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syntax parameters doc formatting
* doc/ref/api-macros.texi (Syntax Parameters): Some copy-editing on Ian's lovely syntax-parameters documentation.
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@c -*-texinfo-*-
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@c This is part of the GNU Guile Reference Manual.
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@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010, 2011
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@c Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003, 2004, 2009, 2010, 2011, 2012
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@c Free Software Foundation, Inc.
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@c See the file guile.texi for copying conditions.
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@ -865,31 +865,32 @@ wrapping in @code{#'} syntax forms.
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@node Syntax Parameters
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@subsection Syntax Parameters
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Syntax parameters@footnote{Described in the paper @cite{Keeping it Clean with
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Syntax Parameters} by Barzilay, Culpepper and Flatt.} are a mechanism for rebinding a macro
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definition within the dynamic extent of a macro expansion. It provides
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a convenient solution to one of the most common types of unhygienic
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macro: those that introduce a unhygienic binding each time the macro
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is used. Examples include a @code{lambda} form with a @code{return} keyword, or
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class macros that introduce a special @code{self} binding.
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Syntax parameters@footnote{Described in the paper @cite{Keeping it Clean
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with Syntax Parameters} by Barzilay, Culpepper and Flatt.} are a
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mechanism for rebinding a macro definition within the dynamic extent of
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a macro expansion. This provides a convenient solution to one of the
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most common types of unhygienic macro: those that introduce a unhygienic
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binding each time the macro is used. Examples include a @code{lambda}
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form with a @code{return} keyword, or class macros that introduce a
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special @code{self} binding.
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With syntax parameters, instead of introducing the binding
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unhygienically each time, we instead create one binding for the
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keyword, which we can then adjust later when we want the keyword to
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have a different meaning. As no new bindings are introduced, hygiene
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is preserved. This is similar to the dynamic binding mechanisms we
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have at run-time like @ref{SRFI-39, parameters} or
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@ref{Fluids and Dynamic States, fluids}, except that the dynamic
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binding only occurs during macro expansion. The code after macro
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expansion remains lexically scoped.
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unhygienically each time, we instead create one binding for the keyword,
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which we can then adjust later when we want the keyword to have a
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different meaning. As no new bindings are introduced, hygiene is
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preserved. This is similar to the dynamic binding mechanisms we have at
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run-time (@pxref{SRFI-39, parameters}), except that the dynamic binding
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only occurs during macro expansion. The code after macro expansion
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remains lexically scoped.
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@deffn {Syntax} define-syntax-parameter keyword transformer
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Binds @var{keyword} to the value obtained by evaluating @var{transformer}. The
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@var{transformer} provides the default expansion for the syntax parameter,
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and in the absence of @code{syntax-parameterize}, is functionally equivalent
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to @code{define-syntax}. Usually, you will just want to have the @var{transformer}
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throw a syntax error indicating that the @var{keyword} is supposed to be
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used in conjunction with another macro, for example:
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Binds @var{keyword} to the value obtained by evaluating
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@var{transformer}. The @var{transformer} provides the default expansion
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for the syntax parameter, and in the absence of
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@code{syntax-parameterize}, is functionally equivalent to
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@code{define-syntax}. Usually, you will just want to have the
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@var{transformer} throw a syntax error indicating that the @var{keyword}
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is supposed to be used in conjunction with another macro, for example:
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@example
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(define-syntax-parameter return
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(lambda (stx)
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@ -899,31 +900,30 @@ used in conjunction with another macro, for example:
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@deffn {Syntax} syntax-parameterize ((keyword transformer) @dots{}) exp @dots{}
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Adjusts @var{keyword} @dots{} to use the values obtained by evaluating
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their @var{transformer} @dots{}, in the expansion of the @var{exp} @dots{}
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forms. Each @var{keyword} must be bound to a
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syntax-parameter. @code{syntax-parameterize} differs from
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@code{let-syntax}, in that the binding is not shadowed, but adjusted,
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and so uses of the keyword in the expansion of exp forms use the new
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transformers. This is somewhatsimilar to how @code{parameterize}
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adjusts the values of regular parameters, rather than creating new
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bindings.
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their @var{transformer} @dots{}, in the expansion of the @var{exp}
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@dots{} forms. Each @var{keyword} must be bound to a syntax-parameter.
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@code{syntax-parameterize} differs from @code{let-syntax}, in that the
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binding is not shadowed, but adjusted, and so uses of the keyword in the
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expansion of @var{exp} @dots{} use the new transformers. This is
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somewhat similar to how @code{parameterize} adjusts the values of
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regular parameters, rather than creating new bindings.
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@example
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(define-syntax lambda^
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(syntax-rules ()
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[(lambda^ argument-list body bodies ...)
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[(lambda^ argument-list body body* ...)
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(lambda argument-list
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(call-with-current-continuation
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(lambda (escape)
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;; in the body we adjust the 'return' keyword so that calls
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;; to 'return' are replaced with calls to the escape continuation
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;; In the body we adjust the 'return' keyword so that calls
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;; to 'return' are replaced with calls to the escape
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;; continuation.
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(syntax-parameterize ([return (syntax-rules ()
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[(return vals (... ...))
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(escape vals (... ...))])])
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body
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bodies ...))))]))
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body body* ...))))]))
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;; now we can write functions that return early. Here, 'product' will
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;; Now we can write functions that return early. Here, 'product' will
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;; return immediately if it sees any 0 element.
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(define product
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(lambda^ (list)
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