module Util (justWhen,whenNothing,(<.>),deleteByIndices,breakAt
            ,maybeRead,stripWhitespace)
    where

import Control.Applicative ((<$>))
import Control.Exception (assert)
import Data.List (sort)
import Data.Char (isSpace)

justWhen :: Monad m => Maybe a -> (a -> m ()) -> m ()
justWhen maybeThis doThis = case maybeThis of
                              Nothing -> return ()
                              Just a -> doThis a

whenNothing :: Monad m => Maybe a -> m () -> m ()
whenNothing maybeThis doThis = case maybeThis of
                                 Nothing -> doThis
                                 _ -> return ()
    
(<.>) :: Functor f => (a -> b) -> (c -> f a) -> c -> f b
(<.>) p q arg = p <$> (q arg)

deleteByIndices :: [Int] -> [a] -> [a]
deleteByIndices indices =
    let deleteByIndex (_,l) ([],list)  = ([],l:list)
        deleteByIndex (lIndex,l) (i:is,list) =
            if lIndex == i then (is,list)
            else (i:is,l:list)
    in
      assert (indices == (reverse $ sort indices)) . 
             snd . foldr deleteByIndex (indices,[]) . zip [0..]

breakAt :: (Eq a) => a -> [a] -> [[a]]
breakAt partition list =  
    case dropWhile ((==) partition) list of
      [] -> []
      list' -> element : (breakAt partition list'')
        where (element,list'') = break ((==) partition) list'

maybeRead :: (Read a) => String -> Maybe a
maybeRead string = case reads string of
                     [(a,[])] -> Just a
                     _ -> Nothing

stripWhitespace :: String -> String
stripWhitespace = takeWhile (not . isSpace) . dropWhile isSpace