Utilities.hs

module GF.Data.Utilities(module GF.Data.Utilities) where

import Data.Maybe
import Data.List
import Control.Monad (MonadPlus(..),liftM,when)
import qualified Data.Set as Set

-- * functions on lists

sameLength :: [a] -> [a] -> Bool
sameLength [] [] = True
sameLength (_:xs) (_:ys) = sameLength xs ys
sameLength _ _ = False

notLongerThan, longerThan :: Int -> [a] -> Bool
notLongerThan n = null . snd . splitAt n
longerThan    n = not . notLongerThan n

lookupList :: Eq a => a -> [(a, b)] -> [b]
lookupList a [] = []
lookupList a (p:ps) | a == fst p = snd p : lookupList a ps
                    | otherwise  =         lookupList a ps

split :: [a] -> ([a], [a])
split (x : y : as) = (x:xs, y:ys)
    where (xs, ys) = split as
split as = (as, [])

splitBy :: (a -> Bool) -> [a] -> ([a], [a])
splitBy p [] = ([], [])
splitBy p (a : as) = if p a then (a:xs, ys) else (xs, a:ys)
    where (xs, ys) = splitBy p as

foldMerge :: (a -> a -> a) -> a -> [a] -> a
foldMerge merge zero = fm
    where fm [] = zero
          fm [a] = a
          fm abs = let (as, bs) = split abs in fm as `merge` fm bs

select :: [a] -> [(a, [a])]
select [] = []
select (x:xs) = (x,xs) : [ (y,x:ys) | (y,ys) <- select xs ]

updateNth :: (a -> a) -> Int -> [a] -> [a]
updateNth update 0 (a : as) = update a : as
updateNth update n (a : as) = a : updateNth update (n-1) as

updateNthM :: Monad m => (a -> m a) -> Int -> [a] -> m [a]
updateNthM update 0 (a : as) = liftM (:as) (update a)
updateNthM update n (a : as) = liftM (a:)  (updateNthM update (n-1) as)

-- | Like 'init', but returns the empty list when the input is empty.
safeInit :: [a] -> [a]
safeInit [] = []
safeInit xs = init xs

-- | Sorts and then groups elements given an ordering of the
--   elements.
sortGroupBy :: (a -> a -> Ordering) -> [a] -> [[a]]
sortGroupBy f = groupBy (compareEq f) . sortBy f

-- | Take the union of a list of lists.
unionAll :: Eq a => [[a]] -> [a]
unionAll = nub . concat

-- | Like 'lookup', but fails if the argument is not found,
--   instead of returning Nothing.
lookup' :: (Show a, Eq a) => a -> [(a,b)] -> b
lookup' x = fromMaybe (error $ "Not found: " ++ show x) . lookup x

-- | Like 'find', but fails if nothing is found.
find' :: (a -> Bool) -> [a] -> a
find' p = fromJust . find p

-- | Set a value in a lookup table.
tableSet :: Eq a => a -> b -> [(a,b)] -> [(a,b)]
tableSet x y [] = [(x,y)]
tableSet x y (p@(x',_):xs) | x' == x = (x,y):xs
                           | otherwise = p:tableSet x y xs

-- | Group tuples by their first elements.
buildMultiMap :: Ord a => [(a,b)] -> [(a,[b])]
buildMultiMap = map (\g -> (fst (head g), map snd g) )
                 . sortGroupBy (compareBy fst)

-- * equality functions

-- | Use an ordering function as an equality predicate.
compareEq :: (a -> a -> Ordering) -> a -> a -> Bool
compareEq f x y = case f x y of
                             EQ -> True
                             _ -> False

-- * ordering functions

compareBy :: Ord b => (a -> b) -> a -> a -> Ordering
compareBy f = both f compare

both :: (a -> b) -> (b -> b -> c) -> a -> a -> c
both f g x y = g (f x) (f y)

-- * functions on pairs

apFst :: (a -> a') -> (a, b) -> (a', b)
apFst f (a, b) = (f a, b)

apSnd :: (b -> b') -> (a, b) -> (a, b')
apSnd f (a, b) = (a, f b)

apBoth :: (a -> b) -> (a, a) -> (b, b)
apBoth f (x, y) = (f x, f y)

-- * functions on lists of pairs

mapFst = map . apFst
mapSnd = map . apSnd
mapBoth = map . apBoth

-- * functions on monads

-- | Return the given value if the boolean is true, els return 'mzero'.
whenMP :: MonadPlus m => Bool -> a -> m a
whenMP b x = if b then return x else mzero

whenM bm m = flip when m =<< bm

repeatM m = whenM m (repeatM m)

-- * functions on Maybes

-- | Returns true if the argument is Nothing or Just []
nothingOrNull :: Maybe [a] -> Bool
nothingOrNull = maybe True null

-- * functions on functions

-- | Apply all the functions in the list to the argument.
foldFuns :: [a -> a] -> a -> a
foldFuns fs x = foldl (flip ($)) x fs

-- | Fixpoint iteration.
fix :: Eq a => (a -> a) -> a -> a
fix f x = let x' = f x in if x' == x then x else fix f x'

-- * functions on strings

-- | Join a number of lists by using the given glue
--   between the lists.
join :: [a] -- ^ glue
     -> [[a]] -- ^ lists to join
     -> [a]
join g = concat . intersperse g

-- * ShowS-functions

nl :: ShowS
nl = showChar '\n'

sp :: ShowS
sp = showChar ' '

wrap :: String -> ShowS -> String -> ShowS
wrap o s c = showString o . s . showString c

concatS :: [ShowS] -> ShowS
concatS = foldr (.) id

unwordsS :: [ShowS] -> ShowS
unwordsS = joinS " "

unlinesS :: [ShowS] -> ShowS
unlinesS = joinS "\n"

joinS :: String -> [ShowS] -> ShowS
joinS glue = concatS . intersperse (showString glue)



-- | Like 'Data.List.nub', but O(n log n) instead of O(n^2), since it uses a set to lookup previous things.
--   The result list is stable (the elements are returned in the order they occur), and lazy.
--   Requires that the list elements can be compared by Ord.
--   Code ruthlessly taken from &lt;http://hpaste.org/54411&gt;
nub' :: Ord a => [a] -> [a]
nub' = loop Set.empty
    where loop _    []            = []
          loop seen (x : xs)
              | Set.member x seen = loop seen xs
              | otherwise         = x : loop (Set.insert x seen) xs


-- | Replace all occurences of an element by another element.
replace :: Eq a => a -> a -> [a] -> [a]
replace x y = map (\z -> if z == x then y else z)