#lang racket

(require racket/fixnum)

(provide interp-R1 interp-exp env-head-value env-head-symbol eval-symbol add-binding)

(define (interp-R1 env)
  (lambda (p)
    (match p
       [`(program ,data ,exp) ((interp-exp env) exp)])))

(define (interp-exp env)
  (lambda (sexp)
    (match sexp
      [(? fixnum?) sexp]
      [(? symbol?) (eval-symbol env sexp)]
      [`(read) (read-fixnum)]
      [`(- ,e) (fx- 0 ((interp-exp env) e))]
      [`(+ ,e1 ,e2) (fx+ ((interp-exp env) e1) ((interp-exp env) e2))]
      [`(let ([,var ,rexp]) ,body)
        (let ([value ((interp-exp env) rexp)])
          ((interp-exp (add-binding env (list var value))) body))])))

(define (read-fixnum)
  (let ([r (read)])
    (cond
       [(fixnum? r) r]
       [else (error "invalid fixnum in input: " r)])))

(define (env-head-symbol env)
  (car (car env)))

(define (env-head-value env)
  (cadr (car env)))

(define (add-binding env binding)
  (cons binding env))
    
(define (eval-symbol env s)
  (cond
    [(equal? env `())                 (error "Symbol " s " not found")]
    [(equal? (env-head-symbol env) s) (env-head-value env)]
    [else                             (eval-symbol (cdr env) s)]))