 80 or indent characters other than spaces (`0x20`) they shall be compelled to present some science or be subject to imprisonment. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Being impure within a 'pure' function
Hello, imagine the following situation: You want to implement e.g. Dijkstra's algorithm to find a shortest path between nodes u and v in a graph. This algorithm relies heavily on mutating arrays, so the type signature would look something like getDistance :: Graph - Node - Node - IO Int Not mutating the underlying arrays would probably result in poor performance. BUT: For a constant graph, the distance between two nodes stays the same all the time, so in fact getDistance should be a pure function! So here is my question: Is there a way to write functions in Haskell that do some IO internally, but that are guaranteed to be side-effect free? Of course one would have to make sure that the array that is mutated inside getDistance must not be accessible from outside the function. Is that possible? If not, wouldn't that be desirable? If not, why not? Thanks Daniel ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Trouble with type signatures and type families
Hi, consider the following code: class Foo f where type Bar f :: * type Baz f :: * from :: Bar f - Baz f to :: Baz f - Bar f data Tree a b c = Empty | Tree b c (Tree a b c) (Tree a b c) --singleton :: (Foo f) = Bar f - Tree a (Bar f) (Baz f) singleton x = Tree x (from x) Empty Empty what type does 'singleton' have? ghci-6.10.2 says: :t singleton singleton :: forall f a. (Foo f) = Bar f - Tree a (Bar f) (Baz f) which is no surprise. But when I uncomment the type signature in the above code sample, loading it to ghci gives the following error: :r [1 of 1] Compiling Main ( T.hs, interpreted ) T.hs:14:22: Couldn't match expected type `Baz f1' against inferred type `Baz f' In the second argument of `Tree', namely `(from x)' In the expression: Tree x (from x) Empty Empty In the definition of `singleton': singleton x = Tree x (from x) Empty Empty T.hs:14:27: Couldn't match expected type `Bar f' against inferred type `Bar f1' In the first argument of `from', namely `x' In the second argument of `Tree', namely `(from x)' In the expression: Tree x (from x) Empty Empty Failed, modules loaded: none. I don't get it... what's wrong, I just copied the inferred type? //Stephan -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Being impure within a 'pure' function
Yes. Use the ST (State Thread) monad. Data.Array.ST, STRef etc. 2009/4/22 Daniel K. anmeldema...@gmail.com: Hello, imagine the following situation: You want to implement e.g. Dijkstra's algorithm to find a shortest path between nodes u and v in a graph. This algorithm relies heavily on mutating arrays, so the type signature would look something like getDistance :: Graph - Node - Node - IO Int Not mutating the underlying arrays would probably result in poor performance. BUT: For a constant graph, the distance between two nodes stays the same all the time, so in fact getDistance should be a pure function! So here is my question: Is there a way to write functions in Haskell that do some IO internally, but that are guaranteed to be side-effect free? Of course one would have to make sure that the array that is mutated inside getDistance must not be accessible from outside the function. Is that possible? If not, wouldn't that be desirable? If not, why not? Thanks Daniel ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Eugene Kirpichov Web IR developer, market.yandex.ru ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Trouble with type signatures and type families
You may want to read the comments at http://hackage.haskell.org/trac/ghc/ticket/1897. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Being impure within a 'pure' function
If you want to do raw IO and repackage it as pure, you can use `unsafePerformIO` and friends. It is important to use the `NOINLINE` pragma in that case. -- Jason Dusek |...unsafePerformIO...| http://haskell.org/ghc/docs/latest/html/libraries/base/System-IO-Unsafe.html#v%3AunsafePerformIO ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Being impure within a 'pure' function
On 22 Apr 2009, at 10:38, Daniel K. wrote: Hello, imagine the following situation: You want to implement e.g. Dijkstra's algorithm to find a shortest path between nodes u and v in a graph. This algorithm relies heavily on mutating arrays, so the type signature would look something like getDistance :: Graph - Node - Node - IO Int Not mutating the underlying arrays would probably result in poor performance. BUT: For a constant graph, the distance between two nodes stays the same all the time, so in fact getDistance should be a pure function! So here is my question: Is there a way to write functions in Haskell that do some IO internally, but that are guaranteed to be side- effect free? Of course one would have to make sure that the array that is mutated inside getDistance must not be accessible from outside the function. Is that possible? If not, wouldn't that be desirable? If not, why not? Either, as Eugene suggested, use the ST monad, as is possible in this case (and much better than the solution I'm proposing), or if you *really* can't get out of using IO, use unsafePerformIO. You will though have to provide several guarantees yourself that the type system would normally provide for you (hence the unsafe part - it should really be verifyItsSafeYourselfPerformIO). Bob ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On 22 Apr 2009, at 13:07, Jon Fairbairn wrote: Miguel Mitrofanov miguelim...@yandex.ru writes: Well, the problem is that every implementor does choose a subset of standart to implement. That's what I'm complaining about. And that's exactly what you (or anybody else) can't do anything about (thank God for that). It's much worse in JavaScript - essential features working differently in Internet Explorer, Firefox, Opera, and Safari, and sometimes they even differ between versions; Web programmers still manage. Strange example to choose. Have you any idea how much time is wasted because of the implementation differences in JavaScript? Actually, I have a pretty good idea - I've used to be a Web programmer. Surprisingly, not much. It's arrogant and disrespectful on the part of the implementors to say that they know better than the committee what features should be part of the language. It's arrogant and disrespectful on the part of the committee to say that they know better than the implementors what features should they implement. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re[2]: [Haskell-cafe] Being impure within a 'pure' function
Hello Jason, Wednesday, April 22, 2009, 1:14:49 PM, you wrote: there is no any need in unsafePerformIO for array computations - ST monad is exactly what one need here If you want to do raw IO and repackage it as pure, you can use `unsafePerformIO` and friends. It is important to use the `NOINLINE` pragma in that case. -- Jason Dusek |...unsafePerformIO...| http://haskell.org/ghc/docs/latest/html/libraries/base/System-IO-Unsafe.html#v%3AunsafePerformIO ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
Miguel Mitrofanov miguelim...@yandex.ru writes: Well, the problem is that every implementor does choose a subset of standart to implement. That's what I'm complaining about. It's much worse in JavaScript - essential features working differently in Internet Explorer, Firefox, Opera, and Safari, and sometimes they even differ between versions; Web programmers still manage. Strange example to choose. Have you any idea how much time is wasted because of the implementation differences in JavaScript? (n+k)-patterns are nothing compared to that. Since there is no need for /any/ differences in the implemented part of H98, we can, if we choose, have /the/ language where all this crap about I'd better not use this part of the standard because the MuckWorx compiler doesn't implement it doesn't apply. This thread is really depressing (this is about the rest of the thread, not your post Miguel). We're rehearsing the arguments about n+k patterns that were gone through by the first Haskell committee and then rehashed by the H98 folks, when that's completely irrelevant -- n+k patterns are in H98, so implementors should implement them. It's arrogant and disrespectful on the part of the implementors to say that they know better than the committee what features should be part of the language. I'm reminded of decades ago when people talked about implementing extended subsets of this or that language. Perl is an extended subset of Haskell... Concerning the suggestion that I should implement them, given that I'm against n+k patterns, I hardly think the effort should fall on me -- I'm not in the business of implementing Haskell at all at the moment. Or maybe I should be more pro-active. Here, using my favoured paradigm of Advanced Reactive Software Engineering (a successor to extreme programming and the like) is my Haskell 98 compiler (currently only implements a subset): module Main where main = error (You have used an unimplemented feature of Haskell 98.\n\ \Please submit a test case and patch to correct the deficiency\n) -- Jón Fairbairn jon.fairba...@cl.cam.ac.uk http://www.chaos.org.uk/~jf/Stuff-I-dont-want.html (updated 2009-01-31) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Issues with IO and FFIs
Does anyone have any comments on the following criticism of some difficulties with FFI, including IO, in Haskell: http://groups.google.com/group/comp.lang.functional/msg/6d650c086b2c8a49?hl=en In particular, is it not always possible to write IO libraries safely in Haskell? -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Cabal's default install location
On Wed, 2009-04-22 at 11:33 +0200, david48 wrote: The default should at least be consistent among cabal install, runghc Setup.hs, installing GHC, Gtk2Hs, and so on. If GHC is installed in /home/myusername/local, Where you choose to install ghc is not related. what does cabal install --global ? Global always means /usr/local by default, unless you change it in the cabal config file. By default ghc, gtk2hs also install globally in /usr/local (unless you specify a --prefix.) This tradition of global and /usr/local for ./configure scripts is ok, but for a convenient package manager it's not necessarily ideal. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Issues with IO and FFIs
Jon Harrop wrote: Does anyone have any comments on the following criticism of some difficulties with FFI, including IO, in Haskell: http://groups.google.com/group/comp.lang.functional/msg/6d650c086b2c8a49?hl=en That post conflates two separate questions. 1) binding to foreign libraries that export functions that are semantically pure, but can't be imported as pure functions, for example because they use pointers. unsafePerformIO was added in the FFI spec for exactly this purpose, as you can see for yourself here: http://www.cse.unsw.edu.au/~chak/haskell/ffi/ffi/ffise5.html#x8-240005.1 2) global variables. This has been a topic of long and heated debates, with no clear winner. I don't think another debate will be fruitful. In particular, is it not always possible to write IO libraries safely in Haskell? Flame bait? Yes, it is not always possible, because some interfaces are inherently unsafe, putting the burden of using them safely on the user. unsafePerformIO itself is a prime example for this. Bertram ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On Wed, Apr 22, 2009 at 12:32 AM, Duncan Coutts duncan.cou...@worc.ox.ac.uk wrote: On Tue, 2009-04-21 at 12:31 +0200, david48 wrote: For what it's worth, It's bothered me often enough that cabal doesn't install globally by default that I had to reinstall ghc in order to solve package issues. Do you know what the problem was exactly? It's possible to get problems with overlap between the user and global package dbs, but the exact same problems can also happen just within the global package db. One problem I had was while installing Lehksah. ( It was you who pointed me to the solution, thanks. ) The last problem was for installing wxhaskell from the source. The first part compiled just fine, but the second wouldn't with a package problem. I didn't want to bother searching what the problem was, so I thought it was faster to reinstall ghc and compile wxhaskell then. (it worked) Also, I think it's been a while I managed to do a cabal upgrade which didn't stop on a dependency issue. Since I didn't write down the exact problems I had, I'm attempting a fresh install, and I'll write down what happens as I go. 1) Installing GHC 6.10.2 from the tarball, I decided to give it a try to ./configure --prefix=/home/david/local 2) Adding /home/david/local' to my PATH 3) I find a binary for cabal-install 0.6.0, 4) cabal update 5) cabal install cabal-install Proceeds to download and compile HTTP-4000.0.6, then zlib-0.5.0.0 which fails because I don't have zlib.h on this new system. da...@pcdavid2:~$ sudo apt-get install ... well there is no zlib-dev, libzlib-dev available on Jaunty. there is a zlib1-dev which fails to install, and a zlib1g-dev which works. da...@pcdavid2:~$ cabal-0.6.0 install cabal-install again. This time zlib-0.5.0.0 compiles, but then : /usr/bin/ld: cannot find -lgmp da...@pcdavid2:~$ sudo apt-get install libgmp3-dev da...@pcdavid2:~$ download/cabal-0.6.0 install cabal-install againagain. This time all goes well except that: Installing executable(s) in /home/david/.cabal/bin why the hell would cabal install binaries in a subdirectory of a hidden directory. Why not /home/david/bin or /home/david/local/bin ? Ok so I find out the setting to change in .cabal/configure, but there's already two packages installed and downloaded there, and I don't know how to change them to the correct location. So, deleting .cabal and local, reinstalling ghc. I kept the de-tarred directory around, so it's really quick. cabal-update again, make sure config has the right path. oops. Cabal thinks zlib is still around, I thought I had deleted that. Looks like reinstalling ghc didn't rewrite my package list. removing .ghc and trying again. Now something else. da...@pcdavid2:~$ ghc-pkg check There are problems in package rts-1.0: include-dirs: PAPI_INCLUDE_DIR doesn't exist or isn't a directory The following packages are broken, either because they have a problem listed above, or because they depend on a broken package. rts-1.0 haddock-2.4.2 ghc-prim-0.1.0.0 integer-0.1.0.1 base-4.1.0.0 ... Solution : da...@pcdavid2:~$ ghc-pkg describe rts | sed 's/PAPI_INCLUDE_DIR//' | ghc-pkg update - da...@pcdavid2:~$ download/cabal-0.6.0 install cabal-install Linking dist/build/cabal/cabal ... Installing executable(s) in /home/david/.cabal/bin WTF? da...@pcdavid2:~$ vi .cabal/config install-dirs user -- prefix: /home/david/local -- bindir: $prefix/bin -- libdir: $prefix/lib I give up for now. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Cabal's default install location
