List

List

(++) :: [a] -> [a] -> [a] infixr 5

Append two lists, i.e.,

[x1, ..., xm] ++ [y1, ..., yn] == [x1, ..., xm, y1, ..., yn]
[x1, ..., xm] ++ [y1, ...] == [x1, ..., xm, y1, ...]

If the first list is not finite, the result is the first list.

(\\) :: Eq a => [a] -> [a] -> [a] infix 5

The \\ function is list difference (non-associative). In the result of xs \\ ys, the first occurrence of each element of ys in turn (if any) has been removed from xs. Thus

(xs ++ ys) \\ xs == ys.

>>> "Hello World!" \\ "ell W"
"Hoorld!"

It is a special case of deleteFirstsBy, which allows the programmer to supply their own equality test.

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)

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

break, applied to a predicate p and a list xs, returns a tuple where first element is longest prefix (possibly empty) of xs of elements that do not satisfy p and second element is the remainder of the list:

break (> 3) [1,2,3,4,1,2,3,4] == ([1,2,3],[4,1,2,3,4])
break (< 9) [1,2,3] == ([],[1,2,3])
break (> 9) [1,2,3] == ([1,2,3],[])

break p is equivalent to span (not . p).

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 match, 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")

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

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

A useful recursion pattern for processing a list to produce a new list, often used for "chopping" up the input list. Typically chop is called with some function that will consume an initial prefix of the list and produce a value and the rest of the list.

For example, many common Prelude functions can be implemented in terms of chop:

group :: (Eq a) => [a] -> [[a]]
group = chop (\ xs@(x:_) -> span (==x) xs)

words :: String -> [String]
words = filter (not . null) . chop (span (not . isSpace) . dropWhile isSpace)

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)

cycle :: [a] -> [a]

cycle ties a finite list into a circular one, or equivalently, the infinite repetition of the original list. It is the identity on infinite lists.

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

delete x removes the first occurrence of x from its list argument. For example,

>>> delete 'a' "banana"
"bnana"

It is a special case of deleteBy, which allows the programmer to supply their own equality test.

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

The deleteBy function behaves like delete, but takes a user-supplied equality predicate.

>>> deleteBy (<=) 4 [1..10]
[1,2,3,5,6,7,8,9,10]

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

The deleteFirstsBy function takes a predicate and two lists and returns the first list with the first occurrence of each element of the second list removed.

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

divvy :: Int -> Int -> [a] -> [[a]]

Divides up an input list into a set of sublists, according to n and m input specifications you provide. Each sublist will have n items, and the start of each sublist will be offset by m items from the previous one.

divvy 5 5 [1..20] == [[1,2,3,4,5],[6,7,8,9,10],[11,12,13,14,15],[16,17,18,19,20]]

In the case where a source list's trailing elements do no fill an entire sublist, those trailing elements will be dropped.

divvy 5 2 [1..10] == [[1,2,3,4,5],[3,4,5,6,7],[5,6,7,8,9]]

As an example, you can generate a moving average over a list of prices:

type Prices = [Float]
type AveragePrices = [Float]

average :: [Float] -> Float
average xs = sum xs / (fromIntegral $ length xs)

simpleMovingAverage :: Prices -> AveragePrices
simpleMovingAverage priceList =
  map average divvyedPrices
    where divvyedPrices = divvy 20 1 priceList

drop :: Int -> [a] -> [a]

drop n xs returns the suffix of xs after the first n elements, or [] if n > length xs:

drop 6 "Hello World!" == "World!"
drop 3 [1,2,3,4,5] == [4,5]
drop 3 [1,2] == []
drop 3 [] == []
drop (-1) [1,2] == [1,2]
drop 0 [1,2] == [1,2]

It is an instance of the more general genericDrop, in which n may be of any integral type.

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..]

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

Drops the given prefix from a list. It returns the original sequence if the sequence doesn't start with the given prefix.

dropPrefix "Mr. " "Mr. Men" == "Men"
dropPrefix "Mr. " "Dr. Men" == "Dr. Men"

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

Drops the given suffix from a list. It returns the original sequence if the sequence doesn't end with the given suffix.

dropSuffix "!" "Hello World!"  == "Hello World"
dropSuffix "!" "Hello World!!" == "Hello World!"
dropSuffix "!" "Hello World."  == "Hello World."

