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/⍵}. In this function, the right argument ⍵ is the literal ', and the left argument ⍺ is the string containing the 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]. So this is precisely what the function does : given the representation of a function ⍵ (itself in this case), and a string ⍺(the quote here), it reproduce the structure `⍵ ⍺ ⍵ ⍺ ⍵ ⍵ ⍵ ⍵`. Another quine is '{∊⍵ ⍺ ⍵ ⍺ ⍵ ⍵ ⍵ ⍵}'{∊⍵ ⍺ ⍵ ⍺ ⍵ ⍵ ⍵ ⍵}'''', and the only thing to notice on this one is the use of ∊ so the result is not a nested array of strings, but a flat string. The quite proposed here is very similar, it only replaces the 4 occurences of ⍵ by 4/⍵ and deletes the useless spaces for the purpose of golfing.