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Source for wiki Keywords version 3

author

arcfide

comment

Insert a hygiene statement of the problems with symbolic tests.

ipnr

69.136.5.227

name

Keywords

readonly

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.

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 06:06:41

version

3