I have no idea what you're trying to do here (looks as if you just
want to recreate Unsafe.Coerce.unsafeCoerce?)
However, dummy is first used an 'a' and then as a 'b', so that can't work.
Next, in sizeOf undefined, undefined does not provide enough
information about that type undefined should be. Although the compiler
can infer that sizeOf undefined must be of the same type as sizeOf v,
that's not enough, it needs to know the type of undefined (I guess you
want it to be the same type as v?)
Can be fixed like:
genericCast :: (Storable a, Storable b) => a -> IO b
genericCast v = let
size = max (sizeOf v) (sizeOf $ undefined `asTypeOf` v)
in if False
then return undefined
else allocaBytes size $ \p -> poke p v >> peek (castPtr p)
But... feels insanely hacky, and is not something a beginner should
attempt to play with I guess ;)
2009/10/26 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)
----
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
----
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)?
Thanks, Maurício
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