Next: Named quasi-literals, Previous: Syntax and conditional compilation, Up: Syntax [Contents][Index]
Libraries and top–level programs can define and use new kinds of derived expressions and definitions called syntactic abstractions or macros. A syntactic abstraction is created by binding a keyword to a macro transformer or, simply, transformer.
The transformer determines how a use of the macro (called a macro use) is transcribed into a more primitive form.
Most macro uses have the form:
(keyword datum …)
where keyword is an identifier that uniquely determines the kind of form. This identifier is called the syntactic keyword, or simply keyword. The number of datums and the syntax of each depends on the syntactic abstraction.
Macro uses can also take the form of improper lists, singleton
identifiers, or set! forms, where the second subform of the
set! is the keyword:
(keyword datum … . datum) keyword (set! keyword datum)
The define-syntax, let-syntax and letrec-syntax
forms create bindings for keywords, associate them with macro
transformers, and control the scope within which they are visible.
The syntax-rules and identifier-syntax forms create
transformers via a pattern language. Moreover, the syntax-case
form allows creating transformers via arbitrary Scheme code.
Keywords occupy the same name space as variables. That is, within the same scope, an identifier can be bound as a variable or keyword, or neither, but not both, and local bindings of either kind may shadow other bindings of either kind.
Macros defined using syntax-rules and identifier-syntax
are “hygienic” and “referentially transparent” and thus preserve
Scheme’s lexical scoping.
Macros defined using the syntax-case facility are also hygienic
unless datum->syntax is used.
Kawa supports most of the syntax-case feature.
Syntax definitions are valid wherever definitions are. They have the following form:
The keyword is a identifier, and transformer-spec
is a function that maps syntax forms to syntax forms,
usually an instance of syntax-rules.
If the define-syntax occurs at the top level, then the top-level
syntactic environment is extended by binding the keyword
to the specified transformer, but existing references to any top-level
binding for keyword remain unchanged. Otherwise, it is an
internal syntax definition, and is local to the body
in which it is defined.
(let ((x 1) (y 2))
(define-syntax swap!
(syntax-rules ()
((swap! a b)
(let ((tmp a))
(set! a b)
(set! b tmp)))))
(swap! x y)
(list x y)) ⇒ (2 1)
Macros can expand into definitions in any context that permits them. However, it is an error for a definition to define an identifier whose binding has to be known in order to determine the meaning of the definition itself, or of any preceding definition that belongs to the same group of internal definitions.
(literals) (pattern expr) ... ¶A convenience macro to make it easy to define syntax-case-style macros.
Defines a macro with the given name and list of literals.
Each pattern has the form of a syntax-rules-style pattern,
and it is matched against the macro invocation syntax form.
When a match is found, the corresponding expr is evaluated.
It must evaluate to a syntax form,
which replaces the macro invocation.
(define-syntax-case macro-name (literals) (pat1 result1) (pat2 result2))
is equivalent to:
(define-syntax macro-name
(lambda (form)
(syntax-case form (literals)
(pat1 result1)
(pat2 result2))))
(name lambda-list) form ... ¶This form is deprecated.
Functionally equivalent to defmacro.
This form is deprecated. Instead of
(defmacro (name ...) (let ... `(... ,exp ...)))
you should probably do:
(define-syntax-case name () ((_ ...) (let #`(... #,exp ...))))
and instead of
(defmacro (name ... var ...) `(... var ...))
you should probably do:
(define-syntax-case name () ((_ ... var ...) #`(... var ...))
Defines an old-style macro a la Common Lisp,
and installs (lambda lambda-list form ...)
as the expansion function for name.
When the translator sees an application of name,
the expansion function is called with the rest of the application
as the actual arguments. The resulting object must be a Scheme
source form that is futher processed (it may be repeatedly macro-expanded).
Returns a new (interned) symbol each time it is called.
The symbol names are implementation-dependent.
(This is not directly macro-related, but is often used in conjunction
with defmacro to get a fresh unique identifier.)
