[Haskell-cafe] Richard Bird and the fast nub function
In Richard Bird's Functional Pearls in Algorithm Design there is chapter 10 Removing duplicates which is about a fast and sorting variant of 'nub'. After reading the introduction of the chapter I answered mentally Set.toAscList . Set.fromList - next chapter please. However after the introduction eight pages follow that develop an efficient algorithm for the problem. I suspected there might be the additional difficulty of not using the standard Set type, however on page 70 the common Set API is imported. In the final remarks of the chapter Bird writes: A nagging doubt remains that there might be a much simpler solution to such a simply stated problem. But so far I have not been able to find one. Now I am lost. Can someone tell me, how the task differs from what Set.toAscList . Set.fromList does? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANN: Cabal v1.18.0 released
On Wed, 4 Sep 2013, Johan Tibell wrote: * GHCi support. It's now much easier to use ghci when developing your packages, especially if those packages require preprocessors (e.g. hsc2hs). That's a great feature! How can I configure Cabal to start ghci with certain options? I like to enable more warnings. I could not find a documentation for the Cabal config file. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] sequence causing stack overflow on pretty small lists
On Tue, 27 Aug 2013, John Lato wrote: [1] Most people are physically incapable of reading documents that explain why what they want to do won't work. Even if people did read the documentation, I suspect that the people most in need of the information would be the least likely to understand how it applies to their situation. Plus: I don't expect that programmers read the documentation of 'sequence' and 'mapM' again every time they use the function. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Extending Type Classes
The problem of refinement of type classes annoys me from time to time when I work on the NumericPrelude. It is an experimental type class hierarchy for mathematical types. Sometimes a new data type T shall be implemented and it turns out that you can implement only a part of all methods of a certain class. Then a natural step is to split the class into two classes A and B: 'A' contains the methods we can implement for T and 'B' contains the remaining methods and 'B' is a sub-class of 'A'. First, this means that all client code has to be rewritten. Second, code for instances becomes very lengthy, because over the time code tends to contain one instances for every method. However the many small instances actually carry information: Every instance has its specialised constraints. E.g. you would certainly try to use only Applicative constraints in an Applicative instance and not Monad constraints. However, if there is a way to define Applicative and Monad instances in one go, the Applicative instance may get Monad constraints. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] GHC and backwards compatibility
If you use $ cabal install --constraint=array installed then cabal-install is forced to use the installed version of array. If a package conflicts with this version, then it will report the conflicting packages. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Renumbered mailing list posts
Am 10.08.2013 21:48, schrieb Austin Seipp: Henning, Thanks for the report. I'm currently investigating this, and think it should be possible to keep all of the old URLs intact. Thank you! This would be really really great! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Renumbered mailing list posts
Recently I found that links from Google search results to archive Haskell-Cafe messages are invalid. The messages are still there, but got a different number. E.g. the search result says: http://www.haskell.org/pipermail/haskell-cafe/2011-February/089455.html But the message is at http://www.haskell.org/pipermail/haskell-cafe/2011-February/088146.html Also links from Haskell-Wiki articles to the Haskell-Cafe archive are invalid now. This is very very very bad, since I used tons of such URLs in Wiki articles and it is very hard to find the message I referred to, if the URL does not work anymore. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANN: Monad.Reader Issue 22
On Wed, 7 Aug 2013, Edward Z. Yang wrote: I am pleased to announce that Issue 22 of the Monad Reader is now available. http://themonadreader.files.wordpress.com/2013/08/issue22.pdf Issue 22 consists of the following two articles: * Generalized Algebraic Data Types in Haskell by Anton Dergunov * Error Reporting Parsers: a Monad Transformer Approach by Matt Fenwick and Jay Vyas * Two Monoids for Approximating NP-Complete Problems by Mike Izbicki That is, there are three kinds of Haskellers: The ones who can count and the others who cannot. :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] [ANN] Initial release of the threepenny-gui library, version 0.1.0.0
On Sun, 21 Jul 2013, Sergey Mironov wrote: Hi, I have a Path problem when installing threepenny-gui from Hackage. Probably somtething trivial. I have written a small script cabal-upload that tries to compile a package before uploading it to Hackage. That helps to assert that all required files are registered in the cabal file. http://hackage.haskell.org/package/cabal-scripts ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] meaning of referential transparency
Can someone enlighten me about the origin of the term referential transparency? I can lookup the definition of referential transparency in the functional programming sense in the Haskell Wiki and I can lookup the meaning of reference and transparency in a dictionary, but I don't know why these words were chosen as name for this defined property. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] closed world instances, closed type families
Recently I needed to define a class with a restricted set of instances.
After some failed attempts I looked into the DataKinds extension and in
Giving Haskell a Promotion I found the example of a new kind Nat for
type level peano numbers. However the interesting part of a complete case
analysis on type level peano numbers was only sketched in section 8.4
Closed type families. Thus I tried again and finally found a solution
that works with existing GHC extensions:
data Zero
data Succ n
class Nat n where
switch ::
f Zero -
(forall m. Nat m = f (Succ m)) -
f n
instance Nat Zero where
switch x _ = x
instance Nat n = Nat (Succ n) where
switch _ x = x
That's all. I do not need more methods in Nat, since I can express
everything by the type case analysis provided by switch. I can implement
any method on Nat types using a newtype around the method which
instantiates the f. E.g.
newtype
Append m a n =
Append {runAppend :: Vec n a - Vec m a - Vec (Add n m) a}
type family Add n m :: *
type instance Add Zero m = m
type instance Add (Succ n) m = Succ (Add n m)
append :: Nat n = Vec n a - Vec m a - Vec (Add n m) a
append =
runAppend $
switch
(Append $ \_empty x - x)
(Append $ \x y -
case decons x of
(a,as) - cons a (append as y))
decons :: Vec (Succ n) a - (a, Vec n a)
cons :: a - Vec n a - Vec (Succ n) a
The technique reminds me on GADTless programming. Has it already a name?
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Re: [Haskell-cafe] closed world instances, closed type families
On Tue, 2 Apr 2013, Daniel Peebles wrote: It seems very similar to Ryan Ingram's post a few years back (pre-TypeNats): http://www.haskell.org/pipermail/haskell-cafe/2009-June/062690.html The main difference is that he introduces the knowledge about zero vs. suc as a constraint, and you introduce it as a parameter. In fact, his induction function (which is probably what I'd call it too) is almost identical to your switch. The answer to his post by Miguel Mitrofanov contains a caseNat that is exactly my 'switch'. I see I am four years late. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANN: psqueue-benchmarks - benchmarks of priority queue implementations
On Fri, 29 Mar 2013, Niklas Hambüchen wrote: (This is a slightly detailed email. If you are the maintainer of one of the packages benchmarked here, you might want to read it though.) Could you please put your experiences the Wiki? This would help others to choose a package. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANN: psqueue-benchmarks - benchmarks of priority queue implementations
On Fri, 29 Mar 2013, Louis Wasserman wrote: Bearing in mind that I haven't looked at this in several years... Why did you switch from queuelike to pqueue? Because I liked the API better? Could you put the code up somewhere manageable (repo)? I had it up on darcs, but since that's not there any more, I don't have any more source history than you do. Was it on code.haskell.org? Then it might have been moved to a non-web directory after the last attack 2011. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANN: psqueue-benchmarks - benchmarks of priority queue implementations
On Fri, 29 Mar 2013, Niklas Hambüchen wrote: On 29/03/13 20:14, Henning Thielemann wrote: Was it on code.haskell.org? Then it might have been moved to a non-web directory after the last attack 2011. Does that mean the repo is still there without web access I assume that. or gone? If the original author has not deleted it, then it should still be somewhere: http://www.haskell.org/pipermail/haskell-cafe/2011-February/089352.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANN: monad-bool 0.1
On Tue, 22 Jan 2013, John Wiegley wrote: Use 'onlyIf' with AndM and AndMT to guard later statements, which are only evaluated if every preceding 'onlyIf' evaluates to True. For example: foo :: AndM Int foo = do onlyIf (True == True) return 100 onlyIf (True == True) return 150 onlyIf (True == False) return 200 When run with `evalAndM foo (-1)` (where (-1) provides a default value), 'foo' returns 150. Does the And monad fulfill the monad laws? In a proper monad an interim (return x) (without a '-') is a no-op. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Silent error
