- Haskell-Cafe - Haskell.org

implementing a csv reader
by Tamas K Papp 24 Aug '06

24 Aug '06

Hi, Now that I have read the tutorials, I think that the best way to learn Haskell would be to use the language and write something simple yet useful. I noticed that Haskell lacks a module for reading/writing csv (comma separated value) files, so I thought I could implement that. Questions: 1. Please tell me if you know of a csv module, because then I would do something else. 2. I am looking for a parser, but I don't know Haskell parsers. Is Parsec a good choice? Thanks, Tamas

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question on traversing syntax tree
by Xiong Yingfei 24 Aug '06

24 Aug '06

I am writing a compiler using Haskell. After the compiler parses program, the program is stored into an syntax tree stucture defined blew: ...... data Exp = Plus Exp Term | Minus Exp Term | Term Term deriving Show data Term = Times Term Factor | Div Term Factor | Factor Factor deriving Show ...... This is just part of the definition. The full tree contains much more definition than this. Now I want to adjust the syntax-tree. However, I don't need to adjust all the data types, but a small subset of the syntax tree. e.g. I might adjust the "Times" data like the following, but not modify the rest of the syntax tree: transformTerm (Times t f) = Times t (FactorInt 100) However, in order to apply the modification like this, I have to write a series of function to traverse the tree until I get to the Term data type. e.g. I have to define: transformExp (Plus e t) = Plus (transformExp e) (transformTerm t) transformExp (Minus e t) = Minus (transformExp e)(transformTerm t) transformTerm (Term t) = ... This is tedious and error-prone. I want to know if there some means in Haskell to write a single "generic" function to traverse the syntax tree and only stop on the "Term" data type. Can anyone tell me something about it? Thanks a lot. -- Xiong, Yingfei (熊英飞) Ph.D. Student Institute of Software School of Electronics Engineering and Computer Science Peking University Beijing, 100871, PRC. Web: http://xiong.yingfei.googlepages.com

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Cabal question, adding data files
by Neil Mitchell 24 Aug '06

24 Aug '06

Hi, I have a cabal executable, which requires additional data files. How do I do this in Cabal? I have seen extra-source-files, but they are not added at install time to the destination directory, which doesn't help me. I noticed that Alex does this, but with a lot of Cabal calling in Setup.hs with hooks etc, which is not really ideal. Is there any easy way to do the "install these files along with the program, please" ? If anyone needs any concrete examples, take for example HsColour, which needs the files .hscolour and hscolour.css, but currently install doesn't put either of them anywhere useful. Thanks Neil

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Proposal to allow {} instead of () in contexts
by Brian Hulley 24 Aug '06

24 Aug '06

Hi - Disregarding my last proposal which involved the use of {} in types, I am wondering if anyone would agree with me that it would be a good idea to use {} instead of () when writing out the context ie: foo :: (Num a, Bar a) => a -> a would become: foo :: {Num a, Bar a} => a -> a and the same for every other situation in the language where a context appears. My reasons are twofold: 1) The context is supposed to be understood (afaiu) as a *set* of constraints not a *tuple* of constraints (ie what relevance does the ordering implied by the tuple notation have here?), so the using of set braces seems mathematically more appropriate 2) It would allow an editor to give the correct fontification for an incomplete type expression. At the moment, if you'd just typed: foo :: (Bar the editor would have no way of knowing if "Bar" is a classcon or a tycon, whereas with: foo :: {Bar the opening brace immediately informs the editor that the following text should be parsed as a context and so it could fontify Bar appropriately. For backwards compatibility the use of () could still be supported - braces don't occur in H98 in the positions I'm proposing to allow them so nothing would be broken. Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com

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One thought: Num to 0 as ? to list?
by Marc Weber 24 Aug '06

24 Aug '06

Is there somethinig corresponding to Num concering lists? I mean there is + - /.. defined. + - are not type specific (Int, Double) neither is : [1,2] notation to Elements.. But what about different implementatins of lists? (linked lists? hash lists? array with index? In other words: why not overload (:) ? Then it would be possible to use "blah" for fast packed strings, too Then you could also implement implementations for C arrays and access them. (Sure you can do it anyway but not using syntactic sugar) One part of the answer is patterns? f 'a' = .. f _ = .. Marc

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extreme newbie: hugs prompt vs load module
by George Young 23 Aug '06

23 Aug '06

[linux, ghci 6.4.3.20060820, hugs May 2006] I have just started learning Haskell. I have hugs and ghci under linux, and I'm going through the Gentle Introduction to Haskell<http://www.haskell.org/tutorial>, so far through section 4, "case expressions and pattern matching". I'm a python programmer, with background in maclisp, scheme, T, C, C++, and a little J. I'm confused about what sort of things I can type at the interpreter prompt, and what things have to be loaded as a module. I keep trying to treat the prompt like a lisp or python REPL, which is obviously wrong. Can someone set me straight? Is there another tutorial that might be more appropriate for me? I am finding haskell quite appealing. I hope to start writing real (if small) applications to do some data analysis from our Postgres DB. Any hints? --George Young -- "Are the gods not just?" "Oh no, child. What would become of us if they were?" (C.S. Lewis)

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Re: [Haskell-cafe] extreme newbie: hugs prompt vs load module
by Hans van Thiel 23 Aug '06

23 Aug '06

Hi, I'm a beginner too and I'm quite taken with 'Yet Another Haskell Tutorial' by Hal Daume III, which is available from the Haskell site. It really starts from scratch and it covers almost everything, as far as I can see. Because I'm a lone learner, I got stuck several times, but there are several other short tutorials for beginners, with a slightly different slant, that helped. For monads, which I'm starting on now, I recommend 'All about Monads' by (Jeff?) Newbern. http://www.nomaware.com/monads/html/ But YAHT starts with a chapter on Hugs, GHC and how to use them. At the moment I'm reading the book again, from the beginning, and I'm seeing several things much better than I did the first time(s). Hope this helps, Hans van Thiel -- Don't worry, be happy, learn Haskell.

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Memoizing longest-common-subsequence
by Mark.Carroll＠Aetion.com 23 Aug '06

23 Aug '06

I wanted a longest common subsequence function and a bit of Googling failed to turn up a functional one, except for in a scary bit of darcs. So, I tried making a memoized functional version of the LCS delta algorithm on the problem's Wikipedia page. It's not the fastest, but it's simple and should be polynomial, growing with the product of input sequence lengths. I've not played with unboxing or strictness here yet, but with the help of Data.Array I have: longestCommonSubsequence xs ys = let lastIndex = (length xs, length ys) memo = array ((0, 0), lastIndex) [ ((xi, yi), deltaLCS (xi, x) (yi, y)) | (xi, x) <- zip [0..] (undefined : xs), (yi, y) <- zip [0..] (undefined : ys) ] deltaLCS (0, _) _ = (0, []) deltaLCS _ (0, _) = (0, []) deltaLCS (xl, x) (yl, y) = if (x == y) then let (xylShrunk, xysShrunk) = memo ! (xl-1, yl-1) in (xylShrunk + 1, x : xysShrunk) else let xaShrunk@(xlShrunk, _) = memo ! (xl-1, yl) yaShrunk@(ylShrunk, _) = memo ! (xl, yl-1) in if xlShrunk > ylShrunk then xaShrunk else yaShrunk in reverse (snd (memo ! lastIndex)) I haven't looked much at optimizing this further so there are probably yet some easy wins to be had - I'd be interested to see what they are. Still, I thought it worth sharing as an example of laziness making memoization easy in a simple functional longest common subsequence example. Take this as your cue to point out the much better LCS algorithm that already exists in the standard libraries, that I couldn't find. (-: -- Mark

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debugging space leaks in references
by Andrew Pimlott 22 Aug '06

