- Haskell-Cafe - Haskell.org

building ghc on opensolaris
by Günther Schmidt 19 Aug '10

19 Aug '10

Hi, does anybody know why --with-gcc= ... has no effect when passed to ./configure? This is under OpenSolaris only, under Mac OS X for instances it takes. Günther

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Specific to General to Specific
by Edward Z. Yang 19 Aug '10

19 Aug '10

Imagine that I have the following type-class: class (Monad (m n)) => NetworkMonad m n data NT n a instance NetworkMonad NT n data NQ n a instance NetworkMonad NQ n monadicValue :: NetworkMonad m n => m n () It defines some family of phantom monads, in this case NT n and NQ n, and permits some values to inhabit either monad (resolved by the surrounding code). Suppose, however, that there are multiple possible implementations for the pair (NT, NQ). Then we might like to generalize this typeclass with an associated type. (Suppose that our old implementation was g = Int.) {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleContexts #-} class Network g where data GNT g :: * -> * -> * data GNQ g :: * -> * -> * class (Monad (m g n)) => GenericNetworkMonad g m n type NT n a = GNT Int n a type NQ n a = GNQ Int n a genericMonadicValue :: GenericNetworkMonad g m n => m g n () genericMonadicValue = undefined monadicValue :: GenericNetworkMonad Int m n => m Int n () monadicValue = undefined My question is, how can I write a specialized typeclass NetworkMonad that recaptures the original simple interface? monadicValue' :: NetworkMonad m n => m n () monadicValue' = undefined I've tried something like this: type family GetM (x :: * -> * -> *) :: (* -> * -> * -> *) type instance GetM (m g) = m type family GetG (x :: * -> * -> *) :: * type instance GetG (m g) = g class (GenericNetworkMonad (GetG mg) (GetM mg) n) => NetworkMonad mg n where But this doesn't seem to be enough information to do inference with: monadicValue'' :: NetworkMonad m n => m n () monadicValue'' = genericMonadicValue gives the type error: Test.hs:28:17: Could not deduce (GenericNetworkMonad g m n) from the context (NetworkMonad m1 n) arising from a use of `genericMonadicValue' at Test.hs:28:17-35 Possible fix: add (GenericNetworkMonad g m n) to the context of the type signature for `monadicValue''' In the expression: genericMonadicValue In the definition of `monadicValue''': monadicValue'' = genericMonadicValue Test.hs:28:17: Couldn't match expected type `m1' against inferred type `m g' `m1' is a rigid type variable bound by the type signature for `monadicValue''' at Test.hs:27:31 In the expression: genericMonadicValue In the definition of `monadicValue''': monadicValue'' = genericMonadicValu Am I out of luck without superclass equality constraints? Cheers, Edward

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Failed inference: maybe bug?
by Edward Z. Yang 18 Aug '10

18 Aug '10

On the prompting of napping, I humbly submit the following code to haskell-cafe: ezyang@javelin:~/Dev/haskell/generic-typeclass$ cat > Bar.hs {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE MultiParamTypeClasses #-} {-# LANGUAGE FlexibleContexts #-} data Foo b = Foo class FooTwo m b where type family Ctor a :: (* -> *) type instance Ctor (f a) = f type family CtorVal a :: * type instance CtorVal (f a) = a instance FooTwo Foo Int where class FooTwo (Ctor mb) (CtorVal mb) => FooOne mb where foo :: FooOne (m Int) => m Int foo = fooGeneric fooGeneric :: FooTwo m b => m b fooGeneric = undefined ezyang@javelin:~/Dev/haskell/generic-typeclass$ ghci Bar.hs GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. [1 of 1] Compiling Main ( Bar.hs, interpreted ) Bar.hs:19:6: Could not deduce (FooTwo m Int) from the context (FooOne (m Int)) arising from a use of `fooGeneric' at Bar.hs:19:6-15 Possible fix: add (FooTwo m Int) to the context of the type signature for `foo' or add an instance declaration for (FooTwo m Int) In the expression: fooGeneric In the definition of `foo': foo = fooGeneric Failed, modules loaded: none. Prelude> Leaving GHCi. It seems that GHC is unable to unify the two instances. So, bug or expected behavior? Cheers, Edward