On Mon, 21 Jan 2013, mip...@meta.ua wrote: The program looks like this: --*** factorial :: Int-Int factorial n = product [1..n] main = do print $ factorial 50 --*** And that yields 0 (no errors). Is it a bug or feature? :) This question is certainly better posted to haskell-cafe. The answer is: Int has limited bitsize (32 ord 64 bit depending on your machine), thus it computes factorial modulo 2^32 or 2^64. You can get the correct result by replacing Int by Integer. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Importing Repa library
On Thu, 3 Jan 2013, Емануела Моллова wrote: Hello! :) I think you should better post your question to haskell-cafe@haskell.org. Then I put this script import qualified Data.Array.Repa as R :m +Data.Array.Repa Z into the file file.hs, This is GHCi syntax, but it is not a valid Haskell module as indicated by the .hs filename extension. opened WinGHCi and loaded the file, and then evaluated it, but what I get is: Could not find module `Data.Array.Repa' Perhaps you meant Data.Array.Base (from array-0.4.0.0) Data.Array.IO (from array-0.4.0.0) Data.Array.ST (from array-0.4.0.0) Use -v to see a list of the files searched for. Failed, modules loaded: none. I wonder why it loads file.hs at all. I would expect that it gives a syntax error. Also ghc-pkg list repa says: WARNING: cache is out of date: C:/Program Files/Haskell Platform/2012.4.0.0\lib\package.conf.d\package.cache use 'ghc-pkg recache' to fix. C:/Program Files/Haskell Platform/2012.4.0.0\lib\package.conf.d: C:\Users\Faery\AppData\Roaming\ghc\i386mingw32-7.4.2\package.conf.d: Since 'repa' is not listed, it was not installed successfully. This would be consistent with the Could not find module message of GHCi. You may post the output of the 'cabal install repa' run. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mtl-2.1 severly broken, cabal needs blacklisting
On Wed, 14 Nov 2012, Jean-Philippe Bernardy wrote: On Tue, Nov 13, 2012 at 11:39 PM, Andreas Abel andreas.a...@ifi.lmu.de wrote: On 13.11.12 11:13 PM, Jean-Philippe Bernardy wrote: Blacklisting equals releasing a bugfix. Not quite. I propose to *define* blacklisting as such. package-X.Y.Z.W is blacklisted if there exists package-X.Y.Z.V where V W (maybe I'm off by one position in the version number scheme here, but you get the idea) This rule is correct for packages following the Package Versioning Policy. Currently cabal-install and Cabal make no assumptions about the employed versioning scheme. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] mtl-2.1 severly broken, cabal needs blacklisting
On Tue, 13 Nov 2012, Bas van Dijk wrote: On 13 November 2012 17:27, Andreas Abel andreas.a...@ifi.lmu.de wrote: This calls for a means of blacklisting broken or malicious packages. cabal update should also pull a blacklist of packages that will never be selected by cabal install (except maybe by explicit user safety overriding). Maybe we can use the existing preferred-versions file that cabal-install uses: http://hackage.haskell.org/packages/archive/preferred-versions It is also possible to deprecate a package. Is it possible to deprecate a single version? I'm afraid that the user is also not warned if he installs a deprecated package. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANN: data-dword: Long binary words from short ones
On Thu, 11 Oct 2012, Mikhail Vorozhtsov wrote: I'm pleased to announce my new little library, data-dword[1]. It provides Template Haskell utilities for defining binary word data types from low and high halves, e.g. data Word96 = Word96 Word32 Word64 -- strictness is configurable data Int96 = Int96 Int32 Word64 What is the advantage over 'largeword' which does the same with plain Haskell 98? http://hackage.haskell.org/package/largeword ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] total Data.Map.! function
Hi Joachim, On Wed, 5 Oct 2012, Joachim Breitner wrote: On Wed, 3 Oct 2012, Henning Thielemann wrote: I wondered whether there is a brilliant typing technique that makes Data.Map.! a total function. That is, is it possible to give (!) a type, such that m!k expects a proof that the key k is actually present in the dictionary m? How can I provide the proof that k is in m? Same question for 'lab' (import Data.Graph.Inductive(lab)). That is, can there be a totalLab, with (totalLab gr = fromJust . lab gr) that expects a proof that the node Id is actually contained in a graph? I think it is possible to do this using the same trick that ST is using, i.e. rank 2 types. The problem is that this code is likely to be unwieldy to use, as the variable needs to change with every change to the map – but I guess if you create your map once and then use it in various places without modification, this can actually work. Thank you for your detailed answer! I thought about such a tagging method but was missing your idea of updating tags of existing keys in order to reflect the knowledge about newly added keys. Your solution will certainly work for the set of methods you have implemented. All of your methods extend the set of keys or preserve it. But what about deletion? The certificate that key k is contained in a Map must be invalidated by the deletion of k. How could I add this to your approach? Maybe I should track the operations applied to the keys of a map and provide algebraic simplifications, like so: insert :: Ord k = Tagged s k - v - Tagged m (Map k v) - Tagged (Insert s m) (Map k v) lookup :: Ord k = Tagged s k - Tagged (Insert s m) (Map k v) - v -- * example simplifications commuteInsert :: Tagged (Insert s0 (Insert s1 m)) (Map k v) - Tagged (Insert s1 (Insert s0 m)) (Map k v) simplifyInsertInsert :: Tagged (Insert s (Insert s m)) (Map k v) - Tagged (Insert s m) (Map k v) simplifyInsertDelete :: Tagged (Insert s (Delete s m)) (Map k v) - Tagged (Insert s m) (Map k v) simplifyDeleteInsert :: Tagged (Delete s (Insert s m)) (Map k v) - Tagged (Delete s m) (Map k v) example = lookup k0 $ commuteInsert $ insert k1 for $ insert k0 bar m I also thought about something like the exceptions trick. That is, a context like (ContainsKeyA k, ContainsKeyB k) = Map k x might tell that certain keys are in the Map. However, this would not only mean that I need a type class for every insertion, it would also not work reliably for deletion. If keyA = keyB, then deletion of keyA must also remove the ContainsKeyB constraint. :-( ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] total Data.Map.! function
I wondered whether there is a brilliant typing technique that makes Data.Map.! a total function. That is, is it possible to give (!) a type, such that m!k expects a proof that the key k is actually present in the dictionary m? How can I provide the proof that k is in m? Same question for 'lab' (import Data.Graph.Inductive(lab)). That is, can there be a totalLab, with (totalLab gr = fromJust . lab gr) that expects a proof that the node Id is actually contained in a graph? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] A riddle...
On Mon, 16 Jul 2012, Felipe Almeida Lessa wrote: On Mon, Jul 16, 2012 at 12:33 PM, Vo Minh Thu not...@gmail.com wrote: It seems like the infered type (and thus bounds) is different when you force the result to be a Color or not. Just give explicit type signatures and conversion functions. Actually, just *always* give explicit type signatures. Additionally, always compile with '-Wall' and follow the warnings. This way GHC will also warn about missing type signatures and it will even suggest type signatures. (and use haskell-cafe for riddles, please :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Sifflet type checker
On Mon, 9 Jul 2012, gdwe...@iue.edu wrote: data Type = TypeVar TypeVarName -- named type variable | TypeCons TypeConsName [Type] -- constructed type deriving (Eq) Do you still think my type checker would be useful to you, or to Haskellers generally? I see. Then it is probably not very useful for me. :-( [1] http://www.youtube.com/watch?v=sXywCHR9WwE Ah, I enjoyed the performance! Nice to hear that you like it! ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANNOUNCE: Sifflet visual programming language, release 2.0.0.0
On Thu, 5 Jul 2012, gdwe...@iue.edu wrote: Sifflet and sifflet-lib 2.0.0.0, now available on Hackage! This version introduces a type checker and partial support for higher order functions in Sifflet, the visual, functional programming language and support system for students learning about recursion. You have implemented your own type-checker, right? I plan to add a type-checker to our live-sequencer project. [1] So far I have thought about using the Helium type checker but I have not done it so far. If you want to make your type-checker useful for other projects, you may put it into a separate package without the hard to install dependencies on cairo and glib. [1] http://www.youtube.com/watch?v=sXywCHR9WwE ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANNOUNCE: lens-family-th 0.1.0.0
On Sat, 7 Jul 2012, rocon...@theorem.ca wrote:
On Fri, 6 Jul 2012, Dan Burton wrote:
Following the announcement of lens-family, I'm pleased to announce
lens-family-th 0.1.0.0, a Template Haskell library supplying macros to
generate
lens-family lenses for fields of data types declared with record syntax.
Be warned that currently, type signatures are *not* generated alongside the
lens definitions. Type inference should correctly determine the type of the
generated lenses, but I have structured the library code so that in the
future, type signatures can also be generated. Patches welcome!
http://hackage.haskell.org/package/lens-family-th
I cannot help but wonder if it is better to *not* generate type signatures
(or at least have an option not to).
In data-accessor-template we generate type signatures, also because it
avoids warnings for missing type signatures. However it needed some
fine-tuning before it worked in all cases.
At the moment one can write:
import Lens.Family2.Stock
import Lens.Family2.TH
data Foo a = Foo { _bar :: Int, _baz :: a }
deriving (Show, Read, Eq, Ord)
$(mkLenses ''Foo)
-- | My documentation for the 'bar' lens.
bar :: Lens (Foo a) Int
-- | My documentation for the 'baz' lens.
baz :: LensFamily (Foo a) (Foo a') a a'
I don't know if it is possible to add haddock to functions whose type
signatures are generated by template haskell.
Could the documentation be an argument of mkLenses?
