Ah thanks, I get confused with instance and declaration level. But why I can call g with Just: let g :: h a b -> h a b; g a = a g Just but Just is a->Maybe a On 11/25/2017 03:34 PM, Francesco Ariis wrote:
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 (* -> *).
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