compile/datum

Syntax: (datum EXPR), (datum EXPR POSITION), inside a macro body.

Behaviour: Expands to (call datum EXPR [POSITION]), wrapping the value in a Datum that carries a source position. In a procedural macro, a symbol the body computes is emitted through datum so hygiene treats it as user syntax rather than a fresh template identifier, and so the compiler can position errors in it.

Examples:

>>> (macroexpand-1 '(datum foo))
(call datum foo None)

syntax job:
  macro-rules []:
    case (job tp rest ...):
      ;; rewrite (job kafka ...) to (job/kafka ...)
      (cons (datum (symbol f"job/{tp.datum.base}")) *: rest nil)

The type tag is a label, so the name is built from its base; a tag a template wrote carries the expansion’s mark, which str would show.

rest is the captured tail, spliced with *: into the variadic cons and closed with nil. Passed as a single argument it would be consed on as one element.

See also: Macros and hygiene, koskari.hygiene.

Also: type datum.