Befunge-93

A purely functional Befunge-93 interpreter, written in final tagless style. Showcasing the beauty and power of compositional design.

The interpreter

Given a Befunge-93 program, usually expressed as a multiline string

val helloWorld = 
    """
>              v
v  ,,,,,"Hello"<
>48*,          v
v,,,,,,"World!"<
>25*,@
   """

you can run it to completion by calling befunge.run(helloWorld), or step through it in debug mode with befunge.debug(helloWorld). Note that both return a cats.effect.IO], which describes their behaviour in a purely functional manner. You can compose the IO computations in a larger program, or, if you want to actually trigger their execution because you are in main or in the repl, call .unsafeRunSync on them.

The befunge object also contains some example programs for you to try out.

The language

Befunge-93 is a stack-based, reflective, bidimensional esoteric programming language. The spec is here

A short table of the commands is as follows:

Command Summary

COMMAND         INITIAL STACK (bot->top)RESULT (STACK)
-------         -------------           -----------------
+ (add)         <value1> <value2>       <value1 + value2>
- (subtract)    <value1> <value2>       <value1 - value2>
* (multiply)    <value1> <value2>       <value1 * value2>
/ (divide)      <value1> <value2>       <value1 / value2> (nb. integer)
% (modulo)      <value1> <value2>       <value1 mod value2>
! (not)         <value>                 <0 if value non-zero, 1 otherwise>
` (greater)     <value1> <value2>       <1 if value1 > value2, 0 otherwise>
> (right)                               PC -> right
< (left)                                PC -> left
^ (up)                                  PC -> up
v (down)                                PC -> down
? (random)                              PC -> right? left? up? down? ???
_ (horizontal if) <boolean value>       PC->left if <value>, else PC->right
| (vertical if)   <boolean value>       PC->up if <value>, else PC->down
" (stringmode)                          Toggles 'stringmode'
: (dup)         <value>                 <value> <value>
\ (swap)        <value1> <value2>       <value2> <value1>
$ (pop)         <value>                 pops <value> but does nothing
. (pop)         <value>                 outputs <value> as integer
, (pop)         <value>                 outputs <value> as ASCII
# (bridge)                              'jumps' PC one farther; skips
                                        over next command
g (get)         <x> <y>                 <value at (x,y)>
p (put)         <value> <x> <y>         puts <value> at (x,y)
& (input value)                         <value user entered>
~ (input character)                     <character user entered>
@ (end)                                 ends program