＿
- The title of this article is not correct because of technical limitations. The correct title is actually _.
_ is an esolang by User:PythonshellDebugwindow.
Commands
| Command | Effect |
|---|---|
% |
Pop a character from the end of the source code |
$ |
Pop a character from the end of the source code and set the next character in the source to the popped character |
^ |
Skip the next command if the last character in the source code is 0
|
\ |
Append the next character to the end of the source code and skip it |
| Anything else | Append the character to the end of the source code |
Examples
Infinite loop
0
Truth-machine
Replace 1 with 0 for input 0.
^1
Computational class
_ is a finite-state automaton.
Let's take the instruction pointer's distance away from the end of the program (i.e. the amount of commands left to run) over the course of a program's execution, and call it k. A program begins with k as the program's length, and the program halts when k hits 0 (or less, in cases such as % being the final instruction). The infinite loop always has a k of 1. Note that only the last k commands of a program matter, since there's no way to access commands that the instruction pointer has already passed.
Now let's consider how each instruction affects k. In each snippet, the last k commands of a program are underlined, i.e. the instruction pointer is at the first underlined command.
% |
$ |
^ (skip) |
^ (no skip) |
\ |
Anything else |
|---|---|---|---|---|---|
...%...x |
...$x...y |
...^x...0 |
...^x...1 |
...\x... |
...x...
|
...%... |
...$y... |
...^x...0 |
...^x...1 |
...\x...x |
...x...x
|
| k → k-2 | k → k-2 | k → k-2 | k → k-1 | k → k-1 | k → k |
Notice that k never increases during the course of the program.
Since every non-command other than 0 cannot be distinguished by the program, a program with a certain k is restricted to one of 6k+6k-1+6k-2+...+62+61+60 states for the rest of its execution. Since a program can only start with a finite k, every _ program must be able to be simulated using only a finite amount of states.
External resources
- An interpreter on GitHub