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Source for ticket #382

cc

changetime

2012-10-12 03:07:14

component

WG1 - Core

description

Submitter's name: John Boyle

Submitter's email: johnthescavenger at gmail.com

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 `if`s 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 two 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 [http://ycombinator.com/arc/tut.txt here], near the phrases "An if" and "cond operator":

id

keywords

milestone

owner

cowan

priority

major

reporter

cowan

resolution

wontfix

severity

status

closed

summary

Formal Comment: Allow "if" to accept arbitrarily many if-then pairs

time

2012-04-23 06:52:13

type

defect

Changes

Change at time 2012-10-12 03:07:14

author

cowan

field

comment

newvalue

oldvalue

raw-time

1349986034491786

ticket

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2012-10-12 03:07:14

Change at time 2012-10-12 03:07:14

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cowan

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2012-10-12 03:07:14

Change at time 2012-10-12 03:07:14

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cowan

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2012-10-12 03:07:14

Change at time 2012-04-23 06:52:26

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cowan

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2012-04-23 06:52:26

Change at time 2012-04-23 06:52:26

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alexshinn

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2012-04-23 06:52:26

Change at time 2012-04-23 06:52:26

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2012-04-23 06:52:26