March 2009 - Haskell-Cafe

Re: Use unsafePerformIO to catch Exception?
by John Lato 27 Mar '09

27 Mar '09

> From: Jules Bean <jules(a)jellybean.co.uk> > wren ng thornton wrote: >> The type of head should not be [a] -> a + Error, it should be (a:[a]) -> >> a. With the latter type the compiler can ensure the precondition will be >> proved before calling head, thus eliminating erroneous calls. > > Yes, but you know and I know that's not haskell. > > I'm talking about haskell. > > In haskell - a language which does not fully support dependent types - > head is both necessary and useful. > I could follow the rest of this, but I don't understand why 'head' is necessary. Couldn't you always replace it with a case statement, with undefined on [] if necessary? I won't deny that it's extremely useful, though! John Lato

3 3

Internationalization of Haskell code
by Vasyl Pasternak 27 Mar '09

27 Mar '09

In my blog http://progandprog.blogspot.com/2009/03/i18n-and-haskell.html, I've described process of internationalization Haskell programms using GNU GetText. I wrote hgettext tool and Text.I18N.GetText library to help with internationalization (latest sources on http://hgettext.googlecode.com/files/hgettext-0.1.2.tar.gz) A bit later I plan to put it to hackage and wiki (waiting for account). -- Best regards, Vasyl Pasternak

4 3

The infamous evil monomorphism restriction (was: A bit of a shock - Memoizing functions)
by Peter Verswyvelen 27 Mar '09

27 Mar '09

>From a previous email in the beginners list I more or less understood that the monomorphism restriction will not exist anymore in Haskell Prime. Is this correct? On Fri, Mar 27, 2009 at 10:32 PM, Jonathan Cast <jonathanccast(a)fastmail.fm>wrote: > On Fri, 2009-03-27 at 14:26 -0700, Kirk Martinez wrote: > > Your powersOfTwo function, since it gets memoized automatically (is > > this the case for all functions of zero arguments?), > > It is the case for all functions which have zero arguments *at the time > they are presented to the code generator*. The infamous evil > monomorphism restriction arises from the fact that overloaded > expressions, such as > > negative_one = exp(pi * sqrt(-1)) > > look like functions of zero arguments, but are not, and hence do not get > memoized. This behavior was considered sufficiently surprising, when it > was discovered in early Haskell compilers, that the construct was > outlawed from the language entirely. > > jcc > > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

1 1

A bit of a shock - Memoizing functions
by Gü?nther Schmidt 27 Mar '09

27 Mar '09

Hi, um, well, I'm not even sure if I have correctly understood this. Some of the memoizing functions, they actually "remember" stuff *between* calls? Günther

9 9

toSelem takes no format parameter
by Kemps-Benedix Torsten 27 Mar '09

27 Mar '09

Sterling, the toSElem function of ToSElem does not take a parameter that allows specification of e.g. the locale that shall be used for rendering a specific attribute. If one wants to change this, one also has to change setAttribute and the like. Do you have any plans to extend HStringTemplate in this way? Schöne Grüße Torsten Kemps-Benedix (Geschäftsführer) SKS Unternehmensberatung GmbH & Co KG Geheimrat-Hummel-Platz 4 D-65239 Hochheim am Main Germany M +49.163.36017.01 T +49.700.36017.000 F +49.700.36017.011 torsten.kemps-benedix(a)sks-ub.de <blocked::mailto:torsten.kemps-benedix@sks-ub.de> http://www.sks-ub.de <blocked::http://www.sks-ub.de/>

2 1

wxHaskell
by Tsunkiet Man 27 Mar '09

27 Mar '09

Hello, I've just installed wxHaskell as follow: - Download wxHaskell from the following page: http://haskell.org/haskellwiki/WxHaskell/Download and downloaded the binairy version for Windows XP. - Registered using the batch file provided. Now I have this small program from my education page that I want to run just to make sure my installation is correct. ( http://www.cs.uu.nl/wiki/pub/FP/Practicum/Balls.hs) I've also got Helium (2003) and winHugs (sept 2006) (I'm so sorry for using the old versions, but I just can't find new versions) My GHC compiler is version: GHC 6.10.1 and my wxHaskell is named like: wxhaskell-bin-msw2.8.9-ghc6.10.1-0.11.0-0.zip. (If you need more information just contact me) Now my error message in winHugs is: And if ERROR file:.\Balls.hs - Can't find imported module "Graphics.UI.WX" And if I run that in Helium I get some error message that the . in "Graphics.UI.WX" is unexpected. (Which means by my views that it didn't import wxHaskell.) And if I run it in ghci it compiles perfect. So I thought let's type in "main" to start it in a interpreter. So I get the following: http://img10.imageshack.us/img10/5118/testzrs.jpg. I don't think that program is running correctly. Can someone point out what went wrong or what I am doing wrong? And some small question, do I always have to compile those programs in order for it to run correctly? Thanks for your time and effort! Tsunkiet Man

