I am tired of having to look this up every time I forget everything I know about SML.
Error: operator and operand don’t agree [tycon mismatch]
operator domain: ‘Z * ‘Y
When you get this error, you have called a function with the wrong argument types.
The operator domain is what the function called expects, and the operand is what the function was called with.
To fix the problem, call the function with the proper arguments.
You can learn more about a variety of SML errors at the SML NJ error document.