Either implements a functional programming idiom that is often used when a function may return either a meaningful value or an error message. By convention, the left variant is used to return an error, usually as a string, and the right variant returns a valid value.
The following example is based on a function that searches for a student with a specific numeric id in a list. find-person-from-id returns Either a valid Person as right or one of two error messages as Strings in left.
import either as E data Person: | student(id :: Number, name :: String) end people = [list: student(001, "Charlie Brown"), student(002, "Sally Brown"), student(003, "Lucy van Pelt"), student(003, "Linus van Pelt")] fun find-person-from-id(p :: List<Person>, i :: Number) -> E.Either: results = p.filter(lam(a): a.id == i end) result-count = results.length() ask: | result-count == 0 then: E.left("Not found error.") | result-count == 1 then: E.right(results.get(0)) | otherwise: E.left("Duplicate ID error.") end where: find-person-from-id(people, 007) is E.left("Not found error.") find-person-from-id(people, 001) is E.right(student(001, "Charlie Brown")) find-person-from-id(people, 003) is E.left("Duplicate ID error.") end
Typically, the Either variants are processed by a cases expression, for example:
fun search(id :: Number): doc: "Hypothetical function calls resulting from either result." result = find-person-from-id(people, id) cases(E.Either) result: | left(s) => display-error-dialog(s) | right(p) => display-person(p) end end