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Programming language

Cat is a functional stack-based programming language, inspired by Joy.

Cat was created by Christopher Diggins, who is still its main developer. The language has a couple of implementations, including the official one written in C#.

Cat has three levels of specification. level-0, or core Cat, is a purely functional language; it provides 4 primitive data types (boolean, integer, dynamically checked variant and list — a collection of variants).

All instructions of the language share data using a single data structure called stack. The stack can contain numbers, strings, functions, lists, or any other kind of data. This results in some interesting features absent in other stack-based languages.

Elements of syntax:

Case-sensitivity level-0 - no, level-1,2 - yes
Function identifier regexp level-0 - [[_a-zA-Z][_a-zA-Z0-9]*]
Block [ ... ]
Physical (shallow) equality eq
Physical (shallow) inequality neq
Comparison gteq, lteq, gt_int, lt_int
Function definition define definition-name (: type-declaration)? metadata? { expression }
Function call definition-name
Function call with no parameters definition-name


Hello, World!:

Example for versions Online Cat 1.3

First line of the example pushes a string “Hello, World!” on the top of the stack. Second line pops the top element from the stack and prints it, followed by a newline.

"Hello, World!"

Fibonacci numbers:

Example for versions Online Cat 1.3

This example uses recursive definition of Fibonacci numbers. First part defines a function fib which works as follows. When it is called, the top element of the stack is index of Fibonacci number to be calculated. First the commands dup and 1 push a copy of this element and a 1 on the stack. Then command <= (which is equivalent of lteq) pops and compares top two elements and pushes true if index is less than or equal to 1, and false otherwise.

Next goes conditional expression: two quotations are pushed on the stack, and then if the third-top element of the stack is true, first of them is evaluated, otherwise second one is. In this case true-quotation is empty (Fibonacci numbers 0 and 1 are equal to 0 and 1, respectively, so no calculations are required), and false-quotation is the following. It duplicates the top element of the stack (which is index N of the number again), decrements the duplicated value (N-1), calculates Fibonacci number N-1 and replaces the index with the number itself, swaps two top elements of the stack (so now they are Fib(N-1) N), subtracts 2 from the top element (N-2) and replaces it with respective Fibonacci number. Finally, two Fibonacci numbers are added to get the number which was required.

Second part of the program is its body which loops over the indices from 1 to 16, calculates numbers and prints them. It uses a while-loop, with body [dup fib write ", " write inc] which duplicates top element of the stack, calculates its Fibonacci number, writes it, writes comma after it and increments the loop counter. [dup 16 lteq] is condition of repeating the loop — while loop counter is less than or equal to 16.

define fib {
    dup 1 <=
        [dup 1 - fib swap 2 - fib +]

    [dup fib write ", " write inc]
    [dup 16 lteq]
"..." writeln


Example for versions Online Cat 1.3

This example uses recursive factorial definition, and works in a similar way to Fibonacci numbers example.

define fact {
    dup 1 <= 
        [pop 1]
        [dup 1 - fact *]

    [dup write "! = " write dup fact writeln inc]
    [dup 16 lteq]