[Haskell-beginners] Fwd: Re: Multiple letters between -> ->

[Francesco Ariis]

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 (* -> *).