Hello Jacques, Tuesday, May 23, 2006, 7:13:33 PM, you wrote:
malloc :: Storable a => IO (Ptr a) malloc = doMalloc undefined where doMalloc :: Storable b => b -> IO (Ptr b) doMalloc dummy = mallocBytes (sizeOf dummy) Is there any reason to not code this as
malloc :: Storable a => IO (Ptr a) malloc = mallocBytes $ sizeof undefined
well 1) in your code type of 'undefined' is unknown. it can be any type, while we need 'undefined' of one concrete type, which is given in function signature. compiler will just complain that type is not fixed here 2) what we really need is to write malloc :: Storable a => IO (Ptr a) malloc = mallocBytes $ sizeof (undefined::a) i.e. fix type of 'undefined' to 'a'. but that is impossible in Haskell-98 3) GHC provides language extension (see above in the thread), that allow to use 'a' in function body if function was declared using 'forall', so this works in GHC malloc :: forall a . Storable a => IO (Ptr a) malloc = mallocBytes $ sizeof (undefined::a) 4) this example is borrowed from std libs, what is common for hugs, ghc and nhc, so they can't use ghc-specific solution. instead, this complex trick is used. it's very unobvious, but at least it works with any H98-compatible compiler -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com