445 lines
17 KiB
Racket
445 lines
17 KiB
Racket
#lang pl
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;;; ==================================================================
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;;; Syntax
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#| The BNF:
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<TOY> ::= <num>
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| <id>
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| { set! <id> <TOY> }
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| { bind {{ <id> <TOY> } ... } <TOY> <TOY> ... }
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| { bindrec {{ <id> <TOY> } ... } <TOY> <TOY> ... }
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| { fun { <id> ... } <TOY> <TOY> ... }
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| { rfun { <id> ... } <TOY> <TOY> ... }
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| { if <TOY> <TOY> <TOY> }
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| { <TOY> <TOY> ... }
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|#
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(define-type BINDINGS = (Listof (Listof Symbol)))
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;; A matching abstract syntax tree datatype:
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(define-type TOY
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[Num Number]
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[Id Symbol]
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[Set Symbol TOY]
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[Bind (Listof Symbol) (Listof TOY) (Listof TOY)]
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[BindRec (Listof Symbol) (Listof TOY) (Listof TOY)]
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[Fun (Listof Symbol) (Listof TOY)]
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[RFun (Listof Symbol) (Listof TOY)]
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[Call TOY (Listof TOY)]
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[If TOY TOY TOY])
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(: unique-list? : (Listof Any) -> Boolean)
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;; Tests whether a list is unique, guards Bind and Fun values.
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(define (unique-list? xs)
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(or (null? xs)
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(and (not (member (first xs) (rest xs)))
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(unique-list? (rest xs)))))
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; Parametric types having to end in '-of' is dumb.
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(define-type (Perhapsof A)
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[Some A]
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[None])
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(: index-of : Symbol (Listof Symbol) -> (Perhapsof Natural))
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(define (index-of name names)
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(: loop : (Listof Symbol) Natural -> (Perhapsof Natural))
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(define (loop remaining idx)
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(cond [(null? remaining) (None)]
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[(symbol=? name (first remaining)) (Some idx)]
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[else (loop (rest remaining) (add1 idx))]))
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(loop names 0))
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(: find-index : Symbol BINDINGS -> (Perhapsof (List Natural Natural)))
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(define (find-index name bindings)
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(: loop : BINDINGS Natural -> (Perhapsof (List Natural Natural)))
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(define (loop rest-bindings binding-idx)
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(cond [(null? rest-bindings) (None)]
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[else
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(let ([name-idx (index-of name (first rest-bindings))])
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(cases name-idx
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[(Some idx-in-scope) (Some (list binding-idx idx-in-scope))]
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[(None) (loop (rest rest-bindings) (add1 binding-idx))]))]))
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(loop bindings 0))
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(: parse-sexpr : Sexpr -> TOY)
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;; parses s-expressions into TOYs
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(define (parse-sexpr sexpr)
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(match sexpr
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[(number: n) (Num n)]
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[(symbol: name) (Id name)]
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[(cons 'set! more)
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(match sexpr
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[(list 'set! (symbol: name) new) (Set name (parse-sexpr new))]
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[else (error 'parse-sexpr "bad `set!' syntax in ~s" sexpr)])]
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[(cons (and binder (or 'bind 'bindrec)) more)
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(match sexpr
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[(list _ (list (list (symbol: names) (sexpr: nameds)) ...)
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body0 body ...)
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(if (unique-list? names)
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((if (eq? 'bind binder) Bind BindRec)
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names
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(map parse-sexpr nameds)
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(map parse-sexpr (cons body0 body)))
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(error 'parse-sexpr "duplicate `~s' names: ~s" binder names))]
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[else (error 'parse-sexpr "bad `~s' syntax in ~s"
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binder sexpr)])]
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[(cons (and funner (or 'fun 'rfun)) more)
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(match sexpr
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[(list _ (list (symbol: names) ...)
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body0 body ...)
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(if (unique-list? names)
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((if (eq? 'fun funner) Fun RFun)
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names
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(map parse-sexpr (cons body0 body)))
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(error 'parse-sexpr "duplicate `~s' names: ~s" funner names))]
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[else (error 'parse-sexpr "bad `~s' syntax in ~s"
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funner sexpr)])]
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[(cons 'if more)
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(match sexpr
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[(list 'if cond then else)
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(If (parse-sexpr cond) (parse-sexpr then) (parse-sexpr else))]
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[else (error 'parse-sexpr "bad `if' syntax in ~s" sexpr)])]
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[(list fun args ...) ; other lists are applications
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(Call (parse-sexpr fun)
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(map parse-sexpr args))]
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[else (error 'parse-sexpr "bad syntax in ~s" sexpr)]))
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(: parse : String -> TOY)
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;; Parses a string containing an TOY expression to a TOY AST.
