Editing Standard ML (section)

====Redundancy checking====
The pattern in the second clause of the following (meaningless) function is redundant:

<syntaxhighlight lang="sml">
fun f (Circle ((x, y), r)) = x + y
  | f (Circle _) = 1.0
  | f _ = 0.0
</syntaxhighlight>

Any value that would match the pattern in the second clause would also match the pattern in the first clause, so the second clause is unreachable. Therefore, this definition as a whole exhibits redundancy, and causes a compile-time warning.

The following function definition is exhaustive and not redundant:

<syntaxhighlight lang="sml">
val hasCorners = fn (Circle _) => false | _ => true
</syntaxhighlight>

If control gets past the first pattern ({{code|Circle}}), we know the shape must be either a {{code|Square}} or a {{code|Triangle}}. In either of those cases, we know the shape has corners, so we can return {{code|lang=sml|true}} without discerning the actual shape.