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":

      1⌽22⍴11⍴'''1⌽22⍴11⍴'''

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

      1⌽,⍨9⍴'''1⌽,⍨9⍴'''

Explanation:

• '''1⌽,⍨9⍴''' the characters '1⌽,⍨9⍴'
• 9⍴ reshape to shape 9, resulting in '1⌽,⍨9⍴''
• ,⍨ concatenate with itself, resulting in '1⌽,⍨9⍴'''1⌽,⍨9⍴''
• 1⌽ rotate one character to the left, getting the characters: 1⌽,⍨9⍴'''1⌽,⍨9⍴'''

Further Entries

In 2022, APL Farm user OsKaR31415 shared the following:

      '{∊⍵⍺⍵⍺,4/⍵}'{∊⍵⍺⍵⍺,4/⍵}''''

Explanation:

In this code, the main function is {∊⍵⍺⍵⍺,4/⍵} 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 ('{∊⍵⍺⍵⍺,4/⍵}').

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 4/⍵ to produce the 4 occurrences of ⍵, and golfs away the spaces, instead opting for Enlist (∊) to flatten the intermediary nested array.