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(define (parse str)
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(parse-sexpr (string->sexpr str)))
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;;; ==================================================================
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;;; Values and environments
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(define-type ENV = (Listof (Listof (Boxof VAL))))
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(define-type VAL
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[BogusV]
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[RktV Any]
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[FunV (Listof Symbol) (ENV -> VAL) ENV Boolean] ; `byref?' flag
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[PrimV ((Listof VAL) -> VAL)])
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;; a single bogus value to use wherever needed
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(define the-bogus-value (BogusV))
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(: extend : (Listof VAL) ENV -> ENV)
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;; extends an environment with a new frame (given plain values).
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(define (extend names values env)
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(cons (map (inst box VAL) values) env))
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(: extend-rec : (Listof (ENV -> VAL)) ENV -> ENV)
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;; extends an environment with a new recursive frame (given compiled
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;; expressions).
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(define (extend-rec compiled-exprs env)
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(define new-boxes
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(map (λ (_) (box the-bogus-value)) compiled-exprs))
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(define new-frame new-boxes)
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(define new-env (cons new-frame env))
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;; note: no need to check the lengths here, since this is only
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;; called for `bindrec', and the syntax make it impossible to have
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;; different lengths
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(for-each (lambda ([boxed-val : (Boxof VAL)] [compiled : (ENV -> VAL)])
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(set-box! boxed-val (compiled new-env)))
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new-boxes compiled-exprs)
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new-env)
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(: framerefbox : (Listof (Boxof VAL)) Natural -> (Boxof VAL))
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;; Gets you the box at idx in the frame.
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(define (framerefbox frame idx)
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(if (null? frame)
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(error 'framerefbox "no binding found for index ~s" idx)
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(if (zero? idx)
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(first frame)
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(framerefbox (rest frame) (sub1 idx)))))
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(: envrefbox : ENV Natural Natural -> (Boxof VAL))
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;; Gets you the box at (frameidx, keyidx) in an environment.
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(define (envrefbox env frameidx keyidx)
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(if (null? env)
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(error 'envrefbox "no binding found for frame index ~s" frameidx)
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(if (zero? frameidx)
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(framerefbox (first env) keyidx)
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(envrefbox (rest env) (sub1 frameidx) keyidx))))
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(: unwrap-rktv : VAL -> Any)
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;; helper for `racket-func->prim-val': unwrap a RktV wrapper in
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;; preparation to be sent to the primitive function
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(define (unwrap-rktv x)
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(cases x
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[(RktV v) v]
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[else (error 'racket-func "bad input: ~s" x)]))
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(: racket-func->prim-val : Function -> VAL)
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;; converts a racket function to a primitive evaluator function which
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;; is a PrimV holding a ((Listof VAL) -> VAL) function. (the
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;; resulting function will use the list function as is, and it is the
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;; list function's responsibility to throw an error if it's given a
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;; bad number of arguments or bad input types.)
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(define (racket-func->prim-val racket-func)
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(define list-func (make-untyped-list-function racket-func))
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(PrimV (lambda (args)
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(RktV (list-func (map unwrap-rktv args))))))
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;; The global environment has a few primitives:
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(: global-environment : (Listof (List Symbol VAL)))
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(define global-environment
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(list (list '+ (racket-func->prim-val +))
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(list '- (racket-func->prim-val -))
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(list '* (racket-func->prim-val *))
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(list '/ (racket-func->prim-val /))
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(list '< (racket-func->prim-val <))
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(list '> (racket-func->prim-val >))
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(list '= (racket-func->prim-val =))
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;; values
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(list 'true (RktV #t))
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(list 'false (RktV #f))))
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(: global-lookup : Symbol -> VAL)
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(define (global-lookup name)
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(let ([cell (assq name global-environment)])
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(if cell
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(second cell)
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(error 'global-lookup "no binding for ~s" name))))
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;;; ==================================================================
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;;; Compilation
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(: compiler-enabled? : (Boxof Boolean))
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;; a global flag that can disable the compiler
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(define compiler-enabled? (box #f))
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(: compile-body : (Listof TOY) BINDINGS -> (ENV -> VAL))
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;; compiles a list of expressions to a single Racket function.
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(define (compile-body exprs bin)
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(unless (unbox compiler-enabled?)
