Question

I’m trying to make a function in standard ml that takes a list of pairs of ints, and I should return a list of int that contains the max element from the pairs. (int * int) list -> int list. Now so far,I’ve written a code but it doesnt work and I cant seem to figure the problem from the error it gave.

Here’s the code:

- fun maxpairs x = 
=   foldr (fn (a, b) => if a > b then a else b) [] x;

And here’s the error I’m getting:

stdIn:15.2-15.50 Error: operator and operand don't agree [overload]
  operator domain: 'Z
  operand:         'Y list
  in expression:
    (foldr (fn (<pat>,<pat>) => if <exp> then <exp> else <exp>)) nil

Answer 1

foldr takes a function of type ('a * 'b) -> 'b, a value of type 'b and a list of type ['a]. In your case the list is a list of pairs and the value of type 'b is an empty list. That means in the function fn (a,b) => ... a will be a pair and b will be a list. You then try to compare a and b using >. Since > can’t be used with a pair as its left operand and a list as its right operand, that doesn’t work. Also you can’t have an if-statement where the then-expression and the else-expression have different types.

If I were you I’d use map for this, which seems to fit the problem better than using a fold.