Syntax Functions

Defining templates can be more trouble than it’s worth particular when you only want to change the shape of an expression. This is called syntax transformation.

In essence, syntax transformers are a form of structured template.

template define-syntax name rules

Associate name with a syntax transformer using rules.

rules is a list:

(syntax-rules (keywords) ((pattern expansion) ...))

• the symbol syntax-rules as the first element

• a list of keywords to be matched for as syntactic structure then discarded

  This is often an empty list.

• a list of (pattern expansion) rules. If the pattern matches then apply the expansion

  The first element of the pattern match will always be the name of the syntax transformer – otherwise we wouldn’t be executing this expander – and is ignored.

  The pattern must have a placeholder for the function name, though, which is often represented as _.

Note

The expansion can produce an instance of the same syntax transformer where one pattern might have an expansion that, say, fills in a default parameter for another pattern to match.

As the first word of the resultant expression of the first expansion is a syntax transformer, the evaluator will loop round again, this time matching against the other pattern and producing its expansion in turn.

Obviously, you should avoid circular loops.

Both pattern and expansion support ellipses. Here, if a term in pattern is followed by ellipses then multiple alike elements are gathered together and spliced in en masse in the corresponding term-ellipses part of expansion. There is an example in define-syntax-rule.

Example:

Define a “wordy” conditional expression which makes use of keywords then and else which are simply syntactic fluff and are discarded:

(define-syntax wordy-if
            (syntax-rules (then else)
              ((_ condition then consequent)
               (and condition consequent))
              ((_ condition then consequent else alternative)
               (cond
                (condition consequent)
                (else alternative)))))

wordy-if (frob a) then b
;; (and (frob a) b)

wordy-if z then #n else (call it)
;; (cond
;;   (z #n)
;;   (else (call it)))

Also note how the expansions are not required to be similar. Here, wordy-if uses two different expansions, one with and, the other with cond.

template define-syntax-rule (name & pattern) expansion

Associate name with a syntax transformer using pattern and expansion.

This is a one-shot syntax transformer.

In effect,

(define-syntax-rule (name & pattern) expansion)

is re-written as

(define-syntax name (syntax-rules () ((name & pattern) expansion)))

Example:

The definitions of when and unless demonstrate the use of ellipses where the expression is saying that body could be multiple expressions, all of which are to be gathered up and put inside a begin form.

define-syntax-rule (when test body ...) (
  if test (begin body ...)
)

define-syntax-rule (unless test body ...) (
  if (not test) (begin body ...)
)

Last built at 2024-12-21T07:10:43Z+0000 from 62cca4c (dev) for Idio 0.3.b.6