- Haskell-Cafe - Haskell.org

Dealing poker hands, redux
by Matthew 25 Aug '12

25 Aug '12

So I've finally got some code which shuffles a deck of cards and deals out an arbitrary number of hands. https://gist.github.com/19916435df2b116e0edc type DealerState = State [Card] [[Card]] deck :: [Card] deck = [ (s, r) | s <- suits, r <- ranks ] shuffleDeck :: Int -> RVar [Card] shuffleDeck n = shuffle $ concat $ replicate n deck deal :: Int -> ([[Card]] -> DealerState) deal n = \xs -> state $ \s -> (xs ++ [take n s], drop n s) -- |Deal a number of hands a number of cards each. dealHands :: Int -> Int -> ([[Card]] -> DealerState) dealHands hs cs = foldr1 (<=<) $ replicate hs (deal cs) First of all, I have no idea if this is any good. The way I end up calling dealHands and getting a "real" result is `runState (dealHands 3 7 []) deck`. And I see that I've got nested lambdas all in `deal`. But hey it took me forever to figure it out and it "works." I'm using `shuffle` from Data.Random.Extras, which results in an RVar, and thus the beginning of my perplexity. I'm happy to end up with RVar inside a State monad, but I'm not sure where to start. To be honest, I'm only barely understanding what I'm doing with the State monad as it is. :) Happy for any help whatsoever! - Matthew

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haskell cryptogaphic libraries
by marcmo 25 Aug '12

25 Aug '12

In my day job I currently need to deal with a lot of cryptographic processing. For the tests I need to perform various cryptographic functions in haskell. these include: * AES Encryption/Decryption (CBC-Mode) * RSA Public Key Encryption/Decryption * Certificate Handling I'm having some difficulties finding the libraries to use and using them correctly. Performance is *not* my primary concern...rather *correctness* and* ease of use*. Haskell has some great cryptographic libs but for me it seems hard to judge what to use. The ones that seem appropriate are: * The AES package (for symmetric encryption) * The RSA package (for public key cryptography) certificate handling is s.th. I haven't found out about * how to deal with Certificates (e.g. extract the public key from X.509 certificates) is there a recommended package (packages) that suite my purpose? thnx

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Combining State and List Monads
by Henry Laxen 25 Aug '12

25 Aug '12

Dear Cafe, It seems to me there should be some simple way of doing this, but thus far it eludes me. I am trying to combine the State and List monads to do the following: countCalls = do a <- [1..2] b <- [1..2] modify (+1) return (a,b) where with some combination of ListT, StateT, List, State, or who knows what would result in: ([(1,1),(1,2),(2,1),(2,2)],4) assuming we initialize the state to 0 Is there any way to make this happen? Thanks in advance. Henry Laxen

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How to implement instance of MonadBaseControl IO
by yi huang 25 Aug '12

25 Aug '12

I have a `newtype Yun a = Yun { unYun :: ReaderT YunEnv (ResourceT IO) a }` , and i need to define an instance of `MonadBaseControl IO` for it. Newtype instance deriving don't work here. I guess the answer is simple, i just can't figure it out, hope anybody can lightening me. Best regards. Yihuang.

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ANNOUNCE: readable-0.1
by MightyByte 24 Aug '12

24 Aug '12

I'm happy to announce a new package called readable: http://hackage.haskell.org/package/readable This package provides simple reading of data types from ByteString and Text using MonadPlus to handle failure. It doesn't seem like there's a standard way to do simple reading from ByteString and Text, so hopefully this package will fill the gap. Suggestions/improvements are most welcome.

