Am Montag 26 Oktober 2009 23:24:26 schrieb Maurício CA:
import Foreign import Foreign.C
genericCast :: (Storable a, Storable b) => a -> IO b genericCast v = let dummy = undefined size = max (sizeOf v) (sizeOf dummy) in if False then return dummy else allocaBytes size $ \p -> poke p v >> peek (castPtr p)
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Code above gives me this:
Ambiguous type variable `a' in the constraint: `Storable a' arising from a use of `sizeOf' at src/Bindings/C.hs:28:27-38
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It seems to refer to '(sizeOf dummy)'. 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 {-# LANGUAGE ScopedTypeVariables #-} -- This is unsafe, don't use genericCast :: forall a b. (Storable a, Storable b) => a -> IO b genericCast v = let dummy :: b dummy = undefined ...
Thanks, Maurício