On Wed, Apr 22, 2009 at 12:06 PM, Duncan Coutts duncan.cou...@worc.ox.ac.uk wrote: On Wed, 2009-04-22 at 11:33 +0200, david48 wrote: The default should at least be consistent among cabal install, runghc Setup.hs, installing GHC, Gtk2Hs, and so on. If GHC is installed in /home/myusername/local, Where you choose to install ghc is not related. What about the package list ? In other words, why would I need a separate user and global package list if ghc is installed in my home directory ? David ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Issues with IO and FFIs
Hello Jon, Wednesday, April 22, 2009, 1:54:58 PM, you wrote: Does anyone have any comments on the following criticism of some difficulties with FFI, including IO, in Haskell: http://groups.google.com/group/comp.lang.functional/msg/6d650c086b2c8a49?hl=en In particular, is it not always possible to write IO libraries safely in Haskell? i think that this letter is true on factual part but he exaggerate this problem 1) you can develop any pure-haskell imperative library with safe (no global state) interface and implementation. well, you can do the same with any other language too :) 2) if you are going to hardware level, it usually has von Neumann architecture, i.e. global state. so when providing some service to application, you have to deal with global state at some level. if it doesn't handled at C level (in his example, MVar may be maintained at C side), you need to to this at Haskell level 3) haskell language still doesn't provide features to create global variables, although it was proposed to add syntax he uses: globalLock - newMVar False -- the same as globalLock = unsafePerformIO (newMVar False) -- but with guarantees of no sharing so we use unsafePerformIO hack instead that's all. if we sometimes deal with global-state C libraries, we may need to use global vars too. anyway, at some level (be it C or Haskell) we need to create safe interface too unsafe von Neumann hardware -- Best regards, Bulatmailto:bulat.zigans...@gmail.com ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: Elerea, another FRP library
Am Dienstag, 21. April 2009 17:18 schrieb Patai Gergely: What about evaluation time? If I remember correctly, the values of signals depend on the time when the signal expressions are evaluated. So evaluating them multiple times might lead to different behavior. Is this correct? It is. However, there is really only one construct that needs extra care here: the latcher. All the others create top-level nodes that get evaluated just once during the first attempt to sample the output of the network. Therefore, duplication and merging of identical expressions only affects the performance unless they are hidden in the input signal of a latcher. But isn’t the latter a fundamental problem? To make things a bit more concrete, look at the following code snippet: time :: Signal DTime time = stateful 0 (+) timeSwitching :: Signal Bool - Signal DTime timeSwitching control = latcher time control (pure time) If the time signal is evaluated only once then for any signal c, timeSwitching c should be equivalent to time. But what if I replace time by stateful 0 (+) in the definition of timeSwitching? If stateful 0 (+) is evaluated everytime a switch occurs then timeSwitching would always switch back to time 0. So one first has to answer the question what the intended semantics should be. Should signals start at the beginning or should they start every time they are switched into? Implementing the first semantics is difficult since the system would have to know what signals will be used later. I think this is impossible in general because of undecidability issues. (Grapefruit’s approach is to force the user to specify what signals are used later.) Implementing the second semantics would require a single signal having possibly different values when started at different times. This in turn would disallow caching of signal values in mutable variables. Maybe I just misunderstood something, so please correct me if I’m wrong. Best wishes, Wolfgang ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Cabal's default install location
On Wed, 2009-04-22 at 12:26 +0200, david48 wrote: On Wed, Apr 22, 2009 at 12:06 PM, Duncan Coutts duncan.cou...@worc.ox.ac.uk wrote: On Wed, 2009-04-22 at 11:33 +0200, david48 wrote: The default should at least be consistent among cabal install, runghc Setup.hs, installing GHC, Gtk2Hs, and so on. If GHC is installed in /home/myusername/local, Where you choose to install ghc is not related. What about the package list ? In other words, why would I need a separate user and global package list if ghc is installed in my home directory ? Sure, you don't strictly need two in that case. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Cabal's default install location
The default should at least be consistent among cabal install, runghc Setup.hs, installing GHC, Gtk2Hs, and so on. If GHC is installed in /home/myusername/local, what does cabal install --global ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: breaking too long lines
Maurício wrote: according the several style guides, lines shouldn't be too long (longer than 78 characters). Since Haskell is usually nice to parse, wouldn't it be interesting to replace a pretty printer program for layout manuals? I saw in your first link that the teacher provided a tool to check for non-compliant layout, but wouldn't it be easier to provide a pretty-printer (with line size as one parameter to the command line)? Yes, this tool http://www.cs.caltech.edu/courses/cs11/material/haskell/misc/haskell_style_check is quite good. It should have been written in Haskell! (Btw, it warns wrongly about a comma not followed by a space in a character literal ',') My other favorite tool is hlint http://community.haskell.org/~ndm/darcs/hlint/hlint.htm Surely, I would prefer a single tool. There are certainly some style elements that can be corrected automatically, like replacing tabs, deleting trailing whitespace, putting a final newline at the of a file, shrinking more than 2 consecutive blank lines to just two blank lines, etc. However, parsing and pretty-printing code _and_ all comments (!) is not that easy to implement. Also tools like Haddock and programatica have limitations (and are written for different purposes). Some comments may refer to line numbers! (This is quite common in teaching documents and maybe correctly and better achieved by labels.) A conversion program could support different style options, but it would need to make sure that the resulting text is still valid haskell and produces equivalent code. But also a speed-up may mess-up your overall behavior! Looking at hlint, it makes suggestions that require additional imports or use different function. All these may cause ambiguities, although unlikely. Some suggestions (eta-reduce) cause the monomorphism- restriction to jump in. The current pretty-printer happened to put a lambda sign \ as the last character on a line causing cpp to fail, etc. Yet, such a (difficult to implement) tool would be useful. Cheers Christian ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On Wed, 2009-04-22 at 12:21 +0200, david48 wrote: Do you know what the problem was exactly? It's possible to get problems with overlap between the user and global package dbs, but the exact same problems can also happen just within the global package db. One problem I had was while installing Lehksah. ( It was you who pointed me to the solution, thanks. ) The last problem was for installing wxhaskell from the source. The first part compiled just fine, but the second wouldn't with a package problem. I didn't want to bother searching what the problem was, so I thought it was faster to reinstall ghc and compile wxhaskell then. (it worked) Also, I think it's been a while I managed to do a cabal upgrade which didn't stop on a dependency issue. Since I didn't write down the exact problems I had, I'm attempting a fresh install, and I'll write down what happens as I go. 1) Installing GHC 6.10.2 from the tarball, I decided to give it a try to ./configure --prefix=/home/david/local 2) Adding /home/david/local' to my PATH 3) I find a binary for cabal-install 0.6.0, 4) cabal update 5) cabal install cabal-install Proceeds to download and compile HTTP-4000.0.6, then zlib-0.5.0.0 which fails because I don't have zlib.h on this new system. da...@pcdavid2:~$ sudo apt-get install ... well there is no zlib-dev, libzlib-dev available on Jaunty. there is a zlib1-dev which fails to install, and a zlib1g-dev which works. da...@pcdavid2:~$ cabal-0.6.0 install cabal-install again. This time zlib-0.5.0.0 compiles, but then : /usr/bin/ld: cannot find -lgmp The ghc installer should really check for this at install time rather than us waiting for the first time you compile something for it to fail. da...@pcdavid2:~$ sudo apt-get install libgmp3-dev da...@pcdavid2:~$ download/cabal-0.6.0 install cabal-install againagain. This time all goes well except that: Installing executable(s) in /home/david/.cabal/bin why the hell would cabal install binaries in a subdirectory of a hidden directory. Why not /home/david/bin or /home/david/local/bin ? Yes, this is clearly suboptimal but getting agreement on where to put it has not proved easy. There are users that will scream and shout if we install to $HOME/bin by default. Please add your thoughts on the best default behaviour to this ticket: http://hackage.haskell.org/trac/hackage/ticket/289 In my opinion, what we should do is something like: * By default use symlink-bindir: ~/bin if that directory is on the path (creating it if necessary -- but only if the dir was already on the $PATH). In this case we would inform users that's what we've done and about the location of the config file if they want to change it. * If the ~/bin directory is not on the $PATH then we should give a warning that binaries will be installed in ~/.cabal/bin and that the user should either put that on the $PATH or should change the config file to specify a symlink-bindir directory that is on the $PATH. The symlink-bindir feature is safe in the sense that we never overwrite files that are not already symlinks to the location where the actual binaries are installed (eg usually ~/.cabal/bin). So in particular we never overwrite any actual binaries you installed there yourself. da...@pcdavid2:~$ ghc-pkg check There are problems in package rts-1.0: include-dirs: PAPI_INCLUDE_DIR doesn't exist or isn't a directory That's a known bug in ghc-6.10.2 sadly. It means for the 6.10.2 release that ghc-pkg check is not helpful (unless you fix it the way you did). da...@pcdavid2:~$ download/cabal-0.6.0 install cabal-install Linking dist/build/cabal/cabal ... Installing executable(s) in /home/david/.cabal/bin WTF? da...@pcdavid2:~$ vi .cabal/config install-dirs user -- prefix: /home/david/local -- bindir: $prefix/bin -- libdir: $prefix/lib I give up for now. Lines starting with -- are comments. You need to uncomment the prefix line for it to have an effect. The latest version of cabal-install makes a config file with these instructions at the top: -- This is the configuration file for the 'cabal' command line tool. -- The available configuration options are listed below. -- Some of them have default values listed. -- Lines (like this one) beginning with '--' are comments. -- Be careful with spaces and indentation because they are -- used to indicate layout for nested sections. Unfortunately I think you mentioned that you grabbed a binary of an older version so you missed out on the improved instructions. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On Wed, Apr 22, 2009 at 1:01 PM, Duncan Coutts duncan.cou...@worc.ox.ac.ukwrote: On Wed, 2009-04-22 at 12:21 +0200, david48 wrote: Lines starting with -- are comments. You need to uncomment the prefix line for it to have an effect. Man do I feel dumb now :) David. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Cabal's default install location
Am Mittwoch 22 April 2009 12:06:37 schrieb Duncan Coutts: On Wed, 2009-04-22 at 11:33 +0200, david48 wrote: The default should at least be consistent among cabal install, runghc Setup.hs, installing GHC, Gtk2Hs, and so on. If GHC is installed in /home/myusername/local, Where you choose to install ghc is not related. what does cabal install --global ? Global always means /usr/local by default, unless you change it in the cabal config file. By default ghc, gtk2hs also install globally in /usr/local (unless you specify a --prefix.) But when I install a binary, it always ends up in /usr/lib and /usr/bin, not in /usr/local. How come? Fortunately, once I've got the first binary, it's source installs from then on :) This tradition of global and /usr/local for ./configure scripts is ok, but for a convenient package manager it's not necessarily ideal. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Cabal's default install location
On Wed, Apr 22, 2009 at 2:06 PM, Duncan Coutts duncan.cou...@worc.ox.ac.ukwrote: On Wed, 2009-04-22 at 11:33 +0200, david48 wrote: The default should at least be consistent among cabal install, runghc Setup.hs, installing GHC, Gtk2Hs, and so on. If GHC is installed in /home/myusername/local, Where you choose to install ghc is not related. what does cabal install --global ? Global always means /usr/local by default, unless you change it in the cabal config file. By default ghc, gtk2hs also install globally in /usr/local (unless you specify a --prefix.) This tradition of global and /usr/local for ./configure scripts is ok, but for a convenient package manager it's not necessarily ideal. Ubuntu/Debian policy seems to be installation into /var/lib/cabal . So it's clear that the whole hierarchy is managed by single tool cabal. Drawback is that you should add /var/lib/cabal/bin into your PATH. -- Victor Nazarov ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Getting the x out
On Wed, Apr 22, 2009 at 2:49 AM, michael rice nowg...@yahoo.com wrote: How do I get the x out of Just x? Hi Michael, in your code you're using Maybe to inform the caller of safeDivision about an exceptional situation. This way, you made a full coverage of all the input cases and nothing is left out, i.e. you created a total function (which is GOOD). If you introduced the Nothing case, you just don't want to ignore it. Also, the type system is forcing you to take the Nothing case into account so you can handle it properly. Hence, you might try something like the maybe function, which accounts for the Nothing case. If you'd use fromJust, the Nothing case would remain uncovered, leading you to an unhandled exception, which conflicts with your safeDivision definition. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Non-atomic atoms for type-level programming
