Swapper

Swapper is a simple self-modifying reversible language invented by User:Tailcalled in February of 2012.

Syntax

Swapper programs consist of a list of statements separated by semicolons:

program = statement [';' program]
statement = <atom> '<->' <atom>
statement = '(' <#> ')' ('<+->' | '<*/>') '[' <#> ']'
statement = '<rev>'
statement = '<prtmd>'

where <atom> is a string of characters in the groups A-Z, a-z, or 0-9, and <#> is just in the group 0-9.

Programming

Integers

Integer literals are created by putting an integer in parentheses.

(42)

Constant integer literals are created with brackets instead. These have the property of remaining unchanged during a pseudo-operation.

[42]

Operators

Operators are enclosed in angle brackets and self-modify the program when executed. They cannot be modified themselves.

Syntax	Action
A <-> B;	swaps all occurrences of A and B, where both are alphanumeric patterns delimited by ; and <->. Invalid characters (spaces, newlines, symbols, etc.) are skipped in the pattern. Text inside [] or <> remains unchanged during the swap.
(x) <+-> [y];	replaces (x) with (x+y) if the program flows forward, and (x-y) if backward.
(x) <*/> [y];	replaces (x) with (x*y) if the program flows forward, and (x/y) if backward, rounding down.
<rev>;	reverses program flow.
<prtmd>;	toggles "print mode". In this mode, any number that <->'s with out is printed to STDOUT.

Additional operators can be added at the programmer's discretion. User:Tailcalled encourages this notion to add power to the language while maintaining its reversibility.

Sample Programs

Operator Demo

This program prints 1 to STDOUT.

i <-> (0);
i <+-> [1];
<prtmd>;
out <-> i;
<prtmd>;

Step by step:

1. All is are swapped with (0):

(0) <-> i;
(0) <+-> [1];
<prtmd>;
out <-> (0);
<prtmd>;

2. All (0)s are incremented.

(1) <-> i;
(1) <+-> [1];
<prtmd>;
out <-> (1);
<prtmd>;

3. Enter print mode.

4. 1 printed because it is in print mode.

5. Exit print mode.

Print the ASCII #s of Hello, world!

<prtmd>;
out <-> (72);
out <-> (101);
out <-> (108);
out <-> (108);
out <-> (111);
out <-> (44);
out <-> (32);
out <-> (119);
out <-> (111);
out <-> (114);
out <-> (108);
out <-> (100);
out <-> (33);
out <-> (10);