October 2006 - Haskell-Cafe

A restricted subset of CPP included in a revision of Haskell 98
by Brian Smith 05 Nov '06

05 Nov '06

Hi, 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. According to the following document, and my own limited experience in reading Haskell code, CPP is the most frequently used extension: http://hackage.haskell.org/trac/haskell-prime/wiki/HaskellExtensions I think that if we accepted that CPP was part of the language, we could then place some restrictions on its use to facilitate easier parsing. Here are some suggestions, off the top of my head: * #define can only be used for parameterless definitions * #define'd symbols are only visible to the preprocessor * #define can only give a symbol a value that is a valid preprocessor expression * #define can only appear above the module declaration * a preprocessor symbol, once defined, cannot be undefined or redefined * #include and #undef are prohibited * The preprocessor can only be used at the top level. In particular, a prepropcessor conditional, #error, #warn, #line would not be allowed within the export list or within a top-level binding. * A Haskell program must assume that any top-level symbol definitions are constant over the entire program. For example, a program must not depend on having one module compiled with one set of command-line preprocessor symbol bindings and another module defined with a different set of bindings. * preprocessor directives must obey Haskell's layout rules. For example, an #if cannot be indented more than the bindings it contains. The result would be: * Syntax can be fully checked without knowing the values of any preprocessor symbols. * Preprocessor syntax can be added easily to a Haskell parser's BNF description of Haskell. * No tool will need to support per-file/module preprocessor symbol bindings. Again, all this is just off the top of my head. I am curious about what problems these restrictions might cause, especially for existing programs. I know that GHC itself uses some features that would be prohibited here. But, GHC is really difficult for tools to handle even with these restrictions on its source code. For now, I am more interested in the libraries in FPTOOLS and users' programs. What libraries/programs cannot easily be reorganizated to meet these restrictions? I suspect "#define'd symbols are only visible to the preprocessor" would be the most troublesome one. Thanks, Brian

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Simple GADT parser for the eval example
by Greg Buchholz 01 Nov '06

01 Nov '06

I'm trying to create a simple parser for the GADT evaluator from the wobbly types paper, and I need a little help. Here's the GADT and the evaluator... > data Term a where > Lit :: Int -> Term Int > Inc :: Term Int -> Term Int > IsZ :: Term Int -> Term Bool > If :: Term Bool -> Term a -> Term a -> Term a > Pair :: Term a -> Term b -> Term (a,b) > Fst :: Term (a,b) -> Term a > Snd :: Term (a,b) -> Term b > > eval :: Term a -> a > eval (Lit i) = i > eval (Inc t) = eval t + 1 > eval (IsZ t) = eval t == 0 > eval (If b t e) = if eval b then eval t else eval e > eval (Pair a b) = (eval a, eval b) > eval (Fst t) = fst (eval t) > eval (Snd t) = snd (eval t) > ...and instead of writing my own read instance, I thought I'd take a shortcut and just try converting a regular data type to our generalized one... > data Expr = ELit Int > | EInc Expr > | EIsZ Expr > | EPair Expr Expr > | EIf Expr Expr Expr > | EFst Expr > | ESnd Expr > deriving (Read,Show) > > my_read' :: Expr -> Term a > my_read' (ELit a) = Lit a > my_read' (EInc a) = Inc (my_read' a) > my_read' (EIsZ a) = IsZ (my_read' a) > my_read' (EPair a b) = Pair (my_read' a) (my_read' b) > my_read' (EIf p t e) = If (my_read' p) (my_read' t) (my_read' e) > my_read' (EFst a) = Fst (my_read' a) > my_read' (ESnd a) = Snd (my_read' a) ...That looks nice and simple, but it doesn't type check. GHCi-6.6 complains... Couldn't match expected type `a' (a rigid variable) against inferred type `Int' `a' is bound by the type signature for `my_read' at eval_gadt_wobbly.hs:45:24 Expected type: Term a Inferred type: Term Int In the expression: Lit a In the definition of `my_read': my_read (ELit a) = Lit a ...No surprise there, since there is no way to fail in the event of a maltyped "Expr". The next thing to try is a type class solution... > class MyRead a where > my_read :: Expr -> Term a > > instance MyRead Int where > my_read (ELit a) = Lit a > my_read (EInc a) = Inc (my_read a) > my_read (EIf p t e) = If (my_read p) (my_read t) (my_read e) > my_read (EFst a) = Fst (my_read a :: MyRead b => Term (Int,b)) > -- my_read (ESnd a) = Snd (my_read a) > instance MyRead Bool where > my_read (EIsZ a) = IsZ (my_read a) > my_read (EIf p t e) = If (my_read p) (my_read t) (my_read e) > -- my_read (EFst a) = Fst (my_read a) > -- my_read (ESnd a) = Snd (my_read a) > instance (MyRead a, MyRead b) => MyRead (a,b) where > my_read (EPair a b) = Pair (my_read a) (my_read b) > my_read (EIf p t e) = If (my_read p) (my_read t) (my_read e) > -- my_read (EFst a) = Fst (my_read a) > -- my_read (ESnd a) = Snd (my_read a) This just about works, except for the definitions for the "Fst" and "Snd" constructors. The compiler complains... Ambiguous type variable `b' in the constraint: `MyRead b' arising from use of `my_read' at eval_gadt_wobbly.hs:65:28-36 Probable fix: add a type signature that fixes these type variable(s) ...Of course, that makes perfect sense, since, if we're applying "Fst" to a term, then we don't care about the other branch of the "Pair". You can get it accepted by the type checker by making the types more concrete... my_read (EFst a) = Fst (my_read a :: Term (Int,Int)) ...or... my_read (EFst a) = Fst (my_read a :: Term (Int,Bool)) ...but how does a person fix this to work in the more general case? Or is this even the right way to build a parser for the GADT evaluator example? Notice the repetition needed: the "If", "Fst", and "Snd" defintions have to be copied to all three instances. Also, feel free to comment on this example, and the fact that it will evaluate with no problems. > static_vs_laziness = eval (my_read (EIf (EIsZ (ELit 0)) > (ELit 9) > (EIsZ (ELit 42)))::Term Int) Thanks, Greg Buchholz

