December 2010 - Haskell-Cafe

Re: [Haskell-cafe] Missing Parsec library in latest stable GHC
by Antoine Latter 22 Dec '10

22 Dec '10

ghc doesn't, by default, go searching for packages to link in to the resulatant executable.. If you try 'ghc --make scheme.hs' your example will work better. You can also specify what to link manually, but --make works pretty well most of the time. In ghc version 7 '--make' is the default, but until that's more widely distributed you'll want to get used to using the switch. Take care, Antoine On Wed, Dec 22, 2010 at 10:36 AM, Aaron Gray <aaronngray.lists(a)gmail.com> wrote: > On 22 December 2010 16:27, Antoine Latter <aslatter(a)gmail.com> wrote: >> >> What commands did you enter to produce this error? >> > > ghc scheme.hs > Aaron > >> >> Ahanks, >> Antoine >> >> On Wed, Dec 22, 2010 at 9:54 AM, Aaron Gray <aaronngray.lists(a)gmail.com> >> wrote: >> > Missing Parsec library :- >> > scheme.o(.text+0x4fa):fake: undefined reference to >> > >> > `parseczm2zi1zi0zi0_TextziParserCombinatorsziParsecziCombinator_skipMany1_closure' >> > scheme.o(.text+0x501):fake: undefined reference to >> > `parseczm2zi1zi0zi0_TextziPaserCombinatorsziParsecziChar_space_closure' >> > scheme.o(.text+0x5c2):fake: undefined reference to >> > `parseczm2zi1zi0zi0_TextziParserCombinatorsziParsecziChar_oneOf_closure' >> > scheme.o(.text+0x63a):fake: undefined reference to >> > `mtlzm1zi1zi0zi0_ControlziMonadziTrans_zdf1_closure' >> > GHC from :- >> > http://www.haskell.org/ghc/dist/stable/dist/ >> > ghc-7.0.1.20101221-i386-windows.exe >> > and earlier. >> > Aaron >> > >> > _______________________________________________ >> > Haskell-Cafe mailing list >> > Haskell-Cafe(a)haskell.org >> > http://www.haskell.org/mailman/listinfo/haskell-cafe >> > >> > > >

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Re: [Haskell-cafe] Functor => Applicative => Monad
by Isaac Dupree 22 Dec '10

22 Dec '10

On 12/14/10 03:13, John Smith wrote: > I would like to formally propose that Monad become a subclass of > Applicative, with a call for consensus by 1 February. The change is > described on the wiki at > http://haskell.org/haskellwiki/Functor-Applicative-Monad_Proposal, That page isn't written as a proposal yet, it's written as a bunch of ideas. I would be happy to see something along the lines of Bas van Dijk's work http://permalink.gmane.org/gmane.comp.lang.haskell.libraries/14740 . -Isaac

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Rendering of <hask> in new wiki (MSIE6)
by Dimitry Golubovsky 22 Dec '10

22 Dec '10

Hi, In MSIE6, <hask> tags are rendered like this (from the Monad_Transformers page): transformers: provides the classes MonadTrans and MonadIO , as well as concrete monad transformers such as StateT ... etc. The Wiki source: [http://hackage.haskell.org/package/transformers transformers]: provides the classes <hask>MonadTrans</hask> and <hask>MonadIO</hask>, as well as concrete monad transformers such as <hask>StateT</hask>. HTML (a small piece of it): provides the classes <div class="inline-code"><div dir="ltr" style="text-align: left;"><div class="source-haskell" style="font-family: monospace;">MonadTrans</div></div></div> Words MonadTrans, MonadIO, StateT etc are enclosed in <hask> tags. They show up in monospace, each starting a new line. Is this only MSIE6, or this is how it is supposed to render? Thanks. PS I am not attaching a screenshot to the mailing list; if anyone needs to see it I'll send via personal e-mail. -- Dimitry Golubovsky Anywhere on the Web

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Latest Haskell Platform for Windows
by Aaron Gray 22 Dec '10

22 Dec '10

Could someone please point me at a copy of the latest Haskell platform or a working GHC please. Many thanks in advance, Aaron

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Windows Haskell Platform download link is broken
by Aaron Gray 22 Dec '10

22 Dec '10

Windows Haskell Platform download link goes nowhere :- http://hackage.haskell.org/platform/windows.html The "Download Haskell for Windows" is broken :- http://lambda.galois.com/hp-tmp/2010.2.0.0/HaskellPlatform-2010.2.0.0-setup… Aaron

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Problem with class Control.Monad.Error noMsg usage
by Aaron Gray 21 Dec '10

