racket/step2_eval.rkt

   1 #!/usr/bin/env racket
   2 #lang racket
   3
   4 (require "types.rkt" "readline.rkt" "reader.rkt" "printer.rkt")
   5
   6 ;; read
   7 (define (READ str)
   8   (read_str str))
   9
  10 ;; eval
  11 (define (eval-ast ast env)
  12   (cond
  13     [(symbol? ast)
  14      (or (hash-ref env ast
  15                    (lambda () (raise (string-append "'"
  16                                                     (symbol->string ast)
  17                                                     "' not found")))))]
  18     [(_sequential? ast) (_map (lambda (x) (EVAL x env)) ast)]
  19     [(hash? ast) (make-hash
  20                   (dict-map ast (lambda (k v) (cons k (EVAL v env)))))]
  21     [else ast]))
  22
  23 (define (EVAL ast env)
  24   (if (not (list? ast))
  25         (eval-ast ast env)
  26
  27         (let* ([el (eval-ast ast env)]
  28                [f (first el)]
  29                [args (rest el)])
  30           (apply f args))))
  31
  32 ;; print
  33 (define (PRINT exp)
  34   (pr_str exp true))
  35
  36 ;; repl
  37 (define repl-env (hash '+ + '- - '* * '/ /))
  38 (define (rep str)
  39   (PRINT (EVAL (READ str) repl-env)))
  40
  41 (define (repl-loop)
  42   (let ([line (readline "user> ")])
  43     (when (not (eq? nil line))
  44       (with-handlers
  45         ([string? (lambda (exc) (printf "Error: ~a~n" exc))]
  46          [blank-exn? (lambda (exc) null)])
  47         (printf "~a~n" (rep line)))
  48       (repl-loop))))
  49 (repl-loop)