a beauty worthy of Scheme.
@deffn {Syntax} syntax-rules literals (pattern template)...
+Create a syntax transformer that will rewrite an expression using the rules
+embodied in the @var{pattern} and @var{template} clauses.
+@end deffn
+
A @code{syntax-rules} macro consists of three parts: the literals (if any), the
patterns, and as many templates as there are patterns.
matches the expression against the patterns, in order, and rewrites the
expression using the template from the first matching pattern. If no pattern
matches, a syntax error is signalled.
-@end deffn
@subsubsection Patterns
@node Syntax Case
@subsection Support for the @code{syntax-case} System
+@code{syntax-case} macros are procedural syntax transformers, with a power
+worthy of Scheme.
+
+@deffn {Syntax} syntax-case syntax literals (pattern [guard] exp)...
+Match the syntax object @var{syntax} against the given patterns, in order. If a
+@var{pattern} matches, return the result of evaluating the associated @var{exp}.
+@end deffn
+
+Compare the following definitions of @code{when}:
+
+@example
+(define-syntax when
+ (syntax-rules ()
+ ((_ test e e* ...)
+ (if test (begin e e* ...)))))
+
+(define-syntax when
+ (lambda (x)
+ (syntax-case x ()
+ ((_ test e e* ...)
+ #'(if test (begin e e* ...))))))
+@end example
+
+Clearly, the @code{syntax-case} definition is similar to its @code{syntax-rules}
+counterpart, and equally clearly there are some differences. The
+@code{syntax-case} definition is wrapped in a @code{lambda}, a function of one
+argument; that argument is passed to the @code{syntax-case} invocation; and the
+``return value'' of the macro has a @code{#'} prefix.
+
+All of these differences stem from the fact that @code{syntax-case} does not
+define a syntax transformer itself -- instead, @code{syntax-case} expressions
+provide a way to destructure a @dfn{syntax object}, and to rebuild syntax
+objects as output.
+
+So the @code{lambda} wrapper is simply a leaky implementation detail, that
+syntax transformers are just functions that transform syntax to syntax. This
+should not be surprising, given that we have already described macros as
+``programs that write programs''. @code{syntax-case} is simply a way to take
+apart and put together program text, and to be a valid syntax transformer it
+needs to be wrapped in a procedure.
+
+Unlike traditional Lisp macros (@pxref{Defmacros}), @code{syntax-case} macros
+transform syntax objects, not raw Scheme forms. Recall the naive expansion of
+@code{my-or} given in the previous section:
+
+@example
+(let ((t #t))
+ (my-or #f t))
+;; naive expansion:
+(let ((t #t))
+ (let ((t #f))
+ (if t t t)))
+@end example
+
+Raw Scheme forms simply don't have enough information to distinguish the first
+two @code{t} instances in @code{(if t t t)} from the third @code{t}. So instead
+of representing identifiers as symbols, the syntax expander represents
+identifiers as annotated syntax objects, attaching such information to those
+syntax objects as is needed to maintain referential transparency.
+
+@deffn {Syntax} syntax form
+Create a syntax object wrapping @var{form} within the current lexical context.
+@end deffn
+
+Syntax objects are typically created internally to the process of expansion, but
+it is possible to create them outside of syntax expansion:
+
+@example
+(syntax (foo bar baz))
+@result{} #<some representation of that syntax>
+@end example
+
+@noindent
+However it is more common, and useful, to create syntax objects when building
+output from a @code{syntax-case} expression.
+
+@example
+(define-syntax add1
+ (lambda (x)
+ (syntax-case x ()
+ ((_ exp)
+ (syntax (+ exp 1))))))
+@end example
+
+It is not strictly necessary for a @code{syntax-case} expression to return a
+syntax object, because @code{syntax-case} expressions can be used in helper
+functions, or otherwise used outside of syntax expansion itself. However a
+syntax transformer procedure must return a syntax object, so most uses of
+@code{syntax-case} do end up returning syntax objects.
+
+Here in this case, the form that built the return value was @code{(syntax (+ exp
+1))}. The interesting thing about this is that within a @code{syntax}
+expression, any appearance of a pattern variable is substitued into the
+resulting syntax object, carrying with it all relevant metadata from the source
+expression, such as lexical identity and source location.
+
+Indeed, a pattern variable may only be referenced from inside a @code{syntax}
+form. The syntax expander would raise an error when defining @code{add1} if it
+found @var{exp} referenced outside a @code{syntax} form.
+
+Since @code{syntax} appears frequently in macro-heavy code, it has a special
+reader macro: @code{#'}. @code{#'foo} is transformed by the reader into
+@code{(syntax foo)}, just as @code{'foo} is tranformed into @code{(quote foo)}.
+
+The pattern language used by @code{syntax-case} is conveniently the same
+language used by @code{syntax-rules}. Given this, Guile actually defines
+@code{syntax-rules} in terms of @code{syntax-case}:
+
+@example
+(define-syntax syntax-rules
+ (lambda (x)
+ (syntax-case x ()
+ ((_ (k ...) ((keyword . pattern) template) ...)
+ #'(lambda (x)
+ (syntax-case x (k ...)