Hi Claus, thanks for your elaborations. I'm still not convinced that a common name (e.g. TT :. Tr :. Tu :. Te) is a better interface than a common import (e.g. TypeLevel.Bool.True). In both cases, the authors of all modules have to actively collaborate, either to define common names, or to define common imports. But I begin to see how type-level atoms could help to, e.g., implement more advanced module system as type-level embedded DSLs in Haskell. Standard ML's answer to that kind of issue is type sharing. Does type sharing help with making modules retroactively compatible? Tillmann ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Getting the x out
It makes sense, once you understand the terminology. Thanks. Michael --- On Wed, 4/22/09, Cristiano Paris fr...@theshire.org wrote: From: Cristiano Paris fr...@theshire.org Subject: Re: [Haskell-cafe] Getting the x out To: michael rice nowg...@yahoo.com Cc: haskell-cafe@haskell.org Date: Wednesday, April 22, 2009, 7:55 AM On Wed, Apr 22, 2009 at 2:49 AM, michael rice nowg...@yahoo.com wrote: How do I get the x out of Just x? Hi Michael, in your code you're using Maybe to inform the caller of safeDivision about an exceptional situation. This way, you made a full coverage of all the input cases and nothing is left out, i.e. you created a total function (which is GOOD). If you introduced the Nothing case, you just don't want to ignore it. Also, the type system is forcing you to take the Nothing case into account so you can handle it properly. Hence, you might try something like the maybe function, which accounts for the Nothing case. If you'd use fromJust, the Nothing case would remain uncovered, leading you to an unhandled exception, which conflicts with your safeDivision definition. Cristiano ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Getting the x out
Hi Neil, I just tried out Hoogle for the first time and it works great. It seems there are a lot of resources available for Haskell; it just takes time to find out about them and how they work. Thanks. Michael --- On Wed, 4/22/09, Neil Mitchell ndmitch...@gmail.com wrote: From: Neil Mitchell ndmitch...@gmail.com Subject: Re: [Haskell-cafe] Getting the x out To: michael rice nowg...@yahoo.com Cc: Tony Morris tonymor...@gmail.com, haskell-cafe@haskell.org Date: Wednesday, April 22, 2009, 1:26 AM Hi It's not too hard. You wanted a function that converted Maybe a - a, you just Hoogle for it: http://haskell.org/hoogle/?hoogle=Maybe+a+-+a Thanks Neil On Wed, Apr 22, 2009 at 2:07 AM, michael rice nowg...@yahoo.com wrote: Got it! I figured there must be some way to unpack it. My goodness, there are so many functions I'm not even aware of. Has anyone ever counted them all? Thanks. Michael --- On Tue, 4/21/09, Tony Morris tonymor...@gmail.com wrote: From: Tony Morris tonymor...@gmail.com Subject: Re: [Haskell-cafe] Getting the x out To: michael rice nowg...@yahoo.com Cc: haskell-cafe@haskell.org Date: Tuesday, April 21, 2009, 8:54 PM You mean, the x out of *Maybe* x even. In the very literal sense, the assumption that there is an x in Maybe x is false -- there may not be one since it is maybe, but not necessarily, x. IT's a bit like the use of null that you might have seen in other languages where you might have a value or you might have null. What you can do however, is say give me the x if there is one, otherwise, use this value. This is the fromMaybe function. Prelude Data.Maybe let safeDivision x y = if y == 0 then Nothing else Just (x/y) Prelude Data.Maybe 3 + (42 `fromMaybe` safeDivision 10 5) 5.0 Prelude Data.Maybe 3 + (42 `fromMaybe` safeDivision 10 0) 45.0 michael rice wrote: How do I get the x out of Just x? Michael = safeDivision :: Float - Float - Maybe Float safeDivision x y = if y == 0 then Nothing else Just (x/y) *Main Data.List safeDivision 10 5 Just 2.0 *Main Data.List 3 + (safeDivision 10 5) interactive:1:0: No instance for (Num (Maybe Float)) arising from a use of `+' at interactive:1:0-22 Possible fix: add an instance declaration for (Num (Maybe Float)) In the expression: 3 + (safeDivision 10 5) In the definition of `it': it = 3 + (safeDivision 10 5) *Main Data.List ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Tony Morris http://tmorris.net/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) --first release
Installing executable(s) in /home/david/.cabal/bin why the hell would cabal install binaries in a subdirectory of a hidden directory. Why not /home/david/bin or /home/david/local/bin ? Yes, this is clearly suboptimal but getting agreement on where to put it has not proved easy. There are users that will scream and shout if we install to $HOME/bin by default. Having learned from experience that user preferences differ wildly, even on similar platforms, not to mention a variety of platforms or, even worse, intentionally different forks of the same platform, and that trying to guess what defaults might be sensible, let alone acceptable, can be a losing game, I'd like to offer an alternative view: if there is no universally acceptable default, do not use a default Next to not being bothered with configurations they agree with, users like to be in control, or at least be informed about what is going on, and precisely how to change it, *before* anything happens that they do not like. cabal install could, on its first invocation, point to its configuration file, explaining precisely the minimum number of changes required to get it working (with potential defaults being present in the config file, commented out and explained, the config file could be a config mini-tutorial). This would depend on few things to be acceptable: - configuration should be straightforward, with explanations of possible consequences being clear and close at hand; if there is no config file, the tool should be able to generate a partial one (with disputed choices commented out) for further editing - configuration should be persistent (never overwrite old config file without user permission; propagate old config to new tool version) That way, nothing would happen until users are satisfied that things will happen exactly as they like it and, once that is done, they won't have to think about this again (until cabal changes substantially and needs to ask for further user advice, which seems better than silently changing behaviour). If you want cabal to be installable in settings where no user is available, you could either generate a full config file before install, or add an --i-really-don't-care-about-config-settings option. This road isn't perfect, but it can be less unacceptable than any arbitrary set of default choices. Claus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Using Data.Complex
Just exploring. How to load? Michael [mich...@localhost ~]$ ghci Data.Complex GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. no location info: module `Data.Complex' is a package module Failed, modules loaded: none. Prelude ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Using Data.Complex
2009/4/22 michael rice nowg...@yahoo.com: Just exploring. How to load? Michael [mich...@localhost ~]$ ghci Data.Complex GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. no location info: module `Data.Complex' is a package module Failed, modules loaded: none. Prelude ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe Try ':module + Data.Complex' from within GHCi. (Or 'import Data.Complex' from within a source file, of course.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Non-atomic atoms for type-level programming
Tillmann Rendel ren...@cs.au.dk wrote: Hi Claus, thanks for your elaborations. I'm still not convinced that a common name (e.g. TT :. Tr :. Tu :. Te) is a better interface than a common import (e.g. TypeLevel.Bool.True). In both cases, the authors of all modules have to actively collaborate, either to define common names, or to define common imports. But I begin to see how type-level atoms could help to, e.g., implement more advanced module system as type-level embedded DSLs in Haskell. Standard ML's answer to that kind of issue is type sharing. Does type sharing help with making modules retroactively compatible? map (\i - rot13 i) import Foo -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
Richard O'Keefe o...@cs.otago.ac.nz wrote: On 21 Apr 2009, at 11:36 pm, Achim Schneider wrote: Richard O'Keefe o...@cs.otago.ac.nz wrote: Some of the right questions are - how many potential whatever users would need to have whatever installed on _some_ machine they do NOT have administrator access to? Irrelevant. How van the question that is the very heart of this thread be irrelevant? This is precisely the situation I'm in, and it's precisely the class of users I'm arguing for. I'm encouraged by the constructive suggestions of package tools (nix, portage) that are said to address some of these issues. Except of course that I have to install them first... It's irrelevant, because I _do_ have root access to my machine, but don't want to get forced into using it by a question that implies that if you have access, you're going to use it. I didn't mean to nit pick, though, I thought you were arguing for the other side... I think the right question is how many people prefer user installs over system installs, wrt. their hackage packages?. I estimate that, concerning developers, who are used to install still-buggy, self-written libraries, as well as install things while working, the percentage is very, very high: At least I don't want my workflow to be broken to deal with the formal requirements of a global install while developing, and I guess many others feel the same way.[1] Endusers, of course, might have other preferences, but cabal doesn't (IMHO) cater to them, directly: It caters to distribution packages (or windows installers, or whatever), so cabal's default behaviour is quite irrelevant for those cases. [1] Thinking of it... is there a way to tell cabal to pretend a package is installed by giving the path to it's source directory? Just like include directories, but with packages. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
That's absurd. You have no way to access private source code, so any decision on what features to exclude from future versions of Haskell must necessarily look at publicly accessible source code. The only alternative is to continuously add, and never remove, features from Haskell, even if no one (that we know) uses them. Moreover, the odds that everyone who is using n + k patterns are doing so only in private is an untestable hypothesis (i.e. unscientific) and extremely unlikely to be true. Regards, John A. De Goes N-BRAIN, Inc. The Evolution of Collaboration http://www.n-brain.net|877-376-2724 x 101 On Apr 21, 2009, at 9:34 PM, Richard O'Keefe wrote: It *is* true that things that *are* used in the commonly available sources should continue to be supported in order to preserve the value of those commonly available sources. It is *not* true that things that are *not* used in the commonly available sources are therefore of no value and safely to be discarded. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Non-atomic atoms for type-level programming
thanks for your elaborations. I'm still not convinced that a common name (e.g. TT :. Tr :. Tu :. Te) is a better interface than a common import (e.g. TypeLevel.Bool.True). In both cases, the authors of all modules have to actively collaborate, either to define common names, or to define common imports. It is not a solution, it is a workaround. All it does is offer users another choice, so they can now say whether they are talking about shared or locally defined labels: module A where import Data.Label data MyLabel x = [$l|label|] y = undefined::MyLabel module B where import Data.Label data MyLabel x = [$l|label|] y = undefined::MyLabel module C where import Data.Label import A import B ok = [A.x,B.x] fails = [A.y,B.y] It does so by offering a meta-level commonality: A and B do not have to agree on a common module to declare all their common types (the author of Data.Label has no idea what labels its importers might use, other than the alphabet the labels are constructed from), they only need to agree on a common way of declaring all their shareable types. But I begin to see how type-level atoms could help to, e.g., implement more advanced module system as type-level embedded DSLs in Haskell. Well, atoms make labels, labels form extensible record fields, extensible records can be used as first-class modules, but there'd still be a lot missing. This is really just a small step, and though it is a useful one, it has no such high aspirations, yet. When I wrote the first-class-labels proposal for Haskell', I was thinking about possible implementations (outlined in the haskell prime wiki page I referred to) but they always looked as if they'd require substantial changes. This workaround suggests that a few well-placed localised changes to GHC might be sufficient to get first-class labels - just split the modifications over two, only losely coupled areas: - provide label constructors - provide label usage (preferably hiding the internal structure) Until someone does that, quasiquoting offers a workaround, so people can resume playing with things like type-level numbers, extensible record libraries with and without label ordering, etc. I've filed a feature request for type-level quasiquoting, in case anyone else has such needs:-) http://hackage.haskell.org/trac/ghc/ticket/3177 Standard ML's answer to that kind of issue is type sharing. Does type sharing help with making modules retroactively compatible? It has been too long since I looked at this in detail, but yes. The way I recall it (and the early example in [1] seems to confirm this, though SML has changed after that paper was published) is that modules have signatures, and type sharing constraints assert that parts of these signatures have to match up (in Haskell-speak: imagine modules as records, with two records R1 a and R2 b, then we can use a type 'a~b = R1 a - R2 b - T' to assert that both records share the same type parameter; only that Haskell modules aren't records and aren't parameterized..). It would be as if one could write modules parameterised by types, instead of declaring them locally, and being able to share a type parameter over several imports: module A(type label) where x = undefined :: label module B(type label) where x = undefined :: label module C(type label) where import A[label] import B[label] ok = [A.x,B.x] Claus [1] http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.14.3595 Robert Harper and Mark Lillibridge, A Type-Theoretic Approach to Higher-Order Modules with Sharing ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) --first release
