Question

Why is there a tycon mismatch operator and operand do not agree error? Any Suggestion for a solution?

fun reve (x:string) = implode o rev o explode x

Answer 1

The problem lies in, that function application binds stronger than o.

That is, it is interpreted as:

fun reve x = implode o rev o (explode x);

Since explode x is a char list and not a function, this will fail.

You can fix this by placing your parentheses properly

fun reve x = (implode o rev o explode) x;

Or writing it in point-free notation:

val reve = implode o rev o explode

It’s also possible to define a right-associative function application operator, usually called $, which does what you want without parentheses:

(* Right-associative function application *)
infixr $
fun f $ x = f x;

fun reve x = implode o rev o explode $ x