January 2007 - Libraries

Is there already a list class?
by Marc Weber 13 May '07

13 May '07

Hello. Is here the right place to request a list class? eg class List l e where (:) :: e -> l e -> l e head :: .. This might be used in Data.Set, Data.Map class StorableAsList l e t where fromList :: l e -> t toList :: t -> l e I'd like to help implementing/ writing it. Do you consider this beeing a useful enhancement? Cheers Marc Weber

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proposal: add 'unsafeCoerce'
by Malcolm Wallace 02 Apr '07

02 Apr '07

Currently, all implementations provide either the 'unsafeCoerce' or 'unsafeCoerce#' functions, but they live in implementation-specific locations. This proposal is to adopt the Haskell'98-compatible name 'unsafeCoerce', and to add it to the standard base library package. Suggested location: Data.Function.Unsafe? http://hackage.haskell.org/trac/ghc/ticket/994 Regards, Malcolm

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Data.IntMap.findMax is missing
by Christian Maeder 18 Mar '07

18 Mar '07

recently I've switched from "Data.Map.Map Int" to "Data.IntMap.IntMap", but I was disappointed because I did not find a function that would yield the maximum key faster than in linear time, i.e. O(log n). In fact the whole "Min/Max" section is missing in Data.IntMap (resp. Data.IntSet) compared to Data.Map (resp. Data.Set). (The section "Indexed" is also only in Data.Map.) Would somebody care to fix this? Cheers Christian

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Proposal: System.Timeout module for base
by Peter Simons 19 Feb '07

19 Feb '07

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Proposal: Make StateT in mtl lazy
by Ian Lynagh 04 Feb '07

04 Feb '07

http://hackage.haskell.org/trac/ghc/ticket/1127 Deadline: 28 February 2007. I propose we make StateT lazy, as I suspect was the intention all along and as has been discussed (without reaching a conclusion) before. The patch is simply (in the definition of (>>=)): hunk ./Control/Monad/State.hs 195 - (a, s') <- runStateT m s + ~(a, s') <- runStateT m s Thanks Ian

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Unsafe aspects of ByteString
by Iavor Diatchki 01 Feb '07

01 Feb '07

Hello, The "packCString" function (and other similar functions) in the ByteString library break referential transperancy, which is one of the big selling points of Haskell (and its libraries). Here is an example: main = do x <- newCString "Hello" let s = packCString x h1 = B.head s print s -- print h1 poke x (toEnum 97) print s let h2 = B.head s print h1 print h2 Output: "Hello" "aello" 97 97 This is already confusing because the "pure" value 's' has magically changed. Also notice that the evaluation order of the program affects the output. If we include the commented out statement (which forces h1 to be evaluated earlier) the output becomes: "Hello" 72 "aello" 72 97 I think that because of this we should either remove these functions, or at least follow the convention of other libraries and give them "unsafe" names. -Iavor

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hackageDB
by Ross Paterson 31 Jan '07

31 Jan '07

The web interface to HackageDB (the repository of releases of Haskell packages) is still fairly crude, and much is yet to be added, but the basic functionality is working: http://hackage.haskell.org/packages/archive/pkg-list.html We now need more early adopters to add their packages and otherwise try it out. All comments welcome.

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Proposal: Add Data.String, containing IsString(fromString), to base
by Ian Lynagh 30 Jan '07

30 Jan '07

http://hackage.haskell.org/trac/ghc/ticket/1126 Proposal to create a new module Data.String, containing IsString(fromString), in the base package. This would be used by the overloaded strings extension (-foverloaded-strings in GHC). Deadline: 28 Feb 2007 Here's the meat of the patch: hunk ./Data/String.hs 1 +{-# OPTIONS_GHC -fno-implicit-prelude #-} +----------------------------------------------------------------------------- +-- | +-- Module : Data.String +-- Copyright : (c) The University of Glasgow 2007 +-- License : BSD-style (see the file libraries/base/LICENSE) +-- +-- Maintainer : libraries(a)haskell.org +-- Stability : experimental +-- Portability : portable +-- +-- Things related to the String type. +-- +----------------------------------------------------------------------------- + +module Data.String ( + IsString(..) + ) where + +#ifdef __GLASGOW_HASKELL__ +import GHC.Base +#endif + +-- | Class for string-like datastructures; used by the overloaded string +-- extension (-foverloaded-strings in GHC). +class IsString a where + fromString :: String -> a + +instance IsString [Char] where + fromString xs = xs Thanks Ian

