Idiomatic conversion of non-nil to explicit t

Question

Is there an idiomatic/commonly used/established convention to convert non-nil values to t that other Elisp programmers are likely to recognize?

Sometimes I have a non-nil value, say from calling member, that I want to return as either t or nil (so that I don't leak implementation details (Cf. Hyrum's Law)).

The Emacs Lisp Coding Conventions, for example, say nothing about this that I can see. The closest I've found is the Elisp manual's section on nil and t, which says "When you need to choose a value that represents true, and there is no other basis for choosing, use t."

Ways I've come up with:

(and val t)

(if val t nil)

(not (not val))

Does one of these have a benefit over the other? Is one "more idiomatic"?

Answer 1

The better option is:

(not (not X))

Some of the advantages are in the ability of the byte-compiler to optimize it away. Also for me it's easy to visually see that it's a "virtual no-op" yet at the same time it makes it clear that it was introduced on purpose.

Answer 2

(and val t)

is the cleanest and clearest.

(if val t nil)

is also clear but is slightly more verbose and less clean.

There may be efficiency advantages to using

(not (not val))

as Stefan mentions. However, efficiency gains are likely minimal in most cases since negation is a cheap operation.

While double negation is clearly intentional, the reason why the programmer would include it is less clear. For example, it could have originally been a single negation and the second one could have been added during debugging and never been removed. There are other possibilities as well.