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

[Marcus Manning]

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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