compile/datumq

Syntax: (datumq FORM), (datumq FORM POSITION), or -'FORM

Behaviour: Wraps FORM in a Datum without evaluating it, recording the source position. It is the quoting counterpart of datum: datum captures the value of its argument, datumq captures the argument form itself, expanding to (datum (quote FORM) POSITION).

Without an explicit POSITION, the position recorded is FORM’s own, or None when the form carries none.

A Datum carries the form on its datum attribute and the position on position. Hygiene leaves what is inside unmarked, and the compiler positions errors in it from the position.

In a template: in a syntax-rules or syntax-match template, (datumq FORM) writes FORM raw: the expansion emits it unmarked, with no pattern variable substituted inside it. A template uses it for a name it shares with the use site, such as a module-level slot another macro reads. It takes exactly one form, and its position is where the template wrote the form. Each expansion consumes one level, so a template that defines another macro writes (datumq (datumq name)), and one that must write a literal datumq call writes ((datumq datumq) name).

-'FORM is reader shorthand for (datumq FORM), and -'-'FORM for the nested form.

Examples:

>>> var d (datumq (+ 1 2))
>>> d.datum
(+ 1 2)

(syntax set-slot
  (syntax-rules [] [(set-slot v) (var -'shared-slot v)]))

(set-slot 7)
shared-slot
;; => 7

See also: datum, compile/quote, Macros and hygiene, Hygiene.