layout/special-match

Symbol: layout/special-match

Use-case: Block syntax for the special-match transformer, normally the inner block of a syntax or extend-syntax block. No literals list; each rule carries an explicit match-style pattern. Converts indented rule blocks into the (special-match [NAME] (PATTERN BODY) ...) form that compile/special-match compiles.

Behaviour: Same rule shape as layout/special-rules, with no literals list in the header: each rule’s pattern column is [COMPILER PATTERN], the first subform naming the compiler parameter the rule body receives and the second being the explicit pattern. A rule 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 is rejected with rule must be a block.

Named header: special-match NAME: 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:

special-match:
  case [COMPILER PATTERN]:
    BODY...

special-match NAME:
  case [COMPILER PATTERN]:
    BODY...

Examples:

syntax say-bye:
  special-match:
    case [c (say-bye (BIND thing))]:
      var t thing.datum
      `(print "bye" ,t)

(say-bye "now")
;; prints: bye now

var compile/say-bye:
  special-match say-bye:
    case [c (say-bye (BIND thing))]:
      var t thing.datum
      `(print "bye" ,t)

(say-bye "now")
;; prints: bye now

Also: special form special-match.