Chapter 4, Metalinguistic Abstraction
Exercise 4.31
It’s an interesting exercise.
I just change make-procedure to contain params-types list too. for eg: if there is a procedure (define (proc (a lazy) b (c lazy-memo)) (+ a b c)), then params-types is a list '(lazy () lazy-memo).
Similar to procedure list-of-delayed-args, I implemented list-of-customized-args which just calls customize-it instead of delay-it.
customize-it is similar to delay-it except that it takes an extra argument for param-type and add tag 'lazy-thunk or 'memo-thunk depending if param-type is lazy or lazy-memo.
Apart from this there are cases added in force-it where memo-thunk case is handled as it was in book’s version.
Well thats it!
The code which contains the changes is below:
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(define (list-of-arg-values exps env)
(if (no-operands? exps)
'()
(cons (actual-value (first-operand exps) env)
(list-of-arg-values (rest-operands exps)
env))))
(define (list-of-customized-args params-types exps env)
(if (no-operands? exps)
'()
(cons (customize-it (car params-types) (first-operand exps) env)
(list-of-customized-args (cdr params-types) (rest-operands exps)
env))))
(define (make-procedure parameters body env)
(define split
(fold-right (lambda(new rem)
(let ((param
(if (pair? new)
(if (or (lazy-param? (cadr new))
(memo-lazy-param? (cadr new)))
new
(error "Wrong parameter type: " (cadr new)))
(list new '()))))
(cons
(cons (car param) (car rem))
(cons (cadr param) (cdr rem)))))
(cons '() '())
parameters))
(list 'procedure (car split) (cdr split) body env))
(define (compound-procedure? p)
(tagged-list? p 'procedure))
(define (procedure-parameters p) (cadr p))
(define (procedure-params-types p) (caddr p))
(define (procedure-body p) (cadddr p))
(define (procedure-environment p) (cadddr (cdr p)))
(define (lazy-param? param-type) (eq? 'lazy param-type))
(define (memo-lazy-param? param-type) (eq? 'memo-lazy param-type))
;; thunks
(define (customize-it param-type exp env)
(cond ((lazy-param? param-type) (list 'lazy-thunk exp env))
((memo-lazy-param? param-type) (list 'memo-thunk exp env))
(else (actual-value exp env))))
;(define (delay-it exp env)
; (list 'thunk exp env))
(define (thunk? obj)
(or (lazy-thunk? obj) (memo-thunk? obj)))
(define (lazy-thunk? obj)
(tagged-list? obj 'lazy-thunk))
(define (memo-thunk? obj)
(tagged-list? obj 'memo-thunk))
(define (thunk-exp thunk) (cadr thunk))
(define (thunk-env thunk) (caddr thunk))
;; "thunk" that has been forced and is storing its (memoized) value
(define (evaluated-thunk? obj)
(tagged-list? obj 'evaluated-thunk))
(define (thunk-value evaluated-thunk) (cadr evaluated-thunk))
;; memoizing version of force-it
(define (force-it obj)
(cond ((memo-thunk? obj)
(let ((result (actual-value
(thunk-exp obj)
(thunk-env obj))))
(set-car! obj 'evaluated-thunk)
(set-car! (cdr obj) result) ; replace exp with its value
(set-cdr! (cdr obj) '()) ; forget unneeded env
result))
((lazy-thunk? obj) (actual-value (thunk-exp obj) (thunk-env obj)))
((evaluated-thunk? obj)
(thunk-value obj))
(else obj)))
Output/Testing
First I tested with a small code:
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;;; L-Eval input:
(define (test a (b lazy) (c memo-lazy)) a b c 'done)
;;; L-Eval value:
ok
;;; L-Eval input:
(test ((lambda() (display 'a-invoked) 'a)) ((lambda() (display 'b-invoked) 'b)) ((lambda() (display 'c-invoked) 'c)))
a-invoked
;;; L-Eval value:
done
Then, I tested it using ex-4.27 code.
Testing each version separately, normal, lazy and memoized by defining argument to procedure id in ex-4.27, as normal, lazy and lazy-memo.
Note there is a difference in how values of count changes between each version.
