类型别名声明中的Haskell类型类

Question

I am trying to develop a module that performs different algorithms over a set of states by receiving the functions to manage them as paremeters. I was defining different type aliases to make it easier to implement; a simple example to understand this would be:

-- Function that expands a state
type Expand = State -> State
-- Function that evaluates a state
type Eval = State -> Float
-- Group of states
type Tree = [State]

And then I realized that I wanted State to be the alias of a type that is equatable: I really don't care if the user's states are (Int, Int) , Float , or String as long as it's possible for him to implement the Expand and Eval functions and for me to compare two different states.

How can I generalize the type State to achieve this? The intuitive thing that comes to my mind is something like type State = (Eq a) => a but that is obviously not possible without an a in scope. Is there an easy way for me to declare functions that treat State as a black box?

Answer 1

You need to include the generic state as a type parameter.

type Expand s = s -> s
type Eval s   = s -> Float
type Tree s   = [s]

It's impossible to completely hide it, because eg Expand for an (Int, Int) state is a completely different type than for a String state.

As for the Eq constraint. Typically, you would include that locally only in the specific function that requires the constraint.