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.
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