How to write a list literal that contains items conditionally? [duplicate]

Question

Given a list literal '(1 2 3)

is it possible to conditionally include parts of the list?

eg:

'(1 2 3
  (when thing-is-true '(10 11 12)))
  4 5 6)

Which would result in

'(1 2 3 4 5 6) or '(1 2 3 10 11 12 4 5 6) dependent on thing-is-true.

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Python for example supports:

[1, 2, 3, *([10, 11, 12] if thing_is_true else []), 4, 5, 6]

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What is the most straightforward way to achieve this using a single expression, without first defining variables and them into a list?

Answer 1

You can use the macro (elisp) Backquote, e.g.,

`(1 2 3 ,@(when nil '(10 11 12)) 4 5 6)
;; => (1 2 3 4 5 6)

`(1 2 3 ,@(when t '(10 11 12)) 4 5 6)
;; => (1 2 3 10 11 12 4 5 6)

if you expand the above macros, they are simply using append

(append '(1 2 3) (when nil '(10 11 12)) '(4 5 6))
;; => (1 2 3 4 5 6)

(append '(1 2 3) (when t '(10 11 12)) '(4 5 6))
;; => (1 2 3 10 11 12 4 5 6)