22 Aug '06

GHC's great space profiling tools don't appear to be much help when your leaked memory is stored in references (IORefs, StablePtrs, etc). I had a real-life case where the allocation profile showed me where the leaked data came from, and I could guess that it was being held by some reference, but I couldn't tell which one. Retainer set profiling showed a big suspicious entry for "SYSTEM", but I couldn't find any way to pinpoint the problem. (It ended up being a missing freeStablePtr in hsgnutls, found by code inspection.) Here's a contrived example that just allocates a bunch of IORefs: import Control.Monad import Data.IORef main = repeatM (newIORef [1,2,3]) repeatM io = liftM2 (:) io (repeatM io) Retainer set profiling shows everything in "SYSTEM". None of the other profiling methods say anything interesting either. What I'd like to get, I think, is (1) your memory is being held in IORefs (2) allocated by this cost center and (3) being retained by this cost center. I guess I'm looking for something like a memory profiler for a traditional language. But I haven't really thought it all out (and I don't even understand everything the existing profiles tell me). Are there some tricks I've missed for this sort of debugging? And perchance would this be an interesting GHC hackathon subject? Andrew

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Haskell wiki: most popular pages
by Bulat Ziganshin 22 Aug '06

22 Aug '06

Hello haskell-cafe, The http://haskell.org/haskellwiki/Special:Popularpages page lists most popular pages on haskell wiki. I think this list is very useful because it shows us what are the questions about Haskell people most interested and gives us hints what should be improved in first place. The only catch is that most of popular pages are just ones listed on the title page. But nevertheless... On the other side, we can simplify title page by omitting links to page that are of little interest for casual readers (such as History). We can also add here links to most popular pages, such as GHC and "Hitchhikers guide". Below is beginning of this list: Haskell (252,505 views) Introduction (50,091 views) Libraries and tools (41,864 views) Books and tutorials (40,040 views) Language and library specification (32,773 views) Haskell in practice (31,698 views) Implementations (24,141 views) GHC (20,634 views) Haskell in 5 steps (16,707 views) Learning (14,088 views) Hitchhikers guide to Haskell (13,191 views) Future (12,754 views) -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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DiffArray is an unsaturated type?
by Neil Mitchell 22 Aug '06

22 Aug '06

Hi, I note that in the base libraries, Data.Array.Diff is defined as: type DiffArray = IOToDiffArray IOArray However, IOToDiffArray takes 3 parameters. I thought this was not allowed in Haskell 98? Its annoying from my point of view because Hoogle wants to know the arity of a type, and if it has to chase down the arity of the RHS to do it, I probably just won't bother. Is there any difference between the above definition and: type DiffArray a b = IOToDiffArray IOArray a b Is the first prefered for any reason? Thanks Neil

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Useful: putCharLn {inspire by the Int->[Char] thread
by Gene A 22 Aug '06

22 Aug '06

The thread on the use of "show" and print to display an Int value, brought up a problem I had early on... the one of cleanly displaying a Char value, on a line all by itself.. My first attempts: This was just plain hard to read: with the character t being where it was: Prelude> putChar $ head "this and that" tPrelude> ----------------------- So I tried this and of course ... type mismatch: Prelude> putStrLn $ head "this and that" <interactive>:1:16: Couldn't match `String' against `Char' Expected type: [String] Inferred type: [Char] In the first argument of `head', namely `"this and that"' In the second argument of `($)', namely `head "this and that"' ---------------------- So I did this... to manually turn it to a string and it does work, but a little cumbersome to work into other function calls: Prelude> putStrLn $ (head "this and that"):[] t ------------- so the definition of putCharLn came to life {may be in some library already and I just haven't found it yet.. but it is in my toolbox now}: Prelude> let putCharLn c = putStrLn (c:[]) Prelude> and an application of it: Prelude> putCharLn $ head "this and that" t --------------- now I also have the char to string conversion alone: c2Str c = c:[] Prelude> let c2Str c = c:[] Prelude> c2Str 'A' "A" ------------------ Now this is almost too trivial a thing to mention these gizmos... what with all the monadic constructions that greater minds toss about on this list.. and I am trying to get understanding of that still, but .... sometimes we just accept the unacceptable little irritants rather than just code a solution, no matter how simple it is. There are probably troves of simple workarounds out there that seem too trivial to mention but hey, share 'em... might hit a guy like me that says. "now why didn't I think to do that?" happy days, gene

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Writing binary files
by Neil Mitchell 22 Aug '06

22 Aug '06

Hi, I'm trying to write out a binary file, in particular I want the following functions: hPutInt :: Handle -> Int -> IO () hGetInt :: Handle -> IO Int For the purposes of these functions, Int = 32 bits, and its got to roundtrip - Put then Get must be the same. How would I do this? I see Ptr, Storable and other things, but nothing which seems directly useable for me. Thanks Neil

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Fw: [Haskell-cafe] A restricted subset of CPP included in a revisionof Haskell 98
by Brian Hulley 22 Aug '06

22 Aug '06

Hi - here is an exchange that was off-list by accident: ---- Original Message ---- From: Brian Smith To: Brian Hulley Sent: Tuesday, August 22, 2006 5:50 AM > (Brian, I see that my last reply only went to you and so I forwarded > it to the list. Since you replied to me directly, I am responding to > you directly. Feel free to forward this to the list if you want). > > > On 8/21/06, Brian Hulley <brianh(a)metamilk.com> wrote: >> The paper "Template meta-programming for Haskell" suggests >> that Template Haskell should be able to do everything that CPP >> is used for at the moment, such as conditional compilation, macros >> etc, so this might be a possible option, though I have to admit apart >> from reading the paper and doing a few experiments with it I don't >> know enough about the current state of TH to know if it would be a >> viable replacement at the moment. > > I am very interested in TH and I hope to give it a thorough workout > on an upcoming project. Until then, I don't know enough about it to > compare it to CPP. But, I think that in an Editor/IDE, a restricted > CPP would be much easier to implement than TH. > > >> I've got a feeling that even allowing #define could break the >> invariant that a module only has one meaning in a program, since >> #define changes the state of a symbol that's not yet been mentioned >> just as #undef changes the state of a defined symbol eg: >> >> >> module P where >> #define Bar >> import R >> >> module R where >> #ifdef Bar >> import S >> #else >> import T >> #endif >> >> module Q where >> import R > > I think that this program will only compile if it is split into three > files, P.hs, R.hs, and Q.hs. There would be two scopes for CPP > macros: global (each macro is bound to exactly one value for all > modules, and it cannot be undefined or redefined) and file/module > (the macro is bound exactly once in a module and that binding is not > visible to other modules). Except for the "exactly once" part, this > is how GHC and cpphs already work. > > So, in your example, let's assume there is no global binding for > "Bar"--if there was, then there would be a redefinition error in > P.hs. Then, P.hs defines Bar, but this binding is in scope only > inside of P.hs--R.hs does not "see" Bar as being defined. Further, > since R.hs does not contain a binding for "Bar," so module R will > always import module T and never module S. > > A more concrete example is this: > > module P where > #ifdef __GHC__ > import P.GHC > #else > import P.Haskell98 > #endif > > In your editor/IDE you might want to support multiple Haskell > implementations. So, sometimes __GHC__ will be defined (when in GHC > "mode") and sometimes it won't. But, I recommend to handle this in > the same way that IntelliJ handles the JDK setting. That is, if the > user wants to switch the implementation he is using (even > temporarily), then when he does so, the whole program gets > reanalyzed. If the user wants to work in Hugs mode and GHC mode in > the same time, then he should create two projects in the IDE. In one > project, the implementation will be set to GHC, and the other one > Hugs. He will have to define output paths, etc. such that the two > project's ouitput files do not overlap even though their sources > will. Then, analyze each project seperately. This technique also > allows the user to build a version of the project against, say, HaXml > 1.14, and other version against 1.15. > > >> Much as I'd like to not have to deal with CPP, I agree that on >> a practical realistic level it's not going to go away immediately, >> but at least the guidelines you posted give a good starting point >> for library authors to gradually try and reduce the dependence >> on the preprocessor, if they are motivated to (which they >> probably aren't ;-) ) > > Right. Plus, if we follow the guidelines I proposed very strictly, > then we can more easily picture what is required of a CPP > replacement. For example, if the guidelines are too prohibitive, then > we will know we need something more powerful like TH. If the > guidelines do not result in any practical loss of functionality, then > we know that we can get by with a much simpler configuaration > language. > > >> I had a wild idea of a tool to eliminate CPP from a source >> tree by first generating the set of N pre-processed source trees >> (ie now pure Haskell) then running some analysis tool over these >> trees to automatically factor out common code, but this would >> probably be a major undertaking. > > That would definitely be an interesting project, and it would be > useful for other things as well (The duplicate code detection part.) > > > >> Cabal seems to help a lot - Edison and ByteString both built on >> my machine with no trouble at all. I think the more everything can >> be done in Haskell the better it will be. > > I agree. But, I also want it to be easy to reuse C-based libraries > whenever possible. That way, we can spend more time writing code in > Haskell that does new things instead of reimplementing old ideas in > it. > > > You mentioned that library maintainers might not be motivated to make > such changes. I think that people will modify their libraries if > there are compelling tools that need those modifications. In fact, I > think people would even be willing to break previous commitments to > backwards compatibility if there was any benefit to doing so--I mean, > who really uses the latest version of HaXml with GHC 5.x or Hugs > 2002-xx-xx? > > If there was an IntelliJ-level IDE for Haskell, and people were > writing "real world" programs in Haskell, then somebody would get > around to modifying libraries to work for "real world" programs built > using such a tool. Or, somebody that had a commercial interest (a > Haskell consultant or an IDE vender) might make these modifications. > > - Brian

