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I'm writing a brainfuck interpreter in Haskell, and I came up with what I believe to be a very interesting description of a program: data Program m = Instruction (m ()) (Program m) | Control (m (Program m)) | Halt However, it's tricky to parse a textual representation of a brainfuck program into this data type. The problem arises with trying to correctly parse square brackets, because there is some knot-tying to do so that the final Instruction inside a loop links to the loop's Control again. A bit more preliminary information. See this version on the github repo for all the details. type TapeM = StateT Tape IO type TapeP = Program TapeM type TapeC = Cont TapeP branch :: Monad m => m Bool -> Program m -> Program m -> Program m branch cond trueBranch falseBranch = Control ((\b -> if b then trueBranch else falseBranch) `liftM` cond) loopControl :: TapeP -> TapeP -> TapeP loopControl = branch (not <$> is0) Here's what I tried: toProgram :: String -> TapeP toProgram = (`runCont` id) . toProgramStep liftI :: TapeM () -> String -> TapeC TapeP liftI i cs = Instruction i <$> toProgramStep cs toProgramStep :: String -> TapeC TapeP toProgramStep ('>':cs) = liftI right cs -- similarly for other instructions toProgramStep ('[':cs) = push (toProgramStep cs) toProgramStep (']':cs) = pop (toProgramStep cs) push :: TapeC TapeP -> TapeC TapeP push mcontinue = do continue <- mcontinue cont (\breakMake -> loopControl continue (breakMake continue)) pop :: TapeC TapeP -> TapeC TapeP pop mbreak = do break <- mbreak cont (\continueMake -> loopControl (continueMake break) break) I figured I could somehow use co

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