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Re: [Haskell-cafe] [Haskell] ANNOUNCE: lens-family-th 0.1.0.0
On Sat, 7 Jul 2012, Dan Burton wrote: Could the documentation be an argument of mkLenses? Does haddock run on the template-haskell expanded code? TH macros must have type Q [Dec]. Dec has no constructor for comments, with the exception of pragmas. This might be feature request worthy, though it is a rather strange case to want to generate comments via a macro. Alternatively, Haddock allows you to document functions in the export list. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] [ANN] GenCheck - a generalized property-based testing framework
On Tue, 19 Jun 2012, Jacques Carette wrote: Its main novel features are: * introduces a number of /testing strategies/ and /strategy combinators/ * introduces a variety of test execution methods * guarantees uniform sampling (at each rank) for the random strategy * guarantees both uniqueness and coverage of all structures for the exhaustive strategy * introduces an /extreme/ strategy for testing unbalanced structures * also introduces a /uniform/ strategy which does uniform sampling along an enumeration * allows different strategies to be mixed; for example one can exhaustively test all binary trees up to a certain size, filled with random integers. * complete separation between properties, generators, testing strategies and test execution methods This sounds very interesting to me since I had a lot of trouble with changed test case distributions when switching from QuickCheck-1 to QuickCheck-2. It was mainly that tested numbers became much bigger in QuickCheck-2 and if I used the numbers as size of lists, then tests could not be run in reasonable time anymore. Thus I think more control over test-case generation is necessary. QuickCheck is Haskell-98 and thus is very portable. I see that GenCheck needs some more extensions - type families, multi-parameter type classes, what else? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] relocation R_X86_64_PC32 against undefined symbol
Just for the record: I compiled with GHC and got the linker error: /usr/bin/ld: dist/build/.../Module.dyn_o: relocation R_X86_64_PC32 against undefined symbol `..._xyz1_closure' can not be used when making a shared object; recompile with -fPIC Problem was that I forgot to declare Module in the Cabal description. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] converting functional dependencies to type families
On Mon, 11 Jun 2012, Simon Peyton-Jones wrote: Thanks. I've linked to it from http://www.haskell.org/haskellwiki/GHC/Type_families#Frequently_asked_questions Thank you! I already added this and another link to a new See also section below. Which one shall we maintain? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] converting functional dependencies to type families
On Thu, 7 Jun 2012, Simon Peyton-Jones wrote: Very useful! Maybe worth turning into a page on the Haskell wiki? I created one: http://www.haskell.org/haskellwiki/Functional_dependencies_vs._type_families ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] HaL7: Haskell-Treffen in Halle/Saale, Freitag 2012-07-13
For our German readers: Es ist wieder so weit: Am Freitag, 13. Juli laden wir zum 7. Haskell-Treffen HaL ein, diesmal nach Halle an der Saale. Wir bieten eine bunte Mischung aus Tutorien zum Mitmachen, Vortraegen zum Anhoeren und Gelegenheiten zum Fachsimpeln, theoretisches wie praktisches, ernstes wie heiteres. Das Programm im Schnelldurchlauf ist: vormittags 6 Tutorien 09:30 - 11:00 snap repa schule 11:30 - 13:00 reactive generics liveseq nachmittags 4 Vortraege 14:30 - 15:15 Dorn: Schule 15:15 - 16:00 Voigtlaender: ADP 16:30 - 17:15 Goergens: XenClient 17:15 - 18:00 Eijkel: Hommage sonstiges: 09:00 - 11:30 Installationshilfen 11:00 - 11:30 Kaffeepause 13:00 - 14:30 Mittagspause 16:00 - 16:30 Kaffeepause 19:00 - Ende Abendessen und Fachsimpelei im Restaurant Die Details finden Sie hier: http://iba-cg.de/hal7.html und die Anmeldung da: http://sim.mathematik.uni-halle.de:8080/hal7/ Bitte melden Sie sich bis zum 4. Juli an. Wir freuen uns auf Ihr zahlreiches Erscheinen! für das Programmkomitee Henning Thielemann ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] converting functional dependencies to type families
Hi all, when I reported a typechecker performance problem related to functional dependencies http://hackage.haskell.org/trac/ghc/ticket/5970 I promised to try to convert from functional dependencies to type families. Thus I converted my code and the llvm package to type-families: http://code.haskell.org/~thielema/llvm-tf/ Here are some of my experiences: == Advantages of TypeFamilies == * Speed For what I did the type families solution was considerably faster than the functional dependencies code at least in GHC-7.4.1. Thus the bug in ticket 5970 does no longer hurt me. (In GHC-6.12.3 the conversion to type families made the compilation even slower.) * Anonymous type function values One of the most annoying type classes of the llvm package was the IsSized class: class (LLVM.IsType a, IsPositive size) = IsSized a size | a - size where size is a type-level decimal natural number. Many llvm functions require that an LLVM type has a size where the particular size is not important. However, I always have to name the size type. I also cannot get rid of it using a subclass, like class (IsSized a size) = IsAnonymouslySized a where The 'size' type is somehow sticky. The conversion of this type class to type families is straightforward: class (IsType a, PositiveT (SizeOf a)) = IsSized a where type SizeOf a :: * Now I have to use SizeOf only if needed. I can also easily define sub-classes like class (IsSized a) = C a where * No TypeSynonymInstances At the right hand side of a 'type instance' I can use type synonyms like type instance F T = String without the TypeSynonymInstance extension. This feels somehow more correct than refering to a type synonym in a class instance head like in instance C T String where The compiler does not need to analyze String in order to find the correct instance. * No FlexibleInstances The same applies to instance C (T a) (A (B a)) which is a flexible instance that is not required for type instance F (T a) = A (B a) * No MultiParamTypeClass, No UndecidableInstances I have some type classes that convert a type to another type and a tuple of types to another tuple of types where the element types are converted accordingly. With functional dependencies: class MakeValueTuple haskellTuple llvmTuple | haskellTuple - llvmTuple where instance (MakeValueTuple ha la, MakeValueTuple hb lb) = MakeValueTuple (ha,hb) (la,lb) The class is a multi-parameter type class and the instance is undecidable. This is much simpler with type families: class MakeValueTuple haskellTuple where type ValueTuple haskellTuple :: * instance (MakeValueTuple ha, MakeValueTuple hb) = MakeValueTuple (ha,hb) where type ValueTuple (ha,hb) = (ValueTuple ha, ValueTuple hb) Thus summarized: Type families may replace several other type extensions. If I ignore the associated type functions then many classes become Haskell 98 with Haskell 98 instances. This is good because those instances prevent instance conflicts with other non-orphan instances. == Disadvantage of TypeFamilies == * Redundant instance arguments I have to write the type arguments both in the instance head and in the function argument. This is especially annoying in the presence of multi-parameter type classes with bidirectional dependencies. E.g. class (a ~ Input parameter b, b ~ Output parameter a) = C parameter a b where type Input parameter b :: * type Output parameter a :: * process :: Causal p (parameter, a) b instance (...) = C (FilterParam a) v (FilterResult v) where type Input (FilterParam a) (FilterResult v) = v type Output (FilterParam a) v = FilterResult v With functional dependencies it was: class C parameter a b | parameter a - b, parameter b - a where process :: Causal p (parameter, a) b instance (...) = C (FilterParam a) v (FilterResult v) where * Bidirectional dependencies In GHC-6.12.3 it was not possible to write class (a ~ Back b, b ~ Forth a) = C a b where Fortunately, this is now allowed in GHC-7. But bidirectional dependencies are still cumbersome to work with as shown in the example above. * Equality constraints are not supported for newtype deriving Not so important, just for completeness: http://hackage.haskell.org/trac/ghc/ticket/6088 == Confusions == * Upper case type function names Why are type function names upper case, not lower case? They are not constructors after all. Maybe this is one reason, why I forget from time to time that type functions are not injective. Sure, lower-case type variables are implicitly forall quantified in Haskell 98. In the presence of lower-case type functions we would need explicit forall quantification. * Why can associated types not be exported by C(AssocType) syntax? Why must they be exported independently from the associated class? * FlexibleContexts The context (Class (TypeFun a)) requires
[Haskell-cafe] HaL-7, 2012-07-13, Call for submissions