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function types as instances of Num
by Greg Buchholz 01 Nov '06

01 Nov '06

Let's say we've got a little stack language, where you compute things by transformations of stacks, using compositions of functions from stacks to stacks (represented here as nested tuples). (See also Chris Okasaki's "Techniques for Embedding Postfix Languages in Haskell" www.eecs.harvard.edu/~nr/ cs252r/archive/chris-okasaki/hw02.ps ) For example, the simple program below calculates the square of 4... > {-# OPTIONS -fglasgow-exts #-} > > main = print $ test () > > test = square . (lit 4) > > lit :: Integer -> a -> (Integer,a) > lit val stack = (val, stack) > > dup (a, b) = (a, (a, b)) > mult (a, (b, c)) = (b*a, c) > square = mult . dup ...now let's say I find that using the function "lit" to annotation numeric literals ugly. What I really want is something like... > test' = square . 4 ...Seems simple enough, I'll just make an appropriate instance of Num and I'll be able to use fromInteger... > instance Eq (a -> (Integer, a)) > instance Show (a -> (Integer, a)) > instance Num (a -> (Integer, a)) where > fromInteger = lit ...but now when I try it, GHC complains... No instance for (Num (a -> (Integer, t))) arising from the literal `4' at final.hs:15:17 Possible fix: add an instance declaration for (Num (a -> (Integer, t))) In the second argument of `(.)', namely `4' In the expression: square . 4 In the definition of `test'': test' = square . 4 ...so it seems that (a -> (Integer, t)) can't be unified with (a -> (Integer, a)), or somesuch. Any thoughts on how to get this to work? Thanks, Greg Buchholz

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More documentation: how to create a Haskell project
by dons＠cse.unsw.edu.au 01 Nov '06

01 Nov '06

There's been a bit of discussion on irc, lists and privately about about documenting publically the best practice for creating a new Haskell project -- be that a library or an application. Some advice is now available here: http://haskell.org/haskellwiki/How_to_write_a_Haskell_program Suggestions include: * use darcs * use cabal But we could do with more information on: * where to host a haskell project * integration of testsuites * anything about Hackage? * portability issues? So have a look at the page and make some suggestions, so it will be easier for newcomers in the future to create and contribute new projects, that will be readily accesible, useable and adopted by the community. -- Don

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Accumulating related XML nodes using HXT
by Daniel McAllansmith 31 Oct '06