+ ((dummy . pattern) #'template)
+ ...))))))
+@end example
+
+And that's that.
+
+@subsubsection Why @code{syntax-case}?
+
+The examples we have shown thus far could just as well have been expressed with
+@code{syntax-rules}, and have just shown that @code{syntax-case} is more
+verbose, which is true. But there is a difference: @code{syntax-case} creates
+@emph{procedural} macros, giving the full power of Scheme to the macro expander.
+This has many practical applications.
+
+A common desire is to be able to match a form only if it is an identifier. This
+is impossible with @code{syntax-rules}, given the datum matching forms. But with
+@code{syntax-case} it is easy:
+
+@deffn {Scheme Procedure} identifier? syntax-object
+Returns @code{#t} iff @var{syntax-object} is an identifier.
+@end deffn
+
+@example
+(define-syntax add1!
+ (lambda (x)
+ (syntax-case x ()
+ ((_ var) (identifier? #'var)
+ #'(set! var (add1 var))))))
+
+(define foo 0)
+(add1! foo)
+foo @result{} 1
+(add1! "not-an-identifier") @result{} error
+@end example
+
+With @code{syntax-rules}, the error for @code{(add1! "not-an-identifier")} would
+be something like ``invalid @code{set!}''. With @code{syntax-case}, it will say
+something like ``invalid @code{add1!}'', because we attach the @dfn{guard
+clause} to the pattern: @code{(identifier? #'var)}. This becomes more important
+with more complicated macros. It is necessary to use @code{identifier?}, because
+to the expander, an identifier is more than a bare symbol.
+
+Note that even in the guard clause, we reference the @var{var} pattern variable
+within a @code{syntax} form, via @code{#'var}.
+
+Another common desire is to introduce bindings into the lexical context of the
+output expression. One example would be in the so-called ``anaphoric macros'',
+like @code{aif}. Anaphoric macros bind some expression to a well-known
+identifier, often @code{it}, within their bodies. For example, in @code{(aif
+(foo) (bar it))}, @code{it} would be bound to the result of @code{(foo)}.
+
+To begin with, we should mention a solution that doesn't work:
+
+@example
+;; doesn't work
+(define-syntax aif
+ (lambda (x)
+ (syntax-case x ()
+ ((_ test then else)
+ #'(let ((it test))
+ (if it then else))))))
+@end example
+
+The reason that this doesn't work is that, by default, the expander will
+preserve referential transparency; the @var{then} and @var{else} expressions
+won't have access to the binding of @code{it}.
+
+But they can, if we explicitly introduce a binding via @code{datum->syntax}.
+
+@deffn {Scheme Procedure} datum->syntax for-syntax datum
+Create a syntax object that wraps @var{datum}, within the lexical context
+corresponding to the syntax object @var{for-syntax}.
+@end deffn
+
+For completeness, we should mention that it is possible to strip the metadata
+from a syntax object, returning a raw Scheme datum:
+
+@deffn {Scheme Procedure} syntax->datum syntax-object
+Strip the metadata from @var{syntax-object}, returning its contents as a raw
+Scheme datum.
+@end deffn
+
+In this case we want to introduce @code{it} in the context of the whole
+expression, so we can create a syntax object as @code{(datum->syntax x 'it)},
+where @code{x} is the whole expression, as passed to the transformer procedure.
+
+Here's another solution that doesn't work:
+
+@example
+;; doesn't work either
+(define-syntax aif
+ (lambda (x)
+ (syntax-case x ()
+ ((_ test then else)
+ (let ((it (datum->syntax x 'it)))
+ #'(let ((it test))
+ (if it then else)))))))
+@end example
+
+The reason that this one doesn't work is that there are really two environments
+at work here -- the environment of pattern variables, as bound by
+@code{syntax-case}, and the environment of lexical variables, as bound by normal
+Scheme. Here we need to introduce a piece of Scheme's environment into that of
+the syntax expander, and we can do so using @code{syntax-case} itself:
+
+@example
+;; works, but is obtuse
+(define-syntax aif
+ (lambda (x)
+ (syntax-case x ()
+ ((_ test then else)
+ ;; invoking syntax-case on the generated
+ ;; syntax object to expose it to `syntax'
+ (syntax-case (datum->syntax x 'it) ()
+ (it
+ #'(let ((it test))
+ (if it then else))))))))
+
+(aif (getuid) (display it) (display "none")) (newline)
+@print{} 500
+@end example
+
+However there are easier ways to write this. @code{with-syntax} is often
+convenient:
+
+@deffn {Syntax} with-syntax ((pat val)...) exp...
+Bind patterns @var{pat} from their corresponding values @var{val}, within the
+lexical context of @var{exp...}.
+
+@example
+;; better
+(define-syntax aif
+ (lambda (x)
+ (syntax-case x ()
+ ((_ test then else)
+ (with-syntax ((it (datum->syntax x 'it)))
+ #'(let ((it test))
+ (if it then else)))))))
+@end example
+@end deffn
+
+As you might imagine, @code{with-syntax} is defined in terms of
+@code{syntax-case}. But even that might be off-putting to you if you are an old
+Lisp macro hacker, used to building macro output with @code{quasiquote}. The
+issue is that @code{with-syntax} creates a separation between the point of
+definition of a value and its point of substitution.