Call for submissions and Save the date for our local Haskell Workshop in Halle/Saale, Germany. Tutorials, talks, demonstrations ... everything welcome. Workshop language is German (mainly), and English (by request). Submission deadline: May, 21, Workshop date: July, 13 Workshop homepage: http://iba-cg.de/hal7.html The complete call in German: - Aufruf zum Einreichen von Beiträgen und Hinweis zum Vormerken des Termins Was: Haskell-Treffen HaL-7 Wann: Freitag, 13.07.2012 Wo: Institut für Informatik an der Martin-Luther-Universität in Halle an der Saale Wir suchen Vorträge zu Haskell im Besonderen und der funktionalen Programmierung im Allgemeinen, zum Beispiel zu den Themen * Neues von Sprache, Bibliotheken, Werkzeugen, * Anwendungen von Kunst bis Industrie, * Lehre an Schulen und Hochschulen, gerne aber auch zu anderen Themen. Die Beiträge können präsentiert werden als * Tutorium (60 .. 90 min) * Vortrag (30 min) * Demonstration, künstlerische Aufführung Die Veranstaltungssprache ist Deutsch, in begründeten Ausnahmen Englisch. Presentations will be given in German but we can switch to English if requested. Bitte reichen Sie Kurzfassungen der Beiträge ein (2 bis 4 Seiten), die dem Programmkomitee eine Einschätzung ermöglichen. Die Kurzfassung soll mit einer Zusammenfassung (10 Zeilen) beginnen und einem Literaturverzeichnis enden. Teilnehmer des Workshops sind Interessenten (keine Erfahrung mit Haskell/FP), Anfänger (wenig Erfahrung) und Experten. Wir bitten die Vortragenden, die Zielgruppe des Beitrags anzugeben und die nötigen Vorkenntnisse zu beschreiben. Bei Tutorien sollen Teilnehmer auf eigenen Rechnern arbeiten. Bitte beschreiben Sie dazu die vorher zu installierende Software. Schicken Sie Beitragsvorschläge als PDF-Dokument bis zum 21.05.2012 per Mail an hal-committee at iba-cg punkt de oder an ein Mitglied des Programmkomitees. Programmkomitee * Henning Thielemann - Univ. Halle (Vorsitzender), * Petra Hofstedt - BTU Cottbus, * Alf Richter - iba CG Leipzig, * Uwe Schmidt - FH Wedel, * Janis Voigtländer - Univ. Bonn, * Johannes Waldmann - HTWK Leipzig. Mit besten Grüßen Henning Thielemann ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANNOUNCE: notcpp-0.0.1
On Fri, 13 Apr 2012, Ben Millwood wrote: But I never liked using CPP: it completely defeats haskell-src-exts and hence things like SourceGraph, and anyway it's not designed for Haskell and doesn't at all understand its structure, or fit with its syntax. With a little thought, I wondered if creative use of template haskell might not achieve the same goal. It turned out it did, and emboldened with this knowledge I set out to write a new package making this technique available to others. I never liked CPP in Haskell, too. The problem is: I also do not like Template Haskell. :-) Whereever possible I use a Cabal flag and a conditional Hs-Source-Dirs. http://www.haskell.org/haskellwiki/Cabal/Developer-FAQ#Adapt_to_different_systems_without_CPP ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Higher types in contexts
moved to haskell-cafe On Mon, 5 Mar 2012, Barney Hilken wrote: Is there any deep reason why I can't write a polymorphic type in a context? I think the record update problem can be (sort of) solved if you could write: class Has r Rev (forall a. [a] - [a]) = HRClass r where setHRClass :: (forall a.[a] - [a]) - r - r but polymorphic types are not allowed in contexts. Is this one of the problems SPJ considers Hard or is it a feasible extension? I don't know what you want to do, but you may wrap the (forall a. [a] - [a]) in an existential type: data ListFunc = forall a. ListFunc ([a] - [a]) class Has r Rev ListFunc = HRClass r where setHRClass :: ListFunc - r - r ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Support tools for development with Cabal and Darcs
On Fri, 10 Feb 2012, Duncan Coutts wrote: Would you mind giving me a breif explanation of the compiling multiple local packages in the right order feature? I'd like to understand what is missing in cabal-install in this respect. Currently with cabal-install you can say: cabal install ./a ./b ./c and it will install the packages in those directories in the right order. (It is also possible to list other install targets including local and remote tarballs, and of course packages from hackage). I believe that the first version of cabal-sort is older than this cabal-install feature. Nonetheless, cabal-sort can be used for other purposes, too. E.g. you can use plain Cabal installation, when 'cabal install' does not work due to problems in constraint solving. You can upload packages in topologically sorted order to Hackage, such that building on Hackage works correctly. Is this also done by cabal-install's upload command? You can also use cabal-sort for stepping into package directories and do some manual updates before installing. However, I will add references to the new cabal-install features in the documentation of cabal-sort. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Support tools for development with Cabal and Darcs
On Fri, 10 Feb 2012, Duncan Coutts wrote: I wonder if you have any suggestions for UI changes/improvements for cabal-install for working with related sets of local packages (something it's fairly weak on at the moment). Are there things in cabal-sort you think we should just lift directly into cabal-install or variations etc? Sure, you can add a command 'sort' that outputs the given packages in topologically sorted order. :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Support tools for development with Cabal and Darcs
On Fri, 10 Feb 2012, Henning Thielemann wrote: On Fri, 10 Feb 2012, Duncan Coutts wrote: I wonder if you have any suggestions for UI changes/improvements for cabal-install for working with related sets of local packages (something it's fairly weak on at the moment). Are there things in cabal-sort you think we should just lift directly into cabal-install or variations etc? Sure, you can add a command 'sort' that outputs the given packages in topologically sorted order. :-) I missed something: The reason for the current upload was a new cabal-sort feature that emits the package dependencies as a Makefile in order to allow building packages in parallel using 'make -j'. Of course, a '--jobs' option would also be a nice addition to 'cabal install'. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANNOUNCE multiarg - parser combinators for command line parsing
On Sun, 29 Jan 2012, Simon Meier wrote: I'm currently using Neil Mitchell's cmdargs package [1]. How does your package compare to that? Last time I checked cmdargs it was not referential transparent. Is multiarg better in this respect? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] black Wikipedia (Was: PhD program at Portland State accepting applications)
On Wed, 18 Jan 2012, Nathan Collins wrote: - Portland is a very popular US city, known for beer, bikes, music, and street food: http://en.wikipedia.org/wiki/Portland_Oregon (wikipedia is blacked out today) Maybe it is only a JavaScript trick. In Firefox (with JavaScript) I see the complete a page before it is overwritten by the protest page. In Konqueror of KDE 3 (with and without JavaScript) I can read the Wiki pages without problems. Edit however is really disabled. Sometimes I am glad to have the old technology available. :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] black Wikipedia
On Wed, 18 Jan 2012, Andrew Butterfield wrote: Just add ?banner=none to the url if you really have to read the page Maybe the intention was to demonstrate that censorship (in this case self-censorship) is mostly a problem for average users but not for advanced users. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] compare lengths of lists lazily
Shortest and most obfuscating solution I found is: import Data.Ord (comparing) import Control.Applicative (($)) compareLength :: [a] - [a] - Ordering compareLength = comparing (()$) :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] Dutch National FP Day 2012
On Wed, 16 Nov 2011, Sean Leather wrote: (Sent on behalf of Doaitse Swierstra) Despite some last minute changes to the planning we are happy to announce that the next Dutch functional programming day will take place on January 6, 2012, at the university campus De Uithof of Utrecht University. In case you want to give a presentation please send title, speaker and abstract to doai...@swierstra.net before Dec 1. What is the language of the talks and the participants? English or Dutch? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANN: global-variables-1.0
On Wed, 12 Oct 2011, Jean-Marie Gaillourdet wrote: * Simplify the configuration problem - at least for code in the IO monad. Note, that this library does not encourage sloppy software design by re-introducing all bad effects of global variables. But isn't this kind of solution for the configuration problem an example for sloppy software design? If a global configuration would be passed around, then you could easily use the program functions with different configurations simultaneously. In contrast to that, with the global IORef this is not possible. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] fyi GHC 7.2.1 release on the benchmarks game
On 12.08.2011 18:44, austin seipp wrote: Hello Isaac, On Fri, Aug 12, 2011 at 10:18 AM, Isaac Gouyigo...@yahoo.com wrote: 1) Some of the GHC programs contributed to the benchmarks game have problems with recent GHC releases - meteor-contest #5 - Ambiguous occurrence `permutations' http://shootout.alioth.debian.org/u64q/program.php?test=meteorlang=ghcid=5#log This can be fixed by changing the line: import Data.List to: import Data.List hiding (permutations) ... and enabling the warning -fwarn-missing-import-lists in order to be warned about such imports ... ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ANNOUNCE: yap-0.0 - yet another prelude
On Thu, 11 Aug 2011, Sebastian Fischer wrote: [switched to Cafe] On Wed, Aug 10, 2011 at 11:46 PM, Henning Thielemann lemm...@henning-thielemann.de wrote: On Wed, 10 Aug 2011, Paterson, Ross wrote: Yet another restructuring of the Prelude numeric classes on algebraic lines, proposed for a revision of the Haskell Prelude: http://hackage.haskell.org/package/yap-0.0 A nice lightweight design, both in terms of the use of type extensions and import dependencies, that should people encourage to use it, when they are afraid of changing to a more radical approach like numeric-prelude. I would have prefered the name AdditiveGroup to AbelianGroup, since with '+' and '-' and '0' I associate more than just the laws of an Abelian group. The multiplicative group of rational numbers is abelian, too. I'm curious: what laws do you have in mind for '+', '-', and '0' that do not hold in the multiplicative group of rational numbers with (+) = (*); (-) = (/); 0 = 1 ? I do not associate more laws, but I expect to use zero for things that are somehow related to numbers. And I also do not want to use (+) and (-), say, for rational numbers to mean (*) and (/). That's not an algebraical reason, but a matter of error prevention and expectations. It's the same reason, why I would not want to define an Ord instance for Complex numbers. Complex numbers can be ordered, but a (b::Complex Rational) will be a mistake in many cases, I guess. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] How to use cabal macros in .hsc files
