Util.hs

-----------------------------------------------------------------------------
--
-- Module      :  Data.Attoparsec.Util
-- Copyright   :  (c) 2012-16 Brian W Bush
-- License     :  MIT
--
-- Maintainer  :  Brian W Bush <consult@brianwbush.info>
-- Stability   :  Stable
-- Portability :  Portable
--
-- | Utilities related to the <https://hackage.haskell.org/package/attoparsec attoparsec> package.
--
-----------------------------------------------------------------------------


{-# LANGUAGE Safe          #-}
{-# LANGUAGE TupleSections #-}


module Data.Attoparsec.Util (
  double'
) where


import Control.Applicative ((<|>), many)
import Data.Attoparsec.Text (Parser, char, digit, inClass, many1, satisfy, space)


-- | Parse a double.
double' :: Parser Double
double' = many space *> parseSigned parseFloat'


parseSigned :: Real a => Parser a -> Parser a
parseSigned p = (negate <$> (char '-' *> p)) <|> p


parseFloat' :: (RealFrac a) => Parser a
parseFloat' =
  do
    (x, y) <-
      ((,) <$> many1 digit) <* char '.' <*> many digit
      <|>
      (("0", ) <$ char '.' <*> many1 digit)
      <|>
      ((, "") <$> many1 digit)
    expo <-
      (satisfy (inClass "eE")
        *> (
          (char '+' *> many1 digit)
          <|>
          (((:) <$> char '-') <*> many1 digit)
          <|>
          many1 digit)
      )
      <|>
      return "0"
    return
      $ fromRational
      $ toInt (x ++ y) * 10^^(toInt' expo - length y)


toInt :: RealFrac a => String -> a
toInt = fromIntegral . toInt'


toInt' ::String -> Int
toInt' = read