57 lines
2.0 KiB
Markdown
57 lines
2.0 KiB
Markdown
# Brainfuck2Scheme
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A simple compiler from Brainfuck to R5RS. The compiler will output a Scheme
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list that is a lambda with three arguments:
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* `data`: The data vector.
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* `dptr`: The initial data pointer.
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* `debugger`: Debugger function
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To turn the list into an executable Scheme function, just give it to
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`eval`. `(execute scheme len)` will run the Scheme code in `scheme`
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with a data vector of length `len`.
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This dialect of Brainfuck supports `#` to call the `debugger` procedure
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with `data` and `dptr` as arguments.
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Since Brainfuck programs become Scheme procedures, you can modularize
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Brainfuck code and (ab)use the debugger for things like procedure calls
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and foreign libraries.
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## How It Works
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Brainfuck is a very simple Harvard architecture computer. The data is
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stored as a vector `data` and the data pointer is an integer `dptr`.
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The big idea is that all the code is compiled to a big lambda form, but
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there are some wrinkles due to jumps.
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Data access brainfuck instructions are translated like
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* `+` -> `(vector-set! data dptr (+ 1 (vector-ref data dptr)))`
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* `-` -> `(vector-set! data dptr (+ -1 (vector-ref data dptr)))`
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* `>` -> `(set! dptr (+ dptr 1))`
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* `<` -> `(set! dptr (- dptr 1))`
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* `.` -> `(display (vector-ref data dptr))`
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* `,` -> `(vector-set! data dptr (read-char))`
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* `#` -> `(debugger data dptr)`
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Branches are trickier. Basically, all code that will be executed in a block
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is in a lambda. Given `[code...]`, the `code...` will be compiled to a
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lambda in a `letrec`, with a conditional at the end that will tail-call the
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lambda if the current data pointer is not zero.
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The transformation then goes like
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`[ CODE ... ] REST ...` ->
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(letrec ((between (lambda ()
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(TRANSLATE CODE ...)
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(if (not (zero? vector-ref data dptr)
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(between))))))
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(if (not (zero? (vector-ref data dptr)))
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(between)))
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(TRANSLATE REST ...)
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where `(TRANSLATE CODE ...)` translates `CODE` to Scheme instructions
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like above.
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