constantp

(constantp form)

Returns t if form is a constant object and nil otherwise. This is a lite implementation: it recognizes self-evaluating objects (numbers, strings, characters, keywords, t, and nil) and (quote x) forms. Anything else -- including a plain symbol or a function-call form -- yields nil. False negatives are harmless (a consumer just defers the work to runtime). Available on all backends except --no-gc.