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Description of Haskell extensions used by FPTOOLS
by Brian Smith 22 Aug '06

22 Aug '06

Is there any design document for the FPTOOLS libraries or some description of language features that are (allowed to be) used in them? I am going to be taking some significant time off from my normal jobs in the upcoming months. During part of that time, I would like to do some work to improve the Haskell toolchain. This involves creating or improving tools that parse and analyze Haskell code. My goal is to have these tools support enough of Haskell to be able to handle at least the most important libraries used by Haskell programmers. In particular, this includes all or most of the libraries in FPTOOLS. Plus, I want these tools to operate on Darcs as it is an obvious poster-child for Haskell. Thus, I need to support Haskell 98 plus all the extensions being used in Darcs and FPTOOLS as of approx. March, 2007 (as I intened to start working again at that time). It would be very nice if there was some document that described "Haskell 98 plus all the extensions being used in Darcs and FPTOOLS as of March, 2007." Besides being useful to me, it would be a useful guide for potential contributors to FPTOOLS. Regards, Brian

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Fwd: [Haskell-cafe] A restricted subset of CPP included in a revisionof Haskell 98
by Brian Smith 22 Aug '06

22 Aug '06

On 8/20/06, Brian Hulley <brianh(a)metamilk.com> wrote: > Henning Thielemann wrote: > > On Thu, 17 Aug 2006, Brian Smith wrote: > > > >> I find it strange that right now almost every Haskell program > >> directly or indirectly (through FPTOOLS) depends on CPP, yet there > >> is no effort to replace CPP with something better or standardize its > >> usage in Haskell. > > > > I think there should be more effort to avoid CPP completely. > > I agree, especially as I'm trying to write an editor for Haskell which > will > certainly not cope with CPP at all! ;-) I agree with this sentiment. But, CPP also has some advantages over the other suggestions presented here. Using CPP, we can create libraries that work with GHC 5.x and old versions of Hugs. If an alternative to CPP were chosen, would we update fptools to use this alternative? If it involves new syntax, then updating FPTOOLS to use the new syntax would break backward compatibility. And, if (the vast majority of) FPTOOLS does not use the solution then it is not useful. The reason it would not cope is that CPP turns what would otherwise be one > program/module/library into several programs/modules/libraries which > simultaneously co-exist in the same text in a rather uneasy and vague > relationship, and what's even worse: the same module can have multiple > meanings in the *same* program depending on use of #ifdef #undef etc, thus > making code navigation quite impossible: the meaning of each module now > depends on how you got there and might even be different the second time > round... Notice that with my suggestions none of these problems apply, because all external "command line" parameters have a single static value over the whole program (two modules cannot depend on differing values of a single macro, #undef is not allowed). Changing the bindings for these macros would invalidate any cached information the editor has. But, the editor has to support updating its caches anyway, to deal with switching libraries in/out, etc. I won't use an IDE effectively if it can't handle the libraries in FPTOOLS, which liberally use the preprocessor. Either the IDE has to handle the preprocessor or the libraries have to stop using them, or they have to meet somewhere in the middle. I think the acid test would be to reach a point where anyone can download > the source for some large program such as GHC and just type ghc --make > Main > and expect the program to be built in one pass with no problems. I agree with this sentiment as well. Most of the little data processing programs I have written are built exactly like that already. How can we get the existing libraries to build like that though? - Brian

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iterative algorithms: how to do it in Haskell?
by Tamas K Papp 21 Aug '06

21 Aug '06

Hi, I am a newbie learning Haskell. I have used languages with functional features before (R, Scheme) but not purely functional ones without side-effects. Most of the programming I do is numerical (I am an economist). I would like to know how to implement the iterative algorithm below in Haskell. f is an a->a function, and there is a stopping rule goOn(a,anext) :: a a -> Bool which determines when to stop. The algorithm looks like this (in imperative pseudocode): a = ainit while (true) { anext <- f(a) if (goOn(a,anext)) a <- anext else stop and return anext } For example, f can be a contraction mapping and goOn a test based on the metric. I don't know how to do this in a purely functional language, especially if the object a is large and I would like it to be garbage collected if the iteration goes on. Thank you, Tamas

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Haddock/Cabal/base bug?
by Neil Mitchell 21 Aug '06

21 Aug '06

Hi, I want to generate documentation for the base libraries, so I darcs get the base libraries, and a very basic (the sample default) Setup.hs, and try: runhaskell Setup configure runhaskell Setup build Pretty much no luck, but I'll leave that for someone else to sort out :) runhaskell Setup haddock First task is to remove errors about -fno-implicit-bangs, which are in a few files, relatively easy to do. After that it gets quite far (invokes haddock a fair bit) before failing with: haddock.exe: Main.all_subs_of_qname: unexpected unqual'd name:IOMode This is on Windows, cabal, base and haddock all fresh from darcs, GHC 6.4.2 Can anyone point out what the problem is, whose fault it is, and what can be done to fix it? Thanks Neil

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type gurus, can you please help?
by Bulat Ziganshin 21 Aug '06

21 Aug '06

Hello haskell, i'm started to write article about type classes. can you, type gurus, please check this initial text for correctness in explaining differences between classes and type classes? at this moment C++/C#/Java languages has classes and templates/generics. what is a difference? with a class, type information carried with object itself while with templates it's outside of object and is part of the whole operation for example, if == operation is defined in a class, the actual procedure called for a==b may depend on run-time type of 'a' but if it is defined in template, actual procedure depends only on template instantiated (and determined at compile time) Haskell's objects don't carry run-time type information. instead, class constraint for polymorphic operation passed in form of "dictionary" implementing all operations of the class (there are also other implementation techniques, but this don't matter). For example, eqList :: (Eq a) => [a] -> [a] -> Bool translated into: type eqDictionary a = (a->a->Bool, a->a->Bool) eqList :: eqDictionary a -> [a] -> [a] -> Bool where first parameter is "dictionary" containing implementation of "==" and "/=" operations for objects of type 'a'. If there are several class constraints, dictionary for each is passed. If class has base class(es), the dictionary tuple also includes tuples of base classes dictionaries: class Eq a => Cmp a where cmp :: a -> a -> Comparision cmpList :: (Cmp a) => [a] -> [a] -> Comparision turns into: type cmpDictionary a = (eqDictionary a, a -> a -> Comparision) cmpList :: cmpDictionary a -> [a] -> [a] -> Bool Comparing to C++, this is like the templates, not classes! As with templates, typing information is part of operation, not object! But while C++ templates are really form of macro-processing (like Template Haskell) and at last end generates non-polymorphic code, Haskell's using of dictionaries allows run-time polymorphism (explanation of run-time polymorphism?). Moreover, Haskell type classes supports inheritance. Run-time polymorphism together with inheritance are often seen as OOP distinctive points, so during long time i considered type classes as a form of OOP implementation. but that's wrong! Haskell type classes build on different basis, so they are like C++ templates with added inheritance and run-time polymorphism! And this means that usage of type classes is different from using classes, with its own strong and weak points. -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com

