Pattern Matching with Functions

The syntax we've been using so far for functions is also a special case of the full syntax that OCaml permits. That syntax is:

let f p1 ... pn = e1 in e2   (* function as part of let expression *)
let f p1 ... pn = e          (* function definition at toplevel *)
fun p1 ... pn -> e           (* anonymous function *)

The truly primitive syntactic form we need to care about is fun p -> e. Let's revisit the semantics of anonymous functions and their application with that form; the changes to the other forms follow from those below:

Static semantics.

  • Let x1..xn be the pattern variables appearing in p. If by assuming that x1:t1 and x2:t2 and ... and xn:tn, we can conclude that p:t and e:u, then fun p -> e : t -> u.

  • The type checking rule for application is unchanged.

Dynamic semantics.

  • The evaluation rule for anonymous functions is unchanged.

  • To evaluate e0 e1:

    1. Evaluate e0 to an anonymous function fun p -> e, and evaluate e1 to value v1.

    2. Match v1 against pattern p. If it doesn't match, raise the exception Match_failure. Otherwise, if it does match, it produces a set \(b\) of bindings.

    3. Substitute those bindings \(b\) in e, yielding a new expression e'.

    4. Evaluate e' to a value v, which is the result of evaluating e0 e1.

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