Re: [Haskell-cafe] Mystery of an Eq instance
On ghc 7.6.3: Prelude 3.16227766016837956 3.1622776601683795 So if you specify a number with greater-than-available precision, it will be truncated. This isn't an issue with (==), but with the necessary precision limitations of Double. On Fri, 20 Sep 2013, damodar kulkarni wrote: Hello, There were some recent discussions on the floating point support in Haskell and some not-so-pleasant surprises people encountered. There is an Eq instance defined for these types! So I tried this: *Main sqrt (10.0) ==3.1622776601683795 True *Main sqrt (10.0) ==3.16227766016837956 True *Main sqrt (10.0) ==3.1622776601683795643 True *Main sqrt (10.0) ==3.16227766016837956435443343 True It seems strange. So my doubts are: 1. I wonder how the Eq instance is defined in case of floating point types in Haskell? 2. Can the Eq instance for floating point types be considered meaningful? If yes, how? In general, programmers are **advised** not to base conditional branching on tests for **equality** of two floating point values. 3. Is this particular behaviour GHC specific? (I am using GHC 6.12.1) If there are references on this please share. Thanks and regards, -Damodar Kulkarni -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Bytestring map/zipWith rationale
Hello all, Something's always bothered me about map and zipWith for ByteString. Why is it map :: (Word8 - Word8) - ByteString - ByteString but zipWith :: (Word8 - Word8 - a) - ByteString - ByteString - [a] ? Obviously they can be transformed into each other with pack/unpack, and as I understand it, the compiler performs sufficient optimizations so that there's no performance hit to doing things like (pack $ zipWith xor a b), but it still seems inconsistent. Is there a deep reason for this? -- Scott Lawrence Linux baidar 3.10.9-1-ARCH #1 SMP PREEMPT Wed Aug 21 13:49:35 CEST 2013 x86_64 GNU/Linux ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Bytestring map/zipWith rationale
On Thu, 12 Sep 2013, Tom Ellis wrote: On Thu, Sep 12, 2013 at 09:21:20AM -0400, Scott Lawrence wrote: Something's always bothered me about map and zipWith for ByteString. Why is it map :: (Word8 - Word8) - ByteString - ByteString but zipWith :: (Word8 - Word8 - a) - ByteString - ByteString - [a] Well, what if you wanted to zipWith a function of type Word8 - Word8 - Foo instead of Word8 - Word8 - Word8? Then I would do what I do with map, and call `unpack` first. Either of the two options is usable: map :: (Word8 - Word8) - ByteString - ByteString zipWith :: (Word8 - Word8 - Word8) - ByteString - ByteString - ByteString (or) map :: (Word8 - a) - ByteString - [a] zipWith :: (Word8 - Word8 - a) - ByteString - ByteString - [a] I just don't understand why we have one from each. -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] remote-build-reporting: cabal phoning home?!
Hello, I think (and a quick reading of source seems to bear this out) that that only happens when you run cabal report. Which isn't quite undocumented - see cabal report --help. On Wed, 1 May 2013, Ertugrul Söylemez wrote: Hello there, could somebody please shed some light on the following line that appeared in my ~/.cabal/config? remote-build-reporting: anonymous The option doesn't seem to be documented anywhere, and I'm very nervous about undocumented remote reporting features. I expect cabal-install to communicate only to fetch the latest package index and to upload and download packages, the former only when i actually use the upload command. If it performs any other communication, please tell me how to disable it. In that case I'd also be interested in an explanation for why this was enabled in the first place. I certainly didn't enable it myself, because it would be a serious security breach in my case. Thank you. Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad. -- Scott Lawrence___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy object deserialization
