I have no idea what you're trying to do here (looks as if you just want to recreate Unsafe.Coerce.unsafeCoerce?)
Yes, except that it works with Storables only and is safe against runtime corruption. Evil, but can save you from worst FFI hacks.
However, dummy is first used an 'a' and then as a 'b', so that can't work.
Where is dummy used as 'a'? The only place with a specific type I used it is in 'return dummy', and it's there exactly to "get" that 'b' type.
Although the compiler can infer that sizeOf undefined must be of the same type as sizeOf v, (...)
size = max (sizeOf v) (sizeOf $ undefined `asTypeOf` v)
No, it's not the same type! The idea is that 'size' value should be the bigger of 'a' and 'b' sizes, so that I guarantee enough memory will be allocated for the cast.
But... feels insanely hacky, and is not something a beginner should attempt to play with I guess ;)
I have an unusual experience with Haskell. I used it a lot, but mainly with FFI. So, I'm an expert in FFI -- I have my own package of hsc2hs macros :) -- but I have no understanding of the type system, except for just the basics. Maurício
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)