The description of macro transformers in section 11.19 states:
Each <syntax rule> must have the following form:
(<srpattern> <template>)
An <srpattern> is a restricted form of <pattern>,
namely, a nonempty <pattern> in one of four
parenthesized forms below whose first subform
is an identifier or an underscore _.
The restriction is sensible for a language such as R5RS in which
transformers can be applied only to syntax representing a macro
instance (a non-empty parenthesized form, whose first subform is a
syntactic keyword). This is no longer true in the current draft of
R6RS since one can write:
(import (rnrs))
((syntax-rules ---) ---)
Restricting the domain of procedures created with syntax-rules is
artificial and needless. The restriction should be dropped and
<srpattern> should be replaced simply with <pattern>. It's a trivial
simplification to the syntax-case definition of syntax-rules given in
12.8 (libraries). Dropping the restriction allows the following
examples to work as expected:
(import (rnrs))
(syntax-rules () (x x)) ;; identity function on syntax.
((syntax-rules ()
(#t 'true)
(#f 'false))
(syntax #t)) ;; => 'true
David
Received on Fri Jul 13 2007 - 20:47:04 UTC