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Source for wiki Keywords version 4

author

cowan

comment

ipnr

198.185.18.207

name

Keywords

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0

text

There seems to be a serious issue with how we handle keywords (by which we mean, things like the `else` in `cond`). R5RS was vague on the matter, and each choice of implementation approach seems to have issues.

 * Is `syntax-rules` a keyword in this sense?  Some Schemes say yes, others no.

The R6RS standard decided to use explicitly exported and defined auxiliary syntax values, where every syntax-rules literals clause matched hygienically using `free-identifier=?`.

Let's summarise the issues with different approaches.

== Keywords as symbols ==

One approach is to say that the implementation of `cond` must match a symbol called `else` - so it's purely symbolic equality, rather than bothering about lexical environments.

Problems: See this thread: http://lists.scheme-reports.org/pipermail/scheme-reports/2011-May/000632.html

Particularly, symbolic equality bypasses the normal identifier equality tests used by `syntax-rules`, thereby violating referential transparency and hygiene in some cases.

== Keywords as Bindings ==

Under this scheme, `else` is bound to something (a value? a macro? a pineapple?) along with the definition of `cond`, and we check that the same binding is in place when `else` is used in the wild.

This means that if we do:

{{{
(let ((else #f))
(cond
     (else 1))
}}}

...we'll get an error, not 1, as we've rebound `else`; that arm of the `cond` will evaluate `else` and get `#f`, so no arm of the `cond` matches.

However, it has another issue. Andy Wingo, I believe, gave an example of a module that exposes both `compile` (a procedure) and `eval-when` (a macro that uses `compile` as a keyword). It's then impossible to expose `eval-when` into a sandbox, still able to use the `compile` keyword, without then also giving them the `compile` procedure.

== Any others? ==

Please add alternative implementation techniques here, and discuss their problems and characteristics.

time

2011-05-05 15:47:34

version

4