Let’s write an evaluator, in the form of an interpreter, for our arithmetic language.
數(shù)據(jù)類型和parser都和上一篇的一樣豌骏，interp還是naive形態(tài)的垢啼。。测摔。
#lang plai-typed
(define-type ArithC
[numC (n : number)]
[plusC (l : ArithC) (r : ArithC)]
[multC (l : ArithC) (r : ArithC)])
(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 ArithC a
[numC (n) n]
[plusC (l r) (+ (interp l) (interp r))]
[multC (l r) (* (interp l) (interp r))]))
(define (driver [s : s-expression]) : number
(interp (parse s)))
(driver '(+ 1 1))