The result of evaluating form is treated as a Scheme expression, syntax-expanded to internal form, and then converted back to (roughly) the equivalent expanded Scheme form.
This can be useful for debugging macros.
To access this function, you must first (require 'syntax-utils).
(require 'syntax-utils) (expand '(cond ((> x y) 0) (else 1))) ⇒ (if (> x y) 0 1)
A transformer-spec is an expression that evaluates to a
transformer procedure, which takes an input form and returns a
resulting form. You can do general macro-time compilation with such a
procedure, commonly using syntax-case (which is documented
in the R6RS library specification).
However, when possible it is better to use the simpler
pattern language of syntax-rules:
transformer-spec ::=(syntax-rules (tr-literal*)syntax-rule*)|(syntax-rulesellipsis(tr-literal*)syntax-rule*)| expression syntax-rule ::=(list-pattern syntax-template)tr-literal ::= identifier ellipsis ::= identifier
An instance of syntax-rules produces a new
macro transformer by specifying a sequence of hygienic
rewrite rules. A use of a macro whose keyword is associated
with a transformer specified by syntax-rules is matched
against the patterns contained in the syntax-rules
beginning with the leftmost syntax rule . When a match is
found, the macro use is transcribed hygienically according
to the template.
The optional ellipsis species a symbol used to indicate
repetition; it defaults to ... (3 periods).
syntax-pattern ::= identifier | constant | list-pattern | vector-pattern list-pattern ::=(syntax-pattern*)|(syntax-pattern syntax-pattern*.syntax-pattern)|(syntax-pattern* syntax-pattern ellipsis syntax-pattern*)|(syntax-pattern* syntax-pattern ellipsis syntax-pattern*.syntax-pattern)vector-pattern ::=#(syntax-pattern*)|#(syntax-pattern* syntax-pattern ellipsis syntax-pattern*)
An identifier appearing within a pattern can be an underscore
(_), a literal identifier listed in the list of tr-literals,
or the ellipsis. All other identifiers appearing within a
pattern are pattern variables.
The outer syntax-list of the pattern in a syntax-rule must start with an identifier. It is not involved in the matching and is considered neither a pattern variable nor a literal identifier.
Pattern variables match arbitrary input elements and are used to refer to elements of the input in the template. It is an error for the same pattern variable to appear more than once in a syntax-pattern.
Underscores also match arbitrary input elements but are not pattern variables and so cannot be used to refer to those elements. If an underscore appears in the literals list, then that takes precedence and underscores in the pattern match as literals. Multiple underscores can appear in a syntax-pattern.
Identifiers that appear in (tr-literal*) are interpreted
as literal identifiers to be matched against corresponding
elements of the input. An element in the input matches a
literal identifier if and only if it is an identifier and either
both its occurrence in the macro expression and its occurrence
in the macro definition have the same lexical binding,
or the two identifiers are the same and both have no lexical
binding.
A subpattern followed by ellipsis can match zero or more elements of the input, unless ellipsis appears in the literals, in which case it is matched as a literal.
More formally, an input expression E matches a pattern P if and only if:
_); or
(P1 ... Pn) and
E is a list of n elements
that match P1 through Pn, respectively; or
(P1 ... Pn . Pn+1) and
E is a list or improper list of n or more elements that
match P1 through Pn, respectively,
and whose nth tail matches Pn+1; or
(P1 ... Pk Pe ellipsis Pk+1 ... Pk+l) where E is a proper list of n elements,
the first k of which match P1 through Pk,
respectively, whose
next n-k-l elements each match Pe, and whose remaining
l elements match Pk+1 through Pk+l; or
(P1 ... Pk Pe ellipsis Pk+1 ... Pk+l . Px)
where E is a list or improper list of n elements,
the first k of which match P1 through Pk,
whose next n-k-l elements each match Pe, and whose
remaining l elements match Pk+1 through Pk+l,
and whose nth and final cdr matches Px; or
#(P1 ... Pn) and E is a
vector of n elements that match P1 through Pn; or
#(P1 ... Pk Pe ellipsis Pk+1 ... Pk+l)
where E is a vector of n elements the first
k of which match P1 through Pk,
whose next n-k-l elements each match Pe,
and whose remaining l elements match Pk+1
through Pk+l; or
equal? procedure.
