From: Ramakrishnan Muthukrishnan <vu3rdd@gmail.com> Date: Wed, 18 Jan 2012 15:58:46 +0000 (+0530) Subject: solution to 4.26 X-Git-Url: https://git.rkrishnan.org/%5B/frontends/$rel_link?a=commitdiff_plain;h=f225ca5e374ba315b4fefb9d24174cb942c59036;p=sicp.git solution to 4.26 --- diff --git a/src/sicp/ex4_26.rkt b/src/sicp/ex4_26.rkt new file mode 100644 index 0000000..01c33af --- /dev/null +++ b/src/sicp/ex4_26.rkt @@ -0,0 +1,19 @@ +#lang racket + +#| + +Yes, if 'unless' were implemented in an applicative order language, then +it would be implemented as a special form and would be translated into +other functions before evaluation. + +Now, if such a special form is used in a higher order function like 'map' +for instance: + +(map unless '((p-list) (usual-list) (exception-list))) + +then, unless gets expanded before evaluation. But map expects a function +as its second argument. + +To see the implementation of unless, see metacircular2.rkt + +|# diff --git a/src/sicp/metacircular2-test.rkt b/src/sicp/metacircular2-test.rkt index 1644f95..cda363a 100644 --- a/src/sicp/metacircular2-test.rkt +++ b/src/sicp/metacircular2-test.rkt @@ -77,7 +77,8 @@ 1 (* (factorial (- n 1)) n))) env1) - (check-equal? (eval '(factorial 10) env1) 3628800 "factorial test")))) + (check-equal? (eval '(factorial 10) env1) 3628800 "factorial test") + (check-equal? (eval '(unless true "true" "false") env1) "false" "unless test 1")))) (run-tests metacircular2-tests) \ No newline at end of file diff --git a/src/sicp/metacircular2.rkt b/src/sicp/metacircular2.rkt index c45a5e7..ef387d4 100644 --- a/src/sicp/metacircular2.rkt +++ b/src/sicp/metacircular2.rkt @@ -227,6 +227,7 @@ [`(define ,(? (lambda (x) (not (pair? x))) var) ,b) (define-variable! var (eval b env) env)] [`(define ,(? pair? var) ,b ..1) (define-variable! (car var) (eval (make-lambda (cdr var) b) env) env)] [`(if ,pred ,consequent ,alternative) (if (true? (eval pred env)) (eval consequent env) (eval alternative env))] + [`(unless ,condition ,consequent ,alternative) (if (true? (eval condition env)) (eval alternative env) (eval consequent env))] [`(lambda ,parameters ,body ..1) (make-procedure parameters body env)] [`(begin ,exp ...) (eval-sequence exp env)] [`(cond ,clauses ...) (eval (cond->if clauses) env)]