CS200: Computer Science, Spring 2002
Notes: Friday 29 March 2002
- Before 3 April: GEB, Aria with Diverse Variations and Chapter XIII: BlooP and FlooP and GlooP.
- Friday, 5 April: Problem Set 7
Code changes from meval.ss:(define (thunk? expr) (tagged-list? expr 'thunk)) (define (make-thunk expr env) (list 'thunk expr env)) (define (thunk-expr thunk) (cadr thunk)) (define (thunk-env thunk) (caddr thunk)) (define (frame-lookup-name name frame) (if (null? frame) (error "Name not found in frame:" name) (if (eq? (car (car frame)) name) (car frame) ;;; was (cdr (car frame)) (frame-lookup-name name (cdr frame))))) (define (environment-get-value! name env) (if (null? env) (error "No binding for" name) (if (frame-contains? name (first-frame env)) (let ((np (frame-lookup-name name (first-frame env)))) (if (thunk? (cdr np)) (set-cdr! np (lazeval (thunk-expr (cdr np)) (thunk-env (cdr np))))) (cdr np)) (environment-get-value! name (enclosing-environment env))))) (define (lazapply procedure operands env) (cond ((primitive-procedure? procedure) (lazapply-primitive procedure (map (lambda (op) (lazeval op env)) operands))) ((compound-procedure? procedure) (lazeval-sequence (procedure-body procedure) (extend-environment (procedure-parameters procedure) (map (lambda (op) (make-thunk op env)) operands) (procedure-environment procedure)))) (else (error "Unknown applicator: " procedure)))) (define (lazeval expr env) (cond ((self-evaluating? expr) expr) ((variable? expr) (environment-get-value! expr env)) ((lambda? expr) (make-procedure (lambda-parameters expr) (lambda-body expr) env)) ((definition? expr) (define-variable! (definition-variable expr) (lazeval (definition-value expr) env) env)) ((application? expr) (lazapply (lazeval (application-operator expr) env) (application-operands expr) env)) (else (error "Unknown expression: " exp))))
Are there procedures we can define in a language with lazy evaluation, that cannot be defined in a language with eager evaluation?
I am no lazier now than I was forty years ago, but that is because I reached the limit forty years ago.
Mark Twain in Eruption
Bertrand Russel, In Praise of Idleness
University of Virginia
Department of Computer Science
CS 200: Computer Science
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