Quine

A quine is a program which takes no input and produces a copy of its own source code as its only output.

In APL, a quine is listed in the FinnAPL idiom library as "an expression giving itself":

In 2019, Nick Nikolov proposed to shorten it using the commute operator:

Explanation:
 * the characters
 * reshape to shape 9, resulting in
 * concatenate with itself, resulting in
 * rotate one character to the left, getting the characters: