Safe Haskell | Safe-Inferred |
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Data.List.Extra
Description
- lower :: String -> String
- upper :: String -> String
- trim :: String -> String
- trimStart :: String -> String
- trimEnd :: String -> String
- word1 :: String -> (String, String)
- line1 :: String -> (String, String)
- dropEnd :: Int -> [a] -> [a]
- takeEnd :: Int -> [a] -> [a]
- splitAtEnd :: Int -> [a] -> ([a], [a])
- breakEnd :: (a -> Bool) -> [a] -> ([a], [a])
- spanEnd :: (a -> Bool) -> [a] -> ([a], [a])
- dropWhileEnd :: (a -> Bool) -> [a] -> [a]
- dropWhileEnd' :: (a -> Bool) -> [a] -> [a]
- takeWhileEnd :: (a -> Bool) -> [a] -> [a]
- stripSuffix :: Eq a => [a] -> [a] -> Maybe [a]
- stripInfix :: Eq a => [a] -> [a] -> Maybe ([a], [a])
- stripInfixEnd :: Eq a => [a] -> [a] -> Maybe ([a], [a])
- wordsBy :: (a -> Bool) -> [a] -> [[a]]
- linesBy :: (a -> Bool) -> [a] -> [[a]]
- breakOn :: Eq a => [a] -> [a] -> ([a], [a])
- breakOnEnd :: Eq a => [a] -> [a] -> ([a], [a])
- splitOn :: Eq a => [a] -> [a] -> [[a]]
- split :: (a -> Bool) -> [a] -> [[a]]
- chunksOf :: Int -> [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]
- drop1 :: [a] -> [a]
- mconcatMap :: Monoid b => (a -> b) -> [a] -> b
- groupSort :: Ord k => [(k, v)] -> [(k, [v])]
- groupSortOn :: Ord b => (a -> b) -> [a] -> [[a]]
- groupSortBy :: (a -> a -> Ordering) -> [a] -> [[a]]
- nubOrd :: Ord a => [a] -> [a]
- nubOrdBy :: (a -> a -> Ordering) -> [a] -> [a]
- nubOrdOn :: Ord b => (a -> b) -> [a] -> [a]
- nubOn :: Eq b => (a -> b) -> [a] -> [a]
- groupOn :: Eq b => (a -> b) -> [a] -> [[a]]
- sortOn :: Ord b => (a -> b) -> [a] -> [a]
- disjoint :: Eq a => [a] -> [a] -> Bool
- allSame :: Eq a => [a] -> Bool
- anySame :: Eq a => [a] -> Bool
- repeatedly :: ([a] -> (b, [a])) -> [a] -> [b]
- for :: [a] -> (a -> b) -> [b]
- firstJust :: (a -> Maybe b) -> [a] -> Maybe b
- concatUnzip :: [([a], [b])] -> ([a], [b])
- concatUnzip3 :: [([a], [b], [c])] -> ([a], [b], [c])
- replace :: Eq a => [a] -> [a] -> [a] -> [a]
- merge :: Ord a => [a] -> [a] -> [a]
- mergeBy :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
String operations
lower :: String -> String
Convert a string to lower case.
lower "This is A TEST" == "this is a test" lower "" == ""
upper :: String -> String
Convert a string to upper case.
upper "This is A TEST" == "THIS IS A TEST" upper "" == ""
trim :: String -> String
word1 :: String -> (String, String)
Split the first word off a string. Useful for when starting to parse the beginning of a string, but you want to accurately perserve whitespace in the rest of the string.
word1 "" == ("", "") word1 "keyword rest of string" == ("keyword","rest of string") word1 " keyword\n rest of string" == ("keyword","rest of string") \s -> fst (word1 s) == concat (take 1 $ words s) \s -> words (snd $ word1 s) == drop 1 (words s)
line1 :: String -> (String, String)
Split the first line off a string.
line1 "" == ("", "") line1 "test" == ("test","") line1 "test\n" == ("test","") line1 "test\nrest" == ("test","rest") line1 "test\nrest\nmore" == ("test","rest\nmore")
Splitting
dropEnd :: Int -> [a] -> [a]
Drop a number of elements from the end of the list.
dropEnd 3 "hello" == "he" dropEnd 5 "bye" == "" dropEnd (-1) "bye" == "bye" \i xs -> dropEnd i xs `isPrefixOf` xs \i xs -> length (dropEnd i xs) == max 0 (length xs - max 0 i) \i -> take 3 (dropEnd 5 [i..]) == take 3 [i..]
takeEnd :: Int -> [a] -> [a]
Take a number of elements from the end of the list.
takeEnd 3 "hello" == "llo" takeEnd 5 "bye" == "bye" takeEnd (-1) "bye" == "" \i xs -> takeEnd i xs `isSuffixOf` xs \i xs -> length (takeEnd i xs) == min (max 0 i) (length xs)
splitAtEnd :: Int -> [a] -> ([a], [a])
returns a split where the second element tries to
contain splitAtEnd
n xsn
elements.