Claus Reinke claus.rei...@talk21.com wrote: [...] +1. That, and better error messages: A Verbose-Consequences flag in the config (on by default), resulting in strings like Binaries have been installed to $HOME/.cabal/bin and _not_ symlinked. $HOME/.cabal/bin is not in your $PATH: You will not be able to call them directly from the command line, and programs depending on them might be unable to locate them, resulting in failure or limited functionality. Hell, I never thought I'd ever advertise the usage of disclaimers... Gentoo does this, too, btw: Remember, you have to be in the group games to play games, You chose to build firefox with official branding, distributing the binary (even in your local network) might result in legal problems with the Mozilla Foundation. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: Elerea, another FRP library
network. Therefore, duplication and merging of identical expressions only affects the performance unless they are hidden in the input signal of a latcher. But isn't the latter a fundamental problem? Of course it is, but I said afterwards that this can be resolved by sampling 'more thoroughly'. If the time signal is evaluated only once then for any signal c, timeSwitching c should be equivalent to time. But what if I replace time by stateful 0 (+) in the definition of timeSwitching? If stateful 0 (+) is evaluated everytime a switch occurs then timeSwitching would always switch back to time 0. The value stored in a constant signal is supposed to be evaluated exactly once, therefore in this case the expected behaviour is that timeSwitching is equivalent to time. The meaning of 'pure sig' is a signal that refers to 'sig' at every point. At the present moment, if you express the same by lifting the signal constructor directly, it will also be evaluated exactly once, but only when the control signal is true for the first time. This is obviously wrong, but it can be cured by forcing evaluation at the beginning. This also means that if you want to restart a signal without external dependencies using a latcher, you have to inject some bogus dependency to prevent memoisation. If the new signal depends on some others, latching should behave intuitively. So one first has to answer the question what the intended semantics should be. Should signals start at the beginning or should they start every time they are switched into? Clearing up the semantics is certainly on my todo list. :) Implementing the first semantics is difficult since the system would have to know what signals will be used later. I think this is impossible in general because of undecidability issues. (Grapefruit's approach is to force the user to specify what signals are used later.) Do you have a compact use case that demonstrates this problem? Implementing the second semantics would require a single signal having possibly different values when started at different times. This in turn would disallow caching of signal values in mutable variables. Or at least it would require deep copying some initial snapshot at every restart. But this only applies to completely self-contained signals, since anything that depends on the outer world cannot be restarted by definition. Gergely -- http://www.fastmail.fm - Faster than the air-speed velocity of an unladen european swallow ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Non-atomic atoms for type-level programming
in case anyone stumbles over my ad-hoc notations, that should have been: module A[type label] where x = undefined :: label module B[type label] where x = undefined :: label module C[type label] where import A[label] import B[label] ok = [A.x,B.x] assuming that: - 'module X[types]' means a module parameterized by 'types' - 'import X[types]' means a module import with parameters 'types'. Claus ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
Richard O'Keefe et all wrote: [n+k patterns] I'd like to add my two cents: Assuming that UHC's roadmap strives to be H'-compilant in the future, and n+k patterns aren't going to be in H', why bother implementing them? Also, assuming that current H98 code will be ported to H', shouldn't n+k patterns be removed from existing code, anyway? -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Is 78 characters still a good option? Was: breaking too long lines
Jason Dusek jason.du...@gmail.com wrote: Really, the whole thing makes me wish we had blasphemy laws. If any person, in speaking or in writing, shall indicate a preference for column widths other than 80 or indent characters other than spaces (`0x20`) they shall be compelled to present some science or be subject to imprisonment. I'll definitely add it to the list of questions should I ever conduct a job interview. Just to test how much backing people have for their zeal. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] haxr build failure from cabal
haxr will no longer compile from cabal because of the dependency marked HTTP 1.0. The current version of the library requires HTTP 4000.0.0 as it stands. Can it be updated real quick in hackage? -- Jeff ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is 78 characters still a good option? Was: breaking too long lines
Another reason for the 80 character limit: some developers have very poor eyesight, which can be overcome with large monitors and large fonts. This won't work if you have to scroll the code. Regards, John A. De Goes N-BRAIN, Inc. The Evolution of Collaboration http://www.n-brain.net|877-376-2724 x 101 On Apr 22, 2009, at 8:32 AM, Achim Schneider wrote: Jason Dusek jason.du...@gmail.com wrote: Really, the whole thing makes me wish we had blasphemy laws. If any person, in speaking or in writing, shall indicate a preference for column widths other than 80 or indent characters other than spaces (`0x20`) they shall be compelled to present some science or be subject to imprisonment. I'll definitely add it to the list of questions should I ever conduct a job interview. Just to test how much backing people have for their zeal. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is 78 characters still a good option? Was: breaking too long lines
I've found that some developers have very poor taste in shirts as well, therefore Haskell should have a dress code Sorry I'm not buying 80 characters as a way to address bad eyesight. ;-) I think there's supposed to be technology in the editors to deal with that... just as we can try to find good women to dress us better. On Wed, Apr 22, 2009 at 7:38 AM, John A. De Goes j...@n-brain.net wrote: Another reason for the 80 character limit: some developers have very poor eyesight, which can be overcome with large monitors and large fonts. This won't work if you have to scroll the code. Regards, John A. De Goes N-BRAIN, Inc. The Evolution of Collaboration http://www.n-brain.net|877-376-2724 x 101 On Apr 22, 2009, at 8:32 AM, Achim Schneider wrote: Jason Dusek jason.du...@gmail.com wrote: Really, the whole thing makes me wish we had blasphemy laws. If any person, in speaking or in writing, shall indicate a preference for column widths other than 80 or indent characters other than spaces (`0x20`) they shall be compelled to present some science or be subject to imprisonment. I'll definitely add it to the list of questions should I ever conduct a job interview. Just to test how much backing people have for their zeal. -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: haxr build failure from cabal
I think you can pass --preference=HTTP==3001.1.5 to cabal-install 0.6.2, try cabal install haxr --preference=HTTP==3001.1.5 Cheers Christian Jeff Heard wrote: haxr will no longer compile from cabal because of the dependency marked HTTP 1.0. The current version of the library requires HTTP 4000.0.0 as it stands. Can it be updated real quick in hackage? -- Jeff ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haxr build failure from cabal
Hi Jeff, I have an updated and _seemingly_ working version of the haxr codebase, but haven't had a chance to test it more than a gentle poke at the tires. Will see if I can upload commit the bits. --sigbjorn On 4/22/2009 07:37, Jeff Heard wrote: haxr will no longer compile from cabal because of the dependency marked HTTP 1.0. The current version of the library requires HTTP 4000.0.0 as it stands. Can it be updated real quick in hackage? -- Jeff ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Overriding a Prelude function?
I've been working through this example from: http://en.wikibooks.org/wiki/Haskell/Understanding_monads I understand what they're doing all the way up to the definition of (), which duplicates Prelude function (). To continue following the example, I need to know how to override the Prelude () with the () definition in my file rand.hs. Michael == [mich...@localhost ~]$ cat rand.hs import System.Random type Seed = Int randomNext :: Seed - Seed randomNext rand = if newRand 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 toDieRoll :: Seed - Int toDieRoll seed = (seed `mod` 6) + 1 rollDie :: Seed - (Int, Seed) rollDie seed = ((seed `mod` 6) + 1, randomNext seed) sumTwoDice :: Seed - (Int, Seed) sumTwoDice seed0 = let (die1, seed1) = rollDie seed0 (die2, seed2) = rollDie seed1 in (die1 + die2, seed2) () m n = \seed0 - let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) [mich...@localhost ~]$ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Overriding a Prelude function?
I think import Prelude hiding (()) does that. -Ross On Apr 22, 2009, at 11:44 AM, michael rice wrote: I've been working through this example from: http://en.wikibooks.org/wiki/Haskell/Understanding_monads I understand what they're doing all the way up to the definition of (), which duplicates Prelude function (). To continue following the example, I need to know how to override the Prelude () with the () definition in my file rand.hs. Michael == [mich...@localhost ~]$ cat rand.hs import System.Random type Seed = Int randomNext :: Seed - Seed randomNext rand = if newRand 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 toDieRoll :: Seed - Int toDieRoll seed = (seed `mod` 6) + 1 rollDie :: Seed - (Int, Seed) rollDie seed = ((seed `mod` 6) + 1, randomNext seed) sumTwoDice :: Seed - (Int, Seed) sumTwoDice seed0 = let (die1, seed1) = rollDie seed0 (die2, seed2) = rollDie seed1 in (die1 + die2, seed2) () m n = \seed0 - let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) [mich...@localhost ~]$ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Overriding a Prelude function?
You can try at the top Import Prelude hiding () On Wed, Apr 22, 2009 at 10:44 AM, michael rice nowg...@yahoo.com wrote: I've been working through this example from: http://en.wikibooks.org/wiki/Haskell/Understanding_monads I understand what they're doing all the way up to the definition of (), which duplicates Prelude function (). To continue following the example, I need to know how to override the Prelude () with the () definition in my file rand.hs. Michael == [mich...@localhost ~]$ cat rand.hs import System.Random type Seed = Int randomNext :: Seed - Seed randomNext rand = if newRand 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 toDieRoll :: Seed - Int toDieRoll seed = (seed `mod` 6) + 1 rollDie :: Seed - (Int, Seed) rollDie seed = ((seed `mod` 6) + 1, randomNext seed) sumTwoDice :: Seed - (Int, Seed) sumTwoDice seed0 = let (die1, seed1) = rollDie seed0 (die2, seed2) = rollDie seed1 in (die1 + die2, seed2) () m n = \seed0 - let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) [mich...@localhost ~]$ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Cabal's default install location
On Wed, Apr 22, 2009 at 1:28 PM, Victor Nazarov asviraspossi...@gmail.comwrote: Ubuntu/Debian policy seems to be installation into /var/lib/cabal . So it's clear that the whole hierarchy is managed by single tool cabal. Drawback is that you should add /var/lib/cabal/bin into your PATH. (K)Ubuntu is so far back on GHC releases that I never install anything haskell from the repositories :( ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Problems with Haskell Program Coverage
I want to use hpc to check that the ASN.1 library tests cover all the code. When I run it with a set of tests that I *know* don't test certain things, it reports that they have been covered i.e. there are not coloured in the markup that hpc produces. I would have expected a lot of yellow. It seems to work ok on small amounts of code so I'm not clear what I'm doing wrong. I'd be happy to provide the .tix file or put the .html files somewhere if that would be helpful. Here's what I use: ghc -o NewTest NewTest.hs -fhpc --make NewTest hpc markup NewTest Dominic. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Overriding a Prelude function?
Be aware that the do unsugars to (Prelude.), not your (), even if
you hide (Prelude.):
import Prelude hiding (())
m f = error Call me!
main = putStrLn . show $ do [3,4]
[5]
The desugaring of the do { [3,4]; [5] } is (Prelude.) [3,4] [5] =
[5,5], whereas you might have hoped for [3,4] [5] = error Call me!
Dan
Ross Mellgren wrote:
I think
import Prelude hiding (())
does that.
-Ross
On Apr 22, 2009, at 11:44 AM, michael rice wrote:
I've been working through this example from:
http://en.wikibooks.org/wiki/Haskell/Understanding_monads
I understand what they're doing all the way up to the definition of
(), which duplicates Prelude function (). To continue following
the example, I need to know how to override the Prelude () with the
() definition in my file rand.hs.
Michael
==
[mich...@localhost ~]$ cat rand.hs
import System.Random
type Seed = Int
randomNext :: Seed - Seed
randomNext rand = if newRand 0 then newRand else newRand + 2147483647
where newRand = 16807 * lo - 2836 * hi
(hi,lo) = rand `divMod` 127773
toDieRoll :: Seed - Int
toDieRoll seed = (seed `mod` 6) + 1
rollDie :: Seed - (Int, Seed)
rollDie seed = ((seed `mod` 6) + 1, randomNext seed)
sumTwoDice :: Seed - (Int, Seed)
sumTwoDice seed0 =
let (die1, seed1) = rollDie seed0
(die2, seed2) = rollDie seed1
in (die1 + die2, seed2)
() m n = \seed0 -
let (result1, seed1) = m seed0
(result2, seed2) = n seed1
in (result2, seed2)
[mich...@localhost ~]$
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Re: [Haskell-cafe] Overriding a Prelude function?
True enough -- if you really want to redefine the monadic operator,
you have to use
{-# LANGUAGE NoImplicitPrelude #-}
import Prelude hiding ((), (=), return)
or something like it, although Michael's example didn't appear to be
going quite that far.
-Ross
On Apr 22, 2009, at 12:37 PM, Dan Weston wrote:
Be aware that the do unsugars to (Prelude.), not your (), even
if you hide (Prelude.):
import Prelude hiding (())
m f = error Call me!
main = putStrLn . show $ do [3,4]
[5]
The desugaring of the do { [3,4]; [5] } is (Prelude.) [3,4] [5] =
[5,5], whereas you might have hoped for [3,4] [5] = error Call
me!
Dan
Ross Mellgren wrote:
I think
import Prelude hiding (())
does that.
-Ross
On Apr 22, 2009, at 11:44 AM, michael rice wrote:
I've been working through this example from:
http://en.wikibooks.org/wiki/Haskell/Understanding_monads
I understand what they're doing all the way up to the definition
of (), which duplicates Prelude function (). To continue
following the example, I need to know how to override the Prelude
() with the () definition in my file rand.hs.