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Ix, Random, and overlapping instances
by Lambda The Insignificant 21 Aug '06

21 Aug '06

Greetings Haskellers, I have recently found myself in a situation where I was needing to general tuples of randomized things; in my case, two coordinates and a custom data type, Direction. As it is always better to generalize, I decided to try to write some code which would generalize randomR to any type which is Ix-able: > instance Ix i => Random i where > random = error "No supported instance for random" > randomR r g > = let > r' = range r > (i, g') = randomR (0,length r'-1) g > in > (r' !! i, g') This works splendidly; with my given example above, I could do: > data Dir = N | NE | E | SE | S | SW | W | NW deriving (Eq, Ord, Show, Ix) ghci> newStdGen >>= print . take 10 . randomRs ((1,1,N),(10,10,NW)) [(5,4,S), (3,10,SE), (6,5,N), (6,6,NW), (10,8,SE), (3,10,W), (10,3,E), (1,6,NE), (10,10,SE), (3,3,S)] As you can see, this results in random tuple(s) of these indexable types. My implementation required -fallow-overlapping-instances and - fallow-undecidable-instances for the code to compile. A few questions: 1) How does the compiler determine how to choose a particular instance for those cases where they overlap? I noticed that if I tried to generate a random :: Int, I was getting the error message defined in the above instance, indicating that the compiler was choosing my implementation over the default for Int (obviously because Int is an instance of Ix). Is there some way to exclude classes in the class qualifier, or to provide an instance as a default case without resorting to -fallow-overlapping-instances? 2) Is there a logical overlap between class Ix and class Bounded? It seems that we could use the ranges of data types of Bounded to generate a reasonably sane `random` implementation for all Ix, if we can guarantee that all Ix instances are Bounded. Would adding an additional class constraint be sufficient to catch the major cases? i.e., instance (HasBounds i, Ix i) => Random i where ... 3) Is there some reason that Ix is restricted to the Int datatype? Is there anything preventing us from moving to a genericIx,or something along those lines which support any integral type (say Integer), or is it merely for performance reasons that we use Int? Is the expectation that if you are using spaces larger than what an Int can accommodate, you should be using a different mechanism? Thanks much for any comments you all can give. Cheers, Lambda the Insignificant

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cabal specify a "tested version", ghci target?
by Marc Weber 20 Aug '06

20 Aug '06

1.) I know I can use Build-Depends: lib == <version>, lib2 < version, lib3 >= version and so on. Do you think it would be useful to introducue some notation to indicate a "tested with" ? Reason, purpose: I think its sometimes the case that a author/ mantainer is quite busy with other projects and misses that some dependencies break things.. If you want to try out you're left with some compiler errors and a dependency and have to try out which version works. I would propose using this syntax: lib-1.3 >=1.1 to indicate that lib 1.1 is required at leeast and tested with up to 1.3.. Cabal might then give a warning if you try to use 1.4 or greater "using newer version than tested" or similar.. What do you think? Would this be useful? 2.) Is there a ghci target? I find it quite useful using ghci to debug test/ check types functions of single modules... My hack is using a Setup.hs file like this adding ./setup ghci which runs ghci and adds all dependency as -pakcage arguments: module Main where import Distribution.Simple import Distribution.Simple.Configure import System import System.Process import System.Exit import Monad main= do args <- getArgs -- check args to launch ghci when (length args > 0) $ do when ((args!!0) == "ghci") $ do lbi <- getPersistBuildConfig let packageArgs = (concat.concat) [ [" -package ", showPackageId pkg] | pkg <- packageDeps lbi ] system("ghci " ++ packageArgs) exitWith ExitSuccess defaultMain -- fallback to defaultMain Would this be useful for you, too? Marc W.

3 2

A backwards-compatible record proposal
by Brian Hulley 20 Aug '06

20 Aug '06

Hi - As I've been writing a Haskell program over the past few months the main problem I encounter is that record field names are not local to the record type, and any systematic way of making them local (eg by prepending "_Tycon_") results in names that are just too clunky, and I feel that identifiers *must* use systematic naming conventions to get code that will be easy to understand and maintain. Although it is relatively easy to think up a better record system, it has taken me up till now to discover how such a system could be integrated with Haskell as it is at the moment, since it's unlikely that the existing record system will ever disappear, at least not in the next few years, and in fact, there are some good points about the existing record system that I wouldn't like to lose. I'll motivate the proposal then talk about how it could be implemented. Motivation ======= Consider the following: data Vector3 a = Vector3{x :: a, y::a, z::a } data Normal3 a = Normal3{x :: a, y::a, z::a } We've got a problem because (x) has been introduced twice to the top level namespace. In the above example, it could be argued that I should have written: data Arr3 a = Arr3 {x::a, y::a, z::a} newtype Vector3 a = Vector3 (Arr3 a) newtype Normal3 a = Normal3 (Arr3 a) but for the sake of argument, let's suppose we can't do this - perhaps one of the record types has some different fields as well. A related problem is suppose I have: data Size = {width :: Int, height :: Int} data Rect = Rect{x1, y1, x2, y2::Int} width :: Rect -> Int width Rect{x1,_,x2,_} = x2 - x1 because a record field of Size has the same name as a top level function. This second conflict can be avoided by always using the rule that record fields begin with an underscore and all other variables don't. So applying this to the first problem, we have: data Vector3 a = Vector3{_x :: a, _y::a, _z::a } data Normal3 a = Normal3{_x :: a, _y::a, _z::a } Of course we haven't solved it yet! But now suppose we introduce a new piece of syntactic sugar, and write: data Vector3 a = Vector3{.x :: a, .y::a, .z::a } data Normal3 a = Normal3{.x :: a, .y::a, .z::a } ie putting a '.' before each field name. The intended meaning is that dotted field names do *not* generate top level functions. Instead they allow the compiler to generate instance decls as follows, where we've introduced a new form of identifier, the dotted id, which behaves as a postfix operator which binds more tightly than function application and can also be used as a class name (by the compiler only): class (.x) :: a b | a -> b where (.x) :: a -> b class (.y) :: a b | a -> b where (.y) :: a -> b class (.z) :: a b | a -> b where (.z) :: a -> b For each dotted id, there is a class defined as above, which is available globally to the whole program as if a module containing an infinite set of class decls as above was exported by the Prelude. In the module containing the data decl for the record, the compiler inserts the following: instance (.x) (Vector3 a) a where (.x) v = ... -- compiler generated code to access the field Then within the rest of the program we can write: magSquared :: Num a => Vector3 a -> a magSquared v = v.x*v.x + v.y*v.y + v.z*v.z -- explicit type when specific function is required vec_x = ((.x) :: (Vector3 a -> a)) The advantage of this proposal is that it is completely backwards compatible with records as they are at the moment, and we can choose which fields we want to be dotted and which we want to just keep as normal top level functions. The only extra thing we need to do is put a dot before the field names we want to access via the dotted syntax, but we in any case needed to use an underscore when we wanted a systematic way to avoid conflicts between field names and other top-level names so there is no extra effort involved. Implementation ========== The above could almost be implemented just by parsing a source file containing uses of dotted fields and using a conversion like: data Rec a = Rec { .f :: a} let rec = Rec {.f = 78} -- dot is used here too p = rec.f * rec.f ==> data Rec a = Rec a instance Dot__f (Rec a) a where __dot_f (Rec a) = a let rec = Rec 78 p = __dot_f rec * __dot_f rec So far so good, but the alert reader :-) will have noticed that we now have a *major* problem with abstraction because although we can write: module M (Rec) where ... code in another module can still say rec.f because in Haskell, all instance decls in a module are always exported [1]. This problem may disappear in Haskell' [2]. http://hackage.haskell.org/trac/haskell-prime/ticket/19 Therefore I think the desugaring would need to take place in the compiler so the compiler could avoid exporting the compiler-generated instances when the fields are not present in the module export list. Regards, Brian. [1] http://haskell.org/onlinereport/modules.html#import-instances [2] http://hackage.haskell.org/trac/haskell-prime/ticket/19 -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com