I tried it, but it still goes and reads the whole list. Looking at the `binary` package source code it seems that strict evaluation is hard-coded in a few places, presumably for performance reasons. It also seems to necessarily read the bytestring sequentially, so complex tree-like data structures would presumably encounter problems even if it worked for a list. Ah well. As long as I'm not duplicating someone else's work, I'm more than happy to go at this from scratch. On Wed, 13 Mar 2013, Jeff Shaw wrote: On 3/13/2013 12:15 AM, Scott Lawrence wrote: Hey all, All the object serialization/deserialization libraries I could find (pretty much just binary and cereal) seem to be strict with respect to the actual data being serialized. In particular, if I've serialized a large [Int] to a file, and I want to get the first element, it seems I have no choice but to deserialize the entire data structure. This is obviously an issue for large data sets. There are obvious workarounds (explicitly fetch elements from the database instead of relying on unsafeInterleaveIO to deal with it all magically), but it seems like it should be possible to build a cereal-like library that allows proper lazy deserialization. Does it exist, and I've just missed it? Thanks, I haven't tested this, but I suspect something like this could give you lazy binary serialization and deserialization. It's not tail recursive, though. newtype LazyBinaryList a = LazyBinaryList [a] instance Binary a = LazyBinaryList a where put (LazyBinaryList []) = putWord8 0 put (LazyBinaryList (x:xs)) = putWord8 1 put x put (LazyBinaryList xs) get = do t - getWord8 case t of 0 - return (LazyBinaryList []) 1 - do x - get (LazyBinaryList xs) - get return $ LazyBinaryList (x:xs) -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Lazy object deserialization
Hey all, All the object serialization/deserialization libraries I could find (pretty much just binary and cereal) seem to be strict with respect to the actual data being serialized. In particular, if I've serialized a large [Int] to a file, and I want to get the first element, it seems I have no choice but to deserialize the entire data structure. This is obviously an issue for large data sets. There are obvious workarounds (explicitly fetch elements from the database instead of relying on unsafeInterleaveIO to deal with it all magically), but it seems like it should be possible to build a cereal-like library that allows proper lazy deserialization. Does it exist, and I've just missed it? Thanks, -- Scott Lawrence Linux baidar 3.7.10-1-ARCH #1 SMP PREEMPT Thu Feb 28 09:50:17 CET 2013 x86_64 GNU/Linux ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Makefile for a Haskell Project
On Fri, 28 Dec 2012, xuan bach wrote: Hi everyone, I'm a newbie in Haskell. I'm wondering that if there is any tool support creating Makefile for Haskell project like Ocamlbuild for Ocaml project? Since ghc handles dependencies automatically, I usually just do, all: ghc --make myprog And then make sure `myprog.hs` is your Main module. Thank you, Regards. -- *Le Dinh Xuan Bach* *Tel: 01234711869 or +65 86967149 * *Email: pig28...@gmail.com School of Information and Communication, * *Hanoi University of Science and Technology - レ。ディン。スアン。バイック 電話番号：01234711869 or +65 86967149 メール： pig28...@gmail.com * -- Scott Lawrence___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Makefile for a Haskell Project
On Fri, 28 Dec 2012, xuan bach wrote: Hi Scott, Thanks for your kind comment. By the way, is it possible to use ghc --make for my project including some external libraries and tools such as happy, Parsec and haskell stub calling C library? I'm not sure about happy and foreign interfaces - as long as parsec is installed (via cabal or your distribution's packages), GHC will use it correctly. I also tried out cabal to build my project but it seems quite complicated to use. It's worth it if you want a more elaborate build process than ghc itself can accomodate, or if you intend to distribute your software on hackage. Running cabal init will guide you through creating a stub cabal file, so it's not too bad. Best Regards. On Fri, Dec 28, 2012 at 2:08 PM, Scott Lawrence byt...@gmail.com wrote: On Fri, 28 Dec 2012, xuan bach wrote: Hi everyone, I'm a newbie in Haskell. I'm wondering that if there is any tool support creating Makefile for Haskell project like Ocamlbuild for Ocaml project? Since ghc handles dependencies automatically, I usually just do, all: ghc --make myprog And then make sure `myprog.hs` is your Main module. Thank you, Regards. -- *Le Dinh Xuan Bach* *Tel: 01234711869 or +65 86967149 * *Email: pig28...@gmail.com School of Information and Communication, * *Hanoi University of Science and Technology --**--**- レ。ディン。スアン。バイック 電話番号：01234711869 or +65 86967149 メール： pig28...@gmail.com * -- Scott Lawrence -- *Le Dinh Xuan Bach* *Tel: 01234711869 or +65 86967149 * *Email: pig28...@gmail.com School of Information and Communication, * *Hanoi University of Science and Technology - レ。ディン。スアン。バイック 電話番号：01234711869 or +65 86967149 メール： pig28...@gmail.com * -- Scott Lawrence___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Can cabal be turned into a package manager?