Michael
==
[mich...@localhost ~]$ cat rand.hs
import System.Random
type Seed = Int
randomNext :: Seed - Seed
randomNext rand = if newRand 0 then newRand else newRand +
2147483647
where newRand = 16807 * lo - 2836 * hi
(hi,lo) = rand `divMod` 127773
toDieRoll :: Seed - Int
toDieRoll seed = (seed `mod` 6) + 1
rollDie :: Seed - (Int, Seed)
rollDie seed = ((seed `mod` 6) + 1, randomNext seed)
sumTwoDice :: Seed - (Int, Seed)
sumTwoDice seed0 =
let (die1, seed1) = rollDie seed0
(die2, seed2) = rollDie seed1
in (die1 + die2, seed2)
() m n = \seed0 -
let (result1, seed1) = m seed0
(result2, seed2) = n seed1
in (result2, seed2)
[mich...@localhost ~]$
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[Haskell-cafe] astronomy projects in haskell
I remember reading some website, that dons (probably) posted once. I'd like to find them again for a report I'm doing. So, if you know of any astronomy websites that talk about projects using haskell, please let me know. thanks Michael Litchard ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] astronomy projects in haskell
2009/4/22 Michael Litchard mich...@schmong.org: I remember reading some website, that dons (probably) posted once. I'd like to find them again for a report I'm doing. So, if you know of any astronomy websites that talk about projects using haskell, please let me know. thanks Michael Litchard Could this be what you meant? http://www.absoluteastronomy.com/topics/Haskell_(programming_language) Deniz Dogan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
2009/04/22 Miguel Mitrofanov miguelim...@yandex.ru: It's arrogant and disrespectful on the part of the implementors to say that they know better than the committee what features should be part of the language. It's arrogant and disrespectful on the part of the committee to say that they know better than the implementors what features should they implement. So what is the committee there for? To approve helpful suggestions? -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is 78 characters still a good option? Was: breaking too long lines
2009/04/22 Achim Schneider bars...@web.de: Jason Dusek jason.du...@gmail.com wrote: Really, the whole thing makes me wish we had blasphemy laws. I'll definitely add it to the list of questions should I ever conduct a job interview. Just to test how much backing people have for their zeal. I'm sure you'll find plenty of college grads willing to use long lines. The decline in religiosity in recent years is a well known phenomenon. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haxr build failure from cabal
OK, new release of haxr available via hackage; compilable with ghc-6.10.1 (but may very well have bootstrap issues with 6.10.2 due to 'time' dependency) http://hackage.haskell.org/cgi-bin/hackage-scripts/package/haxr enjoy --sigbjorn Sigbjorn Finne wrote: Hi Jeff, I have an updated and _seemingly_ working version of the haxr codebase, but haven't had a chance to test it more than a gentle poke at the tires. Will see if I can upload commit the bits. --sigbjorn On 4/22/2009 07:37, Jeff Heard wrote: haxr will no longer compile from cabal because of the dependency marked HTTP 1.0. The current version of the library requires HTTP 4000.0.0 as it stands. Can it be updated real quick in hackage? -- Jeff ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] astronomy projects in haskell
On Wed, Apr 22, 2009 at 1:17 PM, Deniz Dogan deniz.a.m.do...@gmail.com wrote: 2009/4/22 Michael Litchard mich...@schmong.org: Could this be what you meant? http://www.absoluteastronomy.com/topics/Haskell_(programming_language) Deniz Dogan That being just a Wikipedia mirror, seems pretty unlikely. (No idea what the desired result could be. There was a pretty widespread technical report on using Scheme to control telescopes, which had some cool tricks in it, but that's obviously not Haskell.) -- gwern ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On 22 Apr 2009, at 21:19, Jason Dusek wrote: 2009/04/22 Miguel Mitrofanov miguelim...@yandex.ru: It's arrogant and disrespectful on the part of the implementors to say that they know better than the committee what features should be part of the language. It's arrogant and disrespectful on the part of the committee to say that they know better than the implementors what features should they implement. So what is the committee there for? To approve helpful suggestions? To give advice(s). Like, for example, W3C. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On Wed, Apr 22, 2009 at 10:19 AM, Jason Dusek jason.du...@gmail.com wrote: 2009/04/22 Miguel Mitrofanov miguelim...@yandex.ru: It's arrogant and disrespectful on the part of the implementors to say that they know better than the committee what features should be part of the language. It's arrogant and disrespectful on the part of the committee to say that they know better than the implementors what features should they implement. So what is the committee there for? To approve helpful suggestions? The fun of sharing code is that you get to deal with the peanut gallery... Oh right, that's why I don't share. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] breaking too long lines
On Mon, Apr 20, 2009 at 8:44 AM, Tillmann Rendel ren...@cs.au.dk wrote: However, I would prefer the following Coq-like syntax: data Maybe a = | Just a | Nothing Of course, Coq's inductive syntax is just GADT form: Inductive Maybe a := | Just : a - Maybe a | Nothing : Maybe a. data Maybe a where Just :: a - Maybe a Nothing :: Maybe a I find GADT syntax much clearer than H98 syntax, so I use it when I'm not being picky about compatibility. :-) (And yes, I know you just meant the optional leading bar) Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Overriding a Prelude function?
You can hide () from the implicit import of Prelude using: import Prelude hiding (()) Kind regards, Thomas michael rice wrote: I've been working through this example from: http://en.wikibooks.org/wiki/Haskell/Understanding_monads I understand what they're doing all the way up to the definition of (), which duplicates Prelude function (). To continue following the example, I need to know how to override the Prelude () with the () definition in my file rand.hs. Michael == [mich...@localhost ~]$ cat rand.hs import System.Random type Seed = Int randomNext :: Seed - Seed randomNext rand = if newRand 0 then newRand else newRand + 2147483647 where newRand = 16807 * lo - 2836 * hi (hi,lo) = rand `divMod` 127773 toDieRoll :: Seed - Int toDieRoll seed = (seed `mod` 6) + 1 rollDie :: Seed - (Int, Seed) rollDie seed = ((seed `mod` 6) + 1, randomNext seed) sumTwoDice :: Seed - (Int, Seed) sumTwoDice seed0 = let (die1, seed1) = rollDie seed0 (die2, seed2) = rollDie seed1 in (die1 + die2, seed2) () m n = \seed0 - let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) [mich...@localhost ~]$ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Getting the x out
michael rice wrote: Got it! I figured there must be some way to unpack it. If you peek at the thread about getting a value out of IO [1], you will see some similarities. If you look at my response [2], you will see that the functions I suggested for IO are exactly the same as the functions you may want to consider for this case! fmap, liftA, liftM, ($) :: Functor f = (a - b) - (f a - f b) (*), ap :: Applicative f = f (a - b) - (f a - f b) (=) :: Monad m = (a - m b) - (m a - m b) So, for Maybe, you have: ($) :: (a - b) - (Maybe a - Maybe b) (*) :: Maybe (a - b) - (Maybe a - Maybe b) (=) :: (a - Maybe b) - (Maybe a - Maybe b) You will find that a *lot* of the functions you learn in Haskell are actually very general, and once you internalize what they *really* do, you will suddenly have entire classes of problems solved by just a few functions. - Jake [1] http://www.haskell.org/pipermail/haskell-cafe/2009-April/059834.html [2] http://www.haskell.org/pipermail/haskell-cafe/2009-April/059852.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Overriding a Prelude function?
Here's what I get:
[mich...@localhost ~]$ ghci
GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
Prelude import Prelude hiding (())
interactive:1:0: parse error on input `import'
Prelude
=
I was passing seed0 to rollDie and getting back (r1,seed1)
passing seed1 to rollDie and getting back (r2,seed2)
passing seed2 to rollDie and getting back (r3,seed3)
Just based on the problem text, I would guess that
passing rollDie and seed0 to () I would get back (r3,seed3),
losing the intermediate random numbers r1 and r2 along the way, at
least that's what I understood it to say.
So, I know that next I'm probably going to have to do something to
remedy that, but I haven't gotten to that next step yet. What is unsugar?
Thanks in advance for your patience.
Michael
--- On Wed, 4/22/09, Dan Weston weston...@imageworks.com wrote:
From: Dan Weston weston...@imageworks.com
Subject: Re: [Haskell-cafe] Overriding a Prelude function?
To: Ross Mellgren rmm-hask...@z.odi.ac
Cc: michael rice nowg...@yahoo.com, haskell-cafe@haskell.org
haskell-cafe@haskell.org
Date: Wednesday, April 22, 2009, 12:37 PM
Be aware that the do unsugars to (Prelude.), not your (), even if you hide
(Prelude.):
import Prelude hiding (())
m f = error Call me!
main = putStrLn . show $ do [3,4]
[5]
The desugaring of the do { [3,4]; [5] } is (Prelude.) [3,4] [5] = [5,5],
whereas you might have hoped for [3,4] [5] = error Call me!
Dan
Ross Mellgren wrote:
I think
import Prelude hiding (())
does that.
-Ross
On Apr 22, 2009, at 11:44 AM, michael rice wrote:
I've been working through this example from:
http://en.wikibooks.org/wiki/Haskell/Understanding_monads
I understand what they're doing all the way up to the definition of (),
which duplicates Prelude function (). To continue following the example, I
need to know how to override the Prelude () with the () definition in my
file rand.hs.
Michael
==
[mich...@localhost ~]$ cat rand.hs
import System.Random
type Seed = Int
randomNext :: Seed - Seed
randomNext rand = if newRand 0 then newRand else newRand + 2147483647
where newRand = 16807 * lo - 2836 * hi
(hi,lo) = rand `divMod` 127773
toDieRoll :: Seed - Int
toDieRoll seed = (seed `mod` 6) + 1
rollDie :: Seed - (Int, Seed)
rollDie seed = ((seed `mod` 6) + 1, randomNext seed)
sumTwoDice :: Seed - (Int, Seed)
sumTwoDice seed0 =
let (die1, seed1) = rollDie seed0
(die2, seed2) = rollDie seed1
in (die1 + die2, seed2)
() m n = \seed0 -
let (result1, seed1) = m seed0
(result2, seed2) = n seed1
in (result2, seed2)
[mich...@localhost ~]$
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[Haskell-cafe] ANNOUNCE: dataenc 0.12.1.0
I've just uploaded version 0.12.1.0 of dataenc with the following (visible) changes compared to the previous version (0.12): - implementation of a bunch of new encodings: - xxencode - hexadecimal - quoted-printable - python escaping - url encoding - squashing of a bug in the yEncoding implementation that only manifested on 32-bit systems - an attempt to conform to the guidelines for the Haskell Platform regarding versioning and dependencies This release brings the full list of encodings to - Base16 - Base32 - Base32Hex - Base64 - Base64Url - Base85 - Hexadecimal - Python string escaping - quoted-printable - URL encoding - uuencoding - xxencoding - yEncoding The hackage page[1] now reads: Versions 0.9, 0.10.1, 0.10.2, 0.11, 0.11.1, 0.12, 0.12.1.0 Dependencies array (=0.2.0 0.3), base (=4.0.0 4.1), containers (=0.2.0 0.3) License BSD3 Copyright Magnus Therning, 2007-2009 AuthorMagnus Therning Maintainermag...@therning.org Category Codec Home page http://www.haskell.org/haskellwiki/Library/Data_encoding Upload date Wed Apr 22 19:40:22 UTC 2009 Uploaded by MagnusTherning /M [1]: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/dataenc -- Magnus Therning(OpenPGP: 0xAB4DFBA4) magnus＠therning．org Jabber: magnus＠therning．org http://therning.org/magnus identi.ca|twitter: magthe Haskell is an even 'redder' pill than Lisp or Scheme. -- PaulPotts signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Overriding a Prelude function?
On Wed, Apr 22, 2009 at 1:47 PM, michael rice nowg...@yahoo.com wrote:
Here's what I get:
[mich...@localhost ~]$ ghci
GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
Prelude import Prelude hiding (())
You know, to avoid this nonsense you could just name the operator something
else, like ~, or ~, or $...@**!. Operators are just names.
Luke
interactive:1:0: parse error on input `import'
Prelude
=
I was passing seed0 to rollDie and getting back (r1,seed1)
passing seed1 to rollDie and getting back (r2,seed2)
passing seed2 to rollDie and getting back (r3,seed3)
Just based on the problem text, I would guess that
passing rollDie and seed0 to () I would get back (r3,seed3),
losing the intermediate random numbers r1 and r2 along the way, at
least that's what I understood it to say.
So, I know that next I'm probably going to have to do something to
remedy that, but I haven't gotten to that next step yet. What is unsugar?
Thanks in advance for your patience.
Michael
--- On *Wed, 4/22/09, Dan Weston weston...@imageworks.com* wrote:
From: Dan Weston weston...@imageworks.com
Subject: Re: [Haskell-cafe] Overriding a Prelude function?
To: Ross Mellgren rmm-hask...@z.odi.ac
Cc: michael rice nowg...@yahoo.com, haskell-cafe@haskell.org
haskell-cafe@haskell.org
Date: Wednesday, April 22, 2009, 12:37 PM
Be aware that the do unsugars to (Prelude.), not your (), even if you
hide (Prelude.):
import Prelude hiding (())
m f = error Call me!
main = putStrLn . show $ do [3,4]
[5]
The desugaring of the do { [3,4]; [5] } is (Prelude.) [3,4] [5] = [5,5],
whereas you might have hoped for [3,4] [5] = error Call me!
Dan
Ross Mellgren wrote:
I think
import Prelude hiding (())
does that.
-Ross
On Apr 22, 2009, at 11:44 AM, michael rice wrote:
I've been working through this example from:
http://en.wikibooks.org/wiki/Haskell/Understanding_monads
I understand what they're doing all the way up to the definition of
(), which duplicates Prelude function (). To continue following the
example, I need to know how to override the Prelude () with the ()
definition in my file rand.hs.
