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author | Ian Price <ianprice90@googlemail.com> | 2012-01-07 01:59:33 +0000 |
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committer | Andy Wingo <wingo@pobox.com> | 2012-01-08 13:28:35 +0100 |
commit | 729b62bd95c61b6e2992f823b469b76bdc483b0d (patch) | |
tree | cfb7667846e6bc4ee629ac08f87ec4225b3925b9 /doc/ref/api-macros.texi | |
parent | 49d09292ac6d92a86b574478a69450b85d2dd8c1 (diff) | |
download | guile-729b62bd95c61b6e2992f823b469b76bdc483b0d.tar.gz |
document syntax parameters
* doc/ref/api-macros.texi (Macros): Add subsection for "Syntax Parameters"
Diffstat (limited to 'doc/ref/api-macros.texi')
-rw-r--r-- | doc/ref/api-macros.texi | 76 |
1 files changed, 76 insertions, 0 deletions
diff --git a/doc/ref/api-macros.texi b/doc/ref/api-macros.texi index 92816ad82..7ff5bf9bc 100644 --- a/doc/ref/api-macros.texi +++ b/doc/ref/api-macros.texi @@ -40,6 +40,7 @@ languages}, or EDSLs.}. * Syntax Case:: Procedural, hygienic macros. * Defmacros:: Lisp-style macros. * Identifier Macros:: Identifier macros. +* Syntax Parameters:: Syntax Parameters * Eval When:: Affecting the expand-time environment. * Internal Macros:: Macros as first-class values. @end menu @@ -861,6 +862,81 @@ wrapping in @code{#'} syntax forms. @end deffn +@node Syntax Parameters +@subsection Syntax Parameters + +Syntax parameters@footnote{Described in the paper @cite{Keeping it Clean with +Syntax Parameters} by Barzilay, Culpepper and Flatt.} are a mechanism for rebinding a macro +definition within the dynamic extent of a macro expansion. It 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 @code{lambda} form with a @code{return} keyword, or +class macros that introduce a special @code{self} binding. + +With syntax parameters, instead of introducing the binding +unhygienically each time, we instead create one binding for the +keyword, which we can then adjust later when we want the keyword to +have a different meaning. As no new bindings are introduced, hygiene +is preserved. This is similar to the dynamic binding mechanisms we +have at run-time like @ref{SRFI-39, parameters} or +@ref{Fluids and Dynamic States, fluids}, except that the dynamic +binding only occurs during macro expansion. The code after macro +expansion remains lexically scoped. + +@deffn {Syntax} define-syntax-parameter keyword transformer +Binds @var{keyword} to the value obtained by evaluating @var{transformer}. The +@var{transformer} provides the default expansion for the syntax parameter, +and in the absence of @code{syntax-parameterize}, is functionally equivalent +to @code{define-syntax}. Usually, you will just want to have the @var{transformer} +throw a syntax error indicating that the @var{keyword} is supposed to be +used in conjunction with another macro, for example: +@example +(define-syntax-parameter return + (lambda (stx) + (syntax-violation 'return "return used outside of a lambda^" stx))) +@end example +@end deffn + +@deffn {Syntax} syntax-parameterize ((keyword transformer) @dots{}) exp @dots{} +Adjusts @var{keyword} @dots{} to use the values obtained by evaluating +their @var{transformer} @dots{}, in the expansion of the @var{exp} @dots{} +forms. Each @var{keyword} must be bound to a +syntax-parameter. @code{syntax-parameterize} differs from +@code{let-syntax}, in that the binding is not shadowed, but adjusted, +and so uses of the keyword in the expansion of exp forms use the new +transformers. This is somewhatsimilar to how @code{parameterize} +adjusts the values of regular parameters, rather than creating new +bindings. + +@example +(define-syntax lambda^ + (syntax-rules () + [(lambda^ argument-list body bodies ...) + (lambda argument-list + (call-with-current-continuation + (lambda (escape) + ;; in the body we adjust the 'return' keyword so that calls + ;; to 'return' are replaced with calls to the escape continuation + (syntax-parameterize ([return (syntax-rules () + [(return vals (... ...)) + (escape vals (... ...))])]) + body + bodies ...))))])) + +;; now we can write functions that return early. Here, 'product' will +;; return immediately if it sees any 0 element. +(define product + (lambda^ (list) + (fold (lambda (n o) + (if (zero? n) + (return 0) + (* n o))) + 1 + list))) +@end example +@end deffn + + @node Eval When @subsection Eval-when |