This SRFI is currently in final status. Here is an explanation of each status that a SRFI can hold. To provide input on this SRFI, please send email to srfi-147@nospamsrfi.schemers.org. To subscribe to the list, follow these instructions. You can access previous messages via the mailing list archive.
begin in transformer specs.)
Each syntax definition assigns a macro transformer to a keyword. The
macro transformer is specified by a transformer spec, which is either
an instance of syntax-rules, an existing syntactic keyword
(including macro keywords and the syntactic keywords that introduce the core forms,
like lambda, if, or define), or
a use of a macro that eventually expands into an instance
of syntax-rules. In the latter case, the keyword of
macro use is called a custom macro transformer.
While the syntax-rules-transformer is in many cases a
simple way to define new keywords in the Scheme language, sometimes more
specialized macro transformers are being called for (e.g.
the computation-rules of SRFI 53). The current report on
the Scheme language, the R7RS, however, does not describe a means to
define custom macro transformers based on top of syntax rules.
Such a means is described in this SRFI, namely allowing macro uses that eventually expand into a transformer spec in place of transformer spec.
A simple example is given below:
(define-syntax syntax-rules*
(syntax-rules ()
((syntax-rules* (literal ...) (pattern . templates) ...)
(syntax-rules (literal ...) (pattern (begin . templates)) ...))
((syntax-rules* ellipsis (literal ...) (pattern . templates) ...)
(syntax-rules ellipsis (literal ...) (pattern (begin . templates)) ...))))
(let-syntax
((foo
(syntax-rules* ()
((foo a b)
(define a 1)
(define b 2)))))
(foo x y)
(list x y)) ⟹ '(1 2)
A fairly large use case of this SRFI is SRFI 148, which defines a custom macro transformer for hygienic composable macros.
A Scheme system supporting this SRFI should apply the requirements
of this SRFI with respect to syntax-rules mutatis
mutandis to all other natively provided macro
transformers, e.g. sc-macro-transformer
or syntax-case.
A Scheme system supporting this SRFI should apply the requirements
of this SRFI with respect
to define-syntax, let-syntax,
and letrec-syntax mutatis mutandis to all other
natively provided binding facilities for
keywords, e.g. let-syntax/splicing
or define-syntax-parameter.
<transformer spec> -> <keyword> -> <macro use> -> (begin <definition>… <transformer spec>)
Whenever a keyword is bound to a macro transformer, and the macro
transformer is given by a transformer spec that is a macro use, the
keyword is bound to the macro transformer given by the transformer
spec that results from transcribing the macro use. It is an error
if the macro use does not expand into a transformer spec (but see
below). In case of transformer specs that appear in the bindings of
the let-syntax binding construct, the expansion of the
transformer spec takes place in a scope between the outer scope of the binding
construct and its inner scope. In case of transformer specs that appear in the bindings of
the letrec-syntax binding
constructs, the expansion of the transformer spec takes place in the
scope of the bindings of the binding construct.
If the transformer spec is a keyword, the keyword bound to
the transformer spec essentially becomes an alias to the syntactic
keyword keyword. (Keyword may also be one of the
core syntactic keywords
like lambda, if, define.)
The same holds if the transformer spec is a macro use that
eventually expands into a (possibly empty) sequence of multiple definitions followed by
the keyword (possibly bound by the introduced definitions).
In order to facilitate writing sophisticated custom macro
transformers, it is allowed that a transformer spec expands into a
sequence introduced by the keyword begin of multiple
definitions eventually followed by a transformer spec (whose
expansion may make use of the introduced definitions).
Note: This form of begin is a fourth form
of begin different from the forms described in sections
4.2.3 and sections 5.6.1 of the R7RS.
(scheme base) library:
define-syntaxlet-syntaxletrec-syntaxsyntax-rules(srfi 147) library. Typical library declarations of a
program or library using (srfi 147) are:
(cond-expand
(custom-macro-transformers
(import (scheme base)))
(else
(import (except (scheme base)
define-syntax
let-syntax
letrec-syntax
syntax-rules))
(import (srfi 147))))
For convenience, the sample implementation provides these library
declaration in the
file custom-macro-transformers.scm
for inclusion via include-library-declarations.
A Scheme system properly supporting this SRFI should ensure that the bindings
of define-syntax, let-syntax,
letrec-syntax, and syntax-rules exported
from (scheme base) and (srfi 147) are
respectively the same. Such a Scheme system should provide the
feature identifier custom-macro-transformers. This
cannot be achieved by a fully portable implementation using only the
R7RS without redefining (scheme base).
syntax-rules-macro facility, which seems to be
an indispensable and defining part of contemporary Scheme.
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