Without lazy or lazy-memo
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;;; L-Eval input:
(define count 0)
;;; L-Eval value:
ok
;;; L-Eval input:
(define (id x)
(set! count (+ count 1))
x)
;;; L-Eval value:
ok
;;; L-Eval input:
(define w (id (id 10)))
;;; L-Eval value:
ok
;;; L-Eval input:
count
;;; L-Eval value:
2
;;; L-Eval input:
w
;;; L-Eval value:
10
;;; L-Eval input:
count
;;; L-Eval value:
2
;;; L-Eval input:
w
;;; L-Eval value:
10
With lazy argument
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;;; L-Eval input:
(define count 0)
;;; L-Eval value:
ok
;;; L-Eval input:
(define (id (x lazy))
(set! count (+ count 1))
x)
;;; L-Eval value:
ok
;;; L-Eval input:
(define w (id (id 10)))
;;; L-Eval value:
ok
;;; L-Eval input:
count
;;; L-Eval value:
1
;;; L-Eval input:
w
;;; L-Eval value:
10
;;; L-Eval input:
count
;;; L-Eval value:
2
;;; L-Eval input:
w
;;; L-Eval value:
10
;;; L-Eval input:
count
;;; L-Eval value:
3
;;; L-Eval input:
w
;;; L-Eval value:
10
;;; L-Eval input:
count
;;; L-Eval value:
4
With memo-lazy:
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;;; L-Eval input:
(define count 0)
;;; L-Eval value:
ok
;;; L-Eval input:
(define (id (x memo-lazy))
(set! count (+ count 1))
x)
;;; L-Eval value:
ok
;;; L-Eval input:
(define w (id (id 10)))
;;; L-Eval value:
ok
;;; L-Eval input:
count
;;; L-Eval value:
1
;;; L-Eval input:
w
;;; L-Eval value:
10
;;; L-Eval input:
count
;;; L-Eval value:
2
;;; L-Eval input:
w
;;; L-Eval value:
10
;;; L-Eval input:
count
;;; L-Eval value:
2
Complete Code:
I have added changed in this file provided by mit.
Note: Sometimes this code: (ge (make-top-level-environment)) at the top does not correctly clear the environment. So, I executed this directly in scheme repl before loading the file.(I am using emacs). I wasted some time in it believing that there is a bug in code but the actual problem was this :(
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;;;;LAZY EVALUATOR FROM SECTION 4.2 OF
;;;; STRUCTURE AND INTERPRETATION OF COMPUTER PROGRAMS
;;;;Matches code in ch4.scm
;;;; Also includes enlarged primitive-procedures list
;;;;This file can be loaded into Scheme as a whole.
;;;;**NOTE**This file loads the metacircular evaluator of
;;;; sections 4.1.1-4.1.4, since it uses the expression representation,
;;;; environment representation, etc.
;;;; You may need to change the (load ...) expression to work in your
;;;; version of Scheme.
;;;;**WARNING: Don't load mceval twice (or you'll lose the primitives
;;;; interface, due to renamings of apply).
;;;;Then you can initialize and start the evaluator by evaluating
;;;; the two lines at the end of the file ch4-mceval.scm
;;;; (setting up the global environment and starting the driver loop).
;;;; resetting the environment so that we can reload the file
(ge (make-top-level-environment))
(restart 1)
;;;; To run without memoization, reload the first version of force-it below
;;**implementation-dependent loading of evaluator file
;;Note: It is loaded first so that the section 4.2 definition
;; of eval overrides the definition from 4.1.1
(load "ch4-mceval.scm")
;;;SECTION 4.2.2
;;; Modifying the evaluator
(define (eval exp env)
(cond ((self-evaluating? exp) exp)
((variable? exp) (lookup-variable-value exp env))
((quoted? exp) (text-of-quotation exp))
((assignment? exp) (eval-assignment exp env))
((definition? exp) (eval-definition exp env))
((if? exp) (eval-if exp env))
((lambda? exp)
(make-procedure (lambda-parameters exp)
(lambda-body exp)
env))
((begin? exp)
(eval-sequence (begin-actions exp) env))
((cond? exp) (eval (cond->if exp) env))
((application? exp) ; clause from book
(apply (actual-value (operator exp) env)
(operands exp)
env))
(else
(error "Unknown expression type -- EVAL" exp))))
(define (actual-value exp env)
(force-it (eval exp env)))
(define (apply procedure arguments env)