On Wed, 12 Dec 2012, Janek S. wrote: In the recent months there was a lot of dicussion about cabal, dependency hell and alike. After reading some of these discussions there is a question I just have to ask: Why not create a package manager (like rpm or apt) for Haskell software? I've been using Linux for years. Software for Linux is mostly written in C and C++. There are thousands of libraries with lots of dependencies and yet: a) Linux distributions manage to have package repositories that are kept in a consistent state b) Linux package managers can avoid dependency hell, automatically update to new packages, etc. Linux people did it! Is there any technical issue that prevents Haskell people from doing exactly the same thing? Or are we just having non-technical problems like lack of money or developers? Linux package managers are so good at avoiding dependency hell because they don't have to - they fetch only from repositories that are carefully maintained and tested by humans, in a centralized fashion. The problem of handling dependencies in a purely automated fashion, with no concerted human effort, isn't solved by any of the major linux distros AFAIK. Which isn't to say that I think it can't be solved; just that I don't know of any shining star we can use as an example. (Incidentally, many linux distros package cabal packages with the same centralized-testing methodology under their own package repos, and it avoids dependency hell quite nicely. But I think there ought to be a better solution.) Janek ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] ANN: format-0.1.0.0
Hey all, I've just uploaded to hackage format-0.1, a small library for handling format strings similar to those used by nginx's log_format directive. The idea is to allow easy parsing of text rendered with such a format string, to make parsing simple, automatically generated text files as easy as possible. So for example, if you want to parse lines consisting of two bits of text, separated by -- , then map (scanFormatString $a -- $b) $ lines text will do so, generating associative arrays with the variable names as keys and their content as values. A `renderFormatString` function, doing the obvious, is also provided. Obligatory blog post with some more details: http://bytbox.net/blog/2012/05/format-haskell.html I hope somebody finds this useful. (This seems the sort of thing which should already have existed, but I couldn't find anything. Did I miss it?) I'm using parsec underneath, both for parsing the format string and the generated parser, and I've payed no attention to optimization. Some simple testing indicates that a reasonably fast computer takes about 1 second to parse 5000 lines with nginx's (and apache's) default format string - pretty slow. If there's demand, I'll be happy to pay more attention to that. -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] static linking with ghc?
Adding -optl-pthread fixes it for me. On Sun, 22 Apr 2012, Johannes Waldmann wrote: Hi. I want to produce a statically linked executable. I am trying 'ghc --make -fforce-recomp -static -optl-static Main' but it gives lots of errors like (.text+0xfa): undefined reference to `pthread_mutex_unlock' collect2: ld returned 1 exit status A similar thing is mentioned here (see Caveat) http://www.haskell.org/haskellwiki/Web/Literature/Static_linking The ghc user guide talks a great length about shared libs http://www.haskell.org/ghc/docs/latest/html/users_guide/using-shared-libs.html but I don't see anything on how to switch this off. - J.W. ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] cabal-install package precedence with --extra-lib-dirs
When running cabal install with --extra-lib-dirs=./lib, if a package is found both in ~/.cabal/lib and ./lib, cabal seems to favor the ~/.cabal/lib one. Is there some way to specify the correct precedence to use? -- Scott Lawrence signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Anonymous, Unique Types, maybe
On 12/04/11 02:25, Stephen Tetley wrote: Umm, an obvious point is that if you really are using lists as streams they should appear infinite to the processing code, so you shouldn't encounter operations that fail due to incompatible lengths. Didn't explain myself quite right, I guess. The lists are infinite; however, when a function (which doesn't call `filter`) produces more than one list, the two lists are related in that the nth elements of each list came from the same source. Pairing every nth element is meaningfull for two such lists/streams. In contrast, a list coming out of `filter` isn't related to the list going in in this way, and shouldn't be re-paired with that list (or a direct derivative). My goal, again, is to represent that distinction in the type system. -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Anonymous, Unique Types, maybe