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regex-compat on latest Haskell Platform
by Kevin Jardine 18 Aug '10

18 Aug '10

I'm running Haskel Platform 2010.2.0.0 and attempting to compile with regex-compat under Windows XP. The link stage fails with a number of lines of the form: D:\Program Files\Haskell Platform\2010.2.0.0\lib\extralibs\regex- posix-0.94.2\ghc-6.12.3/libHSregex-posix-0.94.2.a(Wrap.o):fake:(.text +0x493): undefined reference to `regerror' I have googled for a solution but cannot see a fix anywhere. Any suggestions? Kevin

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Re: [Haskell-cafe] Embedded scripting Language for haskell app
by John Lato 18 Aug '10

18 Aug '10

> > From: Hemanth Kapila <saihemanth(a)gmail.com> > Subject: [Haskell-cafe] Embedded scripting Language for haskell app > > Hi, > > Can some one please give me a suggestion on the best choice for an embedded > scripting Language for a haskell application? > I mean, something like guile/lua for c/c++ and groovy/jruby for java. > I'm sure that the best option depends on the features you want from your embedded scripting language. With no prior experience, it took me about 15 minutes to install hint and add scripting support to an application. This involves interpreting an EDSL which uses associated types, so at least that much appears to work without problems. The example file on the hint project website is terse but sufficient. One question I have regards distributing applications using hint. Does the target computer need ghc/libs installed? I think so, but would like to have this confirmed. If so, this might be a good reason to use Lua. John > For quite some time, I've been using a lisp-like interpreter that I > implemented myself. But this is not going too well - going by this road, I > suspect I will end up with a mule. I am looking for a pony (a declarative > programming language). I am okay with a donkey too. > > baskell[1] seems interesting. And there's hslua[2]. > Can one use hint[3] like this ? > > Thanks > Hemanth K > [1] baskell - http://hackage.haskell.org/package/baskell > [2] hslua - http://hackage.haskell.org/package/hslua > [3] hint - http://hackage.haskell.org/package/hint > >

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Re: [Haskell-cafe] Heavy lift-ing
by aditya siram 18 Aug '10

18 Aug '10

I wouldn't-it was a bad example. My only point was that because of the way (>>=) is implemented for lists the order of the arguments 'a' and 'b' in 'liftM2 f a b' matters. -deech On Sat, Jul 24, 2010 at 1:37 AM, Lennart Augustsson <lennart.augustsson(a)gmail.com> wrote: > Why would you expect swapped operands to (-) ? > > > Sent from my iPad > > On Jul 23, 2010, at 20:12, aditya siram <aditya.siram(a)gmail.com> wrote: > >> Lists are non-deterministic, but the function taken by liftM2 does not >> necessarily generate all possible outcomes. In the case of (+) it >> does, not in the case of (-): >> liftM2 (-) [0,1] [2,3] => [0-1,0-2,1-2,1-3] => [-2,-3,-1,-2] >> if all possible cases were generated between the two lists we have to >> include also: >> [2-0,2-1,3-0,3-1] >> >> -deech >> >> On Fri, Jul 23, 2010 at 3:44 PM, Alex Stangl <alex(a)stangl.us> wrote: >>> On Fri, Jul 23, 2010 at 11:43:08AM -0700, michael rice wrote: >>>> What does it mean to "promote a function to a monad?" >>>> >>>> It would seem that the monad values must understand the function that's being promoted, like Ints understand (+). >>>> >>>> Prelude Control.Monad> liftM2 (+) (Just 1) (Just 1) >>>> Just 2 >>>> >>>> But how does one add [0,1] and [0,2] to get [0,2,1,3]? >>> >>> It depends upon the semantics of the particular monad. List monads >>> represent nondeterminism. So, for example, [0,1] represents a 0 or >>> 1, and [0,2] represents a 0 or 2. >>> >>> When you add 0 or 1 to 0 or 2, your possible answers are [0,2,1,3]. >>> >>> Alex >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> Haskell-Cafe(a)haskell.org >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> _______________________________________________ >> Haskell-Cafe mailing list >> Haskell-Cafe(a)haskell.org >> http://www.haskell.org/mailman/listinfo/haskell-cafe >

