Parametric polymorphism: Difference between revisions

=={{header|Racket}}==

Typed Racket has parametric polymorphism:

<lang racket>

#lang typed/racket

(define-type (Tree A) (U False (Node A)))

(struct: (A) Node

([val : A] [left : (Tree A)] [right : (Tree A)])

#:transparent)

(: tree-map (All (A B) (A -> B) (Tree A) -> (Tree B)))

(define (tree-map f tree)

(match tree

[#f #f]

[(Node val left right)

(Node (f val) (tree-map f left) (tree-map f right))]))

;; unit tests

(require typed/rackunit)

(check-equal?

(tree-map add1 (Node 5 (Node 3 #f #f) #f))

(Node 6 (Node 4 #f #f) #f))

</lang>