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Concept
Joy (programming language)
Summary
The Joy programming language in computer science is a purely functional programming language that was produced by Manfred von Thun of La Trobe University in Melbourne, Australia. Joy is based on composition of functions rather than lambda calculus. It has turned out to have many similarities to Forth, due not to design but to an independent evolution and convergence. It was also inspired by the function-level programming style of John Backus's FP.
Joy is unusual among functional programming languages (except for function-level programming languages and some esoteric ones, such as Unlambda) in its lack of a lambda operator, and therefore lack of formal parameters. To illustrate this with a common example, here is how the square function might be defined in an imperative programming language (C):
int square(int x)
{
return x * x;
}
The variable x is a parameter which is replaced by the argument to be squared when the function is called.
In a functional language (Scheme), the same function could be defined:
(define square
(lambda (x)
(* x x)))
This is different in many ways, but it still uses the parameter x in the same way.
In Joy, the square function is defined:
DEFINE square == dup * .
In Joy, everything is a function that takes a 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:
DEFINE qsort ==
[small]
[]
[uncons [>] split]
[enconcat]
binrec.
"binrec" is one of Joy's many recursive combinators, implementing binary recursion.
Official source
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