- Haskell-Cafe - Haskell.org

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

4 5

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

8 7

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> ...

2 3

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.

1 1

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

4 3

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.

1 0

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

2 1

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

2 1

YAHT: searchAll5
by Magnus Therning 31 Oct '06

31 Oct '06

I can't get searchAll5[1] in Yet Another Haskell Tutorial to run. Ghci complains that it can't find a MonadPlus that satisfies the required type; it needs a MonadPlus. I suspect this is due to the use of 'mzero' and 'mplus', without making StateT a MonadPlus. My thought for this was to "push" mzero into the inner monad and have 'mplus' pass on to the inner monad: instance Monad m => MonadPlus (StateT state m) where mzero = StateT (\s -> return (s, mzero)) (StateT m1) `mplus` (StateT m2) = StateT (\s -> do (s1, a) <- m1 s (s2, b) <- m2 s1 return (s2, a `mplus` b)) What's above isn't accepted by ghci, and I don't even know for sure that my thought makes sense. /M [1]: http://en.wikibooks.org/wiki/Haskell/YAHT/Monads#Monad_Transformers -- Magnus Therning (OpenPGP: 0xAB4DFBA4) magnus(a)therning.org Jabber: magnus.therning(a)gmail.com http://therning.org/magnus Software is not manufactured, it is something you write and publish. Keep Europe free from software patents, we do not want censorship by patent law on written works. Finagle's Fifth Law: Always draw your curves, then plot your readings.

2 1

Deriving class instances using DrIFT
by Daniel McAllansmith 31 Oct '06

31 Oct '06

Hi. I'm trying to derive some instances using DrIFT, but it will only work for me when I'm deriving for types in the current file or in the prelude. For example, this works: >module Test where > >{-! for Maybe derive : Haskell2Xml !-} this works: >module Test where > >data Foo = Foo > >{-! for Foo derive : Haskell2Xml !-} but this doesn't: >module Test where > >import Data.Word > >{-! for Word32 derive : Haskell2Xml !-} It fails to load the first import, giving an error as follows: {- Generated by DrIFT (Automatic class derivations for Haskell) -} {-# LINE 1 "Test.hs" #-} DrIFT: can't find module Data.Word Any ideas as to why that is? Thanks Daniel

3 4