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import Data.List (elemIndex)
type Parser = String -> Either String String
parseMolekyl :: Parser
parseMolekyl cs = parseAtom cs >>= parseNum
parseAtom :: Parser
parseAtom cs = parseBig cs >>= parseSmall
parseBig :: Parser
parseBig (c : cs) =
if c `elem` "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
then Right cs
else Left $ "Saknad stor bokstav vid radslutet " ++ (c : cs)
parseBig "" = Left "Saknad stor bokstav vid radslutet "
parseSmall :: Parser
parseSmall (c : cs) =
Right $
if c `elem` "abcdefghijklmnopqrstuvwxyz"
then cs
else c : cs
parseSmall "" = Right ""
parseNum :: Parser
parseNum "" = Right ""
parseNum cs =
let (x, cs') = parseNum' 0 cs
in if x >= 2
then Right cs'
else Left ("För litet tal vid radslutet " ++ cs')
where
parseNum' n (c : cs) = case c `elemIndex` "0123456789" of
Just x -> parseNum' (n * 10 + x) cs
Nothing -> (n, c : cs)
parseNum' n "" = (n, "")
output (Right _) = "Formeln är syntaktiskt korrekt"
output (Left s) = s
main = interact (unlines . map (output . parseMolekyl) . takeWhile (/= "#") . lines)
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