It is an error to use a macro keyword, within the scope of its binding, in an expression that does not match any of the patterns.
syntax-template ::= identifier | constant |(template-element*)|(template-element template-element*.syntax-template)|(ellipsis syntax-template)template-element ::= syntax-template [ellipsis]
When a macro use is transcribed according to the template of the matching syntax-rule, pattern variables that occur in the template are replaced by the elements they match in the input. Pattern variables that occur in subpatterns followed by one or more instances of the identifier ellipsis are allowed only in subtemplates that are followed by as many instances of ellipsis . They are replaced in the output by all of the elements they match in the input, distributed as indicated. It is an error if the output cannot be built up as specified.
Identifiers that appear in the template but are not pattern
variables or the identifier ellipsis are inserted into the output
as literal identifiers. If a literal identifier is inserted as a
free identifier then it refers to the binding of that identifier
within whose scope the instance of syntax-rules appears.
If a literal identifier is inserted as a bound identifier then
it is in effect renamed to prevent inadvertent captures of
free identifiers.
A template of the
form (ellipsis template) is
identical to template, except that ellipses
within the template have no special meaning.
That is, any ellipses contained within
template are treated as ordinary identifiers. In particular,
the template (ellipsis ellipsis)
produces a single ellipsis. This allows syntactic
abstractions to expand into code containing ellipses.
(define-syntax be-like-begin
(syntax-rules ()
((be-like-begin name)
(define-syntax name
(syntax-rules ()
((name expr (... ...))
(begin expr (... ...))))))))
(be-like-begin sequence)
(sequence 1 2 3 4) ⇒ 4
Return #t if obj is an identifier, i.e., a syntax object
representing an identifier, and #f otherwise.
The identifier? procedure is often used within a fender to verify
that certain subforms of an input form are identifiers, as in the
definition of rec, which creates self–contained recursive
objects, below.
(define-syntax rec
(lambda (x)
(syntax-case x ()
((_ x e)
(identifier? #'x)
#'(letrec ((x e)) x)))))
(map (rec fact
(lambda (n)
(if (= n 0)
1
(* n (fact (- n 1))))))
'(1 2 3 4 5)) ⇒ (1 2 6 24 120)
(rec 5 (lambda (x) x)) ⇒ exception
The procedures bound-identifier=? and free-identifier=?
each take two identifier arguments and return #t if their arguments
are equivalent and #f otherwise. These predicates are used to
compare identifiers according to their intended use as free
references or bound identifiers in a given context.
id1 and id2 must be identifiers.
The procedure bound-identifier=? returns #t if a binding for
one would capture a reference to the other in the output of the
transformer, assuming that the reference appears within the scope of the
binding, and #f otherwise.
In general, two identifiers are bound-identifier=? only if both
are present in the original program or both are introduced by the same
transformer application (perhaps implicitly, see datum->syntax).
The bound-identifier=? procedure can be used for detecting
duplicate identifiers in a binding construct or for other preprocessing
of a binding construct that requires detecting instances of the bound
identifiers.
id1 and id2 must be identifiers.
The free-identifier=? procedure returns #t if and only if
the two identifiers would resolve to the same binding if both were to
appear in the output of a transformer outside of any bindings inserted
by the transformer. (If neither of two like–named identifiers resolves
to a binding, i.e., both are unbound, they are considered to resolve to
the same binding.)
Operationally, two identifiers are considered equivalent by
free-identifier=? if and only the topmost matching substitution
for each maps to the same binding or the identifiers have the same name
and no matching substitution.
The syntax-case and syntax-rules forms internally use
free-identifier=? to compare identifiers listed in the literals
list against input identifiers.
(let ((fred 17))
(define-syntax a
(lambda (x)
(syntax-case x ()
((_ id) #'(b id fred)))))
(define-syntax b
(lambda (x)
(syntax-case x ()
((_ id1 id2)
#`(list
#,(free-identifier=? #'id1 #'id2)
#,(bound-identifier=? #'id1 #'id2))))))
(a fred))
⇒ (#t #f)
The following definition of unnamed let uses
bound-identifier=? to detect duplicate identifiers.
(define-syntax let
(lambda (x)
(define unique-ids?
(lambda (ls)
(or (null? ls)
(and (let notmem? ((x (car ls)) (ls (cdr ls)))
(or (null? ls)
(and (not (bound-identifier=? x (car ls)))
(notmem? x (cdr ls)))))
(unique-ids? (cdr ls))))))
(syntax-case x ()
((_ ((i v) ...) e1 e2 ...)
(unique-ids? #'(i ...))
#'((lambda (i ...) e1 e2 ...) v ...)))))
The argument #'(i ...) to unique-ids? is guaranteed to be
a list by the rules given in the description of syntax above.
With this definition of let:
(let ((a 3) (a 4)) (+ a a)) ⇒ syntax error
However,
(let-syntax
((dolet (lambda (x)
(syntax-case x ()
((_ b)
#'(let ((a 3) (b 4)) (+ a b)))))))
(dolet a))
⇒ 7
since the identifier a introduced by dolet and the
identifier a extracted from the input form are not
bound-identifier=?.
Rather than including else in the literals list as before, this
version of case explicitly tests for else using
free-identifier=?.
(define-syntax case
(lambda (x)
(syntax-case x ()
((_ e0 ((k ...) e1 e2 ...) ...
(else-key else-e1 else-e2 ...))
(and (identifier? #'else-key)
(free-identifier=? #'else-key #'else))
#'(let ((t e0))
(cond
((memv t '(k ...)) e1 e2 ...)
...
(else else-e1 else-e2 ...))))
((_ e0 ((ka ...) e1a e2a ...)
((kb ...) e1b e2b ...) ...)
#'(let ((t e0))
(cond
((memv t '(ka ...)) e1a e2a ...)
((memv t '(kb ...)) e1b e2b ...)
...))))))
With either definition of case, else is not recognized as
an auxiliary keyword if an enclosing lexical binding for else
exists. For example,
(let ((else #f))
(case 0 (else (write "oops")))) ⇒ syntax error
since else is bound lexically and is therefore not the same
else that appears in the definition of case.
Strip all syntactic information from a syntax object and returns the corresponding Scheme datum.
Identifiers stripped in this manner are converted to their symbolic
names, which can then be compared with eq?. Thus, a predicate
symbolic-identifier=? might be defined as follows.
(define symbolic-identifier=?
(lambda (x y)
(eq? (syntax->datum x)
(syntax->datum y))))
template-id must be a template identifier and datum should be a datum value.
The datum->syntax procedure returns a syntax-object
representation of datum that contains the same contextual
information as template-id, with the effect that the syntax object
behaves as if it were introduced into the code when template-id
was introduced.
If srcloc is specified (and neither #f or #!null),
it specifies the file position (including line number) for the result.
In that case it should be a syntax object representing
a list; otherwise it is currently ignored, though future extensions
may support other ways of specifying the position.
The datum->syntax procedure allows a transformer to “bend”
lexical scoping rules by creating implicit identifiers that
behave as if they were present in the input form, thus permitting the
definition of macros that introduce visible bindings for or references
to identifiers that do not appear explicitly in the input form. For
example, the following defines a loop expression that uses this
controlled form of identifier capture to bind the variable break
to an escape procedure within the loop body. (The derived
with-syntax form is like let but binds pattern variables.)
(define-syntax loop
(lambda (x)
(syntax-case x ()
((k e ...)
