Data.List.Extra

This module extends Data.List with extra functions of a similar nature. The package also exports the existing Data.List functions. Some of the names and semantics were inspired by the text package.

Documentation

lower :: String -> String

Documentation about lowercase

 lower "This is A TEST" == "this is a test"
 lower "" == ""

upper :: String -> String

trim :: String -> String

trimStart :: String -> String

trimEnd :: String -> String

dropAround :: (a -> Bool) -> [a] -> [a]

word1 :: String -> (String, String)

drop1 :: [a] -> [a]

list :: b -> (a -> [a] -> b) -> [a] -> b

uncons :: [a] -> Maybe (a, [a])

unsnoc :: [a] -> Maybe ([a], a)

cons :: a -> [a] -> [a]

snoc :: [a] -> a -> [a]

groupSort :: Ord k => [(k, v)] -> [(k, [v])]

groupSortOn :: Ord a => (k -> a) -> [(k, v)] -> [(k, [v])]

nubOn :: Eq b => (a -> b) -> [a] -> [a]

groupOn :: Eq b => (a -> b) -> [a] -> [[a]]

sortOn :: Ord b => (a -> b) -> [a] -> [a]

repeatedly :: ([a] -> (b, [a])) -> [a] -> [b]

Apply some operation repeatedly, producing an element of output and the remainder of the list.

 \xs -> repeatedly (splitAt 3) xs  == chunksOf 3 xs
 \xs -> repeatedly word1 (trim xs) == words xs

for :: [a] -> (a -> b) -> [b]

Flipped version of map.

 for [1,2,3] (+1) == [2,3,4]

disjoint :: Eq a => [a] -> [a] -> Bool

Are two lists disjoint, with no elements in common.

 disjoint [1,2,3] [4,5] == True
 disjoint [1,2,3] [4,1] == False

allSame :: Eq a => [a] -> Bool

Are all elements the same.

 allSame [1,1,2] == False
 allSame [1,1,1] == True
 allSame [1]     == True
 allSame []      == True

anySame :: Eq a => [a] -> Bool

Is there any element which occurs more than once.

 anySame [1,1,2] == True
 anySame [1,2,3] == False

dropEnd :: Int -> [a] -> [a]

takeEnd :: Int -> [a] -> [a]

breakEnd :: (a -> Bool) -> [a] -> ([a], [a])

Break, but from the end.

 breakEnd isLower "youRE" === ("you","RE")
 breakEnd isLower "youre" === ("youre","")
 breakEnd isLower "YOURE" === ("","YOURE")

spanEnd :: (a -> Bool) -> [a] -> ([a], [a])

Span, but from the end.

 spanEnd isUpper "youRE" == ("you","RE")
 spanEnd (not . isSpace) "x y z" == ("x y ","z")
 \f xs-> spanEnd f xs == swap (both reverse (span f (reverse xs)))

dropWhileEnd :: (a -> Bool) -> [a] -> [a]

dropWhileEnd' :: (a -> Bool) -> [a] -> [a]

A version of dropWhileEnd but with different strictness properties. Often outperforms if the list is short or the test is expensive.

takeWhileEnd :: (a -> Bool) -> [a] -> [a]

stripSuffix :: Eq a => [a] -> [a] -> Maybe [a]

Return the prefix of the second string if its suffix matches the entire first string.

Examples:

 stripSuffix "bar" "foobar" == Just "foo"
 stripSuffix ""    "baz"    == Just "baz"
 stripSuffix "foo" "quux"   == Nothing

concatUnzip :: [([a], [b])] -> ([a], [b])

concatUnzip3 :: [([a], [b], [c])] -> ([a], [b], [c])

merge :: Ord a => [a] -> [a] -> [a]

mergeBy :: (a -> a -> Ordering) -> [a] -> [a] -> [a]

replace :: Eq a => [a] -> [a] -> [a] -> [a]

wordsBy :: (a -> Bool) -> [a] -> [[a]]

linesBy :: (a -> Bool) -> [a] -> [[a]]

firstJust :: (a -> Maybe b) -> [a] -> Maybe b

breakOn :: Eq a => [a] -> [a] -> ([a], [a])

Find the first instance of needle in haystack. The first element of the returned tuple is the prefix of haystack before needle is matched. The second is the remainder of haystack, starting with the match.

Examples:

 breakOn "::" "a::b::c" == ("a", "::b::c")
 breakOn "/" "foobar"   == ("foobar", "")

Laws:

 \needle haystack -> let (prefix,match) = breakOn needle haystack in prefix ++ match == haystack

breakOnEnd :: Eq a => [a] -> [a] -> ([a], [a])

Similar to breakOn, but searches from the end of the string.

The first element of the returned tuple is the prefix of haystack up to and including the last match of needle. The second is the remainder of haystack, following the match.

 breakOnEnd "::" "a::b::c" == ("a::b::", "c")

splitOn :: Eq a => [a] -> [a] -> [[a]]

Break a list into pieces separated by the first list argument, consuming the delimiter. An empty delimiter is invalid, and will cause an error to be raised.

Examples:

 splitOn "\r\n" "a\r\nb\r\nd\r\ne" == ["a","b","d","e"]
 splitOn "aaa"  "aaaXaaaXaaaXaaa"  == ["","X","X","X",""]
 splitOn "x"    "x"                == ["",""]
 splitOn "x"    ""                 == [""]

and

 \s x -> s /= "" ==> intercalate s (splitOn s x) == x
 \c x -> splitOn [c] x                           == split (==c) x

split :: (a -> Bool) -> [a] -> [[a]]

Splits a list into components delimited by separators, where the predicate returns True for a separator element. The resulting components do not contain the separators. Two adjacent separators result in an empty component in the output. eg.

 split (=='a') "aabbaca" == ["","","bb","c",""]
 split (=='a') ""        == [""]

chunksOf :: Int -> [a] -> [[a]]

Split a list into chunks of a given size. The last chunk may contain fewer than n elements. The chunk size must be positive.

 chunksOf 3 "my test" == ["my ","tes","t"]
 chunksOf 3 "mytest"  == ["myt","est"]
 chunksOf 8 ""        == []
 chunksOf 0 "test"    == error