Submitter's name: John Boyle
Submitter's email: [email protected]
Relevant draft: r7rs-small-current
Type: enhancement
Priority: major
Relevant section of draft: Conditionals
Summary: Allow "if" to accept arbitrarily many if-then pairs.
Description:
The conditional operator "if" should be made to accept arbitrarily many
if-then pairs, so that:
(if (blip)
(blop)
(flip)
(flop)
(floop))
is equivalent to:
(cond ((blip)
(blop))
((flip)
(flop))
(else (floop)))
This is more terse and provides for a better user experience. (In the case
of 3 arguments, one who writes "if" and later discovers that he needs to
add more cases must either write nested "if"s or go back and rewrite the
whole expression with "cond"; either choice is unpleasant. For precisely
this reason, there are people who preempt it by always using "cond" over
"if" in the first place; these people pay a cost in extra parentheses,
letters typed, and indentation.)
I anticipate reluctance to alter the syntax of a primitive operator. But
this change is entirely backwards compatible with code that uses the
previous "if", and skeptical people are free to continue only writing ifs
with three arguments and using "cond" otherwise. The only thing they will
lose is error-checking if they accidentally give four or more arguments to
"if". I point out that the same objection could be made to "map", +, -, *,
/, and other functions. I add that the Arc language has used this form of
"if" without any "cond" at all, and I didn't find anyone complaining in any
of 14,000 posts on the Arc Forum that they wanted "cond" back.
Here is an example implementation in terms of "cond". (I see no reason not
to allow *zero* or more arguments, rather than 2 or more, but I expect that
is pushing it.)
(define-syntax my-if
(syntax-rules ()
((my-if a b c ...) ;at least two arguments
(expand-if a b c ...))))
(define-syntax expand-if
(syntax-rules ()
((expand-if) (cond (#f #f))) ;the unspecified fall-through result
((expand-if x) x)
((expand-if a b rest ...)
(cond (a b)
(else (expand-if rest ...))))))
This suggestion comes from the Arc language. Further discussion may be
found in the below link, near the phrases "An if" and "cond operator":
http://ycombinator.com/arc/tut.txt
--John Boyle
*Science is what we understand well enough to explain to a computer. Art is
everything else we do.* --Knuth
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