Thanks all; I haven't quite gotten it to work, but I imagine I'll be able to now (after reading up on ExistentialQuantification). -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Anonymous, Unique Types, maybe
(Sorry if this email is rather unclear - I know my desired end result, but neither how to acheive nor explain it well. Here goes.) I'm processing lists, using them sortof as streams. (Whether that's a good idea isn't the issue here - but let me know if it isn't!) Fundamentally, there are two types of operations (at least, that are relevant here) - those that change the length of the list and those that don't. Some operators might take more than one list/stream as an argument, combining them in some way or another. Obviously, if the lists were different lengths, the operator would fail. I don't want that to happen at run time, so I want to check for it statically, presumably via the type system. I could do this manually: type AList = [Event] type BList = [Event] type CList = [Event] myMapish :: AList - AList mySelect :: AList - (Event - Bool) - BList myOtherSelect :: BList - CList but I'd rather not have to manually define a new type for every new list length: myMapish :: List a - List a mySelect :: List a - List ? The '?' would be an anonymous, unique type - unless there's a better way to accomplish this. Hope that was clear, and thanks (as always) for the help (and being awesome). -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Poll: Do you want a mascot?
No -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Parsec: non-greedy 'between'
Hey all,
Trying to match C-style comments, I have:
between (string /*) (string */) $ many anyChar
Which doesn't work, because it is equivalent (ignoring returned values) to
do {string /*; many anyChar; string */}
If the termination criterion was a single character, then I could use
noneOf or (satisfy . not), but that doesn't help here.
So... what am I missing?
Thanks in advance.
--
Scott Lawrence
signature.asc
Description: OpenPGP digital signature
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Parsec: non-greedy 'between'
On 09/11/11 16:45, Alexander Solla wrote: Use manyTill. Ah, but of course. Thanks again! -- Scott Lawrence signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Prelude read 1234 :: String - *** Exception: Prelude.read: no parse
read expects strings to be quoted: Prelude read \1234\ :: String 1234 On Sat, Aug 6, 2011 at 19:57, michael rice nowg...@yahoo.com wrote: Prelude read 1234 :: Int 1234 Prelude read 1234 :: String *** Exception: Prelude.read: no parse Why? Michael ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] GHC7 build problem
It now appears to have been fixed. Thanks. On Sun, Jun 12, 2011 at 01:13, Karel Gardas karel.gar...@centrum.cz wrote: On 06/11/11 09:37 PM, Edward Z. Yang wrote: Yes, the tree was broken for some time between yesterday and today, and you appear to have gotten unlikely. It should have been fixed now, so you should try again. It's probably not fixed yet, since even last night build fails on opensolaris builder: http://darcs.haskell.org/ghcBuilder/builders/kgardas-opensolaris-x86-head/256/7.html Thanks, Karel -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] GHC7 build problem
Trying to compile GHC7 from source (as the ubuntu repository is still on GHC6), I came across the following error in the final phase: libraries/base/GHC/ST.lhs:78:1: You cannot SPECIALISE `forever' because its definition has no INLINE/INLINABLE pragma (or its defining module `Control.Monad' was compiled without -O) This was after: git clone [...] ./sync-all get perl boot ./configure make Is there some problem with trying to build GHC7 on a machine with GHC6, or did I just get the build procedure wrong (the last line of the error looks like a hint that I did). -- Scott Lawrence signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Building Haskell Platform natively for 64bit Windows
On 06/09/2011 01:47 AM, Jason Dagit wrote: Have you checked this by looking at the generated assembly? I generated some assembly from GHC on windows. Here is what it looks ilke: http://hpaste.org/47610 My assembly-fu is not strong enough to tell if it's using 64bit instructions. It would appear to be 32-bit. (pushl instead of pushq no instances of aligning to 8-byte boundaries) signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cons of -XUndecidableInstances