Michael
==
[mich...@localhost ~]$ cat rand.hs
import System.Random
type Seed = Int
randomNext :: Seed - Seed
randomNext rand = if newRand 0 then newRand else newRand + 2147483647
where newRand = 16807 * lo - 2836 * hi
(hi,lo) = rand `divMod` 127773
toDieRoll :: Seed - Int
toDieRoll seed = (seed `mod` 6) + 1
rollDie :: Seed - (Int, Seed)
rollDie seed = ((seed `mod` 6) + 1, randomNext seed)
sumTwoDice :: Seed - (Int, Seed)
sumTwoDice seed0 =
let (die1, seed1) = rollDie seed0
(die2, seed2) = rollDie seed1
in (die1 + die2, seed2)
() m n = \seed0 -
let (result1, seed1) = m seed0
(result2, seed2) = n seed1
in (result2, seed2)
[mich...@localhost ~]$
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Re: [Haskell-cafe] Optimizing unamb by determining the state of a thunk?
On Mon, Apr 20, 2009 at 2:54 PM, Peter Verswyvelen bugf...@gmail.comwrote: I find this very confusing. Is the documentation of seq wrong (should be weak head normal form)? Yes. Weak head normal form is really the only *essential* one. The popular runtimes do not know how to reduce under a lambda, so they can't reduce something to hnf. Anyway, so I guess we would actually need a function: iswhnf :: a - IO Bool But since the value of this iswhnf function depends on when it is called, I feel it has to be in the IO monad; actually multiple threads evaluating it have nothing to do with it? This is an impure function for a few reasons. I.e. not only does it give different answers at different times (depending on evaluation order), but it is not pure in the domain theory; i.e. (\x. x) 42 = 42, but iswhnf gives different answers for these. So yes, definitely in IO, as a runtime extension (I wouldn't even expect this function to be implementable on all runtimes). Luke ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] GADT on the wiki: I'm lost
I was reading the explanation of GADTs on the wikihttp://en.wikibooks.org/wiki/Haskell/GADT , and but can't make any sense of the examples. Sure I understand what a GADT is, but I'm looking for practical examples, and the ones on the wiki seem to show what you *cannot* do with them... For example, the article gives an alternative approach to the safeHead function (see code below) But now that does not work either, since Cons x y never evaluates to MarkedList x Safe, so safeHead (Cons 1 Nil) will give a type error... Am I missing something or is this wikibook just confusing? Does anybody have good links to examples of GADTs? Yampa surely seems a good example, but it's a bit too advanced. data NotSafe data Safe data MarkedList :: * - * - * where Nil :: MarkedList t NotSafe Cons :: t - MarkedList t y - MarkedList t z safeHead:: MarkedList x Safe - x safeHead (Cons x _) = x silly 0 = Nil silly 1 = Cons () Nil silly n = Cons () $ silly (n-1) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GADT on the wiki: I'm lost
On Wed, Apr 22, 2009 at 3:30 PM, Peter Verswyvelen bugf...@gmail.comwrote: I was reading the explanation of GADTs on the wikihttp://en.wikibooks.org/wiki/Haskell/GADT , and but can't make any sense of the examples. Sure I understand what a GADT is, but I'm looking for practical examples, and the ones on the wiki seem to show what you *cannot* do with them... For example, the article gives an alternative approach to the safeHead function (see code below) But now that does not work either, since Cons x y never evaluates to MarkedList x Safe, so safeHead (Cons 1 Nil) will give a type error... Cons :: t - MarkedList t y - MarkedList t z Note the different variables y and z. Cons 42 y has type MarkedList Int a, for any type a, including Safe, NotSafe, and ElephantBanana. Am I missing something or is this wikibook just confusing? Does anybody have good links to examples of GADTs? Yampa surely seems a good example, but it's a bit too advanced. data NotSafe data Safe data MarkedList :: * - * - * where Nil :: MarkedList t NotSafe Cons :: t - MarkedList t y - MarkedList t z safeHead:: MarkedList x Safe - x safeHead (Cons x _) = x silly 0 = Nil silly 1 = Cons () Nil silly n = Cons () $ silly (n-1) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Trouble with type signatures and type families
Gleb Alexeyev wrote: You may want to read the comments at http://hackage.haskell.org/trac/ghc/ticket/1897. Wow that's subtle... Thanks a lot! -- Früher hieß es ja: Ich denke, also bin ich. Heute weiß man: Es geht auch so. - Dieter Nuhr ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] WHNF versus HNF (was: Optimizing unamb by determining the state of a thunk?)
I'm having difficulty to understand the difference between WHNF and HNF. Is this explanationhttp://encyclopedia2.thefreedictionary.com/Weak+Head+Normal+Form the correct one? Or is WHNF and HNF equivalent in Haskell land? The GHC documentation of seq says: Evaluates its first argument to head normal form, and then returns its second argument as the result. Let's try in GHCi *Main let f = trace \\x $ \x - ((trace \\y $ \y - trace y y + trace x x) $ trace 2 2) *Main f `seq` () \x () *Main That did not evaluate anything inside the body of the first lambda, so according to the article, seq reduces to weak head normal form, not hnf... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GADT on the wiki: I'm lost
On Wed, Apr 22, 2009 at 2:30 PM, Peter Verswyvelen bugf...@gmail.com wrote: I was reading the explanation of GADTs on the wiki , and but can't make any sense of the examples. Sure I understand what a GADT is, but I'm looking for practical examples, and the ones on the wiki seem to show what you *cannot* do with them... For example, the article gives an alternative approach to the safeHead function (see code below) But now that does not work either, since Cons x y never evaluates to MarkedList x Safe, so safeHead (Cons 1 Nil) will give a type error... Am I missing something or is this wikibook just confusing? Does anybody have good links to examples of GADTs? Thrists, or type directed lists. You can find them in the darcs source code (darcs doesn't use Gabor's terminology): http://darcs.net/src/Darcs/Ordered.hs And an explanation of how darcs uses them: http://blog.codersbase.com/2009/03/25/type-correct-changes-a-safe-approach-to-version-control-implementation/ And Gabor's explanation of how to use Thrists them: http://www.opendylan.org/~gabor/Thrist-draft-2007-07-16.pdf If you flip to the appendices of my thesis I do have a brief intro to GADTs as well as a listing of some operations we defined for our GADT based lists, and the slides from my defense talk have an example or two. You can get pdfs of both from the blog link above. If you're already familiar with type classes, you'll want to try to figure out how GADTs are similar and different to type classes. For example, a type class is an open set of types that share an interface (the functions in the type class), but once it is defined that interface is (mostly) closed. You can only add things on top of the interface instead of extending it directly. On the other hand, each data constructor of the GADT behaves similarly to a type in a type class. Except, in the GADT case, the set of types (really data constructors) is closed and the interface they share (functions on your GADT) is open. Another interesting aspect of GADTs is the way in which they allow you to combine existentially quantified types with phantom types along side our usual type variables. Keeping in mind the similarity between type classes, you might notice that specifying the relationship between type variables in a GADT data constructor is similar to using multiparameter type classes and functional dependencies. If you're already familiar with the type hackery that people accomplish with type classes then you should also recognize the power of GADTs. Some benefits of GADTs over type class hackery include, GADT type checking is decidable and at run-time GHC doesn't have to pass dictionaries AFAIK like it does with type classes. I hope that helps, Jason ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GADT on the wiki: I'm lost
Yeah that's what I also thought. I tried it first in GHCi, and I got a type error. I tried it again, and it works now. I must have forgotten to reload the file or something, cause I made the wrong conclusion. Duh, mea culpa. On Wed, Apr 22, 2009 at 11:48 PM, Luke Palmer lrpal...@gmail.com wrote: On Wed, Apr 22, 2009 at 3:30 PM, Peter Verswyvelen bugf...@gmail.comwrote: I was reading the explanation of GADTs on the wikihttp://en.wikibooks.org/wiki/Haskell/GADT , and but can't make any sense of the examples. Sure I understand what a GADT is, but I'm looking for practical examples, and the ones on the wiki seem to show what you *cannot* do with them... For example, the article gives an alternative approach to the safeHead function (see code below) But now that does not work either, since Cons x y never evaluates to MarkedList x Safe, so safeHead (Cons 1 Nil) will give a type error... Cons :: t - MarkedList t y - MarkedList t z Note the different variables y and z. Cons 42 y has type MarkedList Int a, for any type a, including Safe, NotSafe, and ElephantBanana. Am I missing something or is this wikibook just confusing? Does anybody have good links to examples of GADTs? Yampa surely seems a good example, but it's a bit too advanced. data NotSafe data Safe data MarkedList :: * - * - * where Nil :: MarkedList t NotSafe Cons :: t - MarkedList t y - MarkedList t z safeHead:: MarkedList x Safe - x safeHead (Cons x _) = x silly 0 = Nil silly 1 = Cons () Nil silly n = Cons () $ silly (n-1) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Functor and Haskell
Here F is the identity functor, and G is the list functor. And yes, C=D= category of (a subset of) Haskell types. Are you saying the function that goes from list functor to singleton funtor is a natural transformation? But aren't they functors to different subset of Haskell Types? The Haskell Wikibooks also sayshttp://en.wikibooks.org/wiki/Haskell/Category_theory#Functors_on_Haskthe same thing: Functors in Haskell are from *Hask* to *func*, where *func* is the subcategory of *Hask* defined on just that functor's types. E.g. the list functor goes from *Hask* to *Lst*, where *Lst* is the category containing only *list types*, that is, [T] for any type T. The morphisms in *Lst* are functions defined on list types, that is, functions [T] - [U] for types T, U. So in your example there is C that is Hask. But there are two D's, D1 that is all List types, and D2 all singleton types. In this example I guess, the Singleton types are subset of List types which are subset of Hask. Is that related to natural transformation or unrelated? Daryoush On Wed, Apr 22, 2009 at 12:18 AM, Kim-Ee Yeoh a.biurvo...@asuhan.comwrote: Daryoush Mehrtash-2 wrote: I am not sure I follow how the endofunctor gave me the 2nd functor. As I read the transformation there are two catagories C and D and two functors F and G between the same two catagories. My problem is that I only have one functor between the Hask and List catagories. So where does the 2nd functor come into picture that also maps between the same C and D catagories? Consider singleton :: a - [a] singleton x = [x] Here F is the identity functor, and G is the list functor. And yes, C=D= category of (a subset of) Haskell types. -- View this message in context: http://www.nabble.com/Functor-and-Haskell-tp23166441p23170956.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Daryoush Weblog: http://perlustration.blogspot.com/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GADT on the wiki: I'm lost
Am Mittwoch 22 April 2009 23:30:35 schrieb Peter Verswyvelen: I was reading the explanation of GADTs on the wikihttp://en.wikibooks.org/wiki/Haskell/GADT , and but can't make any sense of the examples. Sure I understand what a GADT is, but I'm looking for practical examples, and the ones on the wiki seem to show what you *cannot* do with them... For example, the article gives an alternative approach to the safeHead function (see code below) But now that does not work either, since Cons x y never evaluates to MarkedList x Safe, so safeHead (Cons 1 Nil) will give a type error... No, that works perfectly fine. *SafeHead safeHead $ Cons 1 Nil 1 *SafeHead safeHead $ Cons () Nil () You probably tried something like *SafeHead safeHead $ silly 2 interactive:1:11: Couldn't match expected type `Safe' against inferred type `NotSafe' Expected type: MarkedList () Safe Inferred type: MarkedList () NotSafe In the second argument of `($)', namely `silly 2' In the expression: safeHead $ silly 2 ??? Well, *SafeHead :t silly silly :: (Num t) = t - MarkedList () NotSafe The case silly 0 = Nil makes silly have the return type MarkedList t NotSafe (for some unknown t). The other equations for silly fix t as (), but nothing can transform the NotSafe into Safe. Commenting out the first equation of silly yields *SafeHead :t silly silly :: (Num t) = t - MarkedList () z *SafeHead safeHead $ silly 2 () Am I missing something or is this wikibook just confusing? Does anybody have good links to examples of GADTs? Yampa surely seems a good example, but it's a bit too advanced. data NotSafe data Safe data MarkedList :: * - * - * where Nil :: MarkedList t NotSafe Cons :: t - MarkedList t y - MarkedList t z safeHead:: MarkedList x Safe - x safeHead (Cons x _) = x silly 0 = Nil silly 1 = Cons () Nil silly n = Cons () $ silly (n-1) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] WHNF versus HNF (was: Optimizing unamb by determining the state of a thunk?)
Am Mittwoch 22 April 2009 23:57:08 schrieb Peter Verswyvelen: I'm having difficulty to understand the difference between WHNF and HNF. Is this explanationhttp://encyclopedia2.thefreedictionary.com/Weak+Head+Normal+For m the correct one? Or is WHNF and HNF equivalent in Haskell land? The GHC documentation of seq says: Evaluates its first argument to head normal form, and then returns its second argument as the result. Let's try in GHCi *Main let f = trace \\x $ \x - ((trace \\y $ \y - trace y y + trace x x) $ trace 2 2) *Main f `seq` () \x () *Main That did not evaluate anything inside the body of the first lambda, so according to the article, seq reduces to weak head normal form, not hnf... Yes, error in the docs. seq evaluates to WHNF, not HNF (unless the two coincide). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] WHNF versus HNF (was: Optimizing unamb by determining the state of a thunk?)
