<html><head></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Feb 14, 2012, at 12:18 AM, Erik Silkensen wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div>Another option is you could turn change your fib function to be 'mk-fib' that returns a new instance of the fib function each time it's called:<br><br>(define (mk-fib)<br>  (let ([n0 -1] [n1 1])<br>    (lambda ()<br>      (let ([next (+ n0 n1)])<br>        (set! n0 n1)<br>        (set! n1 next))<br>      n1)))<br></div></blockquote><div><br></div>I frequently assign this sort of thing near the end of a first programming course.  First, I have students write a function "next" which returns how many times it's been called.  Then "next-color", which on successive calls returns "red", "orange", "yellow", "green", "blue", "violet", #f, #f, ...  Then "make-lister", which on input (list "red" "orange" "yellow" "green" "blue" "violet") returns a function acting like "next-color".</div><div><br></div><div>One suggestion: I would have mk-fib take in two parameters specifying the initial values of fib, rather than always making them -1 and 1.</div><br><div> <span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: Helvetica; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-align: -webkit-auto; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; font-size: medium; "><div>Stephen Bloch</div><div><a href="mailto:sbloch@adelphi.edu">sbloch@adelphi.edu</a></div><div><br></div></span><br class="Apple-interchange-newline"> </div> <br></body></html>