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

dropWhile p xs returns the suffix remaining after takeWhile p xs:

dropWhile (< 3) [1,2,3,4,5,1,2,3] == [3,4,5,1,2,3]
dropWhile (< 9) [1,2,3] == []
dropWhile (< 0) [1,2,3] == [1,2,3]

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

The dropWhileEnd function drops the largest suffix of a list in which the given predicate holds for all elements. For example:

>>> dropWhileEnd isSpace "foo\n"
"foo"

>>> dropWhileEnd isSpace "foo bar"
"foo bar"

dropWhileEnd isSpace ("foo\n" ++ undefined) == "foo" ++ undefined

Since: base-4.5.0.0

elemIndex :: Eq a => a -> [a] -> Maybe Int

The elemIndex function returns the index of the first element in the given list which is equal (by ==) to the query element, or Nothing if there is no such element.

>>> elemIndex 4 [0..]
Just 4

elemIndices :: Eq a => a -> [a] -> [Int]

The elemIndices function extends elemIndex, by returning the indices of all elements equal to the query element, in ascending order.

>>> elemIndices 'o' "Hello World"
[4,7]

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

Split into chunks terminated by the given subsequence. Equivalent to split . dropFinalBlank . dropDelims . onSublist. For example:

endBy ";" "foo;bar;baz;" == ["foo","bar","baz"]

Note also that the lines function from Data.List is equivalent to endBy "\n".

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

filter, applied to a predicate and a list, returns the list of those elements that satisfy the predicate; i.e.,

filter p xs = [ x | x <- xs, p x]

findIndex :: (a -> Bool) -> [a] -> Maybe Int

The findIndex function takes a predicate and a list and returns the index of the first element in the list satisfying the predicate, or Nothing if there is no such element.

>>> findIndex isSpace "Hello World!"
Just 5

findIndices :: (a -> Bool) -> [a] -> [Int]

The findIndices function extends findIndex, by returning the indices of all elements satisfying the predicate, in ascending order.

>>> findIndices (`elem` "aeiou") "Hello World!"
[1,4,7]

genericDrop :: Integral i => i -> [a] -> [a]

The genericDrop function is an overloaded version of drop, which accepts any Integral value as the number of elements to drop.

genericIndex :: Integral i => [a] -> i -> a

The genericIndex function is an overloaded version of !!, which accepts any Integral value as the index.

genericLength :: Num i => [a] -> i

The genericLength function is an overloaded version of length. In particular, instead of returning an Int, it returns any type which is an instance of Num. It is, however, less efficient than length.

genericReplicate :: Integral i => i -> a -> [a]

The genericReplicate function is an overloaded version of replicate, which accepts any Integral value as the number of repetitions to make.

genericSplitAt :: Integral i => i -> [a] -> ([a], [a])

The genericSplitAt function is an overloaded version of splitAt, which accepts any Integral value as the position at which to split.

genericTake :: Integral i => i -> [a] -> [a]

The genericTake function is an overloaded version of take, which accepts any Integral value as the number of elements to take.

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

The group function takes a list and returns a list of lists such that the concatenation of the result is equal to the argument. Moreover, each sublist in the result contains only equal elements. For example,

>>> group "Mississippi"
["M","i","ss","i","ss","i","pp","i"]

It is a special case of groupBy, which allows the programmer to supply their own equality test.

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

The groupBy function is the non-overloaded version of group.

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

A version of group where the equality is done on some extracted value.

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)

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"]]

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"]]

inits :: [a] -> [[a]]

The inits function returns all initial segments of the argument, shortest first. For example,

>>> inits "abc"
["","a","ab","abc"]

Note that inits has the following strictness property: inits (xs ++ _|_) = inits xs ++ _|_

In particular, inits _|_ = [] : _|_

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

The insert function takes an element and a list and inserts the element into the list at the first position where it is less than or equal to the next element. In particular, if the list is sorted before the call, the result will also be sorted. It is a special case of insertBy, which allows the programmer to supply their own comparison function.