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Rigid skolem type variable escaping scope
by Matthew Steele 24 Aug '12

24 Aug '12

While working on a project I have come across a new-to-me corner case of the type system that I don't think I understand, and I am hoping that someone here can enlighten me. Here's a minimal setup. Let's say I have some existing code like this: {-# LANGUAGE Rank2Types #-} class FooClass a where ... foo :: (forall a. (FooClass a) => a -> Int) -> Bool foo fn = ... Now I want to make a type alias for these (a -> Int) functions, and give myself a new way to call foo. I could do this: type IntFn a = a -> Int bar :: (forall a. (FooClass a) => IntFn a) -> Bool bar fn = foo fn This compiles just fine, as I would expect. But now let's say I want to change it to make IntFn a newtype: newtype IntFn a = IntFn (a -> Int) bar :: (forall a. (FooClass a) => IntFn a) -> Bool bar (IntFn fn) = foo fn I had expected this to compile too. But instead, I get an error like this one (using GHC 7.4.1): Couldn't match type `a0' with `a' because type variable `a' would escape its scope This (rigid, skolem) type variable is bound by a type expected by the context: FooClass a => a -> Int The following variables have types that mention a0 fn :: a0 -> Int (bound at <the line number that implements bar>) In the first argument of `foo', namely `fn' In the expression: foo fn In an equation for `bar': bar (IntFn fn) = foo fn I don't think I am grasping why adding the layer of newtype wrapping/unwrapping, without otherwise changing the types, introduces this problem. Seems to me like the newtype version would also be type-safe if GHC allowed it (am I wrong?), and I'm failing to understand why GHC can't allow it. Is there a simple explanation, or else some reading that someone could point me to? (-: Cheers, -Matt

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explicit annotations on kind polymorphism for data types
by dude 24 Aug '12

24 Aug '12

Hello All: I'm working through Giving Haskell a Promotion. Section 2.4 presents an explicitly annotated data type declaration similar to the following: data EqRefl (a::X)(b::X) where Refl :: forall X. forall (a::X). EqRefl a a Has this been implemented in GHC 7.4.2? 7.8.3 in the GHC User Guide leads me to believe it has not. -- dude

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Handle close on GC leading to wrong reported exit codes?
by Niklas Hambüchen 24 Aug '12

24 Aug '12

According to System.IO, handles are automatically closed once they are garbage-collected. This served me well for files so far, but for processes, it gives me a weird problem: If I don't prevent my stdErr handle from being garbage-collected, the exit code reported by getProcessExitCode is just wrong (for me, it's always ExitSuccess, no matter what the program does). I posted this into #haskell and some poeple could not reproduce it, so I wanted to ask the Cafe what happens if you run this and if you have an idea why it happens to me. Thank you Niklas The following code gives an example. (http://hpaste.org/73593) You can see the real exit code with "./test; echo $?". module Main where import Control.Concurrent (threadDelay) import System.Process import System.IO import System.Exit run :: FilePath -> [String] -> IO (Handle, Handle, Handle, ProcessHandle) run cmd args = do r@(i, o, e, p) <- runInteractiveProcess cmd args Nothing Nothing getProcessExitCode p >>= \me -> case me of Just (ExitFailure 127) -> error $ "command not found: " ++ show cmd _ -> do mapM_ (flip hSetBuffering LineBuffering) [i, o, e] return r main = do (i, o, e, p) <- run "./test" [] putStrLn "spawned" threadDelay 1100000 putStrLn "waited" print =<< getProcessExitCode p -- If you comment these away, the above always returns Just ExitSuccess -- print =<< hGetContents o -- print =<< hGetContents e {- test.c: #include <stdio.h> #include <unistd.h> int main(int argc, char const *argv[]) { sleep (1); printf ("some stdout\n"); fprintf (stderr, "some stderr\n"); fflush (stdout); fflush (stderr); return 2; } -}

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model theory for type classes
by Patrick Browne 24 Aug '12

24 Aug '12

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Bringing knowledge from Haskell to Java
by Thiago Negri 24 Aug '12

24 Aug '12

Hello everyone. I just posted about a fact that happens to everyone who codes in Java while learning Haskell. You end up trading knowledge from both sides. I'll appreciate if you could read and send me feedbacks: http://me-hunz.blogspot.com.br/2012/08/bringing-knowledge-from-haskell-to-j… Thanks, Thiago.

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