On 08.08.2011 12:08, Bas van Dijk wrote: Hello, Currently it's not possible to use cabal macros like MIN_VERSION_base(x,y,z) in .hsc files: http://hackage.haskell.org/trac/hackage/ticket/870 Is there a workaround to get the same effect? Writing a plain Haskell module that contains the code that depends on the particular base-version? Usually I try to stay away from this preprocessor hackery, and this case seems to support my attempts. What I do is to add Hs-Source-Dirs depending on some Cabal flag, and connect it with a field like Build-Depends: base==3.*. http://www.haskell.org/haskellwiki/Cabal/Developer-FAQ#Adapt_to_different_systems_without_CPP ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Help understanding Haskell runtime costs
On 09.08.2011 01:43, Thiago Negri wrote: Hello all, I'm relatively new to Haskell and trying to solve some online judge's problems in it. One of the problems is to say if a given sentence is a tautogram or not. A tautogram is just a sentence with all the words starting with the same letter. My first try (solution is ok) was to do it as haskeller as possible, trying to overcome my imperative mind. But it did bad at performance (0.30 secs of runtime, 4.6 mb of memory): -- code start import Data.Char (toLower) main = getContents= mapM_ (putStrLn . toStr . isTautogram . words) . takeWhile (/= *) . lines That's still imperative! :-) How about 'interact' and using 'unlines' instead of 'putStrLn' ? toStr :: Bool - [Char] You may want to write String instead of [Char] for clarity. toStr True = Y toStr False = N isTautogram :: [[Char]] - Bool isTautogram (x:[]) = True I assume this case is not necessary, since all [] == True anyway. isTautogram (x:xs) = all ((== firstChar) . toLower . head) xs where firstChar = toLower . head $ x It is maybe more elegant, not to compare all words with the first one, but to compare adjacent words in the list: all (zipWith (...) xs (drop 1 xs)) Note that the only thing that changed between the two tries was the main-loop. The second version runs faster (got 0.11 secs) and with less memory (3.6 mb) Can someone explain to me what is really going on? Maybe pointing out how I can achieve these optimizations using profiling information... Interesting observation. I do not see a problem quickly. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANNOUNCE: GHC version 7.2.1
On 09.08.2011 22:01, Ian Lynagh wrote: The GHC Team is pleased to announce a new major release of GHC, 7.2.1. The 7.2 branch is intended to be more of a technology preview than normal GHC stable branches; in particular, it supports a significantly improved version of DPH, as well as new features such as compiler plugins and safe Haskell. The design of these new features may evolve as we get more experience with them. See the release notes for more details of what's new and what's changed. These sound like a lot of exciting news. The release notes also mention a VECTORISE pragma, that is not mentioned in http://www.haskell.org/ghc/docs/7.2.1/html/users_guide/pragmas.html and not in the index. Is it about the planned GHC support for SSE/AltiVec vector units or is it about DPH? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Github support for cabal files
On 24.07.2011 13:11, Joris Putcuyps wrote: On Sat, 23 Jul 2011 19:25:53 -0700 Greg Weberg...@gregweber.info wrote: [1] http://hackage.haskell.org/package/hamlet The cabal file should be compact, and quite often the description field is very lengthy. I would like it alot if I could reference a README.??? (pandoc compatible extension) from cabal. Then it would be compatible with github and give me more information in my haddock html pages which were generated by `cabal haddock`. To this end I sometimes wrote a dummy module containing more documentation. In the Cabal description file I refer to this module with a quotation like Module. This way, I can even organize the documentation in chapters. I added DEPRECATED pragmas to these modules, in order to prevent people from importing them. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] For class Monoid; better names than mempty mappend might have been: mid (mident) mbinop
On 24.07.2011 22:20, Antoine Latter wrote: On Sat, Jul 23, 2011 at 1:41 PM, KCkc1...@gmail.com wrote: It would be easier for beginners to grok. I think that assumes that all beginners have a strong foundation in algebra. Although it does have the advantage that the names are as abstract as the class. As pointed out earlier in this list, the name of the class and the methods are inconsistent. Monoid refers to a general algebraic structure, whereas mempty and mappend refer to certain instances like lists. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] difference between class context and deriving
On Tue, 2 Aug 2011, Patrick Browne wrote: What is the difference between using a class context and deriving in data type declaration? Are there certain situations in which one or the other is preferred? data Eq a = Set1 a = NilSet1 | ConsSet1 a (Set1 a) Note that these contexts might get removed from the language: http://hackage.haskell.org/trac/haskell-prime/wiki/NoDatatypeContexts ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] A language that runs on the JVM or .NET has the advantage of Oracle Microsoft making those layers more parallelizable.
On Sat, 30 Jul 2011, KC wrote: Are there plans a foot (or under fingers) to make a version of Haskell that runs on the JVM? http://www.haskell.org/haskellwiki/GHC:FAQ#Why_isn.27t_GHC_available_for_.NET_or_on_the_JVM.3F ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Properties for Foldable
On Fri, 29 Jul 2011, Conal Elliott wrote: Is there a collection of laws associated with the Foldable class? Or for Traversable? - Conal Recently I asked the same question: http://www.haskell.org/pipermail/libraries/2011-June/016429.html and got the answer: http://www.cs.ox.ac.uk/jeremy.gibbons/publications/iterator.pdf ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Binding of an integer variable
On Thu, 28 Jul 2011, Paul Reiners wrote: I have a question about the following GHCi interaction: Prelude let x = 23 Prelude :show bindings x :: Integer = _ What is the meaning of the underscore in the third line? Why doesn't it say this, instead? x :: Integer = 23 I have never used ':show bindings' before ... in other cases like let x='a' and let x=23::Int the value of 'x' is shown. If 'x' is bound to a function, the underscore is shown. Thus I assume, that ':show bindings' uses the underscore for types without text representation. But it seems not to use the 'Show' class, since in this case Integers could be shown. The documentation does not mention the output of values, at all: http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/ghci-commands.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Binding of an integer variable
On Thu, 28 Jul 2011, Brandon Allbery wrote: On Thu, Jul 28, 2011 at 14:52, Henning Thielemann lemm...@henning-thielemann.de wrote: I have never used ':show bindings' before ... in other cases like let x='a' and let x=23::Int Since you gave explicit types, x is bound to strict values directly; it's only when no type is given that GHCi needs to translate the constant per the Language Report to an invocation of fromInteger. let x=23::Integer is left unevaluated, too. I do not understand, how fromInteger may change the state of evaluation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Approximate String Matching
On Wed, 27 Jul 2011, dokondr wrote: Hi, I am looking for Haskell libraries to do approximate string matching: http://en.wikipedia.org/wiki/Approximate_string_matching I need this to reduce a set of English word variants with spelling errors to a single canonical dictionary entry. Any libraries to work with spelling will also help. In a quick investigation I found: http://hackage.haskell.org/package/edit-distance ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Stack space overflow in HaskellNet
On Mon, 25 Jul 2011, Manfred Lotz wrote: Hi there, If I take example imap.hs import System.IO import Network.HaskellNet.IMAP import Text.Mime import qualified Data.ByteString.Char8 as BS import Control.Monad -- the next lines were changed to fit to my local imap server imapServer = imap.mail.org user = pass = main = do con - connectIMAP imapServer login con user pass mboxes - list con mapM print mboxes This should be mapM_ and 'ghc -Wall' spots this problem since 6.12. select con INBOX msgs - search con [ALLs] mapM_ (\x - print x) (take 4 msgs) forM_ (take 4msgs) (\x - fetch con x = print) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Idiomatic ways to make all instances of a certain class also instances of another?
On Tue, 26 Jul 2011, Tim Cowlishaw wrote: For instance, for a typeclass representing the interface that any Order type should implement: class Order o where price :: o - Int size :: o - Int I'd like to be able to specify an Eq instance for all types of class Order in a manner similar to this: instance (Order o) = Eq o where o1 == o2 = (price o1 == price o2) (size o1 == size o2) You may define once: orderEq :: Order o = o - o - Bool orderEq o1 o2 = (price o1 == price o2) (size o1 == size o2) and then define instances like instance Order A where ... instance Eq A where (==) = orderEq instance Order B where ... instance Eq B where (==) = orderEq I don't think there is an easier and still predictable way of defining the Eq instances. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Library for Sparse Vectors?
On Tue, 26 Jul 2011, Alexander Solla wrote: Given two (IntMap Double)s a and b, I would compute the projection of a along b as cosineSimilarity :: IntMap Double - IntMap Double - Double cosineSimilarity a b = (dot a b) / ((norm a) * (norm b)) where dot = sum . elems . intersectionWith (*) norm = (**0.5) . sum . fmap (**2) . elems Never write (**2) and (**0.5)! Use (^2) and sqrt! http://www.haskell.org/haskellwiki/Power_function ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Library for Sparse Vectors?