(cond ((primitive-procedure? procedure)
(apply-primitive-procedure
procedure
(list-of-arg-values arguments env))) ; changed
((compound-procedure? procedure)
(eval-sequence
(procedure-body procedure)
(extend-environment
(procedure-parameters procedure)
(list-of-customized-args (procedure-params-types procedure) arguments env) ; changed
(procedure-environment procedure))))
(else
(error
"Unknown procedure type -- APPLY" procedure))))
(define (list-of-arg-values exps env)
(if (no-operands? exps)
'()
(cons (actual-value (first-operand exps) env)
(list-of-arg-values (rest-operands exps)
env))))
(define (list-of-customized-args params-types exps env)
(if (no-operands? exps)
'()
(cons (customize-it (car params-types) (first-operand exps) env)
(list-of-customized-args (cdr params-types) (rest-operands exps)
env))))
(define (make-procedure parameters body env)
(define split
(fold-right (lambda(new rem)
(let ((param
(if (pair? new)
(if (or (lazy-param? (cadr new))
(memo-lazy-param? (cadr new)))
new
(error "Wrong parameter type: " (cadr new)))
(list new '()))))
(cons
(cons (car param) (car rem))
(cons (cadr param) (cdr rem)))))
(cons '() '())
parameters))
(list 'procedure (car split) (cdr split) body env))
(define (compound-procedure? p)
(tagged-list? p 'procedure))
(define (procedure-parameters p) (cadr p))
(define (procedure-params-types p) (caddr p))
(define (procedure-body p) (cadddr p))
(define (procedure-environment p) (cadddr (cdr p)))
(define (eval-if exp env)
(if (true? (actual-value (if-predicate exp) env))
(eval (if-consequent exp) env)
(eval (if-alternative exp) env)))
(define input-prompt ";;; L-Eval input:")
(define output-prompt ";;; L-Eval value:")
(define (driver-loop)
(prompt-for-input input-prompt)
(let ((input (read)))
(define stime (get-universal-time))
(let ((output
(actual-value input the-global-environment)))
;; (newline)
;; (display (list "Time Taken: " (- (get-universal-time) stime)))
;; (newline)
(announce-output output-prompt)
(user-print output)))
(driver-loop))
;;; Representing thunks
;; non-memoizing version of force-it
;; (define (force-it obj)
;; (if (thunk? obj)
;; (actual-value (thunk-exp obj) (thunk-env obj))
;; obj))
(define (lazy-param? param-type) (eq? 'lazy param-type))
(define (memo-lazy-param? param-type) (eq? 'memo-lazy param-type))
;; thunks
(define (customize-it param-type exp env)
(cond ((lazy-param? param-type) (list 'lazy-thunk exp env))
((memo-lazy-param? param-type) (list 'memo-thunk exp env))
(else (actual-value exp env))))
;(define (delay-it exp env)
; (list 'thunk exp env))
(define (thunk? obj)
(or (lazy-thunk? obj) (memo-thunk? obj)))
(define (lazy-thunk? obj)
(tagged-list? obj 'lazy-thunk))
(define (memo-thunk? obj)
(tagged-list? obj 'memo-thunk))
(define (thunk-exp thunk) (cadr thunk))
(define (thunk-env thunk) (caddr thunk))
;; "thunk" that has been forced and is storing its (memoized) value
(define (evaluated-thunk? obj)
(tagged-list? obj 'evaluated-thunk))
(define (thunk-value evaluated-thunk) (cadr evaluated-thunk))
;; memoizing version of force-it
(define (force-it obj)
(cond ((memo-thunk? obj)
(let ((result (actual-value
(thunk-exp obj)
(thunk-env obj))))
(set-car! obj 'evaluated-thunk)
(set-car! (cdr obj) result) ; replace exp with its value
(set-cdr! (cdr obj) '()) ; forget unneeded env
result))
((lazy-thunk? obj) (actual-value (thunk-exp obj) (thunk-env obj)))
((evaluated-thunk? obj)
(thunk-value obj))
(else obj)))
;; A longer list of primitives -- suitable for running everything in 4.2
;; Overrides the list in ch4-mceval.scm
(define primitive-procedures
(list (list 'car car)
(list 'cdr cdr)
(list 'cons cons)
(list 'null? null?)
(list 'list list)
(list '+ +)
(list '- -)
(list '* *)
(list '/ /)
(list '= =)
(list 'newline newline)
(list 'display display)
;; more primitives
))
(define the-global-environment (setup-environment))
(driver-loop)
'LAZY-EVALUATOR-LOADED