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An implementation of syntax-rules and a hygienic low-level macro system.


(require-extension syntactic-closures)


                  A Syntactic Closures Macro Facility
                            by Chris Hanson
                            9 November 1991
  This document describes "syntactic closures", a low-level macro
facility for the Scheme programming language.  The facility is an
alternative to the low-level macro facility described in the `Revised^4
Report on Scheme.' This document is an addendum to that report.
  The syntactic closures facility extends the BNF rule for TRANSFORMER
SPEC to allow a new keyword that introduces a low-level macro
     TRANSFORMER SPEC := (sc-macro-transformer EXPRESSION)
  Additionally, the following procedures are added:
  The description of the facility is divided into three parts.  The
first part defines basic terminology.  The second part describes how
macro transformers are defined.  The third part describes the use of
"identifiers", which extend the syntactic closure mechanism to be
compatible with `syntax-rules'. Terminology
This section defines the concepts and data types used by the syntactic
closures facility.
   * "Forms" are the syntactic entities out of which programs are
     recursively constructed.  A form is any expression, any
     definition, any syntactic keyword, or any syntactic closure.  The
     variable name that appears in a `set!' special form is also a
     form.  Examples of forms:
          (+ x 4)
          (lambda (x) x)
          (define pi 3.14159)
   * An "alias" is an alternate name for a given symbol.  It can appear
     anywhere in a form that the symbol could be used, and when quoted
     it is replaced by the symbol; however, it does not satisfy the
     predicate `symbol?'.  Macro transformers rarely distinguish
     symbols from aliases, referring to both as identifiers.
   * A "syntactic" environment maps identifiers to their meanings.
     More precisely, it determines whether an identifier is a syntactic
     keyword or a variable.  If it is a keyword, the meaning is an
     interpretation for the form in which that keyword appears.  If it
     is a variable, the meaning identifies which binding of that
     variable is referenced.  In short, syntactic environments contain
     all of the contextual information necessary for interpreting the
     meaning of a particular form.
   * A "syntactic closure" consists of a form, a syntactic environment,
     and a list of identifiers.  All identifiers in the form take their
     meaning from the syntactic environment, except those in the given
     list.  The identifiers in the list are to have their meanings
     determined later.  A syntactic closure may be used in any context
     in which its form could have been used.  Since a syntactic closure
     is also a form, it may not be used in contexts where a form would
     be illegal.  For example, a form may not appear as a clause in the
     cond special form.  A syntactic closure appearing in a quoted
     structure is replaced by its form. Transformer Definition
This section describes the `sc-macro-transformer' special form and the
procedures `make-syntactic-closure' and `capture-syntactic-environment'.
 -- Syntax: transformer expression
     Syntax: It is an error if this syntax occurs except as a
     Semantics: The EXPRESSION is evaluated in the standard transformer
     environment to yield a macro transformer as described below.  This
     macro transformer is bound to a macro keyword by the special form
     in which the `sc-macro-transformer' expression appears (for example,
     A "macro transformer" is a procedure that takes two arguments, a
     form and a syntactic environment, and returns a new form.  The
     first argument, the "input form", is the form in which the macro
     keyword occurred.  The second argument, the "usage environment",
     is the syntactic environment in which the input form occurred.
     The result of the transformer, the "output form", is automatically
     closed in the "transformer environment", which is the syntactic
     environment in which the `sc-macro-transformer' expression occurred.
     For example, here is a definition of a push macro using
          (define-syntax  push
            (syntax-rules ()
              ((push item list)
               (set! list (cons item list)))))
     Here is an equivalent definition using `sc-macro-transformer':
          (define-syntax push
             (lambda (exp env)
               (let ((item
                      (make-syntactic-closure env '() (cadr exp)))
                      (make-syntactic-closure env '() (caddr exp))))
                 `(set! ,list (cons ,item ,list))))))
     In this example, the identifiers `set!' and `cons' are closed in
     the transformer environment, and thus will not be affected by the
     meanings of those identifiers in the usage environment `env'.
     Some macros may be non-hygienic by design.  For example, the
     following defines a loop macro that implicitly binds `exit' to an
     escape procedure.  The binding of `exit' is intended to capture
     free references to `exit' in the body of the loop, so `exit' must