On Wed, 27 Jul 2011, dokondr wrote: In your example: 1) I would use: dot = dot' * dot' dot' = sum . elems . intersectionWith (*) norm = sum . fmap (**2) . elems instead of: dot = sum . elems . intersectionWith (*) norm = (**0.5) . sum . fmap (**2) . elems 2) I don't understand the syntax: cosineSimilarity $ lookup x space * lookup y space What are $ and *? They are from Control.Applicative and are applied to a Maybe type here. It means that cosineSimilarity is applied to the result of looking up 'x' and 'y' in space. If 'x' or 'y' is not in space, then cosineSimilarity $ lookup x space * lookup y space is Nothing. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Stack space overflow in HaskellNet
On Tue, 26 Jul 2011, Manfred Lotz wrote: main = do s con - connectIMAP imapServer login con user pass mboxes - list con mapM print mboxes This should be mapM_ and 'ghc -Wall' spots this problem since 6.12. The compiler (7.04) doesn't tell me anything about it. It seems that it is no longer part of -Wall. But since this mistake is very common, I think it would be better. Problem is, that several libraries like parser libraries are designed for silently throwing away results. You have to switch on -fwarn-unused-do-bind, according to http://www.haskell.org/ghc/docs/7.0-latest/html/users_guide/options-sanity.html Also in case this does not fix your stack space overflow, (mapM_ print) is the correct (space-efficient) way. I'm not quite sure I understand what you mean. Stack overflow comes from this: forM_ msgs (\x - fetch con x = print) If I change it to: mapM_ (\x - fetch con x = print) msgs there is the same stack overflow. forM_ and mapM_ are equal in this respect, the underscore is important. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] NetSNMP; FreeBSD 8.1
On Wed, 20 Jul 2011, Комар Максим wrote: I want to install package NetSNMP: cabal install NetSNMP Resolving dependencies... Configuring NetSNMP-0.1.6... cabal: Missing dependency on a foreign library: * Missing C library: netsnmp This problem can usually be solved by installing the system package that provides this library (you may need the -dev version). If the library is already installed but in a non-standard location then you can use the flags --extra-include-dirs= and --extra-lib-dirs= to specify where it is. cabal: Error: some packages failed to install: NetSNMP-0.1.6 failed during the configure step. The exception was: ExitFailure 1 Package net-mgmt/net-snmp already installed: pkg_info | grep snmp net-snmp-5.5_4 An extendable SNMP implementation As Cabal tells, the developer files are needed, too, and are usually found in packages ending on '-dev' or '-devel', that is, in your case net-snmp-dev. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] haskell zlip read position
On Wed, 20 Jul 2011, rahul wrote: Hi Guys, I am trying to parse a binary stream with the format for one entry [headers, zlib compressed content] , with multiple entries. I can use the Zlib library to get the content for the first entry after the headers, but I cannot find a way to get the offset to start parsing for the second entry. Is there a way I can get this information out of ZLib? or is there a better approach to doing this? Any pointers would be very much appreciated. As far as I know, these are compressors for single files. Multiple files can be compressed in connection with TAR, that can be manipulated from Haskell using the 'tar' package. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] NetSNMP; FreeBSD 8.1
On Wed, 20 Jul 2011, Brandon Allbery wrote: Ports is build-from-source, so every package is what Linux calls a -dev package. The OP might need to make sure that cabal knows to look in /usr/local for libraries. I see. If net-snmp supports pkg-config then all such problems should be solvable by using the pkg-config support of Cabal. So you might run $ pkg-config --list-all on your machine, see whether net-smnp is listed, and if yes, encourage the author of NetSNMP bindings to use the PkgConfig field in his Cabal package description. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsing cabal files to calculate average number of dependencies
On 02.07.2011 01:26, Gwern Branwen wrote: Another thing you can do along the same lines is generate a script to download all the repos from packages which declare repos. Some ugly code: If 'script' also includes Haskell code, then the 'tar' package could be of help to walk throught the TAR archive. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Compiler support for orphan instances
Yesterday I tried to install fix-imports and ran into another example of
problems with changing transitive closures of orphan instance propagation:
$ cabal install fix-imports
Resolving dependencies...
Configuring fix-imports-0.1.2...
Preprocessing executables for fix-imports-0.1.2...
Building fix-imports-0.1.2...
[1 of 6] Compiling Types( src/Types.hs,
dist/build/FixImports/FixImports-tmp/Types.o )
[2 of 6] Compiling Util ( src/Util.hs,
dist/build/FixImports/FixImports-tmp/Util.o )
[3 of 6] Compiling Index( src/Index.hs,
dist/build/FixImports/FixImports-tmp/Index.o )
src/Index.hs:48:15:
No instance for (Monad (Either [Char]))
arising from a use of `sequence' at src/Index.hs:48:15-40
Possible fix:
add an instance declaration for (Monad (Either [Char]))
...
It is a repeating problem that you use an instance without noting it (here
Monad (Either [Char])) and import it without noting it (you import a
module, that possibly by accident imported and thus re-exported that
instance). If the modules you import, decide to no longer use the Monad
Either instance, then your imports will ignore the instance.
I thought that some compiler support could reduce this problem.
Proposal 1
GHC might warn, if you use an orphan instance but import it from a module,
that does not define it.
Example:
module A where
instance Msg s = Monad (Either s) ...
module B where
import A
module C where
{-
B does not define instance Monad Either,
but propagates that instance from A.
-}
import B
test :: Either String Int
test = do
fail bla
return 5
{-
Here GHC should emit a warning,
since the Monad Either instance is imported from B
but not directly from A.
If B drops the 'import A' this will no longer work.
-}
Proposal 2
Sometimes a module is actually intended for re-exporting an instance.
Sometimes an instance is moved from module A to module Z, but A still
re-exports the instance in order to maintain compatibility. In these cases
proposal 1 would yield false alarms. So maybe there should be a pragma for
tagging instances that are intentionally re-exported. Problem: How to
identify instances? This is certainly related to earlier ideas of explicit
named instance imports.
module A where
instance Msg s = Monad (Either s) ...
module B where
import A
{-
Explicitly propagate the instance from A,
which is as good as defining it here.
If the instance is not in scope,
then compilation should fail at this pragma.
This way it is asserted, that the instance is really exported.
-}
{-# INSTANCE-EXPORT Monad (Either s) #-}
module C where
import B
test :: Either String Int
test = do
fail bla
return 5
{-
No warning here, because B intentionally propagated the Monad Either
instance.
-}
Proposal 3
Forget about all those proposals and implement named instances. However
the proposals 1 and 2 let you write code that works on all Haskell 98
compilers. Compilers may just not warn or ignore the INSTANCE-EXPORT
pragma. Proposal 3 requires a language extension and using it will work
only on the compilers that support that extension.
Proposal 4
Forget about flexible, undecidable, overlapping, orphan and other
instances and create a system, where classes are replaced by explicit
method dictionaries and some rules for inserting these dictionaries, where
the rules can be declared by the package author in the modules.
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Re: [Haskell-cafe] ANN: fix-imports-0.1.2
On 12.07.2011 09:59, Evan Laforge wrote: I meant more in the Before: ... After: ... sense ;-) (i.e. visually being able to tell what your program does, rather than just a description). Oh, ok, I guess I can do that too: Before: module M where import ZZ.Out.Of.Order import qualified No.Longer.Needed as Needed -- x = Needed.y -- just removed this y = New.something -- just added this After: module M where import qualified Heavily.Nested.New as New import ZZ.Out.Of.Order -- x = Needed.y -- just removed this y = New.something -- just added this I also spend some time on managing imports. I'll try this program. However, usually I have a mix of qualified and unqualified imports, since some libraries are designed for unqualified imports. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Inconsistent trailing comma in export list and record syntax
On Mon, 11 Jul 2011, Johan Tibell wrote: On Mon, Jul 11, 2011 at 3:54 PM, Henning Thielemann thunderb...@henning-thielemann.de wrote: Johan Tibell wrote: I've found this quite annoying, especially when using CPP to conditionally include something in a list, as it might force you to reorder the list to make the commas appear correctly when the conditional section is enabled/disabled. In this case I use the colon like a terminator. http://www.haskell.org/haskellwiki/List_notation Sorry, I wasn't being clear. I meant for import/export lists. But in import/export lists you can use the trailing comma, and thus you can conditionally export or import identifiers at any position. However, the trailing comma is forbidden in enumerations of constructors. E.g. import M (R, S, T, ) is allowed, but not import M (T(A,B,C,)) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Call for GUI examples - Functional Reactive Programming
On Sun, 10 Jul 2011, Felipe Almeida Lessa wrote: On Sat, Jul 9, 2011 at 7:58 AM, Henning Thielemann lemm...@henning-thielemann.de wrote: My stream processors are not Arrows, because 'first' cannot be implemented. However, 'arr' and '.' can be implemented. ... Since I have no Arrow instance I even have to write my own combinators that are counterparts to () and friends. If you have arr, (.) and (), then you have first as well: f g = (arr fst f) (arr snd g) first' f = f arr id Am I missing something here? =) I have only something like (), but not precisely (). I have for instance parallel :: (Monoid b) = T a b - T a b - T a b The arrow (T a b) emits a value of type 'b' for every incoming event of type 'a', but may also emit a 'b' without any input trigger. Monoid.mappend is used to merge the events if both input arrows generate an output at the same time. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Call for GUI examples - Functional Reactive Programming
On Sun, 10 Jul 2011, Heinrich Apfelmus wrote: Question: how would you actually like to describe the guitar simulator at a high-level? Did you already wish for some specific combinators? Assume that you had something like reactive-banana available and imagine that there were a benevolent djinn granting you three new primitive combinators of your choice. If I would know of appropriate combinators, I would just implement them and not ask the djinn. :-) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Call for GUI examples - Functional Reactive Programming
