PL: Lecture #15  Tuesday, February 25th
(text)

The Toy Language

Not in PLAI

A quick note: from now on, we will work with a variation of our language — it will change the syntax to look a little more like Racket, and we will use Racket values for values in our language and Racket functions for built-ins in our language.

Main highlights:

The following is the complete implementation.

#lang pl

;;; ----------------------------------------------------------------
;;; Syntax

#| The BNF:
  <TOY> ::= <num>
          | <id>
          | { bind {{ <id> <TOY> } ... } <TOY> }
          | { fun { <id> ... } <TOY> }
          | { if <TOY> <TOY> <TOY> }
          | { <TOY> <TOY> ... }
|#

;; A matching abstract syntax tree datatype:
(define-type TOY
  [Num  Number]
  [Id  Symbol]
  [Bind (Listof Symbol) (Listof TOY) TOY]
  [Fun  (Listof Symbol) TOY]
  [Call TOY (Listof TOY)]
  [If  TOY TOY TOY])

(: unique-list? : (Listof Any) -> Boolean)
;; Tests whether a list is unique, guards Bind and Fun values.
(define (unique-list? xs)
  (or (null? xs)
      (and (not (member (first xs) (rest xs)))
          (unique-list? (rest xs)))))

(: parse-sexpr : Sexpr -> TOY)
;; parses s-expressions into TOYs
(define (parse-sexpr sexpr)
  (match sexpr
    [(number: n)    (Num n)]
    [(symbol: name) (Id name)]
    [(cons 'bind more)
    (match sexpr
      [(list 'bind (list (list (symbol: names) (sexpr: nameds))
                          ...)
          body)
        (if (unique-list? names)
          (Bind names (map parse-sexpr nameds) (parse-sexpr body))
          (error 'parse-sexpr "duplicate `bind' names: ~s" names))]
      [else (error 'parse-sexpr "bad `bind' syntax in ~s" sexpr)])]
    [(cons 'fun more)
    (match sexpr
      [(list 'fun (list (symbol: names) ...) body)
        (if (unique-list? names)
          (Fun names (parse-sexpr body))
          (error 'parse-sexpr "duplicate `fun' names: ~s" names))]
      [else (error 'parse-sexpr "bad `fun' syntax in ~s" sexpr)])]
    [(cons 'if more)
    (match sexpr
      [(list 'if cond then else)
        (If (parse-sexpr cond)
            (parse-sexpr then)
            (parse-sexpr else))]
      [else (error 'parse-sexpr "bad `if' syntax in ~s" sexpr)])]
    [(list fun args ...) ; other lists are applications
    (Call (parse-sexpr fun)
          (map parse-sexpr args))]
    [else (error 'parse-sexpr "bad syntax in ~s" sexpr)]))

(: parse : String -> TOY)
;; Parses a string containing an TOY expression to a TOY AST.
(define (parse str)
  (parse-sexpr (string->sexpr str)))

;;; ----------------------------------------------------------------
;;; Values and environments

(define-type ENV
  [EmptyEnv]
  [FrameEnv FRAME ENV])

;; a frame is an association list of names and values.
(define-type FRAME = (Listof (List Symbol VAL)))

(define-type VAL
  [RktV  Any]
  [FunV  (Listof Symbol) TOY ENV]
  [PrimV ((Listof VAL) -> VAL)])

(: extend : (Listof Symbol) (Listof VAL) ENV -> ENV)
;; extends an environment with a new frame.
(define (extend names values env)
  (if (= (length names) (length values))
    (FrameEnv (map (lambda ([name : Symbol] [val : VAL])
                    (list name val))
                  names values)
              env)
    (error 'extend "arity mismatch for names: ~s" names)))

(: lookup : Symbol ENV -> VAL)
;; lookup a symbol in an environment, frame by frame,
;; return its value or throw an error if it isn't bound
(define (lookup name env)
  (cases env
    [(EmptyEnv) (error 'lookup "no binding for ~s" name)]
    [(FrameEnv frame rest)
    (let ([cell (assq name frame)])
      (if cell
        (second cell)
        (lookup name rest)))]))

(: unwrap-rktv : VAL -> Any)
;; helper for `racket-func->prim-val': unwrap a RktV wrapper in
;; preparation to be sent to the primitive function
(define (unwrap-rktv x)
  (cases x
    [(RktV v) v]
    [else (error 'racket-func "bad input: ~s" x)]))