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(error 'compile-body "compiler disabled"))
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(let ([compiled-1st (compile (first exprs) bin)]
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[rest (rest exprs)])
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(if (null? rest)
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compiled-1st
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(let ([compiled-rest (compile-body rest bin)])
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(lambda (env)
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(define ignored (compiled-1st env))
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(compiled-rest env))))))
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(: compile-get-boxes : (Listof TOY) BINDINGS -> (ENV -> (Listof (Boxof VAL))))
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;; utility for applying rfun
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(define (compile-get-boxes exprs bin)
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(: compile-getter : TOY -> (ENV -> (Boxof VAL)))
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(define (compile-getter expr)
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(cases expr
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[(Id name)
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(cases (find-index name bin)
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[(Some location) (λ ([env : ENV])
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(envrefbox env (first location) (second location)))]
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[(None) (if (assq name global-environment)
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(λ ([_ : ENV]) (error 'call "rfun can't use global ~s" name))
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(error 'compile-get-boxes "no binding for ~s" name))])]
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[else
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(lambda ([_ : ENV])
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(error 'call "rfun application with a non-identifier ~s"
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expr))]))
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(unless (unbox compiler-enabled?)
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(error 'compile-get-boxes "compiler disabled"))
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(let ([getters (map compile-getter exprs)])
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(lambda (env)
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(map (lambda ([get-box : (ENV -> (Boxof VAL))]) (get-box env))
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getters))))
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(: compile : TOY BINDINGS -> (ENV -> VAL))
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;; compiles TOY expressions to Racket functions.
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(define (compile expr bin)
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;; convenient helper for running compiled code
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(: caller : ENV -> ((ENV -> VAL) -> VAL))
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(define (caller env)
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(lambda (compiled) (compiled env)))
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(unless (unbox compiler-enabled?)
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(error 'compile "compiler disabled"))
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(: compile* : TOY -> (ENV -> VAL))
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(define (compile* expr*) (compile expr* bin))
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(cases expr
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[(Num n) (lambda ([env : ENV]) (RktV n))]
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[(Id name)
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(cases (find-index name bin)
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[(Some location) (λ ([env : ENV]) (unbox (envrefbox env (first location) (second location))))]
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[(None) (let ([global-val (global-lookup name)])
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(λ ([_ : ENV]) global-val))])]
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[(Set name new)
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(define compiled-new (compile new bin))
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(cases (find-index name bin)
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[(Some location)
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(λ ([env : ENV])
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(set-box! (envrefbox env (first location) (second location))
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(compiled-new env))
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the-bogus-value)]
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[(None)
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(let ([_ (global-lookup name)])
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(λ ([_ : ENV]) (error 'compile "can't set flobal binding for ~s" name)))])]
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[(Bind names exprs bound-body)
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(define compiled-exprs (map compile* exprs))
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(define compiled-body (compile-body bound-body (cons names bin)))
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(lambda ([env : ENV])
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(compiled-body
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(extend (map (caller env) compiled-exprs) env)))]
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[(BindRec names exprs bound-body)
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(: ah : (Listof (Listof Symbol)))
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(define ah (cons names bin))
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(define compiled-exprs (map (λ ([x : TOY]) (compile x ah)) exprs))
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(define compiled-body (compile-body bound-body ah))
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(lambda ([env : ENV])
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(compiled-body (extend-rec compiled-exprs env)))]
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[(Fun names bound-body)
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(define compiled-body (compile-body bound-body (cons names bin)))
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(lambda ([env : ENV]) (FunV names compiled-body env #f))]
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[(RFun names bound-body)
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(define compiled-body (compile-body bound-body (cons names bin)))
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(lambda ([env : ENV]) (FunV names compiled-body env #t))]
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[(Call fun-expr arg-exprs)
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(define compiled-fun (compile fun-expr bin))
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(define compiled-args (map compile* arg-exprs))
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(define compiled-boxes-getter (compile-get-boxes arg-exprs bin))
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(lambda ([env : ENV])
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(define fval (compiled-fun env))
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;; delay evaluating the arguments
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(define arg-vals (lambda () (map (caller env) compiled-args)))
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(cases fval
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[(PrimV proc) (proc (arg-vals))]
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[(FunV names compiled-body fun-env byref?)
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(compiled-body (if byref?
