March 2008 - Haskell-Cafe

Video / Slides: Type-driven testing in Haskell (Simon Peyton Jones)
by Robert Hulme 13 Mar '08

13 Mar '08

I was lucky enough to attend a lecture by SPJ yesterday. You're lucky that I taped it and have put it all online :-) http://www.foomongers.org.uk/videos/spj-typedriventestinginhaskell.html If you're a redditer please vote it up on programming.reddit! http://reddit.com/r/programming/info/6byuy/comments/ -Rob -- ----------------------------------------------------- "She's flushing me out with dogs and grenades, and a sincere desire to spend time with me" --Mark Corrigan "That's the way things are these days, let's just put a zip here, a swastika there. Who the hell even knows what these things were used for. Who cares?" --Mark Corrigan http://www.robhulme.com/ http://robhu.livejournal.com/

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Exception handling when using STUArray
by Xiao-Yong Jin 13 Mar '08

13 Mar '08

Dear list, I'm using STUArray in some of my time critical number crunching code. I would like to know some way to catch the exceptions raised in the ST monad, ie. ArrayException. Looking through the Control.Exception module, I understand that those functions can only be used within IO monad. So my question is, what the best way to do exception handling in STUArray is. Plus, it would be helpful if I can also raise an exception in ST monad, but how? Thanks, Xiao-Yong -- c/* __o/* <\ * (__ */\ <

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Bug with GADT in function Patterns?
by Hugo Pacheco 13 Mar '08

13 Mar '08

Hi guys, I have found a bug on the compiler (at least ghc >6.8.2). For some module (yes, the example does nothing at all): *module Test where data Type a where Func :: Type a -> Type b -> Type (a -> b) PF :: Type a -> Type (PF a) data PF a where ID :: PF (a -> a) test :: Type a -> a -> a test (PF (Func _ _)) ID = ID* I get the impossible: *$ ghci Test.hs -fglasgow-exts GHCi, version 6.9.20080303: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. [1 of 1] Compiling Test ( Test.hs, interpreted ) ghc-6.9.20080303: panic! (the 'impossible' happened) (GHC version 6.9.20080303 for i386-apple-darwin): Coercion.splitCoercionKindOf $co${tc aog} [tv] <pred>t_ao8{tv} [tau] ~ a{tv aob} [sk] -> a{tv aob} [sk] Please report this as a GHC bug: http://www.haskell.org/ghc/reportabug* However, the following implementations of *test* compile ok: *test :: Type a -> a -> a test (PF _) ID = ID test :: Type a -> a -> a test (PF (Func _ _)) x = x* It has something to do with mixing different GADTs contructors. Should this be submitted as a bug as it is? Cheers, hugo

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IO () and IO [()]
by Paulo J. Matos 13 Mar '08

13 Mar '08

Hello all, I find it funny that IO () is different from IO [()]. For example, if I define a function to output some lines with mapT, I would do: outputLines :: Int -> IO () outputLines i = mapM (putStrLn . show) (take i $ iterate ((+) 1) 1) However, this is in fact outputLines :: Int -> IO [()] I would like to know if in fact there's any difference in practice between (), [()], i.e. if in practice the difference matters. My first guess is that this is just a consequence of the Haskell type system and so that everything fits this really needs to be like this. Because mapM :: (Monad m) => (a -> m b) -> [a] -> m [b] So I guess that it makes sense that you get IO [()] instead of IO (), and adding an exception just to say that [()] == () isn't good. By the way, as a consequence can you possibly get IO (()) or IO ([()]) and are these all different from each other? Cheers, -- Paulo Jorge Matos - pocm at soton.ac.uk http://www.personal.soton.ac.uk/pocm PhD Student @ ECS University of Southampton, UK Sponsor ECS runners - Action against Hunger: http://www.justgiving.com/ecsrunslikethewind

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An offer to any haskell projects out there.
by michael＠schmong.org 13 Mar '08

13 Mar '08

Hello, My name is Michael Litchard. I'm a techie living in silicon valley, and I want to move into tech writing. I've got the background, now I need a portfolio. I figured the best way to go is to attach myself to some open source projects, and haskell has had my heart for a few years now. I am by no means an expert at haskell. My expertise is writing. So I make this offer. If you need documentation written for your haskell project, I'm your man. Whatever it is, I'll write it or edit it. Thanks for your time. P.S If this was the wrong list, or if anyone has other ideas about how to propogate my proposal, please let me know. Michael

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Reflective capabilities of Haskell (cont'd)
by Martin Hofmann 13 Mar '08