4 9

rand* why not of type State g a
by Marc Weber 20 Aug '06

20 Aug '06

the rand* function are examples for a typical state usage, arent' they? Is there any reasoon why they are not defined RandomGen g => State g a rather than RandomGen g => (a,a) -> g -> (a,g) ? Marc

4 3

automatic instance derivation
by Greg Fitzgerald 19 Aug '06

19 Aug '06

How do you automatically derive an instance for a data type defined in an imported module? -- automatic instance derivation directly after type declaration data Val1 = V1 Int deriving Show -- manual instance derivation can be in separate file data Val2 = V2 Int instance Show Val2 where show (V2 i) = show i -- Is something like this possible? data Val1 = V1 Int ... derive Show Val2 Thanks, Greg

2 2

Problem getting started with graphics library in GHC on Windows
by Tomi Maila 19 Aug '06

19 Aug '06

Hi, I'm a Haskell newbie. I've been studying Haskell by reading the book The Haskell School of Expression. Most of the example in the book use graphics library SOEGraphics. Windows GHC didn't find the library, but I assumed that the new name for the library is Graphics.SOE which seemed to properly loaded. The problem is that I fail to get the Graphics library to work properly. Instead GHC freezes if I use the graphics library in interactive mode and if I compile into a binary the binary freezes. The problem is not only with Graphics.SOE but also with more general Graphics.HGL. If I execute the Hello World program presented in Graphics.HGL documentation front page, GHC interactive mode freezes after opening the window but before drawing any text on the window. Compiled binary never even opens a window but freezes before that. Examples in HSOE (the book) behave the exactly the same way. I get not error messages what so ever. The example I refer to above is below. module Main where import Graphics.HGL main :: IO () main = runGraphics $ withWindow_ "Hello World Window" (300, 200) $ \ w -> do drawInWindow w $ text (100, 100) "Hello World" drawInWindow w $ ellipse (100, 80) (200, 180) getKey w The question is what I could be doing wrong or is Graphics library just incompatible with my environment. I'm using GHC 6.4.2 on Windows XP SP2. I've been developing using Eclipse 3.2 with EclipseFP plugin. However I've also tried to compile on command line. Could somebody help me over (no I'm not moving to Linux...) Cheers, Tomi -- Tomi Maila Research scientist Bio- ja ympäristötieteiden laitos Department of Biological Biotieteellinen tiedekunta and Environmental Sciences PL 65 (Viikinkaari 1) P.O. Box 65 (Viikinkaari 1) 00014 HELSINGIN YLIOPISTO 00014 UNIVERSITY OF HELSINKI puhelin (09) 191 59837 tel. +358-9-191 59837 matkapuh. 040 554 9024 mobile. +358-40-554 9024 http://www.helsinki.fi/~tmaila/

2 1

Re: [Haskell-cafe] HTTPS in Haskell
by Adam Peacock 19 Aug '06

19 Aug '06

On 8/18/06, Pasqualino 'Titto' Assini <tittoassini(a)gmail.com> wrote: > > Is there any implementation of HTTPS/SSL in Haskell? > > > > This would seem to be critical to develop commercial web applications. WASH has a run time function ( runWithOptions ) that has type: runWithOptions :: CGIOptions -> CGI () -> IO () with... type CGIOptions = [CGIOption] data CGIOption = .... AutoHttps -- autodetect Https by checking for port number 443 and env var HTTPS ... Taken from: http://www.informatik.uni-freiburg.de/~thiemann/WASH/doc/WASH-CGI-CGI.html Adam.

2 1

Haskell & LLVM
by Ryan Cleven 19 Aug '06

19 Aug '06

Has anyone ever investigated using LLVM as a target for a haskell compiler? Perhaps a backend for GHC? Thanks, Ryan.

1 0

Haskell genetic algorithm
by Maurício 18 Aug '06

18 Aug '06

Hi, Does anyone knows of nice genetic algorithms implementations in Haskell? Thanks, Maurício

1 0

how to generate XHTML 1.1?
by Tamas K Papp 18 Aug '06

18 Aug '06

Hi, I would like to use Haskell to generate static HTML webpages, preferably using XHTML 1.1. How can I do this? I found libraries to generate XHTML 1.0 transitional, but not 1.1. Scheme has LAML [1], which parses the DTD, uses S-expressions and validates the XHTML, I am looking for something similarly elegant (even if without the S-expressions, which I haven't seen in Haskell yet). Thanks, Tamas [1] http://www.cs.aau.dk/~normark/laml/

3 2

More threading confusion
by Creighton Hogg 17 Aug '06

17 Aug '06

Good afternoon Haskellers, So I'm trying to understand how STM works, and wrote a quick 'eating philosophers' example to see if I understood how it's supposed to work. The problem is that while it executes, it doesn't appear to *do* anything. Did I completely write things wrongheadedly or am I being bitten by something more subtle? Thanks. import Control.Concurrent.STM import Control.Concurrent import Data.Array import System.Random think :: IO () think = do ms <- randomRIO (20,1000) threadDelay ms data Philosopher = Philosopher {left::Bool,right::Bool,neighbors::(Int,Int)} deriving Show makeInitPhilosopher a = Philosopher {left=False,right=False,neighbors=a} initPhilosophers = listArray (0,4) (map makeInitPhilosopher [(1,4),(2,0),(3,1),(4,2),(0,3)]) main = do z <- atomically $ newTVar initPhilosophers mapM_ (\x -> forkIO (loop x z 0 10000)) [0,1,2,3,4] loop n tps c l | c > l = (atomically (readTVar tps)) >>= (\x -> print x) | otherwise = do think atomically $ eat n tps loop n tps (c+1) l eat :: Int -> TVar (Array Int Philosopher) -> STM () eat n tps = do takeLeft n tps takeRight n tps releaseLeft n tps releaseRight n tps takeLeft :: Int -> TVar (Array Int Philosopher) -> STM () takeLeft n tps = do ps <- readTVar tps let p = ps ! n if right (ps ! (fst $ neighbors p)) == False then (writeTVar tps $ ps // [(n,p{left=True})]) else retry takeRight :: Int -> TVar (Array Int Philosopher) -> STM () takeRight n tps = do ps <- readTVar tps let p = ps ! n if left (ps ! (snd $ neighbors p)) == False then (writeTVar tps $ ps // [(n,p{right=True})]) else retry releaseLeft n tps = do ps <- readTVar tps let p = ps ! n writeTVar tps $ ps // [(n,p{left=False})] releaseRight n tps = do ps <- readTVar tps let p = ps ! n writeTVar tps $ ps // [(n,p{right=False})]

3 2

Samba/FTP bindings
by Ivan Tarasov 17 Aug '06

17 Aug '06

Is there some Haskell library which provides Samba bindings and some FTP client library bindings (e.g. ftplib3)? I started writing the bindings for some of the functions myself for my project but it looks like it is a lot of work and probably someone has done it already. -- Best regards, Ivan Tarasov

2 1

Last statement in 'do' must be an expression error.
by Szymon Ząbkiewicz 17 Aug '06

17 Aug '06

Hi. When trying to compilke this code: {...} 8.if (a == 0) && (b == 0) 9. then do 10. nr1 <- read (prompt "enter 1. number: ") 11. nr2 <- read (prompt "enter 2. number: ") 12. else do 13. let nr1 = a 14. nr2 = b {...} The compiler tells me thats there's an error on line 10: "The last statement in a 'do' construct mesy be an expression" Could you tell me how to change it so that the "declaration" of the first nr1 and nr2 is still in the "then" block.

