Quine

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

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

This expression is one of the earliest examples of a quine within the APL community, being listed in a 1977 APL Press article.

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

Explanation
This shorter version is explain as follows:
 * the characters
 * reshape to shape 9, resulting in
 * concatenate with itself, resulting in
 * rotate one character to the left, getting the characters:

Based on replicating quote
In May 2022, APL Farm user OsKaR31415 shared the following:

Explanation
In this code, the main function is  with the right argument   being single quote literal (spelled with four quotes, , because single quotes delimit APL character literals, and quotes inside such must be doubled), and the left argument   being the character vector representation of the function.

The idea behind this quine is that the only thing you need to have a string containing the original code is to reproduce this structure: [quote] [function] [quote] [function] [quote] [quote] [quote] [quote]. This is precisely what the function does. Given the representation of a function  (itself in this case), and character   (here, the quote), it produces the structure. A more explicit version would be  which simply concatenates together the required parts. The original quine is very similar, only using the shorter  to produce the 4 occurrences of , and golfs away the spaces, instead opting for Enlist  to flatten the intermediary nested array.

Based on code point of quote
In Jun 2022, Jay Foad shared the following:

Explanation
In this code, the main function is  with the argument   being the function itself which avoids duplication of quotes by generating the quote from its Universal Character Set code point.

The idea behind this quine is that the only thing you need to have a string containing the original code is to reproduce this structure: [function] [quote] [function] [quote]. This is precisely what the function does. Given the representation of a function  (itself in this case), it generates a quote and concatenates it on the right of the function definition, giving [function] [quote]. It then uses self-concatenation to repeat the two parts into the required four.