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Diffstat (limited to 'doc/ref/api-macros.texi')
-rw-r--r-- | doc/ref/api-macros.texi | 78 |
1 files changed, 77 insertions, 1 deletions
diff --git a/doc/ref/api-macros.texi b/doc/ref/api-macros.texi index 92816ad82..e60864ba3 100644 --- a/doc/ref/api-macros.texi +++ b/doc/ref/api-macros.texi @@ -1,6 +1,6 @@ @c -*-texinfo-*- @c This is part of the GNU Guile Reference Manual. -@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010, 2011 +@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010, 2011, 2012 @c Free Software Foundation, Inc. @c See the file guile.texi for copying conditions. @@ -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. 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 @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 (@pxref{SRFI-39, parameters}), 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 @var{exp} @dots{} use the new transformers. This is +somewhat similar 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 body* ...) + (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 body* ...))))])) + +;; 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 |