03-01-interpreter.rkt

#lang plai-typed

; Type Arithmetic Calculator
(define-type ArithC 
  [numC (n : number)]
  [plusC (l : ArithC) (r : ArithC)]
  [multC (l : ArithC) (r : ArithC)])

; Parsing function
; string -> Expr
; returns an Expr tree
(define (parse [s : s-expression]) : ArithC
  (cond
    [(s-exp-number? s) (numC (s-exp->number s))]
    [(s-exp-list? s)
     (let ([sl (s-exp->list s)])
       (case (s-exp->symbol (first sl))
         [(+) (plusC (parse (second sl)) (parse (third sl)))]
         [(*) (multC (parse (second sl)) (parse (third sl)))]
         [else (error 'parse "invalid list input")]))]
    [else (error 'parse "invalid input")]))

(define (interp [a : ArithC]) : number
  ; type-case matches the type
  ; them it bindings the constructor parameter with vars
  ; the last expression is the returned func with the values
  (type-case ArithC a
    [numC (n) n]
    [plusC (l r) (+ (interp l) (interp r))]
    [multC (l r) (* (interp l) (interp r))]))

#;(parse '(+ (* 1 2) (+ 2 3)))