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functional-programmingpattern-matchingsmlsmlnj

exhaustive patterns

I'm learning ML, can somebody please explain what does it mean exhaustive patterns?

Solution

  • A pattern match is exhaustive if it can not fail. I.e. all cases that could possibly occur are covered by a pattern.

    For example the following pattern match is not exhaustive because it doesn't cover the case that the list is empty:

    fun sum (x::xs) = x + sum xs

    The following is exhaustive because both cases are covered:

    fun sum (x::xs) = x + sum xs
  | sum [] = 0

    Generally a pattern match on an algebraic data type is exhaustive if and only if there is a default case or there is a case for all constructors and the match for each sub-pattern is exhaustive.