2 2

want to upgrade to 6.10.1 ghc
by Vasili I. Galchin 27 Mar '09

27 Mar '09

Hello, I am running Ubuntu Linux. I have forgotten what is the preferred means of upgrading. Download the source and compile it using my current compiler .. 6.8.2? Regards, Vasili

2 1

ANN: HackMail 0.0 -- Procmail + Monads = Awesome!
by Joe Fredette 27 Mar '09

27 Mar '09

I'm happy to announce my very second Hackage upload!* HackMail is a Procmail-alike, though it doesn't (yet) support promail syntax. It dynamically loads a haskell source file (literate or vanilla, eithers okay) and then sits as a daemon watching a directory for new emails. The source file contains a function which sorts email and delivers it to some directory. As well as some configuration data. Not everything is working just yet -- the program is operational, but still lacking some of the features I want, namely a pipe mode, and some other things. It also uses a homebrew email parser, and lacks Haddock and Tests, but I wanted to release it mostly for comments/questions/helpful criticism from interested parties. Also, I wanted to upload something to hackage... It's a good feeling. Questions/Comments/Donations of Money, Vestal Virgins, or Cookies welcome! Package is on Hackage @ http://hackage.haskell.org/cgi-bin/hackage-scripts/package/HackMail Source is on Patch-tag @ http://patch-tag.com/repo/Hackmail /Joe Fredette * (I uploaded a little tool called addLicenseInfo to test out the procedure, so... yah.)

1 0

Arch Linux now provides more than 1000 Haskell libraries and apps
by Don Stewart 27 Mar '09

27 Mar '09

A small milestone in the packaging business: http://archhaskell.wordpress.com/2009/03/27/arch-haskell-news-mar-14-2009-1… More than 1000 Haskell packages packaged up for Arch Linux. Hackage now has 1163 (+41) Haskell packages, of which 1007 (+33) have been natively packaged for Arch in AUR. That’s 33 new packages in two weeks, and lots of updates as well. Read this week's updates on the blog. -- Don

