Brainfuck language transpiler to Emacs Lisp

The brainfuck language transpiler to Emacs Lisp consists of ebf macro which expands to the actual Emacs Lisp code. Here is the signature of this macro:

(ebf INPUT-CALLBACK OUTPUT-CALLBACK &rest INSTRUCTIONS)

INPUT-CALLBACK is called on comma instruction and should have zero arguments and return a number.

OUTPUT-CALLBACK is called on dot instruction and should have one argument of an integer type.

INSTRUCTIONS is a list of symbols and vectors of symbols. Vectors are accepted so we don’t need to escape square brackets of our brainfuck program. Symbols’ names should be sequences of valid brainfuck instructions excluding square brackets.

Evaluation of the macro expansion causes the brainfuck program execution.

So the code

(ebf input output \,+++[->+<].)

will be expanded to

(let ((MEMORY68087 (make-vector 100 0))
      (POINTER68088 0)
      (INPUT68085 input)
      (OUTPUT68086 output))
  (aset MEMORY68087 POINTER68088 (funcall INPUT68085))
  (cl-incf (aref MEMORY68087 POINTER68088) 3)
  (while (not (zerop (aref MEMORY68087 POINTER68088)))
    (cl-decf (aref MEMORY68087 POINTER68088) 1)
    (cl-incf POINTER68088 1)
    (while (<= (length MEMORY68087) POINTER68088)
      (let ((memory-length (length MEMORY68087)))
        (setq MEMORY68087
              (vconcat MEMORY68087
                       (make-vector
                        (max 1 (/ memory-length 2))
                        0)))))
    (cl-incf (aref MEMORY68087 POINTER68088) 1)
    (cl-decf POINTER68088 1))
  (funcall OUTPUT68086 (aref MEMORY68087 POINTER68088)))

We collapse several instructions in a row and automatically expand the memory to the right. We plan to add more optimization in the future.

Here is the classical Hello World example with some output:

(require 'ebf)

(let ((result nil))
  (ebf nil #'(lambda (x) (push x result))
       ++++++++++[>+++++++>++++++++++>+++>+<<<<-]>++
       .>+.+++++++..+++.>++.<<+++++++++++++++.>.+++.
       ------.--------.>+.>.)
  (apply #'string (reverse result)));<- put cursor here and press C-x C-e

ebf macro produces the code that doesn’t depend on ebf module itself. That means we can byte-compile our brainfuck programs so they will not require ebf at runtime.

Check Macros and Byte Compilation section of the official Emacs Lisp manual on how to do that.

Basically we will need to wrap our (require 'ebf) with eval-when-compile like

(eval-when-compile
 (require 'ebf))

Copyright (C) 2015 Alexey Kutepov a.k.a rexim

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