splitAtEnd 3 "hello" == ("he","llo") splitAtEnd 3 "he" == ("", "he") \i xs -> uncurry (++) (splitAt i xs) == xs \i xs -> splitAtEnd i xs == (dropEnd i xs, takeEnd i xs)
breakEnd :: (a -> Bool) -> [a] -> ([a], [a])
Break, but from the end.
breakEnd isLower "youRE" == ("you","RE") breakEnd isLower "youre" == ("youre","") breakEnd isLower "YOURE" == ("","YOURE") \f xs -> breakEnd (not . f) xs == spanEnd f xs
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 -> uncurry (++) (spanEnd f xs) == xs \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.
The function dropWhileEnd
can be used on an infinite list and tests the property
on each character. In contrast, dropWhileEnd'
is strict in the spine of the list
but only tests the trailing suffix.
This version usually outperforms dropWhileEnd
if the list is short or the test is expensive.
Note the tests below cover both the prime and non-prime variants.
dropWhileEnd isSpace "ab cde " == "ab cde" dropWhileEnd' isSpace "ab cde " == "ab cde" last (dropWhileEnd even [undefined,3]) == undefined last (dropWhileEnd' even [undefined,3]) == 3 head (dropWhileEnd even (3:undefined)) == 3 head (dropWhileEnd' even (3:undefined)) == undefined
takeWhileEnd :: (a -> Bool) -> [a] -> [a]
A version of takeWhile
operating from the end.
takeWhileEnd even [2,3,4,6] == [4,6]
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
stripInfix :: Eq a => [a] -> [a] -> Maybe ([a], [a])
Return the the string before and after the search string,
or Nothing
if the search string is not present.
Examples:
stripInfix "::" "a::b::c" == Just ("a", "b::c") stripInfix "/" "foobar" == Nothing
stripInfixEnd :: Eq a => [a] -> [a] -> Maybe ([a], [a])
Similar to stripInfix
, but searches from the end of the
string.
stripInfixEnd "::" "a::b::c" == Just ("a::b", "c")
wordsBy :: (a -> Bool) -> [a] -> [[a]]
A variant of words
with a custom test. In particular,
adjacent separators are discarded, as are leading or trailing separators.
wordsBy (== ':') "::xyz:abc::123::" == ["xyz","abc","123"] \s -> wordsBy isSpace s == words s
linesBy :: (a -> Bool) -> [a] -> [[a]]
A variant of lines
with a custom test. In particular,
if there is a trailing separator it will be discarded.
linesBy (== ':') "::xyz:abc::123::" == ["","","xyz","abc","","123",""] \s -> linesBy (== '\n') s == lines s linesBy (== ';') "my;list;here;" == ["my","list","here"]
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.
If you want the remainder without the patch, use stripInfix
.
breakOn "::" "a::b::c" == ("a", "::b::c") breakOn "/" "foobar" == ("foobar", "") \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.
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" "" == [""] \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.
split (== 'a') "aabbaca" == ["","","bb","c",""] split (== 'a') "" == [""] split (== ':') "::xyz:abc::123::" == ["","","xyz","abc","","123","",""] split (== ',') "my,list,here" == ["my","list","here"]
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" == undefined
Basics
list :: b -> (a -> [a] -> b) -> [a] -> b
Non-recursive transform over a list, like maybe
.
list 1 (\v _ -> v - 2) [5,6,7] == 3 list 1 (\v _ -> v - 2) [] == 1 \nil cons xs -> maybe nil (uncurry cons) (uncons xs) == list nil cons xs
uncons :: [a] -> Maybe (a, [a])
If the list is empty returns Nothing
, otherwise returns the head
and the tail
.
uncons "test" == Just ('t',"est") uncons "" == Nothing \xs -> uncons xs == if null xs then Nothing else Just (head xs, tail xs)
unsnoc :: [a] -> Maybe ([a], a)
If the list is empty returns Nothing
, otherwise returns the init
and the last
.
unsnoc "test" == Just ("tes",'t') unsnoc "" == Nothing \xs -> unsnoc xs == if null xs then Nothing else Just (init xs, last xs)
cons :: a -> [a] -> [a]
Append an element to the start of a list, an alias for '(:)'.
cons 't' "est" == "test" \x xs -> uncons (cons x xs) == Just (x,xs)
snoc :: [a] -> a -> [a]
Append an element to the end of a list, takes O(n) time.
snoc "tes" 't' == "test" \xs x -> unsnoc (snoc xs x) == Just (xs,x)
drop1 :: [a] -> [a]
Equivalent to drop 1
, but likely to be faster and a single lexeme.
