#lang plait
(define PI 3.14159)
(* PI 10)                 ; => 31.4159

(define PI2 : Number (* PI PI))

(define (circle-area [r : Number])
  (* PI (* r r)))
(circle-area 10)          ; => 314.159

(define (f [x : Number]) : Number
     (* x (+ x 1)))