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

It's not possible with standard Haskell. It may be possible using the IncoherentInstances or similar extension.

But why do you want to do this? You can't ask a function how many arguments it expects and then use this knowledge to give it precisely that number of arguments. (Unless you're using Template Haskell, in which case, yes, I expect it is possible at compile time. Are you using Template Haskell?)

What's your actual problem that you're trying to solve?