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(cons (compiled-boxes-getter env) fun-env)
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(extend (arg-vals) fun-env)))]
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[else (error 'call "function call with a non-function: ~s"
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fval)]))]
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[(If cond-expr then-expr else-expr)
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(define compiled-cond (compile cond-expr bin))
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(define compiled-then (compile then-expr bin))
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(define compiled-else (compile else-expr bin))
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(lambda ([env : ENV])
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((if (cases (compiled-cond env)
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[(RktV v) v] ; Racket value => use as boolean
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[else #t]) ; other values are always true
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compiled-then
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compiled-else)
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env))]))
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(: run : String -> Any)
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;; compiles and runs a TOY program contained in a string
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(define (run str)
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(set-box! compiler-enabled? #t)
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(let ([compiled (compile (parse str) '(()))])
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(set-box! compiler-enabled? #f)
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(let ([result (compiled '())])
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(cases result
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[(RktV v) v]
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[else (error 'run "the program returned a bad value: ~s"
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result)]))))
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;;; ==================================================================
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;;; Tests
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(test (run "{{fun {x} {+ x 1}} 4}")
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=> 5)
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(test (run "{bind {{add3 {fun {x} {+ x 3}}}} {add3 1}}")
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=> 4)
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(test (run "{bind {{add3 {fun {x} {+ x 3}}}
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{add1 {fun {x} {+ x 1}}}}
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{bind {{x 3}} {add1 {add3 x}}}}")
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=> 7)
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(test (run "{bind {{identity {fun {x} x}}
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{foo {fun {x} {+ x 1}}}}
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{{identity foo} 123}}")
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=> 124)
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(test (run "{bind {{x 3}}
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{bind {{f {fun {y} {+ x y}}}}
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{bind {{x 5}}
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{f 4}}}}")
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=> 7)
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(test (run "{{{fun {x} {x 1}}
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{fun {x} {fun {y} {+ x y}}}}
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123}")
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=> 124)
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;; More tests for complete coverage
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(test (run "{bind x 5 x}") =error> "bad `bind' syntax")
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(test (run "{fun x x}") =error> "bad `fun' syntax")
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(test (run "{if x}") =error> "bad `if' syntax")
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(test (run "{}") =error> "bad syntax")
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(test (run "{bind {{x 5} {x 5}} x}") =error> "duplicate*bind*names")
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(test (run "{fun {x x} x}") =error> "duplicate*fun*names")
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(test (run "{+ x 1}") =error> "no binding for")
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(test (run "{+ 1 {fun {x} x}}") =error> "bad input")
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(test (run "{+ 1 {fun {x} x}}") =error> "bad input")
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(test (run "{1 2}") =error> "with a non-function")
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(test (run "{{fun {x} x}}") =error> "arity mismatch")
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(test (run "{if {< 4 5} 6 7}") => 6)
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(test (run "{if {< 5 4} 6 7}") => 7)
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(test (run "{if + 6 7}") => 6)
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(test (run "{fun {x} x}") =error> "returned a bad value")
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;; assignment tests
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(test (run "{set! {+ x 1} x}") =error> "bad `set!' syntax")
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(test (run "{bind {{x 1}} {set! x {+ x 1}} x}") => 2)
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(test (run "{set! + 1}") =error> "set")
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(test (run "{set! nope 1}") =error> "no binding")
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;; `bindrec' tests
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(test (run "{bindrec {x 6} x}") =error> "bad `bindrec' syntax")
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(test (run "{bindrec {{fact {fun {n}
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{if {= 0 n}
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1
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{* n {fact {- n 1}}}}}}}
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{fact 5}}")
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=> 120)
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;; tests for multiple expressions and assignment
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(test (run "{bind {{make-counter
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{fun {}
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{bind {{c 0}}
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{fun {}
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{set! c {+ 1 c}}
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c}}}}}
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{bind {{c1 {make-counter}}
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{c2 {make-counter}}}
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{* {c1} {c1} {c2} {c1}}}}")
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=> 6)
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(test (run "{bindrec {{foo {fun {}
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{set! foo {fun {} 2}}
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1}}}
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{+ {foo} {* 10 {foo}}}}")
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=> 21)
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;; `rfun' tests
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(test (run "{{rfun {x} x} 4}") =error> "non-identifier")
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(test (run "{bind {{swap! {rfun {x y}
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{bind {{tmp x}}
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{set! x y}
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{set! y tmp}}}}
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{a 1}
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{b 2}}
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{swap! a b}
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{+ a {* 10 b}}}")
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=> 12)
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;; test that argument are not evaluated redundantly
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(test (run "{{rfun {x} x} {/ 4 0}}") =error> "non-identifier")
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(test (run "{5 {/ 6 0}}") =error> "non-function")
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(test (index-of 'k '(a b k c)) => (Some 2))
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(test (index-of 'k '(a b c)) => (None))
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(test (find-index 'k '((a b) (k y) (k z))) => (Some '(1 0)))
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(test (find-index 'a '((a b) (k y))) => (Some '(0 0)))
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(test (find-index 'q '((a b) (k y))) => (None))
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;; test compiler-disabled flag, for complete coverage
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;; (these tests must use the functions instead of the toplevel `run',
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;; since there is no way to get this error otherwise, this indicates
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;; that this error should not occur outside of our code -- it is an
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;; internal error check)
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(test (compile (Num 1) '(())) =error> "compiler disabled")
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(test (compile-body (list (Num 1)) '(())) =error> "compiler disabled")
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(test (compile-get-boxes (list (Num 1)) '(())) =error> "compiler disabled")
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;;; ==================================================================
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