ReadP

型合わせただけのレベルだけども。

% runghc
module Main where
import Text.ParserCombinators.ReadP

degit :: ReadP Char
degit = foldl (+++) (char '0') (map char "123456789")

main :: IO ()
main = do
  let parser = do seisu <- many degit
                  char '.'
                  syosu <- many degit
                  eof
                  return $ seisu ++ '.' : syosu
  let parsed = readP_to_S parser "123.456"
  print parsed

【実行結果】
[("123.456","")]