layout/special-rules

Symbol: layout/special-rules

Use-case: Block syntax for the special-rules transformer, normally the inner block of a syntax or extend-syntax block. Converts indented rule blocks into the canonical (special-rules [NAME] (LIT ...) (PATTERN BODY) ...) form that compile/special-rules compiles.

Behaviour: The header carries a literals list ([] or [x y]), and may carry a name before it. Each rule’s pattern column is [COMPILER PATTERN]: the first subform names the compiler parameter the rule body receives, the second is the rule 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, ([c (foo x)]):, is rejected with rule must be a block; write case [c (foo x)]: instead.

Named header: special-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:

special-rules [LIT ...]:
  case [COMPILER PATTERN]:
    BODY...

special-rules NAME [LIT ...]:
  COMPILER PATTERN:
    BODY...

Examples:

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

(say-hi "there")
;; prints: hi there

var compile/say-hi:
  special-rules say-hi []:
    c (say-hi thing):
      var t thing.datum
      `(print "hi" ,t)
;; -> (var compile/say-hi
;;      (special-rules say-hi [] ((c (say-hi thing)) (begin ...))))

Also: special form special-rules.