[r6rs-discuss] [Formal] please make square brackets more useful

From: Felix Klock <pfr6rs>
Date: Wed, 20 Jun 2007 10:54:09 -0400

On Jun 20, 2007, at 8:06 AM, Sam TH wrote:

> On 6/20/07, AndrevanTonder <andre at het.brown.edu> wrote:
>> On Tue, 19 Jun 2007, William D Clinger wrote, quoting Jonathan Reese:
>>
>>> Define
>>> [a b c] to be a special reader syntax for the four-element list
>>> (*square-bracketed* a b c)
>
> This also requires that the various special forms defined in the R6RS
> would need to specify if they allowed or required square brackets.
> For example, if it was decided that cond required square brackets, it
> would be much harder to port existing R5RS code to R6RS. If it was
> decided the other way, the large quantities of code written currently
> with square brackets could not be made to work with R6RS.

Your use of the phrase "the other way" here was ambiguous; I infer
that you meant "if it were decided that cond *disallowed* square
brackets, the large quantities of code . . .", but note that there is
the other interpretation of "the other way" (of "if it were decided
that cond *allowed* square brackets") since you stated above that
R6RS would need to specify between "allowed" or "required."

The reality is that there are three choices here; the various special
form would need to specify if they allowed, disallowed, or required
square brackets.

I just wanted to make it crystal clear that if cond were specified to
*allow* (but not require) square brackets (which is implementable as
a syntax-rules macro under the proposal, if a bit ugly) under the
proposal, then there is no portability problem, and there is no
problem with the "large quantities of code written currently with
square brackets."

Furthermore, even if cond were not specified to allow (but instead
require or even disallow) square brackets, I believe that under the
proposal you would still have the freedom to write your own
compatibility library that did allow square brackets and import cond
from that library instead of the (r6rs) library.

The only problem I can see with the proposal is that it does
introduce a burden on the macro writer who wants to emulate the
behavior that square brackets are synonymous with parentheses,
because (I think) they must write several more clauses depending on
how complex the pattern of their macro is. But then again, I imagine
that someone could come up with a helper procedure for syntax case
that would solve even this problem.

Still, until this feature is actually implemented and demonstrated to
the community at large, I would be hesitant to promote its
introduction to the R6RS.

-Felix
Received on Wed Jun 20 2007 - 10:54:09 UTC