;;; Brainfuck for GNU Guile
;; Copyright (C) 2009 Free Software Foundation, Inc.
;; This library is free software; you can redistribute it and/or
;; modify it under the terms of the GNU Lesser General Public
;; License as published by the Free Software Foundation; either
;; version 3 of the License, or (at your option) any later version.
;;
;; This library is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;; Lesser General Public License for more details.
;;
;; You should have received a copy of the GNU Lesser General Public
;; License along with this library; if not, write to the Free Software
;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
;; 02110-1301 USA
;;; Commentary:
;; Brainfuck is a simple language that mostly mimics the operations of a
;; Turing machine. This file implements a compiler from Brainfuck to
;; Guile's Tree-IL.
;;; Code:
(define-module (language brainfuck compile-tree-il)
#:use-module (system base pmatch)
#:use-module (language tree-il)
#:export (compile-tree-il))
;; Compilation of Brainfuck is pretty straight-forward. For all of
;; brainfuck's instructions, there are basic representations in Tree-IL
;; we only have to generate.
;;
;; Brainfuck's pointer and data-tape are stored in the variables pointer and
;; tape, where tape is a vector of integer values initially set to zero. Pointer
;; starts out at position 0.
;; Our tape is thus of finite length, with an address range of 0..n for
;; some defined upper bound n depending on the length of our tape.
;; Define the length to use for the tape.
(define tape-size 30000)
;; This compiles a whole brainfuck program. This constructs a Tree-IL
;; code equivalent to Scheme code like this:
;;
;; (let ((pointer 0)
;; (tape (make-vector tape-size 0)))
;; (begin
;; <body>
;; (write-char #\newline)))
;;
;; So first the pointer and tape variables are set up correctly, then the
;; program's body is executed in this context, and finally we output an
;; additional newline character in case the program does not output one.
;;
;; The fact that we are compiling to Guile primitives gives this
;; implementation a number of interesting characteristics. First, the
;; values of the tape cells do not underflow or overflow. We could make
;; them do otherwise via compiling calls to "modulo" at certain points.
;;
;; In addition, tape overruns or underruns will be detected, and will
;; throw an error, whereas a number of Brainfuck compilers do not detect
;; this.
;;
;; Note that we're generating the S-expression representation of
;; Tree-IL, then using parse-tree-il to turn it into the actual Tree-IL
;; data structures. This makes the compiler more pleasant to look at,
;; but we do lose is the ability to propagate source information. Since
;; Brainfuck is so obtuse anyway, this shouldn't matter ;-)
;;
;; `compile-tree-il' takes as its input the read expression, the
;; environment, and some compile options. It returns the compiled
;; expression, the environment appropriate for the next pass of the
;; compiler -- in our case, just the environment unchanged -- and the
;; continuation environment.
;;
;; The normal use of a continuation environment is if compiling one
;; expression changes the environment, and that changed environment
;; should be passed to the next compiled expression -- for example,
;; changing the current module. But Brainfuck is incapable of that, so
;; for us, the continuation environment is just the same environment we
;; got in.
;;
;; FIXME: perhaps use options or the env to set the tape-size?
(define (compile-tree-il exp env opts)
(values
(parse-tree-il
`(let (pointer tape) (pointer tape)
((const 0)
(apply (primitive make-vector) (const ,tape-size) (const 0)))
,(compile-body exp)))
env
env))
;; Compile a list of instructions to a Tree-IL expression.
(define (compile-body instructions)
(let lp ((in instructions) (out '()))
(define (emit x)
(lp (cdr in) (cons x out)))
(cond
((null? in)
;; No more input, build our output.
(cond
((null? out) '(void)) ; no output
((null? (cdr out)) (car out)) ; single expression
(else `(begin ,@(reverse out)))) ; sequence
)
(else
(pmatch (car in)
;; Pointer moves >< are done simply by something like:
;; (set! pointer (+ pointer +-1))
((<bf-move> ,dir)
(emit `(set! (lexical pointer)
(apply (primitive +) (lexical pointer) (const ,dir)))))
;; Cell increment +- is done as:
;; (vector-set! tape pointer (+ (vector-ref tape pointer) +-1))
((<bf-increment> ,inc)
(emit `(apply (primitive vector-set!) (lexical tape) (lexical pointer)
(apply (primitive +)
(apply (primitive vector-ref)
(lexical tape) (lexical pointer))
(const ,inc)))))
;; Output . is done by converting the cell's integer value to a
;; character first and then printing out this character:
;; (write-char (integer->char (vector-ref tape pointer)))
((<bf-print>)
(emit `(apply (primitive write-char)
(apply (primitive integer->char)
(apply (primitive vector-ref)
(lexical tape) (lexical pointer))))))
;; Input , is done similarly, read in a character, get its ASCII
;; code and store it into the current cell:
;; (vector-set! tape pointer (char->integer (read-char)))
((<bf-read>)
(emit `(apply (primitive vector-set!)
(lexical tape) (lexical pointer)
(apply (primitive char->integer)
(apply (primitive read-char))))))
;; For loops [...] we use a letrec construction to execute the body until
;; the current cell gets zero. The body is compiled via a recursive call
;; back to (compile-body).
;; (let iterate ()
;; (if (not (= (vector-ref! tape pointer) 0))
;; (begin
;; <body>
;; (iterate))))
;;
;; Indeed, letrec is the only way we have to loop in Tree-IL.
;; Note that this does not mean that the closure must actually
;; be created; later passes can compile tail-recursive letrec
;; calls into inline code with gotos. Admittedly, that part of
;; the compiler is not yet in place, but it will be, and in the
;; meantime the code is still reasonably efficient.
((<bf-loop> . ,body)
(let ((iterate (gensym)))
(emit `(letrec (iterate) (,iterate)
((lambda ()
(lambda-case
((() #f #f #f () ())
(if (apply (primitive =)
(apply (primitive vector-ref)
(lexical tape) (lexical pointer))
(const 0))
(void)
(begin ,(compile-body body)
(apply (lexical ,iterate)))))
#f)))
(apply (lexical ,iterate))))))
(else (error "unknown brainfuck instruction" (car in))))))))