type
value
type
value
...

0 -- code for a string value
hello
70 -- code for an int value
10
0
world
20 -- code for a double value
5.20

data KeyValue = forall a. (Typeable a, Show a, Eq a) => KeyValue (Int, a)
instance Eq KeyValue where -- as I need to test equality
    KeyValue (code1, value1) == KeyValue (code2, value2) =
      code1 == code2 && case cast value2 of
                               Just value2' -> value1 == value2
                               Nohing -> False

parser :: Parser [KeyValue]
parser = many' (keyValue <* endOfLine)

keyValue :: Parser KeyValue
keyValue = do
    key <- decimal <* endOfLine
    value <- case key of
               0  -> takeLine
               20 -> double
               70 -> decimal
               _ -> takeLine
    return $ KeyValue (key, value)

takeLine :: Parser Text
takeLine = takeTill isEndOfLine

Couldn't match type Double with Text
Expected type: Parser Text Text
  Actual type: Parser Double
In the expression: double
In a case alternative: 20 -> double