在Haskell中键入`a-> mb`是什么意思？

Question

Example - function runKleisli from Haskell base module

newtype Kleisli m a b = Kleisli { runKleisli :: a -> m b }

That means runKleisli have 1 argument with type a and must return function m , which have 1 argument b ?

Answer 1

It means m is a type of kind * -> * ; that is, m is a type constructor that accepts a single argument, like Maybe or Either String . When you instantiate Kleisli and perform the substitution yourself, you can see how this works out.

For example, consider something like Kleisli Maybe String Integer . The substituted type of runKleisli would be String -> Maybe Integer , and you can see how the m is being used as a type constructor.

For more information about kinds in Haskell, see What exactly is the kind "*" in Haskell? .