Matching

Polymorphism is especially useful when writing functions over parametric types, which include built-in types like lists, sets, and maps.

As an example, we will see how to implement list reversing parametrically on any type, rather than just on lists of a specific type.

Similarly to the polymorphic identity function, we can introduce a type variable that can be generalised. We will write a direct version of the function using an accumulator.

function rev <T>(xs : list<T>) : list<T> {

const rev = <T>([xs, acc] : [list<T>, list<T>]) : list<T> =>

match(xs) {

when([]): acc;

when([y,...ys]): rev([ys, list([y,...acc])])

};

return rev([xs, []]);

};

Note how the type checker was able to infer the types of [] and [y,...ys] in the when clauses (without the need of using [] and [y,...ys]), but in [y,...acc] the cast to list is necessary, because of the rest property that needs to be interpreted as a cons. Similarly, the list in [xs, []] is needed to force the interpretation of [] as the empty list, instead of the empty array ("unit").

See predefined module List.

We use an accumulator variable acc to keep the elements of the list processed, consing each element on it.

As with the identity function, we can then use rev directly with different type instances:

const ints : list<int> = rev([1, 2, 3]);

const nats : list<nat> = rev([1n, 2n, 3n]);

See predefined namespace List.