(with-syntax
((break (datum->syntax #'k 'break)))
#'(call-with-current-continuation
(lambda (break)
(let f () e ... (f)))))))))
(let ((n 3) (ls '()))
(loop
(if (= n 0) (break ls))
(set! ls (cons 'a ls))
(set! n (- n 1))))
⇒ (a a a)
Were loop to be defined as:
(define-syntax loop
(lambda (x)
(syntax-case x ()
((_ e ...)
#'(call-with-current-continuation
(lambda (break)
(let f () e ... (f))))))))
the variable break would not be visible in e ....
The datum argument datum may also represent an arbitrary Scheme
form, as demonstrated by the following definition of include.
(define-syntax include
(lambda (x)
(define read-file
(lambda (fn k)
(let ((p (open-file-input-port fn)))
(let f ((x (get-datum p)))
(if (eof-object? x)
(begin (close-port p) '())
(cons (datum->syntax k x)
(f (get-datum p))))))))
(syntax-case x ()
((k filename)
(let ((fn (syntax->datum #'filename)))
(with-syntax (((exp ...)
(read-file fn #'k)))
#'(begin exp ...)))))))
(include "filename") expands into a begin expression
containing the forms found in the file named by "filename". For
example, if the file flib.ss contains:
(define f (lambda (x) (g (* x x))))
and the file glib.ss contains:
(define g (lambda (x) (+ x x)))
the expression:
(let () (include "flib.ss") (include "glib.ss") (f 5))
evaluates to 50.
The definition of include uses datum->syntax to convert
the objects read from the file into syntax objects in the proper lexical
context, so that identifier references and definitions within those
expressions are scoped where the include form appears.
Using datum->syntax, it is even possible to break hygiene
entirely and write macros in the style of old Lisp macros. The
lisp-transformer procedure defined below creates a transformer
that converts its input into a datum, calls the programmer’s procedure
on this datum, and converts the result back into a syntax object scoped
where the original macro use appeared.
(define lisp-transformer
(lambda (p)
(lambda (x)
(syntax-case x ()
((kwd . rest)
(datum->syntax #'kwd
(p (syntax->datum x))))))))
The message and args are treated similary as for
the error procedure. However, the error is reported
when the syntax-error is expanded.
This can be used as a syntax-rules
template for a pattern that is an invalid use of the
macro, which can provide more descriptive error messages.
The message should be a string literal, and the args
arbitrary (non-evalualted) expressions providing additional information.
(define-syntax simple-let
(syntax-rules ()
((_ (head ... ((x . y) val) . tail)
body1 body2 ...)
(syntax-error "expected an identifier but got" (x . y)))
((_ ((name val) ...) body1 body2 ...)
((lambda (name ...) body1 body2 ...)
val ...))))
This is a procedure that can be called at macro-expansion time
by a syntax transformer function.
(In contrast syntax-error is a syntax form used in the
expansion result.)
The message is reported as a compile-time error message.
The location is used for the source location (file name and
line/column numbers): In general it can be a SourceLocator value;
most commonly it is a syntax object for a sub-list of the input form
that is erroneous.
The value returned by report-syntax-error is an
instance of ErrorExp, which supresses further compilation.
(define-syntax if
(lambda (x)
(syntax-case x ()
((_ test then)
(make-if-exp #'test #'then #!null))
((_ test then else)
(make-if-exp #'test #'then #'else))
((_ e1 e2 e3 . rest)
(report-syntax-error #'rest
"too many expressions for 'if'"))
((_ . rest)
(report-syntax-error #'rest
"too few expressions for 'if'")))))
In the above example, one could use the source form x for the
location, but using #'rest is more accurate. Note that the following
is incorrect, because e1 might not be a pair, in which case
we don’t have location information for it (due to a Kawa limitation):
(syntax-case x ()
...
((_ e1)
(report-syntax-error
#'e1 ;; poor location specifier
"too few expressions for 'if'")))))
The with-syntax form is used to bind pattern variables, just as
let is used to bind variables. This allows a transformer to
construct its output in separate pieces, then put the pieces together.
