Pattern-Matching

Pattern matching means matching values against patterns. In the case where a value matches a pattern, a collection of bindings are generated between the variables in the pattern and certain components in the value. Pattern-matching is performed according to the following set of rules:

• A value that matches a constant pattern must be the same constant, and this matching generates no bindings.

• The void value () only matches the pattern (), and this matching generates no bindings.

• Any value can match the wildcard pattern, and this matching generates no bindings.

• Any value can match a variable pattern, and this matching generates a binding between the variable and the value.

• A tuple value matches a tuple pattern if they are of the same length and each value component in the former matches the corresponding pattern component in the latter, and this matching generates a collection of bindings that is the union of the bindings generated from matching the value components in the tuple value against the pattern components in the tuple pattern.

• A record value matches a record pattern if they have the same field names and each value component in the former matches the corresponding pattern component in the latter, and this matching generates a collection of bindings that is the union of the bindings generated from matching the value components in the record value against the pattern components in the record pattern.

• Given a pattern formed by applying a constructor C to some pattern arguments, a value matches this pattern if the value is formed by applying C to some value arguments matching the pattern arguments, and this matching generates a collection of bindings that is the union of the bindings generated from matching the value arguments against the pattern arguments.

• Given a referenced pattern (x as pat), a value matches the pattern if it matches pat, and this matching generates a collection of bindings that extends the bindings generated from matching the value against pat with a binding from x to the value.

Suppose we have a tuple value (0, 1, 2, 3) and a tuple pattern (0, _, x, y). Then the value matches the pattern and this matching yields bindings from x and y to 2 and 3, respectively.