plai-typed-racket.rkt

#lang typed/racket

(define-syntax deftype
  (syntax-rules ()
    [(_ name [cons-name (field-name : field-type) ...] ...)
     (begin
       (struct cons-name ([field-name : field-type] ...) #:transparent) ...
       (define-type name (U cons-name ...)))]))

(deftype arith-c
  [num-c (n : Number)]
  [plus-c (l : arith-c) (r : arith-c)]
  [mult-c (l : arith-c) (r : arith-c)])

(: eval-arith (arith-c -> Number))
(define (eval-arith expr)
  (match expr
    [(num-c n) n]
    [(plus-c l r) (+ (eval-arith l) (eval-arith r))]
    [(mult-c l r) (* (eval-arith l) (eval-arith r))]))

(: parse-arith (Sexp -> arith-c))
(define (parse-arith expr)
  (match expr
    [(? number? n) (num-c n)]
    [(list + l r) (plus-c (parse-arith l) (parse-arith r))]
    [(list * l r) (mult-c (parse-arith l) (parse-arith r))]))

(eval-arith (parse-arith (+ (* 42 2) (* 2 (+ 1 0)))))