Editing Joy (programming language) (section)

== Overview ==

Functions in Joy lack [[Parameter (computer science)|formal parameters]]. For example, a function that squares a numeric input can be expressed as follows:<ref>{{cite web|title=An informal tutorial on Joy |url=http://www.latrobe.edu.au/phimvt/joy/j01tut.html |url-status=dead |archive-url=https://web.archive.org/web/20111007030359/http://www.latrobe.edu.au/phimvt/joy/j01tut.html |archive-date=October 7, 2011 }}</ref>

 DEFINE square == dup * .

In Joy, everything is a function that takes a [[stack (data structure)|stack]] as an argument and returns a stack as a result. For instance, the numeral '5' does not represent an integer constant, but instead a short program that pushes the number 5 onto the stack.

* The '''dup''' operator simply duplicates the top element of the stack by pushing a copy of it.
* The '''*''' operator pops two numbers off the stack and pushes their product.

So the square function makes a copy of the top element, and then multiplies the two top elements of the stack, leaving the square of the original top element at the top of the stack, with no need for a formal parameter. This makes Joy concise, as illustrated by this definition of [[quicksort]]:<ref>{{cite web|title=Sequence Library |url=http://www.latrobe.edu.au/phimvt/joy/seqlib.joy |url-status=dead |archive-url=https://web.archive.org/web/20111007030359/http://www.latrobe.edu.au/phimvt/joy/seqlib.joy |archive-date=October 7, 2011 }}</ref>

<pre><nowiki>
DEFINE qsort ==
  [small]
  []
  [uncons [>] split]
  [swapd cons concat]
  binrec.
</nowiki></pre>