November 2009 - Haskell-Cafe

Hoogle server error again
by michael rice 28 Nov '09

28 Nov '09

Just tried to use Hoogle and got: 500 Internal Server Error Michael

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implementing recursive let
by Ben Franksen 28 Nov '09

28 Nov '09

I am trying to write an interpreter for a very simple untyped functional language. I have a problem with mutually recursive let expressions, for which my interpreter loops :( This is a code snippet from the eval function: > eval :: Expr -> Eval Value > eval (Let decls body) = mdo > let (names,exprs) = unzip decls > let updateEnv env = foldr (uncurry M.insert) env $ zip names values > (values,result) <- local updateEnv $ liftM2 (,) (mapM eval exprs) (eval body) > return result Module M is Data.Map, the environment is a simple map from strings to values. Values are defined as > data Value = Data String | Function (Value -> Eval Value) The Eval monad is defined as > newtype Eval a = Eval { > unEval :: ErrorT String (StateT Env (Writer [String])) a > } deriving ( > Monad, > MonadFix, > MonadWriter [String], -- for warnings & other messages > MonadState Env, > MonadError String > ) > instance MonadReader Env Eval where > ask = get > local tr act = do > s <- get > modify tr > r <- act > put s > return r When I test this with an extremely simple expression, something like "let x = 1 in x", the code above loops. I don't understand why, especially since in a previous version it worked. In the previous version I had a simpler monad stack that went > newtype Eval a = Eval { > unEval :: ReaderT Env (Writer [String])) a > } deriving ( > Monad, > MonadFix, > MonadWriter [String], > MonadReader Env > ) (Replacing reader with state was done so I can add definitions to the environment at runtime. The ErrorT provides for errors, such as application of a non-function.) Expressions not involving let work fine. Also, if I replace the above definition by one which does not allow recursion (not using mdo, evaluating the defining expressions before the variable gets added to the environment), then non-recursive let-expressions (like the simple example above) work just fine. I am out of ideas as to what causes this problem. Does the addition of ErrorT make my monad too strict? How else can I implement mutual recursion? Cheers Ben

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Partially applied functions
by Casey Hawthorne 28 Nov '09

28 Nov '09

You can pattern match on the right hand side of '|' in a list comprehension, since a list comprehension is just the list monad. add :: Int -> Int -> Int add x y = x + y -- a list of partially applied functions adds = [add 3, add 5, add 7, add 3, add 5, add 8] -- an example usage of the list kP num = map (\ f -> f 10 ) (addPs num) -- add3s = filter (?) adds -- add3s = [add 3, add 3] -- addEvens = filter (?) adds --addEvens = [add 8] addPs num = [ x | x <- adds, x 0 == num ] -- Regards, Casey

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Partially applied functions
by Ozgur Akgun 28 Nov '09

28 Nov '09

Hi cafe, Is such a thing possible, add :: Int -> Int -> Int add x y = x + y -- a list of partially applied functions adds = [add 3, add 5, add 7, add 3, add 5, add 8] -- an example usage of the list k = map (\ f -> f 10 ) adds add3s = filter (?) adds -- add3s = [add 3, add 3] addEvens = filter (?) adds --addEvens = [add 8] I want to have functions in place of the ? signs. I guess one would need a way of extracting the applied value from a partially applied function (or totally, doesn't matter) I came across such a structure while designing an algotihm and used data structures to represent functions and solved the problem. But this doesn't seem very *cute* to me and requires some bolier-plate (if you have a lot of this structure everywhere) , that's why I am asking for your precious suggestions. Best, -- Ozgur Akgun

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cabal-install binary for FreeBSD
by Michael Snoyman 28 Nov '09

28 Nov '09

Hi all, I'm trying out NearlyFreeSpeech.net for hosting my Haskell apps. They use FreeBSD 7.2, but I can't get cabal-install to compile since it runs out of memory during the link phase. So far I haven't had trouble manually installing packages, but it would be nice to just do cabal install... Anyway, I don't have FreeBSD installed locally, and I was wondering if anyone out there might have a cabal binary for FreeBSD that they could send over. Thanks, Michael

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ANNOUNCE: deepseq-1.0.0.0
by Simon Marlow 28 Nov '09

28 Nov '09

I've just uploaded deepseq-1.0.0.0 to Hackage http://hackage.haskell.org/package/deepseq This provides a DeepSeq class with a deepseq method, equivalent to the existing NFData/rnf in the parallel package. I'll be using this in a newly revamped parallel package, which I hope to upload shortly. Cheers, Simon

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Custom cabal languages (hooks?)
by Svein Ove Aas 28 Nov '09

28 Nov '09

I'm currently working on a binding to Brian Gladman's C/assembler AES implementation. This code is sufficiently small that it makes little sense to create a separate C library for it; furthermore, it isn't typically packaged by the various linux distributions, never mind other OSs. As a result, I'd like to make it part of the cabalized AES package. This works well enough with the pure-C variant of the implementation, but it also has assembly-based implementations of the core encryption routines for x86 and x86_64, specifically using yasm, which I would like to use when possible. So, two problems.. - First, I need to make cabal detect yasm's presence, and call out to it when appropriate. The former I can do with a configure script; the latter, I'm not quite sure how to do. Currently, cabal can only do C; basically, I'm looking for a way to let it use any language that can be compiled to .o files. - Second, having manually compiled the C/asm bits to .a and .so form, I did eventually get cabal to link (seemingly) correctly against it, but it then did not proceed to copy libaes.so into ~/.cabal/lib/AES/.... I'm not sure how to make it do that, either. I suppose I'll have to use hooks, but I'd appreciate a little advice on how, exactly Well, I've put a snapshot of my current code at http://brage.info/~svein/aes/, so you can have a look. I'd really appreciate the help. -- Svein Ove Aas

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hoogle down
by Keith Sheppard 28 Nov '09

28 Nov '09

hoogle is down for me: "Internal Server Error" Thanks Keith -- keithsheppard.name

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HWN This week
by Joe Fredette 28 Nov '09

28 Nov '09

Hey folks, I'm at home this weekend, and the internet is somewhat dodgey, so I'm going to delay the special Thanksgiving episode of the HWN till sunday afternoon... I suppose the Turkey-coma isn't helping either, but I'll never admit it... /Joe

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FastCGI under Windows
by Kevin Jardine 28 Nov '09

28 Nov '09

I'm attempting to get FastCGI working with Haskell under Windows XP. However, all the sample code I've tried always fails with: user error (FCGX_Accept_r failed with error code: -9999) unknown listenType (0) Has anyone seen this before? Any ideas on what I could try to get this to work? Kevin

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