document syntax parameters

* doc/ref/api-macros.texi (Macros): Add subsection for "Syntax Parameters"

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