I want to be able to create a function to see if two types are equal. I have a type class called finite defined as follows,

class (Bounded a, Enum a) => Finite a

and want to be able to write a equality comparison function

equals :: (Finite a, Eq b) => (a -> b) -> (a -> b) -> Bool

for functions who's domain is of type Finite.

CodePudding user response：

Basically what you need is to enumerate over all possible values for a, and check if the two functions produce the same result.

You can generate a list of all the items with [minBound ..], and you can use all :: Foldable f => (a -> Bool) -> f a -> Bool to check if for all items a certain predicate is satisfied.

You thus can implement such function with:

equals :: (Finite a, Eq b) => (a -> b) -> (a -> b) -> Bool
equals = all (…) [minBound ..]

where I leave implementing the … part as an exercise. It should call f and g with the element of the list, and return True if and only if the two functions produce the same result.