[R6RS] Enumerations proposal pre-draft
William D Clinger
will at ccs.neu.edu
Wed May 24 11:26:30 EDT 2006
Here is a question concerning define-syntax, syntax-case,
and gensym: How do the editors expect macros such as the
following to be implemented using the R6RS macro system?
With the current draft of the enumerations proposal,
'(black white purple maroon)
needs to expand into something like
(make-enumeration '(black white purple maroon)))
(syntax-rules (black white purple maroon)
((_ black) 'black)
((_ white) 'white)
((_ purple) 'purple)
((_ maroon) 'maroon)))
((_ symbol ...)
(list (color symbol) ...))))))
There are several issues here:
* Obviously define-enumeration can be used only in a definition
context. That's fine.
* If define-syntax can appear only at top-level or library-level,
then define-enumeration can appear only at those levels. The
current draft of syntax.html also allows define-syntax at the
level of internal definitions.
* The define-enumeration macro must generate a unique, fresh,
ideally non-forgeable name. With the macro system we are
considering for R6RS, uniqueness and freshness can only be
accomplished by using syntax-case, if at all, and it even
less clear whether non-forgeability can be accomplished.
* Uniqueness and freshness can be accomplished by syntax-case
only if syntax-case transformers can change the state of the
macro-expansion-time world in which they execute. It is not
clear whether this is allowed by the current draft of the
syntax proposal, or whether it should be allowed.
* The current specification of generate-temporaries says the
temporaries are "guaranteed to be unique, i.e., different
from all other identifiers", but that is a vacuous statement,
true of every identifier.
* What we need to know is whether there is a way to generate
an identifier that will never be generated by any other call
to generate-temporaries, and cannot be forged (e.g. by passing
a string to string->symbol).
* The last paragraph of section 3.9 suggests that the identifiers
returned by generate-temporaries are forgeable, for there does
not seem to be any guarantee that the temporaries generated
during the expansion of one form or library will be distinct
from the temporaries generated during the expansion of another.
* In fact, some existing implementations of syntax-rules generate
the same sequence of temporaries for each form expanded, and
this is legal under the R5RS semantics. If this is supposed
to be illegal under the R6RS semantics, then we need an R6RS
semantics that answers questions of this sort.
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