     be left free when the body is closed:
          (define-syntax loop
             (lambda (exp env)
               (let ((body (cdr exp)))
                   (lambda (exit)
                     (let f ()
                       ,@(map (lambda  (exp)
                                 (make-syntactic-closure env '(exit)
     To assign meanings to the identifiers in a form, use
     `make-syntactic-closure' to close the form in a syntactic
 -- Function: make-syntactic-closure environment free-names form
     ENVIRONMENT must be a syntactic environment, FREE-NAMES must be a
     list of identifiers, and FORM must be a form.
     `make-syntactic-closure' constructs and returns a syntactic closure
     of FORM in ENVIRONMENT, which can be used anywhere that FORM could
     have been used.  All the identifiers used in FORM, except those
     explicitly excepted by FREE-NAMES, obtain their meanings from
     Here is an example where FREE-NAMES is something other than the
     empty list.  It is instructive to compare the use of FREE-NAMES in
     this example with its use in the `loop' example above: the examples
     are similar except for the source of the identifier being left
          (define-syntax let1
             (lambda (exp env)
               (let ((id (cadr exp))
                     (init (caddr exp))
                     (exp (cadddr exp)))
                 `((lambda (,id)
                     ,(make-syntactic-closure env (list id) exp))
                   ,(make-syntactic-closure env '() init))))))
     `let1' is a simplified version of `let' that only binds a single
     identifier, and whose body consists of a single expression.  When
     the body expression is syntactically closed in its original
     syntactic environment, the identifier that is to be bound by
     `let1' must be left free, so that it can be properly captured by
     the `lambda' in the output form.
     To obtain a syntactic environment other than the usage
     environment, use `capture-syntactic-environment'.
 -- Function: capture-syntactic-environment procedure
     `capture-syntactic-environment' returns a form that will, when
     transformed, call PROCEDURE on the current syntactic environment.
     PROCEDURE should compute and return a new form to be transformed,
     in that same syntactic environment, in place of the form.
     An example will make this clear.  Suppose we wanted to define a
     simple `loop-until' keyword equivalent to
          (define-syntax loop-until
            (syntax-rules ()
              ((loop-until id init test return step)
               (letrec ((loop
                         (lambda (id)
                           (if test return (loop step)))))
                 (loop init)))))
     The following attempt at defining `loop-until' has a subtle bug:
          (define-syntax loop-until
             (lambda (exp env)
               (let ((id (cadr exp))
                     (init (caddr exp))
                     (test (cadddr exp))
                     (return (cadddr (cdr exp)))
                     (step (cadddr (cddr exp)))
                      (lambda (exp free)
                        (make-syntactic-closure env free exp))))
                 `(letrec ((loop
                            (lambda (,id)
                              (if ,(close test (list id))
                                  ,(close return (list id))
                                  (loop ,(close step (list id)))))))
                    (loop ,(close init '())))))))
     This definition appears to take all of the proper precautions to
     prevent unintended captures.  It carefully closes the
     subexpressions in their original syntactic environment and it
     leaves the `id' identifier free in the `test', `return', and
     `step' expressions, so that it will be captured by the binding
     introduced by the `lambda' expression.  Unfortunately it uses the
     identifiers `if' and `loop' within that `lambda' expression, so if
     the user of `loop-until' just happens to use, say, `if' for the
     identifier, it will be inadvertently captured.
     The syntactic environment that `if' and `loop' want to be exposed
     to is the one just outside the `lambda' expression: before the
     user's identifier is added to the syntactic environment, but after
     the identifier loop has been added.
     `capture-syntactic-environment' captures exactly that environment
     as follows:
          (define-syntax loop-until
             (lambda (exp env)
               (let ((id (cadr exp))
                     (init (caddr exp))
                     (test (cadddr exp))
                     (return (cadddr (cdr exp)))
                     (step (cadddr (cddr exp)))
                      (lambda (exp free)
                        (make-syntactic-closure env free exp))))
                 `(letrec ((loop
                              (lambda (env)
                                `(lambda (,id)
                                   (,(make-syntactic-closure env '() `if)
                                    ,(close test (list id))
                                    ,(close return (list id))
                                    (,(make-syntactic-closure env '()