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Problems with Cabal
by Marco Túlio Gontijo e Silva 29 Jan '07

29 Jan '07

Hello, I'm having some problems with cabal. I don't know if here is the better place to ask, in http://www.haskell.org/cabal/ sais to ask here. There are two things. One of them is simpler, and it's in haskell-cafe. Shouldn't include-dirs: . be default? The other is about setup clean. It's leaving some things behind. The dist directory is not removed, and is kept empty. Other files that are not removed are the c2hs generated files .h and .chi. Is this a bug? marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ find | grep -v _darcs . ./AUTHORS ./Algebra ./Algebra/Geometric ./Algebra/Geometric/Gpc.chs ./COPYING ./README ./Setup.hs ./ToDo ./haskellgpc.cabal ./send.sh marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ ./Setup.hs configure Configuring HaskellGPC-0.0.1... configure: /usr/bin/ghc-pkg configure: Dependency base-any: using base-2.0 configure: Using install prefix: /usr/local configure: Binaries installed in: /usr/local/bin configure: Libraries installed in: /usr/local/lib/HaskellGPC-0.0.1/ghc-6.6 configure: Private binaries installed in: /usr/local/libexec configure: Data files installed in: /usr/local/share/HaskellGPC-0.0.1 configure: Using compiler: /usr/bin/ghc configure: Compiler flavor: GHC configure: Compiler version: 6.6 configure: Using package tool: /usr/bin/ghc-pkg configure: Using ar found on system at: /usr/bin/ar configure: Using haddock found on system at: /usr/bin/haddock configure: No pfesetup found configure: Using ranlib found on system at: /usr/bin/ranlib configure: Using runghc found on system at: /usr/bin/runghc configure: No runhugs found configure: No happy found configure: No alex found configure: Using hsc2hs: /usr/bin/hsc2hs configure: Using c2hs: /usr/bin/c2hs configure: No cpphs found configure: No greencard found marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ ./Setup.hs build Preprocessing library HaskellGPC-0.0.1... Building HaskellGPC-0.0.1... [1 of 1] Compiling Algebra.Geometric.Gpc ( Algebra/Geometric/Gpc.hs, dist/build/Algebra/Geometric/Gpc.o ) /usr/bin/ar: creating dist/build/libHSHaskellGPC-0.0.1.a marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ ./Setup.hs clean cleaning... marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ find | grep -v _darcs . ./AUTHORS ./Algebra ./Algebra/Geometric ./Algebra/Geometric/Gpc.chi ./Algebra/Geometric/Gpc.chs ./Algebra/Geometric/Gpc.h ./COPYING ./README ./Setup.hs ./ToDo ./dist ./haskellgpc.cabal ./send.sh marcot@quindinho:~/codigo/haskell/haskellgpc/haskellgpc$ Thanks. -- malebria Marco Túlio Gontijo e Silva Correio (MSN): malebria(a)riseup.net Jabber (GTalk): malebria(a)jabber.org Ekiga: malebria(a)ekiga.net IRC: malebria(a)irc.freenode.net malebria(a)irc.oftc.org Skype: marcotmarcot Telefone: 33346720 Celular: 98116720 Endereço: Rua Paula Cândido, 257/201 Gutierrez 30430-260 Belo Horizonte/MG Brasil

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Monad laws for fail
by Ulf Norell 29 Jan '07

29 Jan '07

I tried out the recently released and very nice binary library and tripped up by laziness. Muchly simplified my problem was: data Bad = Bad instance Binary Bad where put = return () get = do fail "this is bad" return Bad Here I expected decoding to Bad to fail, but to my surprise: ghci> decode (encode Bad) :: Bad Bad From the example, it is clear that my brain has the following law for fail built in: fail err >> m === fail err Some testing reveals that this law is violated not only by the Get monad, but also by State, Reader, and Writer. (There was a discussion about the strictness of the state monad a while back, I can't recall if this came up...) So, where am I going wrong? Am I mixing up fail and mzero? Are there other laws for fail that actually hold? / Ulf

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