[Haskell-cafe] Nice addition to Foreign: castAny

[John Meacham]

On Tue, Oct 27, 2009 at 12:48:32AM -0200, Maurí­cio CA wrote:
> This could be beside castPtr, castCharToCChar etc.
>
> ----
>
> castAny :: (Storable a, Storable b) => a -> b
> castAny = unsafePerformIO . genericCast
>   where
>     genericCast :: (Storable a, Storable b) => a -> IO b
>     genericCast v = return undefined >>= \r ->
>       allocaBytes (max (sizeOf v) (sizeOf r)) $ \p ->
>         poke p v >> if False then return r else peek (castPtr p)
>
> ----
>
> GHCi:
>
>    > let a = -1000 :: Int16
>    > castAny a :: Word16  -->
>    64536
>    > castAny a :: Ptr ()
>    0xb4c2fc18
>    > castAny (castAny a :: Ptr ()) :: Int16
>    -1000
>
>    > let b = pi :: CLDouble
>    > b
>    3.141592653589793
>    > castAny b :: CInt
>    1413754136
>    > castAny b :: Ptr ()
>    0x54442d18
>    > castAny b :: CFloat
>    3.3702806e12
>    > castAny b :: Int8
>    24
>
>
> At minimum, this is safer than 'unsafeCoerce'. What do you think?

Try it on a big endian architecture, or one that has alignment
restrictions, or a different size for HsChar or so forth. Casting by
'punning' (as the C folks like to call it) does have uses, but they are
generally hardware dependent and useful only in certain rare
circumstances that a generic cast probably isn't likely to fill.

        John

-- 
John Meacham - ⑆repetae.net⑆john⑈ - http://notanumber.net/