                                     ,(close step (list id)))))))))
                    (loop ,(close init '())))))))
     In this case, having captured the desired syntactic environment,
     it is convenient to construct syntactic closures of the
     identifiers `if' and the `loop' and use them in the body of the
     A common use of `capture-syntactic-environment' is to get the
     transformer environment of a macro transformer:
           (lambda (exp env)
              (lambda (transformer-env)
                ...)))) Identifiers
This section describes the procedures that create and manipulate
identifiers.  Previous syntactic closure proposals did not have an
identifier data type - they just used symbols.  The identifier data
type extends the syntactic closures facility to be compatible with the
high-level `syntax-rules' facility.
  As discussed earlier, an identifier is either a symbol or an "alias".
An alias is implemented as a syntactic closure whose "form" is an
     (make-syntactic-closure env '() 'a)
        => an "alias"
  Aliases are implemented as syntactic closures because they behave just
like syntactic closures most of the time.  The difference is that an
alias may be bound to a new value (for example by `lambda' or
`let-syntax'); other syntactic closures may not be used this way.  If
an alias is bound, then within the scope of that binding it is looked
up in the syntactic environment just like any other identifier.
  Aliases are used in the implementation of the high-level facility
`syntax-rules'.  A macro transformer created by `syntax-rules' uses a
template to generate its output form, substituting subforms of the
input form into the template.  In a syntactic closures implementation,
all of the symbols in the template are replaced by aliases closed in
the transformer environment, while the output form itself is closed in
the usage environment.  This guarantees that the macro transformation
is hygienic, without requiring the transformer to know the syntactic
roles of the substituted input subforms.
 -- Function: identifier? object
     Returns `#t' if OBJECT is an identifier, otherwise returns `#f'.
          (identifier? 'a)
             => #t
          (identifier? (make-syntactic-closure env '() 'a))
             => #t
          (identifier? "a")
             => #f
          (identifier? #\a)
             => #f
          (identifier? 97)
             => #f
          (identifier? #f)
             => #f
          (identifier? '(a))
             => #f
          (identifier? '#(a))
             => #f
     The predicate `eq?' is used to determine if two identifers are
     "the same".  Thus `eq?' can be used to compare identifiers exactly
     as it would be used to compare symbols.  Often, though, it is
     useful to know whether two identifiers "mean the same thing".  For
     example, the `cond' macro uses the symbol `else' to identify the
     final clause in the conditional.  A macro transformer for `cond'
     cannot just look for the symbol `else', because the `cond' form
     might be the output of another macro transformer that replaced the
     symbol `else' with an alias.  Instead the transformer must look
     for an identifier that "means the same thing" in the usage
     environment as the symbol `else' means in the transformer
 -- Function: identifier=? environment1 identifier1 environment2
     ENVIRONMENT1 and ENVIRONMENT2 must be syntactic environments, and
     IDENTIFIER1 and IDENTIFIER2 must be identifiers.  `identifier=?'
     returns `#t' if the meaning of IDENTIFIER1 in ENVIRONMENT1 is the
     same as that of IDENTIFIER2 in ENVIRONMENT2, otherwise it returns
     `#f'.  Examples:
                 (lambda (form env)
                    (lambda (transformer-env)
                      (identifier=? transformer-env 'x env 'x)))))))
            (list (foo)
                  (let ((x 3))
             => (#t #f)
          (let-syntax ((bar foo))
                   (lambda (form env)
                      (lambda (transformer-env)
                        (identifier=? transformer-env 'foo
                                      env (cadr form))))))))
              (list (foo foo)
             => (#f #t)
The syntactic closures facility was invented by Alan Bawden and Jonathan
Rees.  The use of aliases to implement `syntax-rules' was invented by
Alan Bawden (who prefers to call them "synthetic names").  Much of this
proposal is derived from an earlier proposal by Alan Bawden.


Alan Bawden and Chris Hanson, ported to CHICKEN by Taylor Campbell and felix winkelmann


Copyright (c) 1989-91 Massachusetts Institute of Technology

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renamed transformer to sc-macro-transformer for compatibility with MIT-Scheme [suggested by Taylor Campbell]
added SRFI-17 support
removed (probably) overly restrictive check [suggested by Taylor Campbell]
added support for line-number info in debug traces by Alex Shinn
added bugfix for let-syntax by Taylor Campbell, and vector matching by Alex Shinn
added bugfix by Taylor Campbell
added cond-expand
actually works now, thanks to Taylor Campbell
added suppoer for DSSSL lambda lists (but needs let-optionals yet)
Initial release