On Sat, 9 Jul 2011, Heinrich Apfelmus wrote: Oh, I am addressed explicitly, thanks! Yes, GUI for 'streamed' would be nice, too. In the meantime I switched from an approach with lazy lists to one with arrow-like stream processors. This way I could resolve all issues with wrong timing and inappropriate waiting, but now code looks more low-level. There are some examples lying around, that I even not started to implement, because I expect that implementing them in the current low-level way will yield nasty bugs. Thus I am highly interested in more sophisticated MIDI stream editor combinators. I expect that inspiration from other FRP frameworks would help me. Of course, my intention was that you throw away your stream processors and use reactive-banana for everything. ;D I have made sure that it can be used both for real-time and for offline computations. Could you expand a little on your arrow-like stream processors? What do the arrows look like, data SF a b = SF (a - (b, SF a b)) ? My stream processors are not Arrows, because 'first' cannot be implemented. However, 'arr' and '.' can be implemented. As far as I understand, the SF banana can only produce an output if an input event arrives. But my stream processor can set alarms in order to produce a clock signal. This way they can generate output without any input. Currently my stream processors are defined using an explicit state, that is hidden using an existential quantifier, but of course I would prefer a Haskell 98 solution. For example, an arpeggiator stream processor consists of two functions: 1. Receive key up/down events and keep track of the currently pressed keys. 2. Receive a MIDI controller that controls the tempo of the arpeggiator. The stream processor sets the alarm according the current tempo and at every alarm event it sends a single key chosen from the set of the currently pressed keys. This example shows, that a stream processor cannot support Arrow.first, that is extending (arrow a b) to (arrow (a,c) (b,c)), since for an alarm event, we have no input of type c that could be passed through. Currently I have build the two tasks into one stream processor. I would like to split these into two processors. This looks difficult to me, since the first stream processor (for management of pressed keys) must provide a state (the set of pressed keys) whenever the second stream processor (for clock generation) needs it. Which additional primitives did you include, for instance switch :: SF in (out, Maybe t) - (t - SF in out) - SF in out delay :: Time - SF a b - SF a b ? Since I have no Arrow instance I even have to write my own combinators that are counterparts to () and friends. And of course, I am particularly interested in the nasty examples that you came up with. :) For example I want to write a Guitar simulator: You press a set of keys together and the processor converts this into successive tones on a guitar. Technically I like to do it this way: When a key is pressed, collect all key press events in the following 10ms. After this period emit all pressed keys according to a certain pattern. When one of the pressed keys is released, then send key-press(!) events for all currently pressed keys according to another pattern. Repeat this cycle. Manage somehow the keys that are pressed after the first key-down-collecting phase and the keys that are released during this initial phase. Ignore them or do something more sensible about them, but make sure that in the output all key-down events are eventually matched with a key-up event and that for the same key you never send two successive key-down events or two successive key-up events. That is, for the same key, key-up and key-down events must alternate. An exception might be if you receive bogus input. But even then, the number of key-up and key-down events for one note in the output shall match, whenever this is true for the input. For lazy lists you can do time related processing by splitting the list of events and process the prefix differently from the suffix and then concatenate the results. This is easy and declarative, but you have no guarantee that the resulting stream processing is causal and that the timing is correct. (The second problem arises when merging two event lists. Merging implies that when waiting for two events then actually the system waits for the later of the events, but we need to wait for the first of the two events.) ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Call for GUI examples - Functional Reactive Programming
On Fri, 8 Jul 2011, Heinrich Apfelmus wrote: Can GUI programming be liberated from the IO monad? Functional Reactive Programming (FRP) promises as much, and I'm trying to make this dream a reality with my [reactive-banana][] library. Having released version 0.4.0.0, I am now looking for example programs to direct the future evolution of the library. This is the request that I was waiting for. :-) I have some very small wxwidgets example programs that send and receive MIDI messages via ALSA in order to map, e.g. sliders to MIDI controller knobs. I must synchronize the value of a slider with a spin widget and incoming and outgoing MIDI messages and found the way to connect them very low-level. I would like to see this in a more declarative style. http://hackage.haskell.org/package/alsa-gui * I think that Tim Docker's minimal step sequencer [hbeat][] simply makes a great example. I hope that wxHaskell offers a platform-independent way to play sound. I think there are some ways for playing sounds in a portable way, like 'jack' or PortAudio. * Notes of a musical performance can be modeled as event streams (MIDI), as Henning Thielemann has [done with great effect][midi streams]. Surely, reactive-banana should be up to the task, but writing an arpeggiator seems impossible at the moment. Oh, I am addressed explicitly, thanks! Yes, GUI for 'streamed' would be nice, too. In the meantime I switched from an approach with lazy lists to one with arrow-like stream processors. This way I could resolve all issues with wrong timing and inappropriate waiting, but now code looks more low-level. There are some examples lying around, that I even not started to implement, because I expect that implementing them in the current low-level way will yield nasty bugs. Thus I am highly interested in more sophisticated MIDI stream editor combinators. I expect that inspiration from other FRP frameworks would help me. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] function terms
On Wed, 6 Jul 2011, Ian Childs wrote: Suppose I have two terms s and t of type a and b respectively, and I want to write a function that returns s applied to t if a is an arrow type of form b - c, and nothing otherwise. How do i convince the compiler to accept the functional application only in the correct instance? Why can't 's' simply have type 'b - c'? With term do you mean a Haskell expression? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] function terms
On Wed, 6 Jul 2011, Ian Childs wrote: Yes they are Haskell expressions - I called them terms because actually they are GADTs of type Term a and Term b. I can't use type 'b - c' as they are part of a larger pattern. I have a function that returns a witness to 's :: Term a' and 't :: Term b' having the same type, if they do, but I am wondering how to extend this to the first argument of an arrow type. Can you give us a bit more (but not too much) code in order to show the problem? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] function terms
On Wed, 6 Jul 2011, Ian Childs wrote: Term a is meant to be the simply-typed lambda-calculus as a GADT. Then given two terms App (App = l1) r1, and App (App = l2) r2, I want to form App (App = (App l1 l2)) (App r1 r2), but as you can see this will only work if the types of l1 and l2, and r1 and r2, match as detailed previously. does that make sense? What is App? It looks like you apply it once to a string (=) and also to terms(?) like l1, r1. How is the GADT defined? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] ~ type operator
On 24.06.2011 11:10, Daniel Fischer wrote: On Friday 24 June 2011, 10:26:42, Guy wrote: What does the ~ type operator mean? I've sometimes seen types such as (a ~ b) in error messages, but can't understand what GHC is trying to tell me. Type equality, (a ~ b) means that a and b are the same type (rather, that the compiler can prove they're the same). This type operator is needed when using type families. With functional dependencies you can assert a bijection between types by writing class C a b | a - b, b - a where but using type functions you have to tell the compiler somehow that one type function reverses the mapping of another function. This is where (~) is needed. You can use this type operator, when LANGUAGE TypeFamilies is enabled. I assume that the use of (~) in type error messages reflects that GHC's type checker is now internally based on type families. There are also other circumstances, where GHC uses non-Haskell-98 syntax in its messages, e.g. it uses explicit 'forall' quantifiers in error messages also if they are disabled in the affected module. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Period of a sequence
On 26.06.2011 16:16, michael rice wrote: MathWorks has the function seqperiod(x) to return the period of sequence x. Is there an equivalent function in Haskell? seqperiod x = fst $ head $ filter (flip List.isPrefixOf x . snd) $ tail $ zip [0..] $ List.tails x This relies on the fact, that p is a period of x, if isPrefixOf (drop p x) x . ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Printing the empty list.
On 01.07.2011 00:58, Joshua Ball wrote: GHCi seems to be clever about some things: If I try to print the empty list in ghci, I encounter no problems: Prelude [] [] Prelude show [] [] Prelude print [] [] Even though the type of the list is clearly unknown, it must be picking SOME type. (why does it print [] instead of )? If I write a program in a file and load it in main = print [] Then I get the ambiguous type variable error that I would expect. Why doesn't ghci generate this error at the prompt? GHCi will warn you, that type defaulting was used, if you start it with ghci -Wall ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] commutative monoid?
On Fri, 24 Jun 2011, Evan Laforge wrote: So there's a range of possible Monoid instances for each type, and maybe they were chosen by historical happenstance rather than some kind of principle monoid (is there such a thing?). Is there a name for the thing that's like a monoid, but the operator is commutative too? http://en.wikipedia.org/wiki/Commutative_monoid ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] None brute-force BWT algorithm
On Thu, 23 Jun 2011, larry.liuxinyu wrote: I think that version is still a brute-force solution. The only difference is that it uses EOF (sentinel) so that it can sort the suffixes instead of rotations. However, the big-O complexity is the same. Let's take the rbwt for example: rbwt xs = let res = sortBy (\(a:as) (b:bs) - a `compare` b) (zipWith' (:) xs res) in tail . map snd . zip xs $ head res Here we can see that, although the infinity res is lazy-evaluated, it actually sorts the matrix N times, adding one column per evaluation. Did you also compare the actual runtimes? As far as I understand, the matrix in Bertram's code is sorted once. Sharing of data avoids recomputation. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] None brute-force BWT algorithm
On Wed, 22 Jun 2011, larry.liuxinyu wrote: Hi, I read a previous thread about BWT implementation in Haskell: http://www.mail-archive.com/haskell-cafe@haskell.org/msg25609.html and http://sambangu.blogspot.com/2007/01/burrows-wheeler-transform-in-haskell They are all in a `brute-force' way, that is implement based on Burrows-Wheeler's definition like below: I thought that the knot-tying solution by Bertram Felgenhauer in the same thread was both elegant and efficient: http://www.mail-archive.com/haskell-cafe%40haskell.org/msg25692.html ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Simple CSV parser?
On Thu, 16 Jun 2011, Daniel Patterson wrote: Do any of the ones on hackage work for you? http://hackage.haskell.org/package/csv-0.1.2 http://hackage.haskell.org/package/bytestring-csv http://hackage.haskell.org/package/csv-enumerator (note: hackage supports search via google it works reasonably well) How could it miss my lovely package named spreadsheet? It provides a lazy parser. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Simple CSV parser?
On Fri, 17 Jun 2011, Dmitri O.Kondratiev wrote: How to make cabal install all the dependencies? I couldn't find this in the docs at: http://www.haskell.org/haskellwiki/Cabal-Install Usually, 'cabal install' automatically installs all imported packages. But it will certainly not do, if a dependency problem cannot be solved by downloading packages. In your case it may be, that installed packages are compiled with respect to different versions of the same package, say 'transformers'. I found out, that you get more useful cabal messages if you force cabal-install to use a specific version of a package. Say, run $ ghc-pkg list transformers transformers-0.2.2.0 Then call $ cabal install spreadsheet --constraint=transformers==0.2.2.0 What does 'cabal' tell you? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Simple CSV parser?
