Haskell: Function to determine the arity of functions?

Question

Is it possible to write a function arity :: a -> Integer to determine the arity of arbitrary functions, such that

> arity map
2
> arity fol         


        

Answer 1

Yes, it can be done very, very easily:

arity :: (a -> b) -> Int
arity = const 1

Rationale: If it is a function, you can apply it to exactly 1 argument. Note that haskell syntax makes it impossible to apply to 0, 2 or more arguments as f a b is really (f a) b, i.e. not f applied to a and b, but (f applied to a) applied to b. The result may, of course, be another function that can be applied again, and so forth.

Sounds stupid, but is nothing but the truth.