6 5

wxFruit with OpenGL and IO action
by shelarcy 17 Aug '06

17 Aug '06

Hello. Sometimes, a few people are interested in wxFruit. http://zoo.cs.yale.edu/classes/cs490/03-04b/bartholomew.robinson/ But nobady knows current status of wxFruit. Over one and half year ago, I heard that students working on that ... . http://www.haskell.org/pipermail/haskell-cafe/2005-January/008765.html http://www.haskell.org/pipermail/haskell-cafe/2006-March/014998.html So, I'm working on that for subproject of Kamiariduki ... but progress is very slowly. I uploaded current (initial) version on Kamiariduki's file space. File name is "YaPan-initial.tgz". Because this file includes YaPan - Yet another implementation, and reimplementation of Panic (http://www.cse.unsw.edu.au/~sseefried/pan/index.html) using Yampa (and wxFruit). http://sourceforge.net/project/showfiles.php?group_id=168626 I didn't think current verion is good enough to announse [Haskell] list, so I didn't announce that and repraced "YaPan.tgz" file sometimes ... but I know that a few people downloaded my file already. So, I decide to upload initial file on that space. Of cource, "YaPan-initial.tgz" has other wxFuit example. paddle.hs is almost same as original example, except that use menu istead of button. And CFK_GL* is a OpenGL sample code. But current version has some faults. Cabal file support to build and install only YaPan, doesn't support wxFruit. Code is very messy, and YaPan doesn't support all of Panic's features. And I should notice additonal things for older version's user. Now YaPan depends on AutoForms (not for wxFruit now). http://autoforms.sourceforge.net/ And YaPan can't save static one picture, YaPan can save animation by consecutive number files only. Current version, I tested on only Windows with my build of wxWidgets 2.4.2's wxHaskell (I uplaoded Kamiariduki's file space) and afrp-0.4 environment. So I don't know wxFruit and YaPan can works well other environments. Finally, I show current problem of wxFruit with IO action. I extend that button menu allow to regist extra IO action like this. pan pixels = hBox $ proc _ -> do let start = False rec v <- boxSF $ iPre 0 -< 1 + v ev <- getEvent -< () vTrans <- boxSF $ iPre (0,0,1) -< (keyboardEvent 0.1 vTrans) ev panedPixels <- boxSF $ pictureEvent -< (pixels, vTrans, (circle v), v, gameBox) _ <- glpicture (glpsizeWithAttributes gameBox [WXCore.GL_RGBA, WXCore.GL_DOUBLEBUFFER]) -< glppic $ paintGL gameBox panedPixels _ <- menu (mtextWithDialog "&SaveImage") -< mcommandWithDialog $ (\v f -> saveImageHandler (animToDisplayFun (circle)) circle (WX.sizeW gameBox, WX.sizeH gameBox) vTrans 1 v f) v returnA -< () This design is suitable for button and menu's command. But if I want to use IO action out of these control event, modualiry is broken. Because I want to do that action out of control's event, but I have to change control's action. And I must choose using what kind of event handler. (Is interval the best?) So I tried to implement command function that intend to use IO action on reative loop wihtout control in wxFruit.hs ... but action doesn't do registered action or program is not work on my apporoach. (I commented out current command function that occur action.) If anyone have good idea how to use IO action on wxFuit, please tell me that idea. Thanks. -- shelarcy <shelarcy capella.freemail.ne.jp> http://page.freett.com/shelarcy/

1 0

Why does Haskell have the if-then-else syntax?
by Mike Gunter 16 Aug '06

16 Aug '06

I had hoped the "History of Haskell" paper would answer a question I've pondered for some time: why does Haskell have the if-then-else syntax? The paper doesn't address this. What's the story? thanks, -m

18 21

Type hackery help needed!
by Niklas Broberg 16 Aug '06

16 Aug '06

Hi fellow Haskelleers, I have a tricky problem that I just can't seem to solve. The problem is one of unresolved overloading, much like the (show . read) issue, but unlike that particular problem I feel there should be a solution to the one I'm wrestling with. I've tried to strip away all the unnecessary bits so the following will be a bit abstract, but it shows the gist of the problem. I have two overloaded functions - one I call 'build' that creates values from subparts, and one I call 'embed' that turns values into a form appropriate to call 'build' on. What they really represent is creating values of some XML tree structure, with embed making sure that the children are of the right type. Conceptually, we could imagine that these two functions to be defined as > class Build x c | x -> c, c -> x where > build :: String -> [c] -> x > > class Embed a b where > embed :: a -> b They would be used together as in e.g. > p c = build "p" [embed c] This works pretty well, the fundep x -> c tells me what b should be, assuming I infer a proper type for the result of the composition. The type of p is then p :: (Embed a c, Build x c) => a -> x where the c in the middle is determined by x via the fundep. My problems arise because I want to start nesting these to form tree structures, as in > tree = p (p "foo") Expanding the definition of p, the argument to the outer call to build is now embed $ build "p" [embed "foo"] :: (Embed String c, Build x c, Embed x x1) => x1 Through the fundep on the outer build I can infer what x1 should be, but there's no way I can infer x (without inserting an explicit type signature which is out of the question). This problem is probably unsolvable in general, but for my particular problem there are some restrictions that makes me feel there should be a clever way of working with classes and fundeps to make this all work out. I just can't seem to find one. These are the rules that the setup must obey: * A value of any type should be embeddable inside a build expression of any result type, i.e. a -> b or b -> a cannot hold on Embed in general. * The exception to the above is that if an expression using 'build' is embedded inside an outer 'build', as in 'tree' above, the inner build should have the same result type as the outer build (in a sense forming b -> a only for types instantiating Build). In other words, children of an xml element must be of the same type as their parent, even before the call to embed. * However, it would be nice, but probably not crucial, if by using explicit type signatures where I want to, I could actually embed values of "other" xml types than the outer one I am embedding them in, and letting embed convert them to xml of the correct type. I suspect this cannot be. The types of build and embed are negotiable, and certainly the type classes, as long as the two can be used together in roughly the same way as indicated above. The calls to build and embed will be autogenerated by a preprocessor from a HSP-style XML syntax, e.g. <p><% c %></p> <==> build "p" [embed c] and for this reason any solution *must* be syntactically expressible as expressions on a form similar to the above, but those expressions can be arbitrarily convoluted. For instance in my failed attempts so far I have used let expressions around build to form self-recursive versions to "pass a type" down through the evaluation, as in p c = let x = build "p" [embed x c] in x This is an ok solution syntactically, if only it solved the problem, I still can't see how to propagate it to the next level of build. :-( Is there anyone out there with the proper type class fu who can see a way out for me? Is this even possible to do at all? If yes, tell me please, and if not, I would be most interested in seeing why it cannot work. Any and all comments are welcome. Thanks for reading this long :-) /Niklas

2 5

Int->[Char] conversion
by Tamas K Papp 16 Aug '06

16 Aug '06

Hi, I am working through Hal Daume's tutorial, trying to do the exercises. I can't figure out how to output an integer with putStrLn (or any other way), I think I need an Int -> [Char] conversion but couldn't find it. Specifically, in Exercise 3.10, I have the product of numbers in pp, and would like to do putStrLn ("Product: " ++ convertnumbertostring(pp)) but I don't know which function does this... Sorry for the extremely dumb question. Thanks, Tamas