I wrote: type Model a = (Ord a) = Set a -- the set of lexemes - [a] -- the original text to model - [a] -- list of previous lexemes - ProbDist a -- the next lexeme and then entropy :: Model a - Set a - [a] - Double On Mon, Jun 6, 2011 at 03:56, Yitzchak Gale g...@sefer.org wrote: If further down the line you need to write a function that is independent of the model, the types of its arguments will show you what you need to do. Is there a trick that I'm missing? If I want (as a horribly constructed hypothetical example) to write a function later on with type (Model a - Model b) that calls 'entropy' (no idea why), but I want to use the specialized version of 'entropy' for 'Markov' (which requires data that isn't even contained in the first argument, which is really just a function), I don't see any way to do it, without having two entirely different code paths for 'Markov' and other models, starting from the point of decision (user input or some other factor) - an unwieldy solution in case of more than 2 different models (each, presumably, with their own subset of specializations). -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cons of -XUndecidableInstances
On Mon, Jun 6, 2011 at 01:52, Yitzchak Gale g...@sefer.org wrote: Scott Lawrence wrote: You almost never want to use UndecidableInstances when writing practical programs in Haskell. Ah. That's what I wanted to know :P (Although it does seem to me - from looking around docs and the source - that GHC's rules for allowing certain combinations might be a bit too conservative - but then, I have next to no idea what I'm doing, so hey.) When GHC tells you that you need them, it almost always means that your types are poorly designed, usually due to influence from previous experience with OOP. * hides behind book Your best bet is to step back and think again about the problem you are trying to solve. What is the best way to formulate the problem functionally? That will lead you in the right direction. Please feel free to share more details about what you are trying to do. We would be happy to help you work out some good directions. I'm modelling text in a markov-model-like way. I have an actual markov model (albeit one in which X_n depends on a fixed range X_n-1 .. X-n-k). I'm vaguely anticipating the presence of other models: class Model m a | m - a where lexemes :: m - Set a genFunc :: m - [a] - ProbDist a Having that working, I'm trying to estimate the information entropy of a model entropy :: (Model m) = m - Double (This is a slight simplification, since entropy needs a second argument precision to know when to terminate.) Which works well and fine - this function is pretty trivial to implement, on the assumption that Markov (the instance of Model described above) implements genFunc properly. But it happens not to - the array argument to genFunc must be the right size, otherwise an even probability distribution is used. So my OOP-infected mind wants to specialize 'entropy' for Markov: class Entropy d where entropy :: d - Double -- again, simplified Note that it's not (Entropy d a) because the type of the lexeme doesn't matter. Now, the problem code instance (Model m a) = Entropy m where entropy = undefined As you might have picked up, I suspect the part where I want to specialize entropy for Markov is where I mess up - but I'm not sure what to do. (To be clear, I expect to want to specialize entropy for other models too - the general function I have in mind would be horribly slow for many reasonable models.) Thanks. Regards, Yitz -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cons of -XUndecidableInstances
Oops. I can just abandon the Entropy typeclass and put the function directly into Model, eh? Yeah, I think I'll do that... Supposing I didn't want to - any alternatives? Other instances of Entropy I might consider: instance (Eq a) = Entropy [a] instance (Eq a) = Entropy (Tree a) On Mon, Jun 6, 2011 at 02:13, Scott Lawrence byt...@gmail.com wrote: On Mon, Jun 6, 2011 at 01:52, Yitzchak Gale g...