[prev in list] [next in list] [prev in thread] [next in thread] 

List:       haskell-beginners
Subject:    [Haskell-beginners] Re: Why is this type ambiguous?
From:       Maurí­cio_CA <mauricio.antunes () gmail ! com>
Date:       2009-10-27 1:34:23
Message-ID: hc5imv$sro$1 () ger ! gmane ! org
[Download RAW message or body]

 >> But isn't the type of 'dummy' defined by 'return dummy' beeing
 >> a possible return value (and, so, dummy :: b)?

 > No. let-bindings are polymorphic, so dummy :: forall a. a

Oh, I once learned that some code of mine didn't work because
lambda bindings are monomorphic. Now I can use it for good.

    genericCast :: (Storable a, Storable b) => a -> IO b
    genericCast v = return undefined >>= \dummy ->
      allocaBytes (max (sizeOf v) (sizeOf dummy)) $ \p ->
        poke p v >> if False then return dummy else peek (castPtr p)

In ghci:

    > let a = 4 :: WordPtr
    > b <- genericCast a :: IO (Ptr ())
    > b
    0x00000004


Thanks!
Maurício

_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners
[prev in list] [next in list] [prev in thread] [next in thread]