5 4

Questions on threads and IO
by Creighton Hogg 16 Aug '06

16 Aug '06

Hello Haskell'rs, I've been playing with threads and I tried to do a toy example (that used java) from a class. When run, the program should print a prompt and accept commands just like a linux shell. It doesn't have to do anything fancy, just spawn new threads that make system calls when commands are entered at the prompt. The problem is that the UI doesn't work very well. It will seem fine at first, but in order to get back a prompt you have to hit enter one more time than you should. I've tried playing with the buffering settings but it seems to cause the same problem no matter what. The problem seems to be coming from calls of the form (forkIO . system_) "ls /usr/bin" Just entering this into ghci I get the same thing where I need to hit enter *again* in order to get back to the ghci prompt. I'm sure this is something silly on my part, but it is rather confusing. import Control.Concurrent import System import System.IO main = do putStr ">" z <- getLine runCommands z main genWords :: Char -> String -> [String] genWords c s = gwhelper c s [] [] gwhelper :: Char -> String -> [String] -> String -> [String] gwhelper c [] acc temp = acc ++ [(reverse temp)] gwhelper c (x:xs) acc temp | x /= c = gwhelper c xs acc (x:temp) | otherwise = gwhelper c xs (acc++[(reverse temp)]) [] runCommands s = mapM_ (forkIO .system_) (genWords '&' s) system_ :: String -> IO () system_ s = do system s return ()

2 1

Genetic programming Haskell
by Maurício 16 Aug '06

16 Aug '06

Hi, Does anyone know of a package for genetic programming for Haskell? I tried some links from Haskell wiki and some I found on google, and found many interesting papers, but none of them pointed to downloadable code. Does anyone know where can I find a good implementation (even an experimental one)? Thanks, Maurício

6 5

Re: map and fmap
by Duncan Coutts 15 Aug '06

15 Aug '06

On Tue, 2006-08-15 at 12:38 +0200, John Hughes wrote: > One thing I've observed repeatedly is that many students in later > years, who learned functional programming early, have a strong > impression that functional languages are only suitable for toy > programs. Of course, that's because in their first courses they only > SAW toy programs. But perhaps, they also moved on to Java as soon as > they started to build GUIs, and as a result have the impression that > you CAN'T build a GUI in the functional language they learned first. Yes, this was the same impression my colleagues had as undergraduates and it's the same thing I see in the students I now teach. > This is a very negative impression for students to have, and one that > is hard to combat because beginning undergraduates are sceptical to > functional languages from the word go: they "know" they really want to > learn C++ or Java, and wonder initially what we're playing at. > > I combat that--successfully--by showing them early on that Haskell > *is* a "real" programming language, in which one can build real > applications. I tell them about darcs, or whatever new application is > hot at the time. I show them how to build a GUI in wxHaskell--in the > fifth week of their university studies. Yay! I'm doing the same thing, the fifth week in their first term in their first year. Though I'm using Gtk2Hs of course :-) I don't make them build the GUI directly of course. I've got a practical where they do some list processing functions and we provide a GUI module which visualises what they've done. So while still teaching the basics I try to make it clear that that can do GUIs and do them easily and make them look attractive. This year I want to add an optional extension to go 3D, to visualise their stuff with singing dancing rotatey things. I managed to get the Gtk2Hs+HOpenGL stuff working by the end of the practical last year and had lots of students come over to say "wow!". So I'm looking forward to using it this year to motivate the students to do the optional sections of the practical. > I want them to be frustrated at how difficult GUI programming is when > they do it in Java (:-)! It works--some find it very hard, but many of > the good students go much further than my lectures and exercises, and > reappear in the third and fourth years as very expert and enthusiastic > Haskell programmers. This is the first year when people who did my Haskell GUI practical will go on to the OOP course which involves building a GUI in Java. I'll be interested to see how they think it compares. Duncan

1 0

Re: [Haskell-cafe] Re: Why Haskell?
by Johan Tibell 15 Aug '06

15 Aug '06

For me library support, for networking in particular, has been the major hurdle. It gets problematic when too many libraries are still marked as experimental and only partially implements the specification (e.g. protocol) that they are supposed to handle. Also after a quick look at the source for some common (networking) libraries I found a few too many calls to error to feel comfortable. It's not okay for a library to bring down my application behind my back, error should be reported even if this uglifies the code and interfaces. Furthermore I think that Haskell would benefit from moving some commonly used functionality such as HTTP, SQL and XML support into the standard libraries. I haven't seen any news about the progress of HaskellNet but I guess it could solve my problems. I really don't have much problems with tool support, speed and the like. I'm convinced it's a library (and library documentation!) thing. > As for the 'popularity' of Haskell, someone made the very common sense > remark that all successful languages have been promoted by some large > corporation. (Please note 'successful' stands for 'spread of use', not > 'quality' here.) If fp is to reach the mainstream, it will probably be > because of some centralized support from outside the academic world. > Maybe something like a 'Haskell Consortium' or 'Haskell Initiative' to > promote Haskell should be set up. An example could be the promotion of > SystemC for hardware and software co-design by the Open SystemC > Initiative www.systemc.org . Availability of an IEEE standard for > Haskell would also be a good thing for its acceptance, I think. Having someone pay a group of people to hack on Haskell implementations would indeed be desirable. Without knowing the details Ubuntu looks like a promising model. If we could just find a willing billionaire out there... Cheers, Johan

4 8

Where is "Typing Haskell in Haskell"?
by Ralf Lammel 14 Aug '06

14 Aug '06

Hi Mark, Cc Haskell-café, It seems the OGI links for THIH have gone dead. Is there any preferred download location for THIH these days? I couldn't spot anything. Thanks, Ralf

1 0

Why Exotic Languages Are Not Mainstream
by Johan Tibell 14 Aug '06

14 Aug '06

Haskell was mentioned in an article called "Why Exotic Languages Are Not Mainstream" on the blog defmacro.org the other day and I thought maybe someone would be interested (i.e. is procrastinating at work and need an excuse to do something else). Any comments? http://www.defmacro.org/ramblings/not-ready.html

12 20

Re: [Haskell-cafe] Compiling To C And Why Not Compiling To Java Or C#?
by Kaveh Shahbazian 14 Aug '06

14 Aug '06

I do not meant to compile Haskell to MSIL/JVM. I meant to compile Haskell to the Java or C# itself! And GHC will be there for a high performance language (but still O'Caml is better by the time). But Java proves that in enterprise solution, performance is a complex factor of many thing other than speed alone. I love high performance implementation like Haskell. But there must be a way to populize Haskell! Thanks

4 12

Can someone explain this typing restriction?
by Tim Walkenhorst 14 Aug '06

14 Aug '06

I've stumbled over the following (code *extremely* simplified): f :: Bool f = g 1 g :: Num a => a -> Bool g _ = f This results in the following error message (on GHC): Contexts differ in length When matching the contexts of the signatures for f :: Bool g :: forall a. (Num a) => a -> Bool The signature contexts in a mutually recursive group should all be identical A similar problem is described in: darcs.haskell.org/haskore/docs/Tutorial.pdf on page 128. My first question is: 1) Is there a name for this restriction: I can't clearly identify it as some case of Let-Bound Polymorphism or the monomorphism restriction. [http://www.cs.auckland.ac.nz/references/haskell/haskell-intro-html/pitfalls…] 2) What exactly is the problem with it? 3) What's the best workaround? I've first tried to add an abundance of explicit type-annotations like f = (g::Int->Bool) 1 to it, but it doesn't work. Then, I found a solution, but it's awkward: f :: Bool f = g f 1 g :: Num a => Bool -> a -> Bool g f _ = f As you may have guessed, the actual code is a bit more complicated than this. Basically I interpret a symbol in a syntax-tree which can belong to different type-classes. When doing that, other more general symbols must be evaluated. An indirect recursion (potentially) occurs... I hope this hasn't been done to death yet. If it has, please just answer question one so that I can read up on it... Thank you, Tim

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Re: [Haskell] AJAX applications in Haskell
by Joel Björnson 13 Aug '06

