interp.rkt

#lang racket
(provide interp interp-env interp-prim1)
(require "ast.rkt"
         "env.rkt"
         "interp-prims.rkt")

;; type Answer = Value | 'err

;; type Value =
;; | Integer
;; | Boolean
;; | Character
;; | Eof
;; | Void
;; | '()
;; | (cons Value Value)
;; | (box Value)

;; type REnv = (Listof (List Id Value))

;; Expr -> Answer
(define (interp e)
  (interp-env e '()))

;; Expr Env -> Answer
(define (interp-env e r)
  (match e
    [(Int i)  i]
    [(Bool b) b]
    [(Char c) c]
    [(String s) s]
    [(Eof)    eof]
    [(Empty)  '()]
    [(Var x)  (lookup r x)]
    [(Prim0 'void) (void)]
    [(Prim0 'read-byte) (read-byte)]
    [(Prim0 'peek-byte) (peek-byte)]
    [(Prim1 p e)
     (match (interp-env e r)
       ['err 'err]
       [v (interp-prim1 p v)])]
    [(Prim2 p e1 e2)
     (match (interp-env e1 r)
       ['err 'err]
       [v1 (match (interp-env e2 r)
             ['err 'err]
             [v2 (interp-prim2 p v1 v2)])])]
    [(If p e1 e2)
     (match (interp-env p r)
       ['err 'err]
       [v
        (if v
            (interp-env e1 r)
            (interp-env e2 r))])]
    [(Begin e1 e2)
     (match (interp-env e1 r)
       ['err 'err]
       [_    (interp-env e2 r)])]
    [(Let x e1 e2)
     (match (interp-env e1 r)
       ['err 'err]
       [v (interp-env e2 (ext r x v))])]))