@raichoo reports a nasty bug related to text (it's still unclear what exactly is going on here, although I suspect text's RULE framework to be the culprit -- I hope so at least, as the alternative would mean that several GHC releases out there may generate wrong code):

{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE BangPatterns #-}
module Main where

import Prelude hiding (length, filter)
import Data.Text

currencyParser :: Text -> Bool
currencyParser x = cond == 1
  where
    cond = length fltr
    fltr = filter (== ',') x

main :: IO ()
main = print (currencyParser "0,00")

when compiling this with -O0 or running in GHCi, the proper True output is generated. However, when compiling with -O1 the wrong False output is generated.