Warning: This is the manual of the legacy Guile 2.2 series. You may want to read the manual of the current stable series instead.

6.10.6 Identifier Macros

When the syntax expander sees a form in which the first element is a macro, the whole form gets passed to the macro’s syntax transformer. One may visualize this as:

(define-syntax foo foo-transformer)
(foo arg...)
;; expands via
(foo-transformer #'(foo arg...))

If, on the other hand, a macro is referenced in some other part of a form, the syntax transformer is invoked with only the macro reference, not the whole form.

(define-syntax foo foo-transformer)
foo
;; expands via
(foo-transformer #'foo)

This allows bare identifier references to be replaced programmatically via a macro. syntax-rules provides some syntax to effect this transformation more easily.

Syntax: identifier-syntax exp

Returns a macro transformer that will replace occurrences of the macro with exp.

For example, if you are importing external code written in terms of fx+, the fixnum addition operator, but Guile doesn’t have fx+, you may use the following to replace fx+ with +:

(define-syntax fx+ (identifier-syntax +))

There is also special support for recognizing identifiers on the left-hand side of a set! expression, as in the following:

(define-syntax foo foo-transformer)
(set! foo val)
;; expands via
(foo-transformer #'(set! foo val))
;; if foo-transformer is a "variable transformer"

As the example notes, the transformer procedure must be explicitly marked as being a “variable transformer”, as most macros aren’t written to discriminate on the form in the operator position.

Scheme Procedure: make-variable-transformer transformer

Mark the transformer procedure as being a “variable transformer”. In practice this means that, when bound to a syntactic keyword, it may detect references to that keyword on the left-hand-side of a set!.

(define bar 10)
(define-syntax bar-alias
  (make-variable-transformer
   (lambda (x)
     (syntax-case x (set!)
       ((set! var val) #'(set! bar val))
       ((var arg ...) #'(bar arg ...))
       (var (identifier? #'var) #'bar)))))

bar-alias ⇒ 10
(set! bar-alias 20)
bar ⇒ 20
(set! bar 30)
bar-alias ⇒ 30

There is an extension to identifier-syntax which allows it to handle the set! case as well:

Syntax: identifier-syntax (var exp1) ((set! var val) exp2)

Create a variable transformer. The first clause is used for references to the variable in operator or operand position, and the second for appearances of the variable on the left-hand-side of an assignment.

For example, the previous bar-alias example could be expressed more succinctly like this:

(define-syntax bar-alias
  (identifier-syntax
    (var bar)
    ((set! var val) (set! bar val))))

As before, the templates in identifier-syntax forms do not need wrapping in #' syntax forms.