layout/bare-syntax

Symbol: layout/bare-syntax

Use-case: Block syntax for the bare arm of a transformer: the expansion that applies when the transformer’s name is used as a lone atom rather than at the head of a call form. Normally the inner block of a syntax or extend-syntax block. A transformer has at most one bare arm.

Behaviour: The header names the called-by slot, and may carry a name before it. The body is the template, left unexpanded the way a syntax-rules template is; a body of several forms is wrapped in begin. Produces (bare-syntax SLOT TEMPLATE), or (bare-syntax NAME SLOT TEMPLATE) with a name in the header. SLOT is the only binding the template may refer to, and it captures the symbol the expansion was reached through. Does not accept peers. A header with no symbol raises bare-syntax requires a called-by symbol, and an empty body raises bare-syntax requires a template.

Named header: bare-syntax NAME SLOT: puts NAME in the emitted form, which is where the transformer takes its identity. syntax and extend-syntax supply that name themselves, so the named header belongs to a transformer bound directly, as var compile/NAME:.

Syntax:

bare-syntax SLOT:
  TEMPLATE...

bare-syntax NAME SLOT:
  TEMPLATE...

Examples:

syntax marker:
  syntax-rules []:
    case (marker x):
      (print "called" x)

extend-syntax marker:
  bare-syntax slot:
    "bare marker"

(marker 1)
(print marker)
;; prints: called 1
;; prints: bare marker

var compile/marker:
  bare-syntax marker slot:
    "the marker"

(print marker)
;; prints: the marker
;; -> (var compile/marker (bare-syntax marker slot "the marker"))

Also: special form bare-syntax.