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Brainf**k Interpreter
making an interpreter for the Brainf**k programming language
Created ~ 2022-05
Tech Used ~ python

Basic steps of creating a programming language

The phases of an interpreter and their subsequent tools are:

  1. lexical analysis (lexer)
  2. semantic analysis (parser)
  3. evaluation (evaluator)

I learned this basic framework from Andy Balaam’s tutorial with the Cell programming language:

  1. Intro and Lexer
  2. Parser
  3. Evaluator

My general review of this tutorial: Gets a bit hard to follow if you don’t read the code carefully (I would suggest code it and then go through it line by line), a few missing lines of code (will need to refer to his GitLab repository for them), and missing detail regarding native Python functions in Cell so the code won’t work at the end lol. But it gives you a sense of how the code of an interpreter should look like. Overall good tutorial if you’re getting started, I essentially based the BF interpreter code off of this tutorial!

Let’s tell it in a form of a story (note: all the outputs are based off the tutorial which is written in Python, but it is not for any specific language).

  1. You have program.
    x = 3 + 4
  2. You feed program file to lexer, lexer reads character by character, lexer generates tokens.
    Lexer ouputs [('symbol','x'), ('=',''), ('number',3), ('operation', '+'), ('number', 4)]
  3. You feed tokens to parser, parser reads token by token (kind of), parser generates abstract syntax tree (in this case, nested tuples). You can also view it as an actual tree:
    Parser outputs [('assignment', ('symbol','x'), ('operation','+',('number',3), ('number',4)))]
    Paint MS skills on point.
  4. Evaluator goes through the abstract syntax tree and execute the code, using environments to store symbols and their values.

So that’s essentially what the interpreter does, at least what the tutorial shows.

Brainkf**k Programming Language

What makes this language so shocking is that it only compromises of 8 operations each symbolized by only one character. Read more about this language on this wiki.

How this BF interpreter works


Since BF only has 8 operations ( ><+-.,[] ), the lexer retains that simplicity.

# code omitted

match char:
    case '>':
        yield ('shift_right')
    case '<':
        yield ('shift_left')
    case '+':
        yield ('increment')
    case '-':
        yield ('decrement')
    case '.':
        yield ('output')
    case ',':
        yield ('input')
    case '[':
        yield ('start_while')
    case ']':
        yield ('end_while')
    case _:

I could make it like ('shift', 'right'), ('shift', 'left') but it that would just make the code more complicated then it needs to be. I figure the only case where that would be necessary is if you have tokens that have an infinite amount of lexemes, or strings. For instance, the tokens 'number' or 'string'.


The way the Cell parser works is by looking at the previous token to figure out the meaning of the code (and thus determine whether it is valid). In BF’s case, there’s not much to check except the while symbols [ and ].

# code omitted
match typ:

    case 'start_while':
        expressions_list = self._get_multiple_expressions()
        return ('while',expressions_list)

    case 'end_while':
        raise Exception(" ']' does not have matching '[' ")

    case _:
        return (typ,)
# code omitted

So what the code does is it spots the start_while token and then uses a function called _get_multiple_expressions() which continues reading through the tokens to get the expressions within the while loop and stops at end_while token. The code throws exceptions if there is a mismatch with the [ and ]. As for all the other tokens besides the while symbols, it just returns it but in tuple form.


Since there is no concept of scope in BF, there is no need for an environment class. Instead the evaluator is itself a class that will simulate a BF runtime environment.

The BF runtime environment has two things:

  1. An array of size 30,000
  2. A pointer/index
class Evaluator:

    def __init__(self):
        self.arr = [0] * 30000
        self.index = 0

    # code omitted

As the evaluator goes through each expression in the abstract syntax tree that was generated by the parser, it executes the corresponding Python code that would manipulate the values of the arr and index attributes.

# code omitted
match typ:

    case 'shift_right':
        self.index += 1

    case 'shift_left':
        if self.index != 0:
            self.index -= 1
            raise Exception('Negative index')

    case 'increment':
        self.arr[self.index] += 1

    case 'decrement':
        self.arr[self.index] -= 1

    case 'output':
        print(chr(self.arr[self.index]), end='')

    case 'input':
        entered = ''
        while len(entered) != 1:
            entered = input('\nInput[one character only]: ')
        self.arr[self.index] = ord(entered)

    case 'while':
        while self.arr[self.index] != 0:
            expression_list = expr[1]
# code omitted

And that’s it! That was a broad overview of how the BF interpreter works.

View it in action