drop1 "" == "" drop1 "test" == "est" \xs -> drop 1 xs == drop1 xs
mconcatMap :: Monoid b => (a -> b) -> [a] -> b
Version on concatMap
generalised to a Monoid
rather than just a list.
mconcatMap Sum [1,2,3] == Sum 6 \f xs -> mconcatMap f xs == concatMap f xs
List operations
groupSort :: Ord k => [(k, v)] -> [(k, [v])]
A combination of group
and sort
.
groupSort [(1,'t'),(3,'t'),(2,'e'),(2,'s')] == [(1,"t"),(2,"es"),(3,"t")] \xs -> map fst (groupSort xs) == sort (nub (map fst xs)) \xs -> concatMap snd (groupSort xs) == map snd (sortOn fst xs)
groupSortOn :: Ord b => (a -> b) -> [a] -> [[a]]
A combination of group
and sort
, using a part of the value to compare on.
groupSortOn length ["test","of","sized","item"] == [["of"],["test","item"],["sized"]]
groupSortBy :: (a -> a -> Ordering) -> [a] -> [[a]]
A combination of group
and sort
, using a predicate to compare on.
groupSortBy (compare `on` length) ["test","of","sized","item"] == [["of"],["test","item"],["sized"]]
nubOrd :: Ord a => [a] -> [a]
O(n log n). The nubOrd
function removes duplicate elements from a list.
In particular, it keeps only the first occurrence of each element.
Unlike the standard nub
operator, this version requires an Ord
instance
and consequently runs asymptotically faster.
nubOrd "this is a test" == "this ae" nubOrd (take 4 ("this" ++ undefined)) == "this" \xs -> nubOrd xs == nub xs
nubOrdBy :: (a -> a -> Ordering) -> [a] -> [a]
A version of nubOrd
with a custom predicate.
nubOrdBy (compare `on` length) ["a","test","of","this"] == ["a","test","of"]
nubOrdOn :: Ord b => (a -> b) -> [a] -> [a]
A version of nubOrd
which operates on a portion of the value.
nubOrdOn length ["a","test","of","this"] == ["a","test","of"]
nubOn :: Eq b => (a -> b) -> [a] -> [a]
A version of nub
where the equality is done on some extracted value.
nubOn f
is equivalent to nubBy ((==)
, but has the
performance advantage of only evaluating on
f)f
once for each element in the
input list.
groupOn :: Eq b => (a -> b) -> [a] -> [[a]]
A version of group
where the equality is done on some extracted value.
sortOn :: Ord b => (a -> b) -> [a] -> [a]
Sort a list by comparing the results of a key function applied to each
element. sortOn f
is equivalent to sortBy (comparing f)
, but has the
performance advantage of only evaluating f
once for each element in the
input list. This is called the decorate-sort-undecorate paradigm, or
Schwartzian transform.
sortOn fst [(3,"z"),(1,""),(3,"a")] == [(1,""),(3,"z"),(3,"a")]
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 allSame (1:1:2:undefined) == False \xs -> allSame xs == (length (nub xs) <= 1)
anySame :: Eq a => [a] -> Bool
Is there any element which occurs more than once.
anySame [1,1,2] == True anySame [1,2,3] == False anySame (1:2:1:undefined) == True anySame [] == False \xs -> anySame xs == (length (nub xs) < length xs)
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 \xs -> repeatedly line1 xs == lines xs
for :: [a] -> (a -> b) -> [b]
Flipped version of map
.
for [1,2,3] (+1) == [2,3,4]
firstJust :: (a -> Maybe b) -> [a] -> Maybe b
Find the first element of a list for which the operation returns Just
, along
with the result of the operation. Like find
but useful where the function also
computes some expensive information that can be reused. Particular useful
when the function is monadic, see firstJustM
.
firstJust id [Nothing,Just 3] == Just 3 firstJust id [Nothing,Nothing] == Nothing
concatUnzip :: [([a], [b])] -> ([a], [b])
A merging of unzip
and concat
.
concatUnzip [("a","AB"),("bc","C")] == ("abc","ABC")
concatUnzip3 :: [([a], [b], [c])] -> ([a], [b], [c])
A merging of unzip3
and concat
.
concatUnzip3 [("a","AB",""),("bc","C","123")] == ("abc","ABC","123")
replace :: Eq a => [a] -> [a] -> [a] -> [a]
Replace a subsequence everywhere it occurs. The first argument must not be the empty list.
replace "el" "_" "Hello Bella" == "H_lo B_la" replace "el" "e" "Hello" == "Helo" replace "" "e" "Hello" == undefined \xs ys -> not (null xs) ==> replace xs xs ys == ys
merge :: Ord a => [a] -> [a] -> [a]
Merge two lists which are assumed to be ordered.
merge "ace" "bd" == "abcde" \xs ys -> merge (sort xs) (sort ys) == sort (xs ++ ys)