Absurd Brainfuck is a Brainfuck-derivative by Tailcalled. It is called 'absurd' because the side effects flow in the opposite direction of the data. In other words, when you change a cell, you change what it *was*, not what it is. This can lead to undecidable or paradoxical scenarios, of course, especially when using the
Absurd Brainfuck has the commands of Brainfuck, but with a "slightly" different effect.
+ Changes the current cell to be one higher in the past - Changes the current cell to be one lower in the past > Goes to the next cell in the past < Goes to the previous cell in the past . Outputs the current value of the current cell. , Reads a value and puts it in the current cell in the past. [p] Executes p until the value of the current cell is 0.
Absurd Brainfuck has an infinite tape in *both* directions.
Aside from one command, all temporal effects are reversible, so one can calculate the effect of reversing them, and those commands are trivial substitutions for Brainfuck ones. Absurd Brainfuck is a Turing machine with non-deterministic effects when it comes to the input. However, many languages are non-deterministic when it comes to input; the difference is that in Absurd Brainfuck inputs can easily cause undefined behaviour or contradictions, and only specific deterministic inputs cause the program to continue unimpeded.