[Haskell-beginners] Re: Why is this type ambiguous?
mauricio.antunes at gmail.com
Mon Oct 26 19:12:15 EDT 2009
> 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
> 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.
>> 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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