21 Dec '10

I have been trying to build the Scheme in 24 Hours on WikiBooks :- http://en.wikibooks.org/wiki/Write_Yourself_a_Scheme_in_48_Hours http://jonathan.tang.name/files/scheme_in_48/code/listing10.hs But I am getting an error :- scheme.hs:289:6: `noMsg' is not a (visible) method of class `Error' scheme.hs:290:6: `strMsg' is not a (visible) method of class `Error' with the following :- instance Error LispError where noMsg = Default "An error has occurred" strMsg = Default if I try importing them :- import Control.Monad.Error (noMsg, strMsg) I get the following :- scheme.hs:4:29: Module `Control.Monad.Error' does not export `noMsg' scheme.hs:4:36: Module `Control.Monad.Error' does not export `strMsg' Many thanks in advance, Aaron

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Re: [Haskell-cafe] Proof in Haskell
by Daniel Fischer 21 Dec '10

21 Dec '10

I wrote: > On Tuesday 21 December 2010 19:34:11, Felipe Almeida Lessa wrote: >> >> Theorem mirror_mirror : forall A (x : Tree A), mirror (mirror x) = x. >> induction x; simpl; auto. >> rewrite IHx1; rewrite IHx2; trivial. >> Qed. > Since mirror (mirror x) = x doesn't hold in Haskell, I take it that Coq > doesn't allow infinite structures? Oops, mirroring infinite binary trees should work. Ignore above.

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type class design
by Uwe Schmidt 21 Dec '10

21 Dec '10

Dear Haskellers, I've a question about type class design. When developing the set of functions for a class, there are often two or more functions, let's say f and g, where the semantics of g can be expressed by f. When writing down the code, there are two choices for g. First g is included in the type class, second g is defined outside and the signature has a context referencing the class. 1. case class Foo a where f :: ... a ... g :: ... a ... g = expr[f] 2. case class Foo a where f :: ... a ... g :: Foo a => ... a ... g = expr[f] What are the reasons for choosing 1., what are the arguments for 2.? My arguments for 1.: a) Efficiency: g may be overwritten in an instance declaration by a more efficient piece of code. b) Readability: All functions, which are logically connected are grouped together in a single syntactic construct and can be found in one place (similar to an interface in Java). My argument for 2.: c) Correctness: The semantics for g are fix (relative to f), All laws for g hold everywhere, assuming f is implemented correctly. I've sometimes the problem to balance the reasons for 1. and 2 and I doubt, that this list of criteria is complete. In the standard Haskell classes we can find both cases, even within a single class. Eq with (==) as f and (/=) as g belongs to the 1. case, so does Monad and (>>=) as f and (>>) as g. But (>=>) and the Monad class fall into the 2. case. It seem, there is no uniform or consistent strategy in the core Haskell classes. What are your best practice rules for this problem? Expecting your advices, Uwe

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ANN: The attosplit package splits lazy bytestrings lazily
by Yitzchak Gale 21 Dec '10

21 Dec '10

This is a belated announcement of a package I uploaded to hackage last week. When processing a large input stream, one of the most important techniques is to split the input into smaller pieces and process each piece separately. If that large input stream happens to be a lazy bytestring, the attosplit package provides a simple but powerful way to use that technique. The attosplit package exports a single function: split :: Parser Strict.ByteString -> Lazy.ByteString -> [Lazy.ByteString] It splits a lazy bytestring into a lazy list of lazy bytestrings, at boundaries defined by an attoparsec parser. Whenever the parser matches, the result of the match is prepended to the new lazy bytestring which begins at that point. For example, suppose lazyH is a lazy bytestring read from a gigantic HTML file, and you want to process each section beginning with an <H1> tag separately. Each section could itself still be quite large, so you want to process each section lazily as well. Then split (string "<H1>") lazyH is a lazy list of lazy bytestrings. The first lazy bytestring gives you all of the bytes up to the first <H1> tag. The rest of the lazy bytestrings give each section beginning with <H1> separately. This allows you to read the entire file lazily, while processing each individual section lazily on its own. Of course, you are not limited to split only at boundaries defined by a string. You have the full power of attoparsec at your fingertips. http://hackage.haskell.org/package/attosplit Enjoy! -Yitz

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Re: [Haskell-cafe] Do we need Monad fail (or MonadFail)?
by Jonathan Geddes 21 Dec '10

21 Dec '10

I'd be really interested in learning the rationale behind those changes. I'm sure it wasn't done for capricious or arbitrary reasons, but I can't help but see it as a step back. --Jonathan Geddes (sent from android mobile) On Dec 21, 2010 8:47 AM, "Lauri Alanko" <la(a)iki.fi> wrote: On Tue, Dec 21, 2010 at 08:31:08AM -0700, Jonathan Geddes wrote: > I'd love for the compiler to give... You will be interested to know that everything you ask for already was in Haskell ages ago: http://www.cs.auckland.ac.nz/references/haskell/haskell-report-1.4-html/exp… They decided to get rid of it in Haskell 98, for reasons that someone else can probably explain. Lauri _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe(a)haskell.org ...

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