On Wednesday 22 April 2009, Peter Verswyvelen wrote: I'm having difficulty to understand the difference between WHNF and HNF. Is this explanationhttp://encyclopedia2.thefreedictionary.com/Weak+Head+Normal+For m the correct one? Or is WHNF and HNF equivalent in Haskell land? The GHC documentation of seq says: Evaluates its first argument to head normal form, and then returns its second argument as the result. Let's try in GHCi *Main let f = trace \\x $ \x - ((trace \\y $ \y - trace y y + trace x x) $ trace 2 2) *Main f `seq` () \x () *Main That did not evaluate anything inside the body of the first lambda, so according to the article, seq reduces to weak head normal form, not hnf... Hi, Try: Prelude Debug.Trace (f 0) `seq` () \x \y y 2 x () -- Thanks! Marcin Kosiba signature.asc Description: This is a digitally signed message part. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] WHNF versus HNF
Peter Verswyvelen wrote: The GHC documentation of seq says: Evaluates its first argument to head normal form, and then returns its second argument as the result. I think this should be weak head normal form. I don't think you have any means to evaluate under a binder in Haskell. The weak in weak head normal forms means that evaluation does not go under lambda binders. The head in weak head normal form means that evaluation does not take place in argument positions. Note that most programming languages evaluate to weak normal forms, because functions are only evaluated when they are called. So the head part is much more important then the weak part when explaining Haskell semantics, leading to sometimes omitting the weak part. Tillmann ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Functor and Haskell
On Wed, Apr 22, 2009 at 03:14:03PM -0700, Daryoush Mehrtash wrote: The Haskell Wikibooks also says the same thing: Functors in Haskell are from Hask to func, where func is the subcategory of Hask defined on just that functor's types. E.g. the list functor goes from Hask to Lst, where Lst is the category containing only list types, that is, [T] for any type T. The morphisms in Lst are functions defined on list types, that is, functions [T] - [U] for types T, U. That's true, but not a particularly helpful view. Any functor F : C - D can be viewed as a functor from C to the full subcategory of D on objects of the form F A for A an object of C. But then different functors map to different categories and you can't talk about natural transformations between them. Composing functors also becomes impossible. The simple view is that [], Maybe and Id are all functors from Hask to Hask. Then listToMaybe :: [a] - Maybe a is a natural transformation from [] to Maybe, because fmap f . listToMaybe = listToMaybe . map f ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On Wed, 2009-04-22 at 13:20 +0200, david48 wrote: On Wed, Apr 22, 2009 at 1:01 PM, Duncan Coutts duncan.cou...@worc.ox.ac.uk wrote: On Wed, 2009-04-22 at 12:21 +0200, david48 wrote: Lines starting with -- are comments. You need to uncomment the prefix line for it to have an effect. Man do I feel dumb now :) Don't :-) you're not the only one who got tripped up by this. It looked a lot like -- as in --command-line-flags=. The new text in 0.6.2 makes it clearer. Duncan ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Is 78 characters still a good option? Was: [Haskell-cafe] breaking too long lines
On 22 Apr 2009, at 8:09 pm, Jason Dusek wrote: Really, the whole thing makes me wish we had blasphemy laws. If any person, in speaking or in writing, shall indicate a preference for column widths other than 80 or indent characters other than spaces (`0x20`) they shall be compelled to present some science or be subject to imprisonment. I did find a paper that claimed that 95-character lines were significantly faster to read than three shorter lengths. On closer study, it turned out that they were using the paging model rather than the scrolling model: once you came to the end of a (short) page of text, you had to hit the Next button to see the next page. What they had in fact proved was that hitting the Next Page button takes time... Now that I have access to a recent Mac laptop, I've found that in reading plain text, I like to make the lines narrower and to approximate continuous scrolling: read one paragraph, stroke the pad to move the next one up, keep on doing it. There are several important differences between program text and ordinary running natural language text. In particular, program text is two-dimensional in a way that ordinary text is not. By the way, in the era of punched cards, while the *cards* were 80 columns, one's *text area* was not. Typically the last 8 columns were used for a sequence number, so that if you dropped your cards -- yes, this happened -- you could sort them back into order, and to provide editing facilities. So people were really programming with 72-column lines. That seemed to work pretty well, and printers seemed to have no trouble reproducing it in books. I do know a psychologist who has done reading studies; I must see if I can talk him into looking into this. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: Is 78 characters still a good option? Was: [Haskell-cafe] breaking too long lines
Some material I've read on typography -- can't find the reference now -- suggests ~65 is the best number of characters per line. The advice was, if your page is larger than that, you should make columns. If someone has done some studies with specifically program text, I'd of course be really interested to know what they discovered; but I really do suspect the issue of line length is more about having a convention than having the best one. Would people who prefer, for example, 132 character lines switch to 66 if that were empirically the best? Probably not. The 80 column convention sets a clear expectation for all of us; it's not a matter of what anyone likes. -- Jason Dusek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Overriding a Prelude function?
OK, I changed the operator from () to (~). When I try to use it I get this:
[mich...@localhost ~]$ ghci rand
GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main ( rand.hs, interpreted )
Ok, modules loaded: Main.
*Main rollDie ~ (rollDie ~ rollDie)
interactive:1:0:
No instance for (Show (Seed - (Int, Seed)))
arising from a use of `print' at interactive:1:0-32
Possible fix:
add an instance declaration for (Show (Seed - (Int, Seed)))
In a stmt of a 'do' expression: print it
*Main
Michael
--- On Wed, 4/22/09, Luke Palmer lrpal...@gmail.com wrote:
From: Luke Palmer lrpal...@gmail.com
Subject: Re: [Haskell-cafe] Overriding a Prelude function?
To: michael rice nowg...@yahoo.com
Cc: Ross Mellgren rmm-hask...@z.odi.ac, Dan Weston
weston...@imageworks.com, haskell-cafe@haskell.org
haskell-cafe@haskell.org
Date: Wednesday, April 22, 2009, 5:02 PM
On Wed, Apr 22, 2009 at 1:47 PM, michael rice nowg...@yahoo.com wrote:
Here's what I get:
[mich...@localhost ~]$ ghci
GHCi, version 6.10.1: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
Prelude import Prelude hiding (())
You know, to avoid this nonsense you could just name the operator something
else, like ~, or ~, or $...@**!. Operators are just names.
Luke
interactive:1:0: parse error on input `import'
Prelude
=
I was passing seed0 to rollDie and getting back (r1,seed1)
passing seed1 to rollDie and getting back (r2,seed2)
passing seed2 to rollDie and getting back (r3,seed3)
Just based on the problem text, I would guess that
passing rollDie and seed0 to () I would get back (r3,seed3),
losing the
intermediate random numbers r1 and r2 along the way, at
least that's what I understood it to say.
So, I know that next I'm probably going to have to do something to
remedy that, but I haven't gotten to that next step yet. What is unsugar?
Thanks in advance for your patience.
Michael
--- On Wed, 4/22/09, Dan Weston weston...@imageworks.com wrote:
From: Dan Weston weston...@imageworks.com
Subject: Re: [Haskell-cafe] Overriding a Prelude function?
To: Ross Mellgren rmm-hask...@z.odi.ac
Cc: michael rice nowg...@yahoo.com, haskell-cafe@haskell.org
haskell-cafe@haskell.org
Date: Wednesday, April 22, 2009, 12:37 PM
Be aware that the do unsugars to (Prelude.), not your (), even if you hide
(Prelude.):
import Prelude hiding (())
m f = error Call me!
main = putStrLn . show $ do [3,4]
[5]
The desugaring of the do { [3,4]; [5] } is (Prelude.) [3,4] [5] = [5,5],
whereas you might have hoped for [3,4] [5] = error Call me!
Dan
Ross Mellgren wrote:
I think
import Prelude hiding (())
does that.
-Ross
On Apr 22, 2009, at 11:44 AM, michael rice wrote:
I've been working through this example from:
http://en.wikibooks.org/wiki/Haskell/Understanding_monads
I understand what they're doing all the way up to the definition of (),
which duplicates Prelude
function (). To continue following the example, I need to know how to
override the Prelude () with the () definition in my file rand.hs.
Michael
==
[mich...@localhost ~]$ cat rand.hs
import System.Random
type Seed = Int
randomNext :: Seed - Seed
randomNext rand = if newRand 0 then newRand else newRand + 2147483647
where newRand = 16807 * lo - 2836 * hi
(hi,lo) = rand `divMod` 127773
toDieRoll :: Seed - Int
toDieRoll seed = (seed `mod` 6) + 1
rollDie :: Seed - (Int, Seed)
rollDie seed = ((seed `mod` 6) + 1, randomNext seed)
sumTwoDice :: Seed - (Int,
Seed)
sumTwoDice seed0 =
let (die1, seed1) = rollDie seed0
(die2, seed2) = rollDie seed1
in (die1 + die2, seed2)
() m n = \seed0 -
let (result1, seed1) = m seed0
(result2, seed2) = n seed1
in (result2, seed2)
[mich...@localhost ~]$
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[Haskell-cafe] Re: Is 78 characters still a good option? Was: breaking too long lines
P.S. We really need such a well written style guide for haskell. Python has this nice PEP (Python Enhancement Proposals). Should we start making our own HEP? We have one: urchin.earth.li/~ian/style/haskell.html Yes, it's good. We should publicise it more. Just a tought: I would like to see a guide talking about the code itself, not about the presentation. Maybe this is ignored because it's difficult. It's easy to get bad code and make sure it follows strict layout, doesn't resemble imperative code and has comments on all functions. It's still bad code. Good practice, I believe, is more like thinking about every side of your code: how can I change this so that the reader will be guided as naturally as possible to understand the way it works? Maybe prose writers or musicians have something to teach us: how to present characters, how to organize ideas, how not to confuse the reader, how to avoid showing something before the reader has the proper background. Also, when to break the rules, remove clever things that do not fit in the whole, pretend to ignore the theory when it confuses instead of helping. The cons: If we do it well, it makes our code to be undervalued. If you take two months to make a complicated thing look simple, all you can claim is that you wrote a simple program :) Best, Maurício ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Re: Overriding a Prelude function?
michael rice nowg...@yahoo.com wrote: OK, I changed the operator from () to (~). When I try to use it I get this: [mich...@localhost ~]$ ghci rand GHCi, version 6.10.1: http://www.haskell.org/ghc/__ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( rand.hs, interpreted ) Ok, modules loaded: Main. *Main rollDie ~ (rollDie ~ rollDie) interactive:1:0: __ No instance for (Show (Seed - (Int, Seed))) __ arising from a use of `print' at interactive:1:0-32 __ Possible fix: __ add an instance declaration for (Show (Seed - (Int, Seed))) __ In a stmt of a 'do' expression: print it Well, you obviously need an initial seed: rollDie 0xdeadbeef ~ (rollDie ~ rollDie) -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] installing Sqlite3 driver for windows
I'm working from Real World Haskell, trying to install HDBC and the Sqlite3 driver on Windows XP. It was easy enough to use Cabal to install HDBC. However, for the Sqlite3 driver things get fuzzy. I downloaded hdbc-sqlite3_2.1.0.0.zip, but don't know what I'm supposed to do with it. Also, it wasn't clear what to do with the sqlite3.dll file. On this page http://software.complete.org/software/wiki/hdbc/FrequentlyAskedQuestions it seems to imply you need to put it in 'gch --print-libdir'\include and %windir%\system32 but that page is really about building the driver from source. Maybe it only needs to go in the system32 directory. Thanks, Mike ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Overriding a Prelude function?
