December 2008 - Haskell-Cafe

what does "atomically#" mean?
by Daryoush Mehrtash 20 Mar '09

20 Mar '09

Any idea was the atomically# mean in the following code? atomically :: STM a -> IO a atomically (STM m) = IO (\s -> (*atomically# *m) s ) Code is from GHC.Conc module http://www.haskell.org/ghc/docs/6.6/html/libraries/base/GHC-Conc.html daryoush

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Using cabal to build an exportable library?
by Justin Bailey 20 Mar '09

20 Mar '09

I've been following this instructions at http://www.haskell.org/haskellwiki/Calling_Haskell_from_C to build a Haskell library which I can call from a C program. I'd like to use cabal to do the build in the future. Are there any examples showing how to do it?

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Proposal for associated type synonyms in Template Haskell
by Thomas van Noort 19 Mar '09

19 Mar '09

Hello, Recently, we released a library on Hackage for generic rewriting (package "rewriting" if you are curious). The user of the library is expected to define type class instances to enable rewriting on his or her own datatypes. As these instances follow the datatype declarations closely, we tried to generate the instances using Template Haskell. Unfortunately, associated type synonyms are not yet supported by TH. After a presentation at the WGP'08, Simon encouraged us to write a proposal about adding associated type synonyms to TH, so that it can be added to GHC. So, here is our proposal. The TH AST must allow 1) kind declarations of associated type synonyms in class declarations and 2) their definitions in instance declarations. For example, class Foo a where type Bar a :: * instance Foo Int where type Bar Int = String The TH library defines a datatype Dec which contains a constructor for class declarations and instance declarations: data Dec = ... | ClassD Cxt Name [Name] [FunDep] [Dec] | InstanceD Cxt Type [Dec] ... 1) Associated type synonym kind declarations We suggest to add a constructor to the Dec type: ... | AssocTySynKindD Name [Name] (Maybe Kind) ... assocTySynKindD :: Name -> [Name] -> Maybe KindQ -> DecQ The first field is the name of the associated type synonym, the second field is a list of type variables, and the third field is an optional kind. Since kinds are not yet defined in TH, we have to add some kind of kind definition (pun intended): data Kind = StarK | ArrowK Kind Kind type KindQ = Q Kind starK :: KindQ arrowK :: KindQ -> KindQ -> KindQ We explicitly choose not to reuse the Type type to define kinds (i.e., type Kind = Type as in GHC) since we think a separation between the two worlds is much clearer to the users of TH. 2) Associated type synonym definitions We suggest to add another constructor to the Dec type: ... | AssocTySynD Name [Type] Type ... assocTySynD :: Name -> [TypeQ] -> TypeQ -> DecQ The first field is the name of the type synonym, the second field is a list of type arguments, and the third field is the body of the type synonym. We would like to hear your comments to this proposal. Regards, Thomas

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What *not* to use Haskell for
by Dave Tapley 19 Mar '09

19 Mar '09

Hi everyone So I should clarify I'm not a troll and do "see the Haskell light". But one thing I can never answer when preaching to others is "what does Haskell not do well?" Usually I'll avoid then question and explain that it is a 'complete' language and we do have more than enough libraries to make it useful and productive. But I'd be keen to know if people have any anecdotes, ideally ones which can subsequently be twisted into an argument for Haskell ;) Cheers, Dave

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Iteratee-based IO and lightweight monadic regions in the wild
by Artyom Shalkhakov 15 Feb '09

15 Feb '09

Hello, Is anybody planning to use these shiny new ways for doing IO? I'm also interested in a fair comparison of ByteString/Binary with iteratee-based IO. Has anybody done this? Regards, Artyom Shalkhakov.

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The Haskell re-branding exercise
by Paul Johnson 12 Feb '09

12 Feb '09

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Cabal and more than one version
by Jason Dusek 03 Feb '09

03 Feb '09

I'd like to be able to do something like: if (template-haskell < 2.3) cpp-options: -D TH_THE_YOUNGER else cpp-options: -D TH_THE_ELDER I guess this kind of thing is not possible at present? -- _jsn

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Re: Haskell on ARM (was Re: ANN: Topkata)
by Jeremy Apthorp 30 Jan '09

30 Jan '09

2008/6/25 Braden Shepherdson <Braden.Shepherdson(a)gmail.com>: > Jeremy Shaw wrote: >> >> If any does attempt to build for the ARM, please, please, please, >> document what you do in the wiki. And, if you do, edit the wiki as you >> go, you definitely won't be able to remember what you did after the >> fact. (Or, at the very least, use some program to capture everything >> you do so that you can refer to it later). >> >> And, finally, I heard vague rumors a while ago that made me think that >> GHC 6.10 might have a pure ANSI-C backend? Is there any truth to this? >> That would make porting much easier if it meant you did not have to do >> steps 1-4. >> >> j. >> ps. I would be happy to try to answer any questions if someone tries a >> port. I would still like to run GHC on my 770, and I hope to own an >> iPhone if they fix a few ommisions (bluetooth keyboard support, and >> similar stuff). >> > > > I recently acquired the ARM-based Nokia N810 (and <3 it), powered by Maemo. > Running a uname -a on it: > > Linux Nokia-N810-42-19 2.6.21-omap1 #2 Fri Nov 16 16:24:58 EET 2007 armv6l > unknown > > I would love a working GHC implementation on it, if for nothing else than > how awesome it would be. Whether that means using a C back-end or native > compilation doesn't matter to me so much. > > I might be one to attempt this, as I know C and ARM-ish asm decently well > and have a powerful desktop to compile on. I have no familiarity > with GHC internals, though. Lastly, I won't have anything like the time to > attempt this seriously until mid-September or so. Oops, forgot to Cc the list. Next year I'll be working on a project for my undergraduate computing course at UNSW that will involve getting GHC to target the Nintendo DS. It'll require cross-compilation, because the DS isn't powerful enough to actually run GHC (4M main ram and a 66MHz processor). It'll also require that I significantly strip down the runtime system, as the current RTS won't fit in 4M and leave any left over for the main application. I'll be documenting the whole process, but I won't be starting until next year... Jeremy

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Monades - I've got it! (hopefully)
by Tobias Kräntzer 28 Jan '09

28 Jan '09

Hi, I think I now understand all the stuff about monades (not the mathematics). I have just one further question. Beside of the syntactic sugar of the "do" construct and the fact that the IO monade is an "internal type of haskell": Is haskell aware of the concept of monades? As I now understand it, it is "just" a programming model rather than a concept of the language. If not, I have to start new. One problem understanding monades could be, that everywhere is written, that it is so complicated and therefore I was a bit prestressed. . . . Tobias By the way: Merry Christmas! -- Tobias Kräntzer info(a)tobias-kraentzer.de

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know a workaround for greedy context reduction?
by Nicolas Frisby 20 Jan '09

20 Jan '09

With these three declarations {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} class C a where c :: a class C a => D a where d :: a instance C a => D a where d = c ghci exhibits this behavior: *> :t d d :: (C a) => a Where I would prefer "d :: (D a) => a". In my actual examples, the context is much larger and I can't involve overlapping instances. Is there a known workaround? I didn't find a related bug on the GHC trac, and I don't know if other compilers behave in the same way.

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