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module Parser(parseProgram) where
import AST
parseProgram :: String -> Either String Program
parseProgram src = do
(inss, "") <- parseLoop 0 src
return $ Program $ IStart : inss
parseLoop :: Int -> String -> Either String ([Instruction], String)
parseLoop 0 "" = Right ([], "")
parseLoop _ "" = Left "More '[' than ']'"
parseLoop 0 (']':_) = Left "Unexpected ']'"
parseLoop _ (']':cs) = Right ([], cs)
parseLoop d ('[':cs) = do
(lp, rest) <- parseLoop (d+1) cs
(after, rest') <- parseLoop d rest
return (ILoop lp 0 : after, rest')
parseLoop d (c:cs)
| Just ins <- parseSimple c = do
(lp, rest) <- parseLoop d cs
return (ins:lp, rest)
parseLoop d (_:cs) = parseLoop d cs
parseSimple :: Char -> Maybe Instruction
parseSimple '+' = Just $ IAdd 1 0
parseSimple '-' = Just $ IAdd (-1) 0
parseSimple '>' = Just $ ISlide 1
parseSimple '<' = Just $ ISlide (-1)
parseSimple ',' = Just $ IInput 0
parseSimple '.' = Just $ IOutput 0
parseSimple _ = Nothing
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