>>> insert 4 [1,2,3,5,6,7]
[1,2,3,4,5,6,7]

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

The non-overloaded version of insert.

intercalate :: [a] -> [[a]] -> [a]

intercalate xs xss is equivalent to (concat (intersperse xs xss)). It inserts the list xs in between the lists in xss and concatenates the result.

>>> intercalate ", " ["Lorem", "ipsum", "dolor"]
"Lorem, ipsum, dolor"

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

The intersect function takes the list intersection of two lists. For example,

>>> [1,2,3,4] `intersect` [2,4,6,8]
[2,4]

If the first list contains duplicates, so will the result.

>>> [1,2,2,3,4] `intersect` [6,4,4,2]
[2,2,4]

It is a special case of intersectBy, which allows the programmer to supply their own equality test. If the element is found in both the first and the second list, the element from the first list will be used.

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

The intersectBy function is the non-overloaded version of intersect.

intersperse :: a -> [a] -> [a]

The intersperse function takes an element and a list and `intersperses' that element between the elements of the list. For example,

>>> intersperse ',' "abcde"
"a,b,c,d,e"

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

The isInfixOf function takes two lists and returns True iff the first list is contained, wholly and intact, anywhere within the second.

>>> isInfixOf "Haskell" "I really like Haskell."
True

>>> isInfixOf "Ial" "I really like Haskell."
False

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

The isPrefixOf function takes two lists and returns True iff the first list is a prefix of the second.

>>> "Hello" `isPrefixOf` "Hello World!"
True

>>> "Hello" `isPrefixOf` "Wello Horld!"
False

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

The isSubsequenceOf function takes two lists and returns True if all the elements of the first list occur, in order, in the second. The elements do not have to occur consecutively.

isSubsequenceOf x y is equivalent to elem x (subsequences y).

Examples

>>> isSubsequenceOf "GHC" "The Glorious Haskell Compiler"
True
>>> isSubsequenceOf ['a','d'..'z'] ['a'..'z']
True
>>> isSubsequenceOf [1..10] [10,9..0]
False

Since: base-4.8.0.0

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

The isSuffixOf function takes two lists and returns True iff the first list is a suffix of the second. The second list must be finite.

>>> "ld!" `isSuffixOf` "Hello World!"
True

>>> "World" `isSuffixOf` "Hello World!"
False

iterate :: (a -> a) -> a -> [a]

iterate f x returns an infinite list of repeated applications of f to x:

iterate f x == [x, f x, f (f x), ...]

Note that iterate is lazy, potentially leading to thunk build-up if the consumer doesn't force each iterate. See 'iterate\'' for a strict variant of this function.

iterate' :: (a -> a) -> a -> [a]

'iterate\'' is the strict version of iterate.

It ensures that the result of each application of force to weak head normal form before proceeding.

lookup :: Eq a => a -> [(a, b)] -> Maybe b

lookup key assocs looks up a key in an association list.

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

map f xs is the list obtained by applying f to each element of xs, i.e.,

map f [x1, x2, ..., xn] == [f x1, f x2, ..., f xn]
map f [x1, x2, ...] == [f x1, f x2, ...]

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

O(n^2). The nub function removes duplicate elements from a list. In particular, it keeps only the first occurrence of each element. (The name nub means `essence'.) It is a special case of nubBy, which allows the programmer to supply their own equality test.

>>> nub [1,2,3,4,3,2,1,2,4,3,5]
[1,2,3,4,5]

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

The nubBy function behaves just like nub, except it uses a user-supplied equality predicate instead of the overloaded == function.

>>> nubBy (\x y -> mod x 3 == mod y 3) [1,2,4,5,6]
[1,2,6]

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 ((==) on f), but has the performance advantage of only evaluating f once for each element in the input list.

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"]

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

O(n log n). The nubSort function sorts and removes duplicate elements from a list. In particular, it keeps only the first occurrence of each element.

nubSort "this is a test" == " aehist"
\xs -> nubSort xs == nub (sort xs)

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

A version of nubSort with a custom predicate.

nubSortBy (compare `on` length) ["a","test","of","this"] == ["a","of","test"]

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

A version of nubSort which operates on a portion of the value.

nubSortOn length ["a","test","of","this"] == ["a","of","test"]

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

The partition function takes a predicate a list and returns the pair of lists of elements which do and do not satisfy the predicate, respectively; i.e.,

partition p xs == (filter p xs, filter (not . p) xs)

>>> partition (`elem` "aeiou") "Hello World!"
("eoo","Hll Wrld!")

permutations :: [a] -> [[a]]

The permutations function returns the list of all permutations of the argument.