13 Aug '06

2006/8/9, Pasqualino 'Titto' Assini <tittoassini(a)gmail.com>: > > Hi, > > Those among you who have an interest in AJAX-style web development - that > is > to say the development of web applications that run entirely into the > browser environment, calling back to the server back-end only to get raw > data -- will probably have noticed the recent appearance of the Google Web > Toolkit (http://code.google.com/webtoolkit) ... > Is anyone working on anything similar or that might be interested in such a > project? I am currently working on a Google Summer of Code project relating to some of this... The project is more or less bound to Haskell Server Pages ( http://www.cs.chalmers.se/~d00nibro/hsp/) enabling JavaScript code generation. The main difference compared to the Google Web Toolkit approach is that there are no special compiler magic added for compiling directly into JavaScript. Instead the compiled program, e.g. a cgi program generates the code (JavaScript/HTML/XML) at runtime. However, type safety is gained via an embedded language (HJScript) and a typed abstract representation of JavaScript in Haskell. For more info see http://www.dtek.chalmers.se/~bjornson/soc/ Regards, Joel

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Re: [Haskell-cafe] A lazy question
by Mario Blazevic 13 Aug '06

13 Aug '06

> > sourceToSinkTransducer :: SourceTransducer x y -> SinkTransducer x y > > > > I can't find any way to implement the sourceToSinkTransducer function > > Indeed that cannot be done because it breaks referential transparency. > Ok, thanks for clarifying the reason. > Food for thought: > > You need some sort of side effects (I'm not quite sure which), so > it'd be interesting to try > > newtype Source m x = Source (m (x, Source x)) > newtype Sink m x = Sink (x -> m (Sink x)) > > where m is an appropriate monad. Will the continuation monad work? I'm not sure. I'm trying to find the minimal data types that will let me implement the higher-level combinators. So far, the best I've found is the CPS and the following declarations: data Source i r = Source {get :: Sink i r -> r} data Monoid r => Sink i r = Sink {put :: i -> Source i r -> r, close :: r} newtype Filter x y = Filter { transduce :: forall r. Monoid r => Source x r -> Sink y r -> r } newtype Splitter i = Splitter { split :: forall r. Monoid r => Source i r -> Sink i r -> Sink i r -> r } The 'close' function and result type 'r' are there because the streams are usually finite. The 'Source' type should really be declared as data Source i = Source {get :: forall r. Sink i r -> r} because sources should not depend the result type, but that caused a lot of problems with type checking: the two 'r's in Source and Sink declaration are not unified. I don't see a way to express my intention. Anyway, thanks again for the explanation of why it has to be this complicated.

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A lazy question
by Mario Blazevic 13 Aug '06

13 Aug '06

I was hoping to use Haskell's laziness to implement some of the stuff from "Continuation and Transducer Composition" (http://www-static.cc.gatech.edu/~shivers/papers/fcoro.pdf) a bit more elegantly, without manually coding the coroutine logic in continuation-passing style. Unfortunately, I'm beginning to think there is no workaround. Here's the challenge in a simplified form: newtype Source x = Source (x, Source x) newtype Sink x = Sink (x -> Sink x) type SourceTransducer x y = Source x -> Source y type SinkTransducer x y = Sink y -> Sink x sourceToSinkTransducer :: SourceTransducer x y -> SinkTransducer x y I can't find any way to implement the sourceToSinkTransducer function and its inverse. This is disappointing, because SourceTransducer and SinkTransducer are obviously isomorphic and there are languages like Oz that can accomplish this. Tell me I'm missing something obvious?

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Nested Monads Questions
by Stefan Aeschbacher 12 Aug '06

12 Aug '06

Hi I'm trying to understand Monad Transformers. The code below works as expected but I have the following questions: - why can I use liftIO but not lift in the doSomething function? - why is there no liftSTM function? now to the code: module Main where import Control.Monad.Reader import Control.Monad.Writer import Control.Concurrent.STM type MyM a = WriterT [Entry] (ReaderT MyData IO) a data MyData = MyData {myData::TVar String} data Entry = Log String deriving (Eq, Show) logMsg :: String -> MyM () logMsg s = tell [Log s] doSomething :: MyM Int doSomething = do dataRef <- asks myData logMsg "Writing" liftIO $ do --lift $ do mv <- atomically $ readTVar dataRef putStrLn mv logMsg "Written" return 1 main :: IO () main = do i <- atomically $ newTVar "2" log <- runReaderT (runWriterT doSomething) (MyData i) print log regards and thanks Stefan

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Re: [Haskell] AJAX applications in Haskell
by Jared Updike 11 Aug '06

11 Aug '06

> This would require to: > > - Retarget one of the existing Haskell compilers to generate JavaScript > (other possible targets would be Flash or higher level UI languages such as > OpenLaszlo that in turn compiles down to either Flash or JavaScript/HTML) As I've noticed from experience, you can just write JavaScript itself! (You will end up doing this a lot anyway because any library is bound to be missing some feature you need, say, input textbox history w/ up/down keys, or, say, autoscroll to bottom of iframe when content added, etc. as here): http://www.ugcs.caltech.edu/~lordkaos/calc.cgi (source available here http://www.ugcs.caltech.edu/~lordkaos/calc.tar.gz) > Is anyone working on anything similar or that might be interested in such a > project? I'm definitely interested in this. I would love to be able to deploy a Haskell application in the web browser w/out having to drain server computing resources. Imagine reading a Haskell tutorial and providing interactive applications that run in the reader's browser, etc. Also, I really would love to see something in Haskell that can compete with the Web Services / Web Forms stuff from Visual Studio 2005/C# in terms of simplicity and power and feature-completeness: just write your app and it generates all the HTML/CSS/JavaScript on the client and the XML request stuff on the serve. Perhaps in Haskell we could have something fugdets-like but deployable in any (relatively recent) browser w/ no downloads. Jared. -- http://www.updike.org/~jared/ reverse ")-:"

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A sudoku solver in 707 bytes
by Marco Maggesi 11 Aug '06

11 Aug '06

Hi, just a short message to publicize my summer exercise in Haskell: a stupid sudoku solver (yes, again). Neither fast nor clever, but very short source code (707 bytes). Solves the "nefarious" test board cited in then Haskell Wiki in aprox 100s on my laptop (P4 2GHz). The code and an example session follows. -- Marco Code: (http://web.math.unifi.it/users/maggesi/haskell_sudoku_solver.html) ------------------------------------------------------------------------------- import List type T = (Int,Int) -> [Int] main = do s <- getContents putStr $ unlines $ map disp $ solve [input s] solve :: [T] -> [T] solve s = foldr search s idx where search p l = [mark (p,n) s | s <- l, n <- s p] mark :: ((Int,Int),Int) -> T -> T mark (p@(i,j),n) s q@(x,y) = if p==q then [n] else if x==i || y==j || e x i && e y j then delete n $ s q else s q where e a b = div (a-1) 3==div (b-1) 3 disp :: T -> String disp s = unlines [unwords [show $ head $ s (i,j) | j <- [1..9]] | i <- [1..9]] input :: String -> T input s = foldr mark (const [1..9]) $ [(p,n) | (p,n) <- zip idx $ map read $ lines s >>= words, n>0] idx :: [(Int,Int)] idx = [(i,j) | i <- [1..9], j <- [1..9]] ------------------------------------------------------------------------------- How to use it: Compile with $ ghc -o sudoku sudoku.hs then feed the board in the standard input as follows: $ cat schema 0 5 0 0 6 0 0 0 1 0 0 4 8 0 0 0 7 0 8 0 0 0 0 0 0 5 2 2 0 0 0 5 7 0 3 0 0 0 0 0 0 0 0 0 0 0 3 0 6 9 0 0 0 5 7 9 0 0 0 0 0 0 8 0 1 0 0 0 6 5 0 0 5 0 0 0 3 0 0 6 0 $ ./sudoku < schema 9 5 3 7 6 2 8 4 1 6 2 4 8 1 5 9 7 3 8 7 1 3 4 9 6 5 2 2 8 9 4 5 7 1 3 6 1 6 5 2 8 3 4 9 7 4 3 7 6 9 1 2 8 5 7 9 6 5 2 4 3 1 8 3 1 8 9 7 6 5 2 4 5 4 2 1 3 8 7 6 9

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