93 lines
2.7 KiB
Scheme
93 lines
2.7 KiB
Scheme
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;;;;LAZY EVALUATOR FROM SECTION 4.2 OF
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;;;; STRUCTURE AND INTERPRETATION OF COMPUTER PROGRAMS
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;;;;Matches code in ch4.scm
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;;;; Also includes enlarged primitive-procedures list
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;;;;This file can be loaded into Scheme as a whole.
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;;;;**NOTE**This file loads the metacircular evaluator of
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;;;; sections 4.1.1-4.1.4, since it uses the expression representation,
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;;;; environment representation, etc.
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;;;; You may need to change the (load ...) expression to work in your
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;;;; version of Scheme.
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;;;;**WARNING: Don't load mceval twice (or you'll lose the primitives
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;;;; interface, due to renamings of apply).
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;;;;Then you can initialize and start the evaluator by evaluating
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;;;; the two lines at the end of the file ch4-mceval.scm
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;;;; (setting up the global environment and starting the driver loop).
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;;;; To run without memoization, reload the first version of force-it below
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;;**implementation-dependent loading of evaluator file
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;;Note: It is loaded first so that the section 4.2 definition
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;; of eval overrides the definition from 4.1.1
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;;;SECTION 4.2.2
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;;; Modifying the evaluator
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(define (actual-value exp env)
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(force-it (eval exp env)))
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(define (list-of-arg-values exps env)
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(if (no-operands? exps)
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'()
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(cons (actual-value (first-operand exps) env)
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(list-of-arg-values (rest-operands exps)
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env))))
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(define (list-of-delayed-args exps env)
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(if (no-operands? exps)
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'()
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(cons (delay-it (first-operand exps) env)
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(list-of-delayed-args (rest-operands exps)
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env))))
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(define (eval-if exp env)
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(if (true? (actual-value (if-predicate exp) env))
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(eval (if-consequent exp) env)
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(eval (if-alternative exp) env)))
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(define (force-it obj)
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(if (thunk? obj)
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(actual-value (thunk-exp obj) (thunk-env obj))
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obj))
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;; thunks
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(define (delay-it exp env)
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(list 'thunk exp env))
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(define (thunk? obj)
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(and (pair? obj)
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(= (length obj) 3)
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(tagged-list? obj 'thunk)))
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(define (thunk-exp thunk) (cadr thunk))
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(define (thunk-env thunk) (caddr thunk))
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;; "thunk" that has been forced and is storing its (memoized) value
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(define (evaluated-thunk? obj)
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(tagged-list? obj 'evaluated-thunk))
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(define (thunk-value evaluated-thunk) (cadr evaluated-thunk))
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;; memoizing version of force-it
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(define (force-it obj)
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(cond ((thunk? obj)
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(let ((result (actual-value
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(thunk-exp obj)
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(thunk-env obj))))
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(set-car! obj 'evaluated-thunk)
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(set-car! (cdr obj) result) ; replace exp with its value
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(set-cdr! (cdr obj) '()) ; forget unneeded env
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result))
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((evaluated-thunk? obj)
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(thunk-value obj))
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(else obj)))
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'LAZY-EVALUATOR-LOADED
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