Error in SML code

Question

Hi i am getting compilation errors in the following SML code, can someone help?

Error: operator and operand don't agree [UBOUND match]
  operator domain: 'Z list
  operand:         ''list
  in expression:
    null mylist
stdIn:4.15-4.24 Error: operator and operand don't agree [UBOUND match]
  operator domain: 'Z list
  operand:         ''list
  in expression:
    hd mylist
stdIn:6.19-6.36 Error: operator and operand don't agree [UBOUND match]
  operator domain: 'Z list
  operand:         ''list
  in expression:
    tl mylist
stdIn:6.10-7.21 Error: operator is not a function [circularity]
  operator: 'Z

in expression: (exists_in (item,tl mylist)) exists_in

code:

 fun exists_in (item: ''int, mylist:''list) =
        if null mylist
        then false
        else if  (hd mylist) = item 
        then true
        else exists_in(item, tl mylist)
    exists_in(1,[1,2,3]);

I think you meant either `int` and `int list` or `''a` and `''a list`, not `''int` and `''list`. — molbdnilo, Sep 13 '17 at 11:32

score 4 · Answer 1 · answered Sep 13 '17 at 12:49

What each of the error messages is telling you is that you're applying a function to something of the wrong type. For example, null is of type 'a list -> bool, and so expects to be applied to an argument of type 'a list. In exists_in, you're applying null to something of type ''list on line 4.

SML also provides type inference, so there's no real need to specify the types of your arguments (though it can be useful for debugging). If you do want to specify the types, as molbdnilo commented, the types you are looking for are int and int list, or ''a and ''a list ('' a being a type variable for equality types).

Unrelatedly, perhaps a more idiomatic way to write your function is do define it by case analysis on the structure of your list, rather than using boolean checks. The advantage of this is that you are immediately given the data supporting the boolean check on whether or not the list is empty, rather than checking and then extracting the data. For example:

fun exists_in (item, []) = false
  | exists_in (item, h::t) = (item = h) orelse exists_in (item, t)

Error in SML code

1 Answers1