(: racket-func->prim-val : Function -> VAL)
;; converts a racket function to a primitive evaluator function
;; which is a PrimV holding a ((Listof VAL) -> VAL) function.
;; (the resulting function will use the list function as is,
;; and it is the list function's responsibility to throw an error
;; if it's given a bad number of arguments or bad input types.)
(define (racket-func->prim-val racket-func)
  (define list-func (make-untyped-list-function racket-func))
  (PrimV (lambda (args)
          (RktV (list-func (map unwrap-rktv args))))))

;; The global environment has a few primitives:
(: global-environment : ENV)
(define global-environment
  (FrameEnv (list (list '+ (racket-func->prim-val +))
                  (list '- (racket-func->prim-val -))
                  (list '* (racket-func->prim-val *))
                  (list '/ (racket-func->prim-val /))
                  (list '< (racket-func->prim-val <))
                  (list '> (racket-func->prim-val >))
                  (list '= (racket-func->prim-val =))
                  ;; values
                  (list 'true  (RktV #t))
                  (list 'false (RktV #f)))
            (EmptyEnv)))

;;; ----------------------------------------------------------------
;;; Evaluation

(: eval : TOY ENV -> VAL)
;; evaluates TOY expressions.
(define (eval expr env)
  ;; convenient helper
  (: eval* : TOY -> VAL)
  (define (eval* expr) (eval expr env))
  (cases expr
    [(Num n)  (RktV n)]
    [(Id name) (lookup name env)]
    [(Bind names exprs bound-body)
    (eval bound-body (extend names (map eval* exprs) env))]
    [(Fun names bound-body)
    (FunV names bound-body env)]
    [(Call fun-expr arg-exprs)
    (let ([fval (eval* fun-expr)]
          [arg-vals (map eval* arg-exprs)])
      (cases fval
        [(PrimV proc) (proc arg-vals)]
        [(FunV names body fun-env)
          (eval body (extend names arg-vals fun-env))]
        [else (error 'eval "function call with a non-function: ~s"
                      fval)]))]
    [(If cond-expr then-expr else-expr)
    (eval* (if (cases (eval* cond-expr)
                  [(RktV v) v] ; Racket value => use as boolean
                  [else #t])  ; other values are always true
              then-expr
              else-expr))]))

(: run : String -> Any)
;; evaluate a TOY program contained in a string
(define (run str)
  (let ([result (eval (parse str) global-environment)])
    (cases result
      [(RktV v) v]
      [else (error 'run "evaluation returned a bad value: ~s"
                  result)])))

;;; ----------------------------------------------------------------
;;; Tests

(test (run "{{fun {x} {+ x 1}} 4}")
      => 5)
(test (run "{bind {{add3 {fun {x} {+ x 3}}}} {add3 1}}")
      => 4)
(test (run "{bind {{add3 {fun {x} {+ x 3}}}
                  {add1 {fun {x} {+ x 1}}}}
              {bind {{x 3}} {add1 {add3 x}}}}")
      => 7)
(test (run "{bind {{identity {fun {x} x}}
                  {foo {fun {x} {+ x 1}}}}
              {{identity foo} 123}}")
      => 124)
(test (run "{bind {{x 3}}
              {bind {{f {fun {y} {+ x y}}}}
                {bind {{x 5}}
                  {f 4}}}}")
      => 7)
(test (run "{{{fun {x} {x 1}}
              {fun {x} {fun {y} {+ x y}}}}
            123}")
      => 124)

;; More tests for complete coverage
(test (run "{bind x 5 x}")      =error> "bad `bind' syntax")
(test (run "{fun x x}")        =error> "bad `fun' syntax")
(test (run "{if x}")            =error> "bad `if' syntax")
(test (run "{}")                =error> "bad syntax")
(test (run "{bind {{x 5} {x 5}} x}") =error> "duplicate*bind*names")
(test (run "{fun {x x} x}")    =error> "duplicate*fun*names")
(test (run "{+ x 1}")          =error> "no binding for")
(test (run "{+ 1 {fun {x} x}}") =error> "bad input")
(test (run "{+ 1 {fun {x} x}}") =error> "bad input")
(test (run "{1 2}")            =error> "with a non-function")
(test (run "{{fun {x} x}}")    =error> "arity mismatch")
(test (run "{if {< 4 5} 6 7}")  => 6)
(test (run "{if {< 5 4} 6 7}")  => 7)
(test (run "{if + 6 7}")        => 6)
(test (run "{fun {x} x}")      =error> "returned a bad value")

;;; ----------------------------------------------------------------