Util.hs

{- This file is part of funbot.
 -
 - Written in 2015 by fr33domlover <fr33domlover@rel4tion.org>.
 -
 - ♡ Copying is an act of love. Please copy, reuse and share.
 -
 - The author(s) have dedicated all copyright and related and neighboring
 - rights to this software to the public domain worldwide. This software is
 - distributed without any warranty.
 -
 - You should have received a copy of the CC0 Public Domain Dedication along
 - with this software. If not, see
 - <http://creativecommons.org/publicdomain/zero/1.0/>.
 -}

module FunBot.Util
    ( (!?)
    , replaceMaybe
    , passes
    , passesBy
    )
where

import Data.Maybe (listToMaybe)
import FunBot.Types

-- | List index operator, starting from 0. Like @!!@ but returns a 'Maybe'
-- instead of throwing an exception. On success, returns 'Just' the item. On
-- out-of-bounds index, returns 'Nothing'.
(!?) :: [a] -> Int -> Maybe a
l !? i = listToMaybe $ drop i l

-- | Replace the list item at the given position, with the given new item.
-- Return the resulting list. If the position is out of range, return
-- 'Nothing'.
replaceMaybe :: [a] -> Int -> a -> Maybe [a]
replaceMaybe l i y =
    case splitAt i l of
        (_, [])   -> Nothing
        (b, x:xs) -> Just $ b ++ y : xs

-- | Check whether a value passes a given filter.
passes :: Eq a => a -> Filter a -> Bool
v `passes` (Accept l) = v `elem` l
v `passes` (Reject l) = v `notElem` l

-- | Like 'passes', but using a given predicate to compare items.
passesBy :: (a -> a -> Bool) -> a -> Filter a -> Bool
passesBy p v (Accept l) = any (p v) l
passesBy p v (Reject l) = all (not . p v) l