25 Nov
2017
25 Nov
'17
2:34 p.m.
On Sat, Nov 25, 2017 at 03:03:26PM +0100, Marcus Manning wrote:
Ok,
but what is h in:
f :: h a -> ...
is "h" a data constructor or a type constructor or a normal function? What is j in
f:: j k l -> ...
and hwat is the difference between j and h?
`h` is a type constructor and `h a` denotes a kind of `* -> *`, hence λ> :t f f :: h s -> h s λ> :t f (Just 8) f (Just 8) :: Num s => Maybe s -- because Maybe :: * -> * λ> :t f Bool <interactive>:1:3: error: Data constructor not in scope: Bool :: h s Similarly, `f :: j k l -> j k l` would only work on kinds `* -> * -> *` (tuples, etc.) and not on Maybes (* -> *).