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Brain-Flak is an "Turing-tarpit", e.g. a language which can, in theory compute anything, but in reality is very inconvenient and painful to use. It was heavily inspired by Brainf**k, the original turing-tarpit.

To get started with Brain-Flak, download the project from here, and run ruby brain_flak.rb inputs. Additionally, you can try it online! (Online intrepreter provided thanks to @DennisMitchell)


Brain-Flak has two stacks, known as 'left' and 'right'. The active stack starts at left. If an empty stack is popped, it will return 0. That's it. No other variables. When the program starts, each command line argument is pushed on to the active stack.

The only valid characters in a Brain-Flak program are ()[]{}<>, and they must always be balanced. There are two types of functions: Nilads and Monads. A nilad is a function that takes 0 arguments. Here are all of the nilads:

  • () Evaluates to one.
  • [] Evaluates to the height of the current stack.
  • {} Pop the active stack. Evaluates to the popped value.
  • <> Toggle the active stack. Evaluates to zero.

These commands are added together when they are evaluated. So if we had a '3' on top of the active stack, this snippet:


would evaluate to 1 + 1 + active.pop() which would evaluate to 5.

The monads take one argument, a chunk of Brain-Flak code. Here are all of the monads:

  • (n) Push 'n' on the active stack.
  • [n] Evaluates to negative 'n'
  • {foo} While zero is not on the top of the stack, do foo.
  • <foo> Execute foo, but evaluate it as 0.

The (...) monad will also evaluate to its argument, so


Will push 3 but


Will push 3 twice.

The {...} monad will evaluate to the sum of all runs. So if we had '3' and '4' on the top of the stack:


would evaluate as 7.

When the program is done executing, each value left on the active stack is printed, with a newline between. Values on the other stack are ignored.

That's it. That's the whole language.

Sample code

Here are some full programs that do interesting things.

Adding two numbers:


Multiplying two numbers (Positive only):


Square a number (Positive only):


Print the first N Fibonacci numbers: