139: Syntax parameters139: Syntax parametersby Marc Nieper-Wißkirchen
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Syntax parameters are to the expansion process of a Scheme program what parameters are to the evaluation process of a Scheme program. They allow hygienic implementation of syntactic forms that would otherwise introduce implicit identifiers unhygienically.
Syntax parameters are a mechanism for rebinding a macro definition within the dynamic extent of a macro expansion. This provides a convenient solution to one of the most common types of unhygienic macro: those that introduce a unhygienic binding each time the macro is used. Examples include a lambda form with a return keyword, class macros that introduce a special self binding, or a while loop form that provides an escape procedure as a binding for an auxiliary abort keyword.
With the syntax-rules macro system, it is not possible to introduce bindings unhygienically. With low-level macro systems like the sc-macro-transformer or the syntax-case macro system, one can break hygiene by constructing new identifiers in an existing syntactic closure. While use cases of introducing identifiers unhygienically like in SRFI 99 seem unproblematic, the use cases given above where bindings are based on identifiers that are completely independent of the arguments of the macro are often severely broken.
With syntax parameters, instead of introducing the binding unhygienically each time, one instead creates one binding for the keyword, which is then adjusted later when the keyword to supposed to have a different meaning. As no new bindings are introduced, hygiene is preserved. This is similar to the dynamic binding mechanisms available at run-time (in the form of parameters), except that the dynamic binding only occurs during macro expansion. The code after macro expansion remains lexically scoped.
The first example is certainly not an example of good functional
programming style but it easily demonstrates how syntax parameters
are used and why
call-with-current-continuation is known as the
fundamental control-flow construct.
(import (scheme base)
(scheme write)
(srfi 139))
(define-syntax-parameter abort
(syntax-rules ()
((_ . _)
(syntax-error "abort used outside of a loop"))))
(define-syntax forever
(syntax-rules ()
((forever body1 body2 ...)
(call-with-current-continuation
(lambda (escape)
(syntax-parameterize
((abort
(syntax-rules ()
((abort value (... ...))
(escape value (... ...))))))
(let loop ()
body1 body2 ... (loop))))))))
(define i 0)
(forever
(display i)
(newline)
(set! i (+ 1 i))
(when (= i 10)
(abort)))
The following example is derived from an example given in the Guile reference manual.
(import (scheme base)
(scheme write)
(srfi 1)
(srfi 139))
(define-syntax-parameter return
(syntax-rules ()
((_ . _)
(syntax-error "return used outside of a lambda^"))))
(define-syntax lambda^
(syntax-rules ()
((lambda^ formals body1 body2 ...)
(lambda formals
(call-with-current-continuation
(lambda (escape)
(syntax-parameterize
((return
(syntax-rules ()
((return value (... ...))
(escape value (... ...))))))
body1 body2 ...)))))))
(define product
(lambda^ (list)
(fold (lambda (n o)
(if (zero? n)
(return 0)
(* n o)))
1 list)))
...
(define-syntax-parameter <keyword> <transformer spec>)
Binds <keyword> to the transformer obtained by evaluating <transformer spec>. The transformer provides the default expansion for the syntax parameter, and in the absence of syntax-parameterize, is functionally equivalent to define-syntax. (For example, you may want to have the transformer throw a syntax error indicating that the keyword is supposed to be used in conjunction with another macro as in the examples above).
(syntax-parameterize
((<keyword> <transformer spec>) ...)
<body>)
Adjusts the <keyword>s to use the transformer obtained
by evaluating the corresponding <transformer>s in
the expansion of the <body>. Each keyword must be bound to a
syntax parameter. syntax-parameterize differs from
let-syntax in that the binding is not shadowed, but
adjusted, and so uses of the keyword in the expansion
of <body> use the new transformers. This is
somewhat similar to how parameterize adjusts the values of regular
parameters, rather than creating new bindings.
A sample implementation is provided by the
Unsyntax Scheme implementation,
which implements this SRFI in the (srfi 139) library.
Syntax parameters are also implemented by, at least, Chibi-Scheme, Guile, and Racket.
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