Returns scheme code with the macros and derived expression types of expression expanded to primitive expression types.
macro:evalreturns the value of expression in the current top level environment. expression can contain macro definitions. Side effects of expression will affect the top level environment.
filename should be a string. If filename names an existing file, the
macro:loadprocedure reads Scheme source code expressions and definitions from the file and evaluates them sequentially. These source code expressions and definitions may contain macro definitions. The
macro:loadprocedure does not affect the values returned by
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:
transformer spec := (transformer expression)
Additionally, the following procedures are added:
make-syntactic-closure capture-syntactic-environment identifier? identifier=?
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
This section defines the concepts and data types used by the syntactic closures facility.
set!special form is also a form. Examples of forms:
17 #t car (+ x 4) (lambda (x) x) (define pi 3.14159) if define
symbol?. Macro transformers rarely distinguish symbols from aliases, referring to both as identifiers.
This section describes the
transformer special form and the
Syntax: It is an error if this syntax occurs except as a transformer spec.
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
transformerexpression 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
For example, here is a definition of a push macro using
syntax-rules:(define-syntax push (syntax-rules () ((push item list) (set! list (cons item list)))))
Here is an equivalent definition using
transformer:(define-syntax push (transformer (lambda (exp env) (let ((item (make-syntactic-closure env '() (cadr exp))) (list (make-syntactic-closure env '() (caddr exp)))) `(set! ,list (cons ,item ,list))))))
In this example, the identifiers
consare closed in the transformer environment, and thus will not be affected by the meanings of those identifiers in the usage environment
Some macros may be non-hygienic by design. For example, the following defines a loop macro that implicitly binds
exitto an escape procedure. The binding of
exitis intended to capture free references to
exitin the body of the loop, so
exitmust be left free when the body is closed:(define-syntax loop (transformer (lambda (exp env) (let ((body (cdr exp))) `(call-with-current-continuation (lambda (exit) (let f () ,@(map (lambda (exp) (make-syntactic-closure env '(exit) exp)) body) (f))))))))
To assign meanings to the identifiers in a form, use
make-syntactic-closureto close the form in a syntactic environment.
environment must be a syntactic environment, free-names must be a list of identifiers, and form must be a form.
make-syntactic-closureconstructs 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 environment.
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
loopexample above: the examples are similar except for the source of the identifier being left free.(define-syntax let1 (transformer (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))))))
let1is a simplified version of
letthat 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
let1must be left free, so that it can be properly captured by the
lambdain the output form.
To obtain a syntactic environment other than the usage environment, use
capture-syntactic-environmentreturns 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-untilkeyword 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-untilhas a subtle bug:(define-syntax loop-until (transformer (lambda (exp env) (let ((id (cadr exp)) (init (caddr exp)) (test (cadddr exp)) (return (cadddr (cdr exp))) (step (cadddr (cddr exp))) (close (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
ididentifier free in the
stepexpressions, so that it will be captured by the binding introduced by the
lambdaexpression. Unfortunately it uses the identifiers
lambdaexpression, so if the user of
loop-untiljust happens to use, say,
iffor the identifier, it will be inadvertently captured.
The syntactic environment that
loopwant to be exposed to is the one just outside the
lambdaexpression: before the user's identifier is added to the syntactic environment, but after the identifier loop has been added.
capture-syntactic-environmentcaptures exactly that environment as follows:(define-syntax loop-until (transformer (lambda (exp env) (let ((id (cadr exp)) (init (caddr exp)) (test (cadddr exp)) (return (cadddr (cdr exp))) (step (cadddr (cddr exp))) (close (lambda (exp free) (make-syntactic-closure env free exp)))) `(letrec ((loop ,(capture-syntactic-environment (lambda (env) `(lambda (,id) (,(make-syntactic-closure env '() `if) ,(close test (list id)) ,(close return (list id)) (,(make-syntactic-closure env '() `loop) ,(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
loopand use them in the body of the
A common use of
capture-syntactic-environmentis to get the transformer environment of a macro transformer:(transformer (lambda (exp env) (capture-syntactic-environment (lambda (transformer-env) ...))))
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
As discussed earlier, an identifier is either a symbol or an alias. An alias is implemented as a syntactic closure whose form is an identifier:
(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
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
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.
#tif object is an identifier, otherwise returns
#f. Examples:(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
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
condmacro uses the symbol
elseto identify the final clause in the conditional. A macro transformer for
condcannot just look for the symbol
else, because the
condform might be the output of another macro transformer that replaced the symbol
elsewith an alias. Instead the transformer must look for an identifier that “means the same thing” in the usage environment as the symbol
elsemeans in the transformer environment.
environment1 and environment2 must be syntactic environments, and identifier1 and identifier2 must be identifiers.
#tif the meaning of identifier1 in environment1 is the same as that of identifier2 in environment2, otherwise it returns
#f. Examples:(let-syntax ((foo (transformer (lambda (form env) (capture-syntactic-environment (lambda (transformer-env) (identifier=? transformer-env 'x env 'x))))))) (list (foo) (let ((x 3)) (foo)))) ⇒ (#t #f)(let-syntax ((bar foo)) (let-syntax ((foo (transformer (lambda (form env) (capture-syntactic-environment (lambda (transformer-env) (identifier=? transformer-env 'foo env (cadr form)))))))) (list (foo foo) (foobar)))) ⇒ (#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