On Fri, 17 Jun 2011, Dmitri O.Kondratiev wrote: cabal install spreadsheet --constraint=transformers==0.2.2.0 Resolving dependencies... cabal: cannot configure spreadsheet-0.1. It requires transformers ==0.0.* For the dependency on transformers ==0.0.* there are these packages: transformers-0.0.0.0 and transformers-0.0.1.0. However none of them are available. transformers-0.0.0.0 was excluded because of the top level dependency transformers ==0.2.2.0 transformers-0.0.1.0 was excluded because of the top level dependency transformers ==0.2.2.0 I see. I had fixed the problem locally long time ago, but somehow missed upload to Hackage. Try the updated package, please. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Simple CSV parser?
On Thu, 16 Jun 2011, Felipe Almeida Lessa wrote: [1] http://hackage.haskell.org/package/bytestring-csv [2] http://hackage.haskell.org/package/csv-enumerator [3] http://hackage.haskell.org/package/spreadsheet [4] http://hackage.haskell.org/package/csv [5] http://hackage.haskell.org/package/ssv I have updated http://www.haskell.org/haskellwiki/Spreadsheet ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Best platform for development with GHC?
On Wed, 15 Jun 2011, Dmitri O.Kondratiev wrote: Which platform - Mac OS X, Linux or Win32 is best for development with GHC today? How are things with Ubuntu? It was quite a while already that I used Haskell on Linux. Today I have to code on Win32 and Mac OS X. Installing extra libraries caused most of the pains for me on Win32 - for example GTK and Gnuplot is not easy to install on Win32. Installing GTK on Max OSX is also a hard work, but no problems with Gnuplot at all. Haskell mode for Emacs works fine on Mac OS X, but only to some extent on Win32. I have not managed to start GHCi in Emacs buffer on Win32. Since I maintain the gnuplot binding for Haskell - what are the particular problems with that package on Windows? I understand that Linux should be a much better environment for integrating with external libraries (just because most of them were originally developed for Linux). I use GHC and GHCi on Linux/KDE. Most editors and text viewers know about Haskell and provide syntax highlighting. E.g. if I watch a Haskell module in a remote darcs repository with KDE's browser Konqueror, then the text is shown with highlighted syntax. It is very helpful, to recognize outcommented parts of code quickly. If I edit, I see immediately whether comments are closed appropriately. I once used GHCi on Windows, but I didn't like it, because the Windows command line was no fun to use. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] [Haskell] ANNOUNCE: Parallel and Concurrent programming in Haskell, a tutorial
On Thu, 19 May 2011, Simon Marlow wrote: I've put together a tutorial on Parallel and Concurrent programming in Haskell, here: http://community.haskell.org/~simonmar/par-tutorial.pdf The main reason for writing this was that I needed some lecture notes for a course at the CEFP summer school next month, but I hope the material is useful to the Haskell community in general. Thank you a lot for this overview! Concerning the parallelisation of K-means: It seems to be difficult to choose a size for the pieces of points. Since the 'combine' function is associative something map-reduce-like seems to be applicable. That is, if there is a library function for a parallelised 'reduce' and I promise, that I call it with an associative function as argument, then the library could choose an appropriate size for the pieces. Is there already library support for parallelism of this style? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Errors installing 'Haskell Charts'
On Fri, 10 Jun 2011, Dmitri O.Kondratiev wrote: /var/folders/Bo/BobN2Ap5GZW51e9o-n2Ack+++TI/-Tmp-/cairo-0.12.04207/cairo-0.12.0/Gtk2HsSetup.hs:201:69: Couldn't match expected type `PackageDBStack' against inferred type `PackageDB' In the sixth argument of `registerPackage', namely `packageDb' In the expression: registerPackage verbosity installedPkgInfo pkg lbi inplace packageDb In a case alternative: _ | modeGenerateRegFile - die Generate Reg File not supported | modeGenerateRegScript - die Generate Reg Script not supported | otherwise - registerPackage verbosity installedPkgInfo pkg lbi inplace packageDb This looks like an incompatibility between cabal versions. Try switching between Cabal-1.10 and Cabal-1.8. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Matplotlib analog for Haskell?
On Sat, 11 Jun 2011, Dmitri O.Kondratiev wrote: That's what I get (see below). What else should I do to install gnuplot? ~/wks/haskell-wks/gnuplot/gnuplot-0.4.2/srcghc -v Demo.hs Do you really want to compile the Demo (with ghc)? It is already compiled if 'cabal install -fbuildExamples' was successful. Maybe you want to run 'ghci' for interactive tests. But you need to tell 'ghci' the path to the Paths module. 'make ghci-demo' should call GHCi with the right options on Linux: $ ghci -Wall -i:src:dist/build/autogen:execute/tmp src/Demo.hs Did I already advertise the gnuplot mailing list? http://projects.haskell.org/cgi-bin/mailman/listinfo/gnuplot/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Matplotlib analog for Haskell?
On Sat, 11 Jun 2011, Dmitri O.Kondratiev wrote: Yes, you did, thanks. Yet, please allow me to ask a few more questions here, before I subscribe to gnuplot list. I am really short on time - must have some plots for demo next Monday ( 1) Can current version of gnuplot draw histograms? I need these first of all. 2) It seems that $ ghci -Wall -i:src:dist/build/autogen:execute/tmp src/Demo.hs works on Mac OSX, yet I can't make it produce any output directly on screen. How to generate output and display it immediately in an output window? Calling 'main' from the Demo module should open several gnuplot windows showing some examples. If it does not, then this might be a difference of gnuplot between Mac and Linux. For PNG output use the Terminal.PNG module instead of Terminal.X11. In the latest version from darcs repository, there is also histogram support and a demo of it. darcs get http://code.haskell.org/gnuplot/ ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Input and output of mathematical expressions
On Thu, 9 Jun 2011, Jacek Generowicz wrote: On 2011 Jun 9, at 16:59, Chris Smith wrote: Ae you looking to do this in a web application, or client-side? Since one of your requirements is to display a typeset equation, that makes a bit of difference. In a web-based setting, the best way to do that is probably MathML, whereas a GUI will be a bit harder. Yes, this pretty much summarizes my (ignorant) thoughts on it so far (GUI: hard; MathML: relatively easy), which is why I suspect that running it as a local web application might be the simplest way to go. Concerning a local GUI: How about running LaTeX for Tex-PDF, generate a PNG from PDF by ghostscript and show the PNG? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Replacing stdin from within Haskell
On Thu, 9 Jun 2011, Neil Davies wrote: Anyone out there got an elegant solution to being able to fork a haskell thread and replace its 'stdin' ? Why do I want this - well I'm using the pcap library and I want to uncompress data to feed into 'openOffile' (which will take - to designate read from stdin). Yes I can create a separate command and use the stuff in System.Process or uncompress it into a temporary file - but that all seems in-elegant. I think the clean solution is to have a parameter of type Handle, that the threads use for reading, instead of reading from stdin. Or pass the data from stdin in a lazy String. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Comment Syntax
Nicolas Wu schrieb: This whole discussion is reminding me of Wadler's Law of Language Design [1], it's nice to see that in 15 years things haven't changed much! WADLER'S LAW OF LANGUAGE DESIGN In any language design, the total time spent discussing a feature in this list is proportional to two raised to the power of its position. 0. Semantics 1. Syntax 2. Lexical syntax 3. Lexical syntax of comments [1] http://www.informatik.uni-kiel.de/~curry,/listarchive/0017.html It's the first time, that I see a discussion about comment syntax in Haskell-Cafe. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Matplotlib analog for Haskell?
Malcolm Wallace schrieb: I tried gnuplot: Demo.hs:25:18: Could not find module `Paths_gnuplot': Use -v to see a list of the files searched for. Failed, modules loaded: none. Prelude Graphics.Gnuplot.Simple Where to get `Paths_gnuplot': module? $ cd gnuplot-0.4.2 $ cabal install # this generates the Paths_gnuplot module $ ghci Yes, you must install the package before loading Demo into GHCi, since the package contains a data file that the Demo wants to load. Currently I am trying to interface to gnuplot's histogram feature. You may also to subscribe to http://projects.haskell.org/cgi-bin/mailman/listinfo/gnuplot ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Names for pretty-printing combinators
Casey McCann schrieb: One drastic approach I've used in personal libraries--operator-heavy EDSLs specifically--is to define everything first with alphanumeric names, then put operators in their own modules. In some cases I'd have three such modules: One providing a minimal set of operators that don't clash with anything significant, one providing a larger set of operators that clash with one or more common modules (often deliberately, e.g. duplicating Arrow combinators for some type with similar semantics but no valid Arrow instance), and one providing a bunch of gratuitous Unicode operators that look pretty in my code editor but I don't know how to type in GHCi. I use explicit imports in order to avoid infix operator clashes. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] *GROUP HUG*
Adrien Haxaire schrieb: I fully agree. These are two of the three reasons which made me choose haskell as the functional language to learn. Coding fortran all day, I wanted a new approach on programming. The strong scientific roots of haskell would give me stuff to learn and discover for a lot of time. The atmosphere/halo around haskell was intriguing too: come on! it's fun! i can write foldr with foldl! is not the kind of enthusiasm I was used too :) Really, you can write foldr in terms of foldl? So far I was glad I could manage the opposite direction. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Decimal type-level arithmetic.
On Wed, 1 Jun 2011, Serguey Zefirov wrote: I would like to present my version of type arithmetic with decimal encoding: http://thesz.mskhug.ru/svn/hhdl/TyleA.hs How does it compare to http://hackage.haskell.org/package/type-level ? ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