1 0

typeOf for polymorphic value
by oleg＠okmij.org 27 Mar '09

27 Mar '09

> Does anyone know of a trick to accomplish `typeOf id'? > Using something else than TypeRep as the representation, of course. Yes. The analysis of polymorphic types has been used in the inverse type-checker http://okmij.org/ftp/Haskell/types.html#de-typechecker The enclosed code computes TypeRep of polymorphic values. The code returns real TypeRep. Here are a few examples: tt2 = mytypof not -- Bool -> Bool tt3 = mytypof [True] -- [Bool] tt4 = mytypof id -- any1 -> any1 tt5 = mytypof [] -- [any1] tt6 = mytypof const -- any1 -> any2 -> any1 tt7 = mytypof Just -- any1 -> Maybe any1 tt8 = mytypof map -- (any1 -> any2) -> [any1] -> [any2] tt9 = mytypof maybe -- any1 -> (any2 -> any1) -> Maybe any2 -> any1 The code was tested on GHC 6.8.2. I don't have access to any computer with GHC 6.10, so I can't say how it works with that version of GHC. {-# LANGUAGE ScopedTypeVariables, EmptyDataDecls #-} {-# LANGUAGE FlexibleInstances, FlexibleContexts #-} {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE IncoherentInstances #-} module Typeof where import Data.Typeable -- tests tt1 = mytypof True -- Bool tt2 = mytypof not -- Bool -> Bool tt3 = mytypof [True] -- [Bool] tt4 = mytypof id -- any1 -> any1 tt5 = mytypof [] -- [any1] tt6 = mytypof const -- any1 -> any2 -> any1 tt7 = mytypof Just -- any1 -> Maybe any1 tt8 = mytypof map -- (any1 -> any2) -> [any1] -> [any2] tt9 = mytypof maybe -- any1 -> (any2 -> any1) -> Maybe any2 -> any1 class MyTypeable a where mytypof :: a -> TypeRep -- Gamma is the sequence of type varaibles. It is used to assign -- different indices to different type variables instance (Analyze a result, MyTypeable' HNil gout result) => MyTypeable a where mytypof a = fst $ mytypof' HNil (analyze a) class MyTypeable' gin gout classification | gin classification -> gout where mytypof' :: gin -> classification -> (TypeRep,gout) instance Typeable a => MyTypeable' gin gin (TAtomic a) where mytypof' gin _ = (typeOf (undefined::a), gin) instance Typeable a => MyTypeable' gin gin (TAtomic1 a) where mytypof' gin _ = (typeOf (undefined::a), gin) instance (MyTypeable' gin g a, MyTypeable' g gout b) => MyTypeable' gin gout (TFunction a b) where mytypof' gin _ = let (tr1,g) = mytypof' gin (undefined::a) (tr2,gout) = mytypof' g (undefined::b) in (mkFunTy tr1 tr2,gout) instance (MyTypeable' gin g c, MyTypeable' g gout a) => MyTypeable' gin gout (TConstructed c a) where mytypof' gin _ = let (tr1,g) = mytypof' gin (undefined::c) (tr2,gout) = mytypof' g (undefined::a) in (mkTyConApp (typeRepTyCon tr1) [tr2],gout) instance (HIndex a gin n, MyTypeable'' n gin gout (TVariable a)) => MyTypeable' gin gout (TVariable a) where mytypof' = mytypof'' (undefined::n) class MyTypeable'' n gin gout classification | n gin classification ->gout where mytypof'' :: n -> gin -> classification -> (TypeRep,gout) instance HLen gin n1 => MyTypeable'' Z gin (HCons a gin) (TVariable a) where mytypof'' _ gin _ = (mkany (undefined::S n1), HCons (undefined::a) gin) instance Nat n => MyTypeable'' (S n) gin gin (TVariable a) where mytypof'' _ gin _ = (mkany (undefined::S n), gin) mkany :: Nat n => n -> TypeRep mkany n = mkTyConApp (mkTyCon ts) [] where ts = "any" ++ show (nat n) -- Lookup the index of an item x in the list l -- The index is 1-based. If not found, return 0 class Nat n => HIndex x l n | x l -> n instance HIndex x HNil Z instance (Nat n, TypeEq x a f, HIndex' f x (HCons a l) n) => HIndex x (HCons a l) n class HIndex' f x l n | f x l -> n instance HLen l n => HIndex' HTrue x l n instance HIndex x l n => HIndex' HFalse x (HCons a l) n -- Length of a list class Nat n => HLen l n | l -> n instance HLen HNil Z instance HLen l n => HLen (HCons a l) (S n) -- Analyze a type -- Result of the type analysis data TAtomic a data TVariable a data TFunction a b data TConstructed c a -- only one level: kind * -> * data TAtomic1 a data TVariable1 a data Level1 a data W a = W a -- We can have more levels, cf Typeable2, etc. class Analyze a b | a -> b analyze:: Analyze a b => a -> b analyze = undefined instance TypeCast f (TAtomic Int) => Analyze Int f instance TypeCast f (TAtomic Bool) => Analyze Bool f instance TypeCast f (TAtomic Char) => Analyze Char f instance TypeCast f (TAtomic Int) => Analyze (W Int) f instance TypeCast f (TAtomic Bool) => Analyze (W Bool) f instance TypeCast f (TAtomic Char) => Analyze (W Char) f instance (Analyze (c x) rx, Analyze (d y) ry, TypeCast f (TFunction rx ry)) => Analyze (c x->d y) f instance (Analyze (W x) rx, Analyze (d y) ry, TypeCast f (TFunction rx ry)) => Analyze (x->d y) f instance (Analyze (c x) rx, Analyze (W y) ry, TypeCast f (TFunction rx ry)) => Analyze (c x->y) f instance (Analyze (W x) rx, Analyze (W y) ry, TypeCast f (TFunction rx ry)) => Analyze (x->y) f instance (Analyze (W x) rx, Analyze (W y) ry, TypeCast f (TFunction rx ry)) => Analyze (W (x->y)) f instance (Analyze (Level1 (c ())) rc, Analyze (W x) rx, TypeCast f (TConstructed rc rx)) => Analyze (c x) f instance TypeCast f (TAtomic1 ([] ())) => Analyze (Level1 ([] ())) f instance TypeCast f (TAtomic1 (Maybe ())) => Analyze (Level1 (Maybe ())) f instance TypeCast f (TVariable1 (c ())) => Analyze (Level1 (c ())) f -- the most general instance instance TypeCast f (TVariable a) => Analyze (W a) f -- instance TypeCast f (TVariable a) => Analyze a f -- the most general instance {- ta1 = analyze True -- ta1 :: TAtomic Bool ta2 = analyze not -- ta2 :: TFunction (TAtomic Bool) (TAtomic Bool) ta3 = analyze [True] -- ta3 :: TConstructed (TAtomic1 [()]) (TAtomic Bool) ta4 = analyze id -- ta4 :: TFunction (TVariable GHC.Prim.Any) (TVariable GHC.Prim.Any) ta5 = analyze [] -- ta5 :: TConstructed (TAtomic1 [()]) (TVariable GHC.Prim.Any) ta6 = analyze const -- ta6 :: TFunction -- (TVariable GHC.Prim.Any) -- (TFunction (TVariable GHC.Prim.Any) (TVariable GHC.Prim.Any)) -} -- Standard HList stuff data Z data S a class Nat a where nat :: a -> Int instance Nat Z where nat _ = 0 instance Nat a => Nat (S a) where nat _ = succ (nat (undefined::a)) data HTrue data HFalse instance Show HTrue where show _ = "HTrue" instance Show HFalse where show _ = "HFalse" data HNil = HNil deriving Show data HCons a b = HCons a b deriving Show -- literally lifted from the HList library class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x class TypeEq' () x y b => TypeEq x y b | x y -> b where type'eq :: x -> y -> b type'eq _ _ = undefined::b class TypeEq' q x y b | q x y -> b class TypeEq'' q x y b | q x y -> b instance TypeEq' () x y b => TypeEq x y b instance TypeCast b HTrue => TypeEq' () x x b instance TypeEq'' q x y b => TypeEq' q x y b instance TypeEq'' () x y HFalse

2 1