An Interpreter of the language Befunge-93 written in Python. This language is an esoteric language that was created to demonstrate the difficulty of creating a compiller. Also, the language is simple because it only has a small set of commands, but they are a bit different from commom languages, so the code is weird.

The language is made up of a stack, so you don't store values in variables, you push/pop values into a global stack. In commom language each command has an order and you run the commands one after the other, in a single order. But in Befunge, the code are placed in a 2D grid, so you have four different execution flows, from left to right, from right to left, bottom to top and top to bottom. The below program prints 1 using the normal flow and then prints 2 following the top to down flow.

1 . v
    2
    .
    @

If you want to know how the language work and how to program on it, see the following documentations:

• Befunge - Esolang
• Befunge - Specification

To run a Befunge program you should have an python runtime in your computer. The following command run a "hello world" program:

$ python run.py examples/hello_world.bf

The above code is(examples/hello_world.bf):

64+"!dlroW ,olleH">:#,_@

Jump to the examples/ directory to see more code examples.

The dificult of implementation lies in the self-modification property of the language, this means that the source-code can change itself in runtime, so how to compile a code that only exists in runtime? It's a dificult task. Here's an example of code that change itself:

$ cat examples/self_changing_code.bf 

2  v                         
v  <                      
>> 1+: ~ \ 7p : 28*1+ `v                
                       !    
 ^                     _$v     
v                        <   
   @@@@@@@@@@@@@@@@@   
>#@XXXXXXXXXXXXXXXX@  
   @@@@@@@@@@@@@@@@@   


The X's will be replaced by the commands inserted by the user in runtime.

This code will ask the user for a sequence of commands that will be placed in the X's above, when the user fills all X's the program will direct the flow to the commands entered by the user.

The interpreter support all commands of Befunge 93.

The original specification of Befunge-93 have a fixed limit of cells, which is 80x25. However, this interpreter don't establish a fixed limit of cells, the code can be more than 80x25, thus it's a turing complete implementation.

The language specification and all the commands can be found here: Befunge-93.