On Sep 16, 2011, at 11:52 AM, Shriram Krishnamurthi wrote:
I introduced templates today. Almost as if on cue, one student asked
whether he could use else instead of (cons? l). I told them I was
going to make a MORAL judgment about why it was EVIL, and spent ten
minutes talking about all that.
As class ended, one of my students came up and said,
"So why doesn't the Racket language Web site agree with you?"
http://docs.racket-lang.org/guide/Lists__Iteration__and_Recursion.html
(Look for "[else".)
Interesting.
I'm with you: I tell my students to use "else" to capture true "none
of the above" cases that can't easily be described directly, e.g.
(define (size thing)
(cond [(string? thing) (string-length thing)]
[(real? thing) (abs thing)]
[(image? thing) (* (image-width thing) (image-height thing))]
[else (error 'size "I don't know how to handle that
type.")]))
Stephen Bloch
[email protected]
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