The types tell me that that's not the function in question. Type variables in Haskell mean the caller of the function gets to pick the types. That means the i

The types tell me that that's not the function in question. Type variables in Haskell mean the caller of the function gets to pick the types. That means the implementation doesn't get to - and the compiler makes sure of it. You're pattern matching on a number and returning a string. The function might be handed a function and expected to return a file handle for all you know from inside the function.