This section describes the procedures that create and manipulate
identifiers. The identifier data type extends the syntactic closures
facility to be compatible with the high-level
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
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
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 (sc-macro-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 (sc-macro-transformer (lambda (form env) (capture-syntactic-environment (lambda (transformer-env) (identifier=? transformer-env 'foo env (cadr form)))))))) (list (foo foo) (foo bar)))) => (#f #t)
Sometimes it is useful to be able to introduce a new identifier that is
guaranteed to be different from any existing identifier, similarly to
the way that
generate-uninterned-symbol is used.
Creates and returns and new synthetic identifier (alias) that is guaranteed to be different from all existing identifiers. Identifier is any existing identifier, which is used in deriving the name of the new identifier.
This is implemented by syntactically closing identifier in a special empty environment.