{-# LANGUAGE CPP #-}
{-# LANGUAGE PackageImports #-}
module Data.List.Extra
( partitionM
, mapAccumLM
, iterateNM
, (<:>)
, indexMaybe
, splitAtList
, equalLength
, countEq
, zipEqual
-- * From Control.Monad.Extra
, anyM
, allM
, orM
-- * From "extra"
, module NeilsExtra
) where
import "extra" Data.List.Extra as NeilsExtra
import "extra" Control.Monad.Extra (anyM, allM, orM, partitionM)
import Control.Applicative (liftA2)
#if defined(DEBUG)
import GHC.Stack (HasCallStack)
#endif
-- | Monadic version of 'Data.List.mapAccumL'
mapAccumLM
:: (Monad m)
=> (acc -> x -> m (acc,y))
-> acc
-> [x]
-> m (acc,[y])
mapAccumLM _ acc [] = return (acc,[])
mapAccumLM f acc (x:xs) = do
(acc',y) <- f acc x
(acc'',ys) <- mapAccumLM f acc' xs
return (acc'',y:ys)
-- | Monadic version of 'iterate'. A carbon copy ('iterateM') would not
-- terminate, hence the first argument.
iterateNM
:: Monad m
=> Word
-- ^ Only iterate /n/ times. Note that /n/ is the length of the resulting
-- list, _not_ the number of times the iteration function has been invoked
-> (a -> m a)
-- ^ Iteration function
-> a
-- ^ Start value
-> m [a]
iterateNM 0 _f _a = pure []
iterateNM limit f a = fmap (a:) (go (limit - 1) a)
where
go 0 _a0 = pure []
go n a0 = do
a1 <- f a0
fmap (a1:) (go (n - 1) a1)
infixr 5 <:>
-- | Applicative version of 'GHC.Types.(:)'
(<:>) :: Applicative f => f a -> f [a] -> f [a]
(<:>) = liftA2 (:)
-- | Safe indexing, returns a 'Nothing' if the index does not exist
indexMaybe :: [a] -> Int -> Maybe a
indexMaybe [] _ = Nothing
indexMaybe (x:_) 0 = Just x
indexMaybe (_:xs) n = indexMaybe xs (n-1)
splitAtList :: [b] -> [a] -> ([a], [a])
splitAtList [] xs = ([], xs)
splitAtList _ xs@[] = (xs, xs)
splitAtList (_:xs) (y:ys) = (y:ys', ys'')
where
(ys', ys'') = splitAtList xs ys
equalLength :: [a] -> [b] -> Bool
equalLength [] [] = True
equalLength (_:as) (_:bs) = equalLength as bs
equalLength _ _ = False
-- | Return number of occurrences of an item in a list
countEq
:: Eq a
=> a
-- ^ Needle
-> [a]
-- ^ Haystack
-> Int
-- ^ Times needle was found in haystack
countEq a as = length (filter (== a) as)
-- | Zip two lists of equal length
--
-- NB Errors out for a DEBUG compiler when the two lists are not of equal length
#if !defined(DEBUG)
zipEqual
:: [a] -> [b] -> [(a,b)]
zipEqual = zip
{-# INLINE zipEqual #-}
#else
zipEqual ::
HasCallStack =>
[a] -> [b] -> [(a,b)]
zipEqual = go
where
go [] [] = []
go (a:as) (b:bs) = (a,b) : go as bs
go (_:_) [] = error "zipEqual: left list is longer"
go [] (_:_) = error "zipEqual: right list is longer"
#endif