Jacques Carette wrote:
Bulat Ziganshin 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 ?
Yes. The supreme clevernesss of Bulat's trick is that doMalloc unifies the type variable 'a' with the type of the argument to sizeOf, whereas "sizeOf undefined" would be ambiguous because the type of "undefined" is not constrained there ie: malloc :: Storable a => IO (Ptr a) malloc = doMalloc undefined where doMalloc :: Storable b => b -> IO (Ptr b) doMalloc dummy = mallocBytes (sizeOf dummy) (doMalloc undefined) :: IO (Ptr a) ==> (doMalloc dummy) :: IO (Ptr a) ==> -- doMalloc :: Storable b => b -> IO (Ptr b) dummy :: a whereas: malloc = mallocBytes $ sizeOf undefined malloc :: IO (Ptr a) ==> (mallocBytes (sizeOf undefined)) :: IO (Ptr a) ==> (sizeOf undefined) :: Int ==> undefined :: unconstrained Regards, Brian.