31 Oct '06

Hello. I have some html from which I want to extract records. Each record is represented within a number of <tr> nodes, and all records <tr> nodes are contained by the same parent node. The things I've tried so far end up giving me the cartesian product of record fields, so for the html fragment included below I'd end up with: [ Prod "Television" 17 "/prod17" "A very nice telly." , Prod "Television" 17 "/prod17" "Mind your fillings." , Prod "Cyclotron" 24 "/prod24" "A very nice telly." , Prod "Cyclotron" 24 "/prod24" "Mind your fillings." ] instead of: [ Prod "Television" 17 "/prod17" "A very nice telly." , Prod "Cyclotron" 24 "/prod24" "Mind your fillings." ] How should I go about accumulating related <tr> nodes into individual records? Thanks Daniel HTML fragment follows: ... <tr> <tr> <td>Product:</td> <td><a href="/prod17">Television</a> (code: 17)</td> </tr> <tr> <td>Description:</td> <td>A very nice telly.</td> </tr> <tr> <td><hr color="#00000"></td> </tr> <tr> <td>Product:</td> <td><a href="/prod24">Cyclotron</a> (code: 24)</td> </tr> <tr> <td>Description:</td> <td>Mind your fillings.</td> </tr> <tr> <td><hr color="#00000"></td> </tr> </tr> ...

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hdbc linking errors
by Jeff Polakow 31 Oct '06

31 Oct '06

Hello, When trying to compile a standalone program using hdbc in cygwin, I get many linker errors. I have no problems using my code interactively with ghci. I am using the command line: ghc --make -package HDBC -package HDBC-odbc -O -o testExecute testExecute.hs Am I missing something? thanks Jeff -- This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.

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Why is Haskell not homoiconic?
by Henning Sato von Rosen 31 Oct '06

31 Oct '06

Hi all! I am curious as to why Haskell not is homoiconic? I am interested in the reasons behind that design descision. I ask using this defintion og homiconicity: Homiconic means that "the primary representation of programs is also a data structure in a primitive type of the language itself" -- http://en.wikipedia.org/wiki/Homoiconic Examples: LISP, Rebol, Natural Languages... Thank you/Henning

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Monad.Reader Licensing (Re: [Haskell] The Monad.Reader - Call for Copy)
by Conrad Parker 31 Oct '06

31 Oct '06

Hi, On Tue, Oct 31, 2006 at 12:10:21PM +0000, Wouter Swierstra wrote: > > * License: The entire magazine should be published under a > Creative Commons Attribution 2.5 License. This makes it much easier > to publish, distribute, teach, and share the Reader. This license > allows anyone to use the work in any way they seem fit as long as > credit is given to the original author. Given that articles written in LaTeX are likely to be written in literate Haskell (yay!), it's quite likely that their contents will be incorporated into software. Others have expressed misgivings about the use of the CC Attribution license for software. Usually the issue is that the method of attribution is unclear: the license leaves it up to the original author of each article to specify how they should be attributed (or not), and has some vague terms regarding "comparable authorship credit". A more detailed summary is at http://people.debian.org/~evan/ccsummary which suggests that such works would not be distributable by the Debian project. I suggest that articles in the Monad.Reader be BSD3 licensed, which has similar intent but clearer rules for incorporation in software. Besides, CC-BY-2.5 is not a member of Distribution.License in Cabal ;-) cheers, Conrad.

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Tutorial: Writing a Lisp Interpreter In Haskell
by dons＠cse.unsw.edu.au 31 Oct '06

31 Oct '06

People might be interested in a new tutorial that's just appeared in blogspace, by coffeemug (of #haskell): http://www.defmacro.org/ramblings/lisp-in-haskell.html Also, its on reddit, http://programming.reddit.com/info/oj1w/details An enthusiastic view of the language from a newcomer's perspective. -- Don

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Guidelines for proposing library changes
by dons＠cse.unsw.edu.au 31 Oct '06

31 Oct '06

After some discussion on the libraries list, I've put up the suggested 'best practice' for proposing library changes, here: http://haskell.org/haskellwiki/Library_submissions The idea is to reduce the number of 'bikeshed' discussions we're having, ensure that submissions are not dropped, and generally improve productivity. People should refer to this when proposing new functions for the core libraries, at risk of not having the suggestion adopted or considered ;) Cheers, Don

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