are defined as patterns, is merely syntactic sugar for a case expression:

fun area shape = case shape of Circle (_, r) => Math.pi * square r | Square (_, s) => square s | Triangle p => heron p

Exhaustiveness checking

Pattern-exhaustiveness checking will make sure that each constructor of the datatype is matched by at least one pattern.

The following pattern is not exhaustive:

fun center (Circle (c, _)) = c | center (Square ((x, y), s)) = (x + s / 2.0, y + s / 2.0)