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Different ByteString types
by Henk-Jan van Tuyl 18 Aug '10

18 Aug '10

L.S., I hope someone can help me with the following: I am trying to check the SHA1 hash of a file, using the package cryptohash [0]; the following error message is displayed when I try to compile my program: SHA1check.lhs:35:32: Couldn't match expected type `B.ByteString' against inferred type `Data.ByteString.Internal.ByteString' In the second argument of `(.)', namely `hashlazy' In the second argument of `(.)', namely `B.unpack . hashlazy' In the first argument of `(<$>)', namely `show . B.unpack . hashlazy' The relevant lines of code are: > import qualified Data.ByteString.Lazy as B > import Data.CryptoHash.SHA1 ( hashlazy ) > hash <- show . B.unpack . hashlazy <$> B.hGetContents h There is only one version of the bytestring package installed. Details: - Windows XP - bytestring-0.9.1.7 - cryptohash-0.5.1 - Haskell Platform 2010.2.0.0 Regards, Henk-Jan van Tuyl P.S. A patch is needed if you want to install cryptohash with MinGW [0] http://hackage.haskell.org/package/cryptohash -- http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html --

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Anything like Mythryx in Haskell?
by Martin DeMello 18 Aug '10

18 Aug '10

Mythryl [http://mythryl.org/] is an ML dialect that mostly puts an Algolish syntax and some good posix interoperability atop SML/NJ. More than the language (which doesn't seem to be as tastefully designed as Haskell, particularly in terms of syntax - what do people have against MLish syntax anyway?) I like the goals of the associated Mythryx project [http://mythryl.org/my-Mythryx.html] namely, to develop a well-engineered unixy ecosystem in a strongly typed functional language. Has anyone attempted something like this in Haskell? martin

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Re: [Haskell-cafe] Question about memory usage
by Sebastian Fischer 18 Aug '10

18 Aug '10

[CC-ing café again] On Aug 16, 2010, at 5:52 PM, Daniel Fischer wrote: >> I am a bit concerned about the memory usage. > > Of your implementation of the matrix power algorithm? Yes. > Making the fields of Matrix strict should help: > > data Matrix a = Matrix !a !a !a Making all fields strict makes memory usage worse. When making only the first field strict, memory usage hardly differs (from the current implementation without strictness annotations). Sebastian -- Underestimating the novelty of the future is a time-honored tradition. (D.G.)

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Re: [Haskell-cafe] Re: Handling absent maintainers
by Antoine Latter 18 Aug '10

18 Aug '10

It's a bad call to put any version restriction on the template haskell package in your package description - the cabal solver will still go for the latest version of your package and then try to install the listed version of template haskell from Hackage, which leads to confusing errors. Even if you try to use the version of template haskell to set a flag in your package description, cabal-install will always form an install plan based on the default value of your flag (leaving someone out). Antoine On Aug 18, 2010 3:59 AM, "Jonas Almström Duregård" <jonas.duregard(a)gmail.com> wrote: > I had been in contact with the maintainer of the > package who has prepared a version 0.5 and was ... I can upload a working .4 version of th-lift if needed, but I guess it might be undesirable to depend on it if 0.5 changes the API. I'll have a look at your version and see if I can merge my additions into it. For now, perhaps somebody should re-upload the current version of src-meta with the correct dependencies (TH<2.4)? 2010/8/17 Ben Millwood <haskell(a)benmachine.co.uk>: > 2010/8/17 Jonas Almström Duregård <jonas.duregard(a)gmail.com>: >> Hi, >> >> Has there been any prog...

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