>>> permutations "abc"
["abc","bac","cba","bca","cab","acb"]

repeat :: a -> [a]

repeat x is an infinite list, with x the value of every element.

replicate :: Int -> a -> [a]

replicate n x is a list of length n with x the value of every element. It is an instance of the more general genericReplicate, in which n may be of any integral type.

reverse :: [a] -> [a]

reverse xs returns the elements of xs in reverse order. xs must be finite.

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

scanl is similar to foldl, but returns a list of successive reduced values from the left:

scanl f z [x1, x2, ...] == [z, z `f` x1, (z `f` x1) `f` x2, ...]

Note that

last (scanl f z xs) == foldl f z xs.

scanl' :: (b -> a -> b) -> b -> [a] -> [b]

A strictly accumulating version of scanl

scanl1 :: (a -> a -> a) -> [a] -> [a]

scanl1 is a variant of scanl that has no starting value argument:

scanl1 f [x1, x2, ...] == [x1, x1 `f` x2, ...]

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

scanr is the right-to-left dual of scanl. Note that

head (scanr f z xs) == foldr f z xs.

scanr1 :: (a -> a -> a) -> [a] -> [a]

scanr1 is a variant of scanr that has no starting value argument.

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

The sort function implements a stable sorting algorithm. It is a special case of sortBy, which allows the programmer to supply their own comparison function.

Elements are arranged from from lowest to highest, keeping duplicates in the order they appeared in the input.

>>> sort [1,6,4,3,2,5]
[1,2,3,4,5,6]

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

The sortBy function is the non-overloaded version of sort.

>>> sortBy (\(a,_) (b,_) -> compare a b) [(2, "world"), (4, "!"), (1, "Hello")]
[(1,"Hello"),(2,"world"),(4,"!")]

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.

Elements are arranged from from lowest to highest, keeping duplicates in the order they appeared in the input.

>>> sortOn fst [(2, "world"), (4, "!"), (1, "Hello")]
[(1,"Hello"),(2,"world"),(4,"!")]

Since: base-4.8.0.0

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

span, applied to a predicate p and a list xs, returns a tuple where first element is longest prefix (possibly empty) of xs of elements that satisfy p and second element is the remainder of the list:

span (< 3) [1,2,3,4,1,2,3,4] == ([1,2],[3,4,1,2,3,4])
span (< 9) [1,2,3] == ([1,2,3],[])
span (< 0) [1,2,3] == ([],[1,2,3])

span p xs is equivalent to (takeWhile p xs, dropWhile p 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)))

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"]

splitAt :: Int -> [a] -> ([a], [a])

splitAt n xs returns a tuple where first element is xs prefix of length n and second element is the remainder of the list:

splitAt 6 "Hello World!" == ("Hello ","World!")
splitAt 3 [1,2,3,4,5] == ([1,2,3],[4,5])
splitAt 1 [1,2,3] == ([1],[2,3])
splitAt 3 [1,2,3] == ([1,2,3],[])
splitAt 4 [1,2,3] == ([1,2,3],[])
splitAt 0 [1,2,3] == ([],[1,2,3])
splitAt (-1) [1,2,3] == ([],[1,2,3])

It is equivalent to (take n xs, drop n xs) when n is not _|_ (splitAt _|_ xs = _|_). splitAt is an instance of the more general genericSplitAt, in which n may be of any integral type.

splitAtEnd :: Int -> [a] -> ([a], [a])

splitAtEnd n xs returns a split where the second element tries to contain n 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)

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

Split on the given sublist. Equivalent to split . dropDelims . onSublist. For example:

splitOn ".." "a..b...c....d.." == ["a","b",".c","","d",""]

In some parsing combinator frameworks this is also known as sepBy.

Note that this is the right inverse of the intercalate function from Data.List, that is,

intