layout/macro-rules

Symbol: layout/macro-rules

Use-case: Block syntax for the macro-rules transformer within a syntax block. Same surface as layout/syntax-rules (literals list and indented rules); lowers to (macro-rules [NAME] (LIT ...) (PATTERN BODY) ...). The rule body is a procedural macro body rather than a template, and nested layout blocks within it are expanded.

Behaviour: Same rule shape as layout/syntax-rules; only the emitted head and the treatment of the rule body differ. A body of several forms is wrapped in begin. Does not accept peers.

A rule block header is either case FORM:, where case is followed by exactly one form and that form is the pattern, or a header whose first element is a symbol, in which case the whole header is the pattern. A header that is a lone parenthesized form, (complex pat):, raises LayoutError: rule must be a block; write case (complex pat): instead.

Named header: macro-rules NAME [LIT ...]: puts NAME in the emitted form, which is where the transformer takes its identity for help text and match errors. syntax and extend-syntax supply that name themselves, so the named header belongs to a transformer bound directly, as var compile/NAME:.

Syntax:

macro-rules [LIT ...]:
  PATTERN1:
    BODY1...
  case (PATTERN2):
    BODY2...

macro-rules NAME [LIT ...]:
  case (PATTERN):
    BODY...

Examples:

macro-rules [x]:
  case (foo a):
    (bar a)
    (baz a)
;; -> (macro-rules [x] ((foo a) (begin (bar a) (baz a))))

var compile/twice:
  macro-rules twice []:
    case (twice x):
      `(begin ,x ,x)

(twice (print "hello"))
;; prints: hello
;; prints: hello

Also: special form macro-rules.