Each pattern is identical in form to a syntax-case
pattern. The value of each expression is computed and
destructured according to the corresponding pattern, and pattern
variables within the pattern are bound as with
syntax-case to the corresponding portions of the value within
body.
The with-syntax form may be defined in terms of
syntax-case as follows.
(define-syntax with-syntax
(lambda (x)
(syntax-case x ()
((_ ((p e0) ...) e1 e2 ...)
(syntax (syntax-case (list e0 ...) ()
((p ...) (let () e1 e2 ...))))))))
The following definition of cond demonstrates the use of
with-syntax to support transformers that employ recursion
internally to construct their output. It handles all cond clause
variations and takes care to produce one-armed if expressions
where appropriate.
(define-syntax cond
(lambda (x)
(syntax-case x ()
((_ c1 c2 ...)
(let f ((c1 #'c1) (c2* #'(c2 ...)))
(syntax-case c2* ()
(()
(syntax-case c1 (else =>)
(((else e1 e2 ...) #'(begin e1 e2 ...))
((e0) #'e0)
((e0 => e1)
#'(let ((t e0)) (if t (e1 t))))
((e0 e1 e2 ...)
#'(if e0 (begin e1 e2 ...)))))
((c2 c3 ...)
(with-syntax ((rest (f #'c2 #'(c3 ...))))
(syntax-case c1 (=>)
((e0) #'(let ((t e0)) (if t t rest)))
((e0 => e1)
#'(let ((t e0)) (if t (e1 t) rest)))
((e0 e1 e2 ...)
#'(if e0
(begin e1 e2 ...)
rest)))))))))))
The quasisyntax form is similar to syntax, but it allows
parts of the quoted text to be evaluated, in a manner similar to the
operation of quasiquote.
Within a quasisyntax template, subforms of unsyntax
and unsyntax-splicing forms are evaluated, and everything else is
treated as ordinary template material, as with syntax.
The value of each unsyntax subform is inserted into the output in
place of the unsyntax form, while the value of each
unsyntax-splicing subform is spliced into the surrounding list or
vector structure. Uses of unsyntax and unsyntax-splicing
are valid only within quasisyntax expressions.
A quasisyntax expression may be nested, with each
quasisyntax introducing a new level of syntax quotation and each
unsyntax or unsyntax-splicing taking away a level of
quotation. An expression nested within n quasisyntax
expressions must be within n unsyntax or
unsyntax-splicing expressions to be evaluated.
As noted in abbreviation, #`template is equivalent to
(quasisyntax template), #,template is
equivalent to (unsyntax template), and
#,@template is equivalent to (unsyntax-splicing
template). Note that for backwards compatibility,
you should only use #,template inside a literal #`template form.
The quasisyntax keyword can be used in place of
with-syntax in many cases. For example, the definition of
case shown under the description of with-syntax above can
be rewritten using quasisyntax as follows.
(define-syntax case
(lambda (x)
(syntax-case x ()
((_ e c1 c2 ...)
#`(let ((t e))
#,(let f ((c1 #'c1) (cmore #'(c2 ...)))
(if (null? cmore)
(syntax-case c1 (else)
((else e1 e2 ...)
#'(begin e1 e2 ...))
(((k ...) e1 e2 ...)
#'(if (memv t '(k ...))
(begin e1 e2 ...))])
(syntax-case c1 ()
(((k ...) e1 e2 ...)
#`(if (memv t '(k ...))
(begin e1 e2 ...)
#,(f (car cmore)
(cdr cmore))))))))))))
Note: Any
syntax-rulesform can be expressed withsyntax-caseby making thelambdaexpression andsyntaxexpressions explicit, andsyntax-rulesmay be defined in terms ofsyntax-caseas follows.(define-syntax syntax-rules (lambda (x) (syntax-case x () ((_ (lit ...) ((k . p) t) ...) (for-all identifier? #'(lit ... k ...)) #'(lambda (x) (syntax-case x (lit ...) ((_ . p) #'t) ...))))))
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