On Thu, Nov 23, 2017 at 06:19:51PM +0100, Marcus Manning wrote:

Hi,

Original I thought a Signature like:

f :: h a -> h a

means that h is a higher kinded type just like in Type Classes ( for instance f in Functor f).

But I heard such a meaning is not allowed in normal Haskell functions. What instead is the meaning of h a?

Hello Marcus, you can write that but, since we know nothing about `h` and `a`, the only possible (non-undefined) function to implement that signature is: f :: h a -> h a f = id Any other implementation would require us to know something about h, hence a typeclass-constraint (e.g. Functor h =>) on h.