+
+@pindex quasisyntax
+@pindex unsyntax
+@pindex unsyntax-splicing
+So for cases in which a @code{quasiquote} style makes more sense,
+@code{syntax-case} also defines @code{quasisyntax}, and the related
+@code{unsyntax} and @code{unsyntax-splicing}, abbreviated by the reader as
+@code{#`}, @code{#,}, and @code{#,@@}, respectively.
+
+For example, to define a macro that inserts a compile-time timestamp into a
+source file, one may write:
+
+@example
+(define-syntax display-compile-timestamp
+ (lambda (x)
+ (syntax-case x ()
+ ((_)
+ #`(begin
+ (display "The compile timestamp was: ")
+ (display #,(current-time))
+ (newline))))))
+@end example
+
+Finally, we should mention the following helper procedures defined by the core
+of @code{syntax-case}:
+
+@deffn {Scheme Procedure} bound-identifier=? a b
+Returns @code{#t} iff the syntax objects @var{a} and @var{b} refer to the same
+lexically-bound identifier.
+@end deffn
+
+@deffn {Scheme Procedure} free-identifier=? a b
+Returns @code{#t} iff the syntax objects @var{a} and @var{b} refer to the same
+free identifier.
+@end deffn
+
+@deffn {Scheme Procedure} generate-temporaries ls
+Return a list of temporary identifiers as long as @var{ls} is long.
+@end deffn
+
+Readers interested in further information on @code{syntax-case} macros should
+see R. Kent Dybvig's excellent @cite{The Scheme Programming Language}, either
+edition 3 or 4, in the chapter on syntax. Dybvig was the primary author of the
+@code{syntax-case} system. The book itself is available online at
+@uref{http://scheme.com/tspl4/}.
+
@node Defmacros
@subsection Lisp-style Macro Definitions
-In Lisp-like languages, the traditional way to define macros is very
-similar to procedure definitions. The key differences are that the
-macro definition body should return a list that describes the
-transformed expression, and that the definition is marked as a macro
-definition (rather than a procedure definition) by the use of a
-different definition keyword: in Lisp, @code{defmacro} rather than
-@code{defun}, and in Scheme, @code{define-macro} rather than
-@code{define}.
+The traditional way to define macros in Lisp is very similar to procedure
+definitions. The key differences are that the macro definition body should
+return a list that describes the transformed expression, and that the definition
+is marked as a macro definition (rather than a procedure definition) by the use
+of a different definition keyword: in Lisp, @code{defmacro} rather than
+@code{defun}, and in Scheme, @code{define-macro} rather than @code{define}.
@fnindex defmacro
@fnindex define-macro
The difference is analogous to the corresponding difference between
Lisp's @code{defun} and Scheme's @code{define}.
-@code{false-if-exception}, from the @file{boot-9.scm} file in the Guile
-distribution, is a good example of macro definition using
-@code{defmacro}:
+Having read the previous section on @code{syntax-case}, it's probably clear that
+Guile actually implements defmacros in terms of @code{syntax-case}, applying the
+transformer on the expression between invocations of @code{syntax->datum} and
+@code{datum->syntax}. This realization leads us to the problem with defmacros,
+that they do not preserve referential transparency. One can be careful to not
+introduce bindings into expanded code, via liberal use of @code{gensym}, but
+there is no getting around the lack of referential transparency for free
+bindings in the macro itself.
-@lisp
-(defmacro false-if-exception (expr)
- `(catch #t
- (lambda () ,expr)
- (lambda args #f)))
-@end lisp
+Even a macro as simple as our @code{when} from before is difficult to get right:
-@noindent
-The effect of this definition is that expressions beginning with the
-identifier @code{false-if-exception} are automatically transformed into
-a @code{catch} expression following the macro definition specification.
-For example:
+@example
+(define-macro (when cond exp . rest)
+ `(if ,cond
+ (begin ,exp . ,rest)))
-@lisp
-(false-if-exception (open-input-file "may-not-exist"))
-@equiv{}
-(catch #t
- (lambda () (open-input-file "may-not-exist"))
- (lambda args #f))
-@end lisp
+(when #f (display "Launching missiles!\n"))
+@result{} #f
-@deffn {Scheme Procedure} cons-source xorig x y
-@deffnx {C Function} scm_cons_source (xorig, x, y)
-Create and return a new pair whose car and cdr are @var{x} and @var{y}.
-Any source properties associated with @var{xorig} are also associated
-with the new pair.
-@end deffn
+(let ((if list))
+ (when #f (display "Launching missiles!\n")))
+@print{} Launching missiles!
+@result{} (#f #<unspecified>)
+@end example
+
+Guile's perspective is that defmacros have had a good run, but that modern
+macros should be written with @code{syntax-rules} or @code{syntax-case}. There
+are still many uses of defmacros within Guile itself, but we will be phasing
+them out over time. Of course we won't take away @code{defmacro} or
+@code{define-macro} themselves, as there is lots of code out there that uses
+them.
@node Identifier Macros
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