I'm just copying code from this web page: http://en.wikibooks.org/wiki/Haskell/Understanding_monads Using their initial seed I get this: *Main rollDie 362354 ~ (rollDie ~ rollDie) interactive:1:0: Couldn't match expected type `t - (t1, t2)' against inferred type `(Int, Seed)' In the first argument of `(~)', namely `rollDie 362354' In the expression: rollDie 362354 ~ (rollDie ~ rollDie) In the definition of `it': it = rollDie 362354 ~ (rollDie ~ rollDie) *Main === Passing their initial seed through successive calls to rollDie: *Main rollDie 362354 Loading package old-locale-1.0.0.1 ... linking ... done. Loading package old-time-1.0.0.1 ... linking ... done. Loading package random-1.0.0.1 ... linking ... done. (3,1795116384) *Main rollDie 1795116384 (1,523309185) *Main rollDie 523309185 (4,1311937830) == ISN'T THIS SUPPOSED TO BE THE RESULT? *Main Michael --- On Wed, 4/22/09, Achim Schneider bars...@web.de wrote: From: Achim Schneider bars...@web.de Subject: [Haskell-cafe] Re: Overriding a Prelude function? To: haskell-cafe@haskell.org Date: Wednesday, April 22, 2009, 9:52 PM michael rice nowg...@yahoo.com wrote: OK, I changed the operator from () to (~). When I try to use it I get this: [mich...@localhost ~]$ ghci rand GHCi, version 6.10.1: http://www.haskell.org/ghc/__ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( rand.hs, interpreted ) Ok, modules loaded: Main. *Main rollDie ~ (rollDie ~ rollDie) interactive:1:0: __ No instance for (Show (Seed - (Int, Seed))) __ arising from a use of `print' at interactive:1:0-32 __ Possible fix: __ add an instance declaration for (Show (Seed - (Int, Seed))) __ In a stmt of a 'do' expression: print it Well, you obviously need an initial seed: rollDie 0xdeadbeef ~ (rollDie ~ rollDie) -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Where to Cabal Install (was: Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) --first release)
Claus Reinke wrote: Installing executable(s) in /home/david/.cabal/bin why the hell would cabal install binaries in a subdirectory of a hidden directory. Why not /home/david/bin or /home/david/local/bin ? Yes, this is clearly suboptimal but getting agreement on where to put it has not proved easy. There are users that will scream and shout if we install to $HOME/bin by default. Having learned from experience that user preferences differ wildly, even on similar platforms, not to mention a variety of platforms or, even worse, intentionally different forks of the same platform, and that trying to guess what defaults might be sensible, let alone acceptable, can be a losing game, I'd like to offer an alternative view: if there is no universally acceptable default, do not use a default +1. Given that cabal has a config file already and so users don't have to enter everything on the commandline, this seems like a remarkably straightforward solution. The only downside seems like a minor startup cost (in exchange for the major restart cost from defaults having been bad). -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Overriding a Prelude function?
I forgot the type signiture on the last last: (~) :: (Seed - (a,Seed)) - (Seed - (b,Seed)) - (Seed - (b,Seed)) (~) m n = \seed0 - let (result1, seed1) = m seed0 (result2, seed2) = n seed1 in (result2, seed2) With it I get this: *Main rollDie 362354 ~ (rollDie ~ rollDie) interactive:1:0: Couldn't match expected type `Seed - (a, Seed)' against inferred type `(Int, Seed)' In the first argument of `(~)', namely `rollDie 362354' In the expression: rollDie 362354 ~ (rollDie ~ rollDie) In the definition of `it': it = rollDie 362354 ~ (rollDie ~ rollDie) *Main Michael --- On Wed, 4/22/09, Achim Schneider bars...@web.de wrote: From: Achim Schneider bars...@web.de Subject: [Haskell-cafe] Re: Overriding a Prelude function? To: haskell-cafe@haskell.org Date: Wednesday, April 22, 2009, 9:52 PM michael rice nowg...@yahoo.com wrote: OK, I changed the operator from () to (~). When I try to use it I get this: [mich...@localhost ~]$ ghci rand GHCi, version 6.10.1: http://www.haskell.org/ghc/__ :? for help Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( rand.hs, interpreted ) Ok, modules loaded: Main. *Main rollDie ~ (rollDie ~ rollDie) interactive:1:0: __ No instance for (Show (Seed - (Int, Seed))) __ arising from a use of `print' at interactive:1:0-32 __ Possible fix: __ add an instance declaration for (Show (Seed - (Int, Seed))) __ In a stmt of a 'do' expression: print it Well, you obviously need an initial seed: rollDie 0xdeadbeef ~ (rollDie ~ rollDie) -- (c) this sig last receiving data processing entity. Inspect headers for copyright history. All rights reserved. Copying, hiring, renting, performance and/or quoting of this signature prohibited. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
It's irrelevant, because I _do_ have root access to my machine, How nice to be you. Since the argument is entirely about people who _don't_, your point it? It is clear that the only sensible default is no default. Someone else has said it recently and said it much better. I think the right question is how many people prefer user installs over system installs, wrt. their hackage packages?. No, because the costs are asymmetric. Endusers, of course, might have other preferences, but cabal doesn't (IMHO) cater to them, directly: It caters to distribution packages (or windows installers, or whatever), so cabal's default behaviour is quite irrelevant for those cases. The clear impression I've received on this mailing list is that cabal is _also_ for people who are using Haskell and find that there's a new package reported in HWN that they'd like to have. If you are now telling me that I should ignore it because I'm not making distribution packages or Windows installers, fine, just tell me what to use instead. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: Is 78 characters still a good option? Was: breaking too long lines
Maurício wrote: We have one: urchin.earth.li/~ian/style/haskell.html Yes, it's good. We should publicise it more. Just a tought: I would like to see a guide talking about the code itself, not about the presentation. Maybe this is ignored because it's difficult. It's easy to get bad code and make sure it follows strict layout, doesn't resemble imperative code and has comments on all functions. It's still bad code. You've nailed it: this is ignored because it's difficult. It's nothing special about Haskell, it holds for style guides for every language I've seen. It's difficult because it's not a question of science, but rather a question of aesthetics. And as anyone in the humanities can tell you, when it comes to aesthetics people disagree. But the reason they disagree is because of this fundamental truth: there's no right answer. There are many wrong answers, to be sure, but there's no right answer. To put a finer point on it, not everyone likes the same authors. One person's clear discussion is another's pedantry or another's logorrhea. Consider style guides for writing prose. Many of these are full of discussion about where to place commas, or when to use a different word because of the possibility for confusion, or when to reorder the parts of a sentence. What few of them ever discuss are things like how to structure an argument, how to introduce characters, when a character should be removed, what makes for an interesting plot, etc. Even those that discuss such things are typically vague about it and often make appeals to intuitions and aesthetics they presuppose. Good writers and good coders will learn these things over time but they learn them by practice just like any artisan. Good style guides are like a good instructor: they can teach you how to use a chisel, but they can't teach you how to carve. Over time, with enough experience after carving many different things, the good sculptor will come to an intuitive understanding about what things will work and what things are likely to be problematic. They can pass down the lessons, the horror stories, a codification of their hunches, but what they can't teach is the intuition itself. Don't get me wrong, I'd love to see more guides that take on these sorts of issues. This is why they're so rare, is all. My job over the last year has been trying to teach a group of colleagues ---in person--- about what makes good style. In addition to the issues mentioned above, there's another one that looms large. Most people already have a style ---good, bad, indifferent--- and they often perceive any attempts to change that style as (at best) a frivolity or (at worst) an attack on their abilities. Books and webpages can circumvent this to an extent (thanks to the selection bias of who would be reading them anyways), but any guide that is too challenging will be thrown out, no matter how right it is. Thus it takes someone who's a good writer, speaker, or manager and not just someone who's a good coder. Because the author needs to convey not only the intuition, but also enough of themselves that the reader will accept their experience and listen to what they have to say. (And folks who are good with both personal and technical arenas can earn a lot more elsewhere than writing guides, so they'd have to be charitable and generous as well ;) -- Live well, ~wren ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Where to install cabal
Richard O'Keefe wrote: Endusers, of course, might have other preferences, but cabal doesn't (IMHO) cater to them, directly: It caters to distribution packages (or windows installers, or whatever), so cabal's default behaviour is quite irrelevant for those cases. The clear impression I've received on this mailing list is that cabal is _also_ for people who are using Haskell and find that there's a new package reported in HWN that they'd like to have. If you are now telling me that I should ignore it because I'm not making distribution packages or Windows installers, fine, just tell me what to use instead. distribution packages. Edward ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Functor and Haskell
Daryoush Mehrtash-2 wrote: singleton :: a - [a] singleton x = [x] Here F is the identity functor, and G is the list functor. And yes, C=D= category of (a subset of) Haskell types. Are you saying the function that goes from list functor to singleton funtor is a natural transformation? Here *singleton* is the natural transformation from the identity functor to the list functor. Daryoush Mehrtash-2 wrote: But aren't they functors to different subset of Haskell Types? They're usually treated as endofunctors on Hask, for reasons Ross Paterson has given. -- View this message in context: http://www.nabble.com/Functor-and-Haskell-tp23166441p23189784.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On 23 Apr 2009, at 2:09 am, John A. De Goes wrote: That's absurd. You have no way to access private source code, Right. so any decision on what features to exclude from future versions of Haskell must necessarily look at publicly accessible source code. Wrong. There is no necessarily about it. People made decisions about what to deprecate in the Fortran and COBOL standards without looking at publicly accessible source code. The changes made in producing ECMA Eiffel were _definitely_ done without looking at publicly accessible source code. And the decision to remove n+k patterns from Haskell', wrong though I think it was, was NOT made on a nose-counting basis. Or if it was, there is not the slightest evidence of it in http://hackage.haskell.org/trac/haskell-prime/wiki/RemoveNPlusK The reasons for the removal are, in order - no other data type has it (and how many other data types allow [a,b,c] as well as a:b:c:[]?) - a somewhat bogus claim about how much of the library you need to know how to use it (of COURSE you need to know about integers in order to use an integer operation, what's so bad about that?) - the claim that + doesn't mean + (this is really an argument about the scope of + and could have been dealt with by ruling that n+k is only available when the version of + in scope is the one from the Prelude) - an assertion of personal taste (the side condition that the thing is FOR is called 'ugly') - a pious hope that something might replace them - another assertion of personal taste. That's *it*. There is nothing about the operation being *rare* in any source code, publicly available or otherwise. There is no suggestion that commonly used features would not be proposed for removal. For example, . as the composition operator, and ~ lazy patterns, have both been proposed for removal. The only alternative is to continuously add, and never remove, features from Haskell, even if no one (that we know) uses them. Look again at the Haskell' status page, http://hackage.haskell.org/trac/haskell-prime/wiki/Status Of all the proposed removals, n+k is the ONLY one to have been accepted. It may well be true that someone counted how many publicly accessible modules use those features, but I cannot find any evidence in the Haskell' wiki that this is so, or for that matter that people would have cared very much. (Meyer wasn't bothered by the major incompatibilities between ETL3 Eiffel and ECMA Eiffel.) With this one unique solitary exception, so far Haskell' *is* just adding features to Haskell. So if trawling through publicly available sources is somehow supposed to stop that happening, I'm sorry, but it hasn't done so yet. Moreover, the odds that everyone who is using n + k patterns are doing so only in private is an untestable hypothesis (i.e. unscientific) and extremely unlikely to be true. It's also a straw man argument. Nobody says that. I certainly don't. What I DO say is that - MY code contains n+k - MY code is not publicly available (there are many much brighter people than me working on Haskell and whenever I have a cool idea it has so far always been done, and in any case, much of the Haskell code I've written would be of no interest to anyone else) - so the nose counting process would certainly miss ME - there doesn't seem to be any reason to consider me unique We simply DON'T KNOW how many people are using n+k patterns or anything else that people have proposed for removal from Haskell'. One thing we DO know is that existing Haskell textbooks teach the use of n+k patterns. Is there a simple way to download everything from Hackage? As I've said before, trawling through stuff like that _can_ show that something is used, but if it doesn't show that, we do NOT learn that it isn't, but only that we DON'T KNOW. As someone who is only a Haskell user, I don't really have any *rights* in the matter. Using Haskell at all is a privilege, and if the Haskell committee decide, and Haskell implementors agree, to remove a feature that I like, then I just have to live with it. But let's not pretend that this was ever about how many people used the feature. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Re: ANNOUNCE: Utrecht Haskell Compiler (UHC) -- first release
On 23 Apr 2009, at 2:24 am, Achim Schneider wrote: Richard O'Keefe et all wrote: [n+k patterns] I'd like to add my two cents: Assuming that UHC's roadmap strives to be H'-compilant in the future, and n+k patterns aren't going to be in H', why bother implementing them? Haskell' is a moving target. There are as yet no programs that are *known* to be Haskell' programs. There are as yet no Haskell' textbooks. (I remember someone once wrote a book about Prolog that presented Prolog according to the then-current draft of the standard. The next year the draft lurched back towards then-current practice, and the book was left describing a language that was never implemented.) If you want to support existing code written by people trained on the existing textbooks, you support as much of the existing language as you can. Otherwise you have to rely on people writing code specifically for your compiler. Also, assuming that current H98 code will be ported to H', shouldn't n+k patterns be removed from existing code, anyway? Someone who really believed that would surely be recommending that a compiler accept the feature and generate working code for it but WARN at each occurrence with a warning that can't be switched off. That would be a useful way to help people remove this clarity from their code. No, the UHC people can do what they please. It's _their_ compiler. It's _great_ that there's another almost-Haskell98 compiler. It's a little puzzling that section 3 Language extensions and differences with Haskell98 says nothing whatsoever about n+k patterns. It's only in section 4.1, where we learn also that 'default' isn't there and might never be. We don't yet know whether 'default' will be in Haskell'. We also learn from http://www.cs.uu.nl/wiki/bin/view/Ehc/EhcUserDocumentation that IO is under construction, Directory, Time, Locale, CPUTime, and Random are not available. This is clearly work in progress, and we can only be thankful for something that's intended to be read as well as written. In fact, I've downloaded it, precisely in order to see how much work it will be to add n+k support. The thing is that it really seems bizarre to see this one feature singled out for non-implementation. If I can do the equivalent of n+k patterns by programming in the *type system*, why *not* in a pattern? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
