16.2.2 Macro Transformer Procedures

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► | 16 | Macros

16.1 | Pattern-Based Macros
16.2 | General Macro Transformers
16.3 | Module Instantiations and Visits

▼ | 16.2 | General Macro Transformers

16.2.1 | Syntax Objects
16.2.2 | Macro Transformer Procedures
16.2.3 | Mixing Patterns and Expressions: syntax-case
16.2.4 | with-syntax and generate-temporaries
16.2.5 | Compile and Run-Time Phases
16.2.6 | General Phase Levels
16.2.7 | Tainted Syntax

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##### 16.2.2`` Macro Transformer Procedures🔗ℹ

Any procedure of one argument can be a macro transformer. As it turns out, the syntax-rules form is a macro that expands to a procedure form. For example, if you evaluate a syntax-rules form directly (instead of placing on the right-hand of a define-syntax form), the result is a procedure:

> > (syntax-rules () [(nothing) something])
> #<procedure>

Instead of using syntax-rules, you can write your own macro transformer procedure directly using lambda. The argument to the procedure is a syntax object that represents the source form, and the result of the procedure must be a syntax object that represents the replacement form:

> > (define-syntax self-as-string
> (lambda (stx)
> (datum->syntax stx
> (format "~s" (syntax->datum stx)))))
> > (self-as-string (+ 1 2))
> "(self-as-string (+ 1 2))"

The source form passed to a macro transformer represents an expression in which its identifier is used in an application position (i.e., after a parenthesis that starts an expression), or it represents the identifier by itself if it is used as an expression position and not in an application position.The procedure produced by syntax-rules raises a syntax error if its argument corresponds to a use of the identifier by itself, which is why syntax-rules does not implement an identifier macro.

> > (self-as-string (+ 1 2))
> "(self-as-string (+ 1 2))"
> > self-as-string
> "self-as-string"

The define-syntax form supports the same shortcut syntax for functions as define, so that the following self-as-string definition is equivalent to the one that uses lambda explicitly:

> > (define-syntax (self-as-string stx)
> (datum->syntax stx
> (format "~s" (syntax->datum stx))))
> > (self-as-string (+ 1 2))
> "(self-as-string (+ 1 2))"

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