13 Mar '08

I am trying to port a programme written in Maude, which is a reflective language based on rewriting logic ( http://maude.cs.uiuc.edu/ ), to Haskell. I thought using Template Haskell might be a good idea, but I got stuck and now I am wondering if this is really possible in Haskell. Let me give an example of a Maude module defining the function last over a list of Peano numbers. fmod LAST is sorts Peano PeanoList . op 0 : -> Peano [ctor] . op s : Peano -> Peano [ctor] . op [] : -> PeanoList [ctor] . op cons : Peano PeanoList -> PeanoList [ctor] . op last : PeanoList -> Peano . vars X Y : Peano . var Xs : PeanoList . eq last(cons(X,[])) = X . eq last(cons(X,cons(Y,Xs))) = last(cons(Y,Xs)) . endfm So, last(cons(s(0),cons(s(s(0)),cons(s(s(s(0))),[])))) would reduce to s(s(s(0))). The cool thing about Maude is, that terms, modules, ... can be lifted to the meta level. For example: upModule('LAST, false) would yield fmod 'LAST is including 'BOOL . sorts 'Peano ; 'PeanoList . none op '0 : nil -> 'Peano [ctor] . op '`[`] : nil -> 'PeanoList [ctor] . op 'cons : 'Peano 'PeanoList -> 'PeanoList [ctor] . op 'last : 'PeanoList -> 'Peano [none] . op 's : 'Peano -> 'Peano [ctor] . none eq 'last['cons['X:Peano,'`[`].PeanoList]] = 'X:Peano [none] . eq 'last['cons['X:Peano,'cons['Y:Peano,'Xs:PeanoList]]] = last['cons[ 'Y:Peano,'Xs:PeanoList]] [none] . endfm I also have access, e.g. to the defined type constructors. So upOpDecls('LAST,false). yields op '0 : nil -> 'Peano [ctor] . op '`[`] : nil -> 'PeanoList [ctor] . op 'cons : 'Peano 'PeanoList -> 'PeanoList [ctor] . op 'last : 'PeanoList -> 'Peano [none] . op 's : 'Peano -> 'Peano [ctor] . Given an arbitrary function, I have access to its definition, its types and the type constructors, all as ADTs. Is this possible with Template Haskell, or in some other way? Thanks a lot, Martin

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Re: unification
by Christian Maeder 13 Mar '08

13 Mar '08

andrea wrote: >> data Tipo = Var Int | Const String | Tipo :-> Tipo deriving (Show, Eq) >> > But now how can I do to make the set of Const fixed?? > I want just a few base types, like "int" and "string", how could I > define it? your base types could be: Const "String" :: Tipo Const "Int" :: Tipo and the polymorphic type of the identity function could be: Var 1 :-> Var 1 :: Tipo (and with "infixr :->") the type of binary function like (+) Const "Int" :-> Const "Int" :-> Const "Int" Note: the data type does not support type constructors like "List". I recommend from: http://www.haskell.org/haskellwiki/Research_papers/Type_systems Typing Haskell in Haskell The first few pages will be enough (substitution is on page 7) However, the link http://www.cse.ogi.edu/~mpj/pubs/thih.html does not work for me Could someone correct that link? Christian

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floating point operations and representation
by Don Stewart 13 Mar '08

13 Mar '08

> I am under the restriction that I need to write Haskell programs using > Double which mimic existing C/C++ programs or generated data sets, and > get the same answers. (It's silly, but take it as a given > requirement.) If the C programs are using "log2", then I need "log2" > in the Haskell, or else I run the risk of not producing the same > answers. Hey Jacob, Just to make life super simple, I packaged up a binding to the basic math.h library for Doubles. You can find the library here: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/cmath For example, Prelude> Foreign.C.Math.Double.log10 5 0.6989700043360189 Prelude> log 5 / log 10 0.6989700043360187 Enjoy! -- Don

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Re: Reflective capabilities of Haskell (cont'd)
by oleg＠okmij.org 13 Mar '08

13 Mar '08

Martin Hofmann wrote: > Thanks a lot, this helps a bit, but access to function bodies is exactly > what I need. Then perhaps you might like the method of reconstructing bodies (of possibly compiled) functions http://okmij.org/ftp/Computation/Generative.html#diff-th in the form of AST -- the template Haskell AST. The reconstructed bodies of functions can be arbitrarily manipulated (e.g., _symbolically_ differentiated or algebraically simplified) and then converted `back' to the compiled code.

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[GSoC] A data parallel physics engine
by Roman Cheplyaka 13 Mar '08

13 Mar '08

I'm looking for interesting project to work on during Google Summer of Code. So I found [1]"A data parallel physics engine" ticket and got excited about it. I'd like to know interested mentors and community opinion about the complexity of such project. I have not very deep knowledge about both NDP and physics engines. Is it feasible to learn all the necessary stuff during these 2 month before SoC starts? 1. http://hackage.haskell.org/trac/summer-of-code/ticket/1535 -- Roman I. Cheplyaka :: http://ro-che.info/ ...being in love is totally punk rock...

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