@sefer.org wrote: Scott Lawrence wrote: You almost never want to use UndecidableInstances when writing practical programs in Haskell. Ah. That's what I wanted to know :P (Although it does seem to me - from looking around docs and the source - that GHC's rules for allowing certain combinations might be a bit too conservative - but then, I have next to no idea what I'm doing, so hey.) When GHC tells you that you need them, it almost always means that your types are poorly designed, usually due to influence from previous experience with OOP. * hides behind book Your best bet is to step back and think again about the problem you are trying to solve. What is the best way to formulate the problem functionally? That will lead you in the right direction. Please feel free to share more details about what you are trying to do. We would be happy to help you work out some good directions. I'm modelling text in a markov-model-like way. I have an actual markov model (albeit one in which X_n depends on a fixed range X_n-1 .. X-n-k). I'm vaguely anticipating the presence of other models: class Model m a | m - a where lexemes :: m - Set a genFunc :: m - [a] - ProbDist a Having that working, I'm trying to estimate the information entropy of a model entropy :: (Model m) = m - Double (This is a slight simplification, since entropy needs a second argument precision to know when to terminate.) Which works well and fine - this function is pretty trivial to implement, on the assumption that Markov (the instance of Model described above) implements genFunc properly. But it happens not to - the array argument to genFunc must be the right size, otherwise an even probability distribution is used. So my OOP-infected mind wants to specialize 'entropy' for Markov: class Entropy d where entropy :: d - Double -- again, simplified Note that it's not (Entropy d a) because the type of the lexeme doesn't matter. Now, the problem code instance (Model m a) = Entropy m where entropy = undefined As you might have picked up, I suspect the part where I want to specialize entropy for Markov is where I mess up - but I'm not sure what to do. (To be clear, I expect to want to specialize entropy for other models too - the general function I have in mind would be horribly slow for many reasonable models.) Thanks. Regards, Yitz -- Scott Lawrence -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cons of -XUndecidableInstances
On 06/06/2011 02:57 AM, Yitzchak Gale wrote: Generally, we don't start out with a type class. Type classes are great for the special situations in which they are needed (although you can do pretty well without them even then), but first let's get the basic concepts. Perhaps a model is just a function: type Model a = Ord a = Set a - [a] - ProbDist a or something like that. Erm... yeah, actually. But... this prevents me from storing more information in a Model in the future. While I don't really anticipate needing too (I can see this function covering all likely use cases), it does seem sorta restrictive. Having that working, I'm trying to estimate the information entropy of a model entropy :: (Model m) = m - Double Perhaps just a function: entropy :: Model a - Double I still don't know enough details about what you're doing, so my types are probably off. But I hope you get the idea. No, your types are right. If that's not general enough, you may introduce more functions, or some data types. Those give you a huge amount of power - remember that data types can take multiple type parameters (without any GHC extension), they can have functions as their parameters, etc. Or, perhaps you'll even get to the point where you'll need a type class, but that's pretty far down the road, and what you would need it for is very different than what a class is in OOP - they are different concepts. Oh, I understand the difference between a class and a typeclass. It's the difference between an interface and a typeclass that I apparently haven't grasped. Thanks. Hope this helps, Yitz signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Cons of -XUndecidableInstances
On 06/06/2011 03:13 AM, Scott Lawrence wrote:
I still don't know enough details about what you're doing,
so my types are probably off. But I hope you get the idea.
No, your types are right.
Or not.
type Model a = (Ord a) = Set a -- the set of lexemes
- [a] -- the original text to model
- [a] -- list of previous lexemes
- ProbDist a -- the next lexeme
and then
entropy :: Model a - Set a - [a] - Double
or perhaps more simply
entropy :: [a] - ProbDist a - Double
(Let me know if I'm doing something insane again - thanks.)
But this doesn't allow me to specialize for markov models. Seems to me
that to do that, I'd have to store data - and once I'm using a datatype
for markov models:
data Markov a = Markov
{ lexemeSet :: Set a
, matrix:: Map [a] (ProbDist a)
}
Then in order to get a consistent interface to various models, I'm going
to need a typeclass. (Which is required to use a single function name on
multiple datatypes, yes?)
I suppose the alternative is something like
data Model a = Markov {...} | OtherModel
Is that the functional solution? It seems to preclude the possibility of
separating the markov-specialized code and the other specialized code.
signature.asc
Description: OpenPGP digital signature
___
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Cons of -XUndecidableInstances
According to the haskell-prime wiki[1], -XUndecidableInstances removes checks on the form of instance declaration, and just impose a depth limit to ensure termination (of compilation, I assume?). The listed Con is that this removes the clear boundary between legal and illegal programs, and behaviour may be implementation-dependent as the edge of that boundary is reached. How can I tell when I'm nearing that boundary? (And where are the sorts of things GHC does with types documented? I can't seem to find any good explanation of these things.) More specifically, I have class Model m a | m - a where ... class Entropy d where ... instance (Model m a) = Entropy m where ... The first line requires MultiParamTypeClasses and FunctionalDependencies (the two seem to go together) - the third requires UndecidableInstances (since the type variable 'a' appears on the left but not the right). Is this likely to cause a problem? My guess is it shouldn't, since it's equivalent to class Model m a | m - a where ... class Entropy d a where ... instance (Model m a) = Entropy m a where ... without bothering to actually use 'a' in Entropy - but one never knows... (Actually, a third type variable has to be introduced to Entropy to remove the UndecidableInstances dependency - Constraint is no smaller than the instance head. This only increases the illogic in my humble eyes. These examples seem simple enough for GHC to handle nicely...) [1] http://hackage.haskell.org/trac/haskell-prime/wiki/UndecidableInstances -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
[Haskell-cafe] Lazy Evaluation in Monads
I was under the impression that operations performed in monads (in this case, the IO monad) were lazy. (Certainly, every time I make the opposite assumption, my code fails :P .) Which doesn't explain why the following code fails to terminate: iRecurse :: (Num a) = IO a iRecurse = do recurse - iRecurse return 1 main = (putStrLn . show) = iRecurse Any pointers to a good explanation of when the IO monad is lazy? === The long story === I wrote a function unfold with type signature (([a] - a) - [a]), for generating a list in which each element can be calculated from all of the previous elements. unfold :: ([a] - a) - [a] unfold f = unfold1 f [] unfold1 :: ([a] - a) - [a] - [a] unfold1 f l = f l : unfold1 f (f l : l) Now I'm attempting to do the same thing, except where f returns a monad. (My use case is when f randomly selects the next element, i.e. text generation from markov chains.) So I want unfoldM1 :: (Monad m) = ([a] - m a) - [a] - m [a] My instinct, then, would be to do something like: unfoldM1 f l = do next - f l rest - unfoldM1 f (next : l) return (next : rest) But that, like iRecurse above, doesn't work. signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy Evaluation in Monads
On 05/31/2011 04:20 PM, Artyom Kazak wrote: Suppose iRecurse looks like this: iRecurse = do x - launchMissiles r - iRecurse return 1 As x is never needed, launchMissiles will never execute. It obviously is not what is needed. Prelude let launchMissiles = putStrLn UH OH return 1 Prelude let iRecurse = launchMissiles return 1 Prelude iRecurse UH OH 1 Prelude Looks like launchMissiles /does/ execute, even though x is (obviously) never needed. signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy Evaluation in Monads
On 05/31/2011 04:48 PM, Artyom Kazak wrote: Oh, sorry. I was unclear. I have meant assuming IO is lazy, as Yves wrote. Ah, ok. That makes more sense. And saying some hacks I meant unsafeInterleaveIO, which lies beneath the laziness of, for example, getContents. Which explains why assuming getContents is strict has never worked for me. I'm trying to implement unfoldM1 without using unsafeIO, if possible. Since unfoldM1 f l = do next - f l rest - unfoldM1 f (next : l) return (next : rest) obviously won't work, I've been trying to use fmap unfoldM1 :: (Functor m, Monad m) = ([a] - m a) - [a] - m [a] unfoldM1 f l = do next - f l fmap (next :) $ unfoldM1 f (next : l) Evaluation here also doesn't terminate (or, (head $ unfoldM (return . head)) doesn't), although I can't figure out why. fmap shouldn't need to fully evaluate a list to prepend an element, right? signature.asc Description: OpenPGP digital signature ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
Re: [Haskell-cafe] Lazy Evaluation in Monads
Apparently: Prelude let r = (fmap (1:) r) :: IO [Integer] Prelude fmap (take 5) r *** Exception: stack overflow Thanks - I'll just have to stay out of IO for this, then. On Tue, May 31, 2011 at 17:05, Stephen Tetley stephen.tet...@gmail.com wrote: 2011/5/31 Scott Lawrence byt...@gmail.com: Evaluation here also doesn't terminate (or, (head $ unfoldM (return . head)) doesn't), although I can't figure out why. fmap shouldn't need to fully evaluate a list to prepend an element, right? I'm afriad fmap doesn't get to choose - if the monad is strict then both definitions are equivalent (probably...). ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Scott Lawrence ___ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
