[ML] Error use filter, isupper and string.sub

I've tried various ways to set this up to get only the strings (starting) with capital letters but I can't figure out a way to do it.

This version is the closes I've come but I'm getting this error:

HW3.sml:38.9-38.28 Error: operator and operand don't agree [tycon mismatch]
operator domain: 'Z -> bool
operand: (string -> bool) * string list
in expression:
List.filter (g,sl)

I don't know any Standard ML, but from the error message it looks like the syntax List.filter(g,sl) is applying List.filter to a 2-Tuple (g,sl) instead of applying it first to g, then to sl. Have you checked whatever reference you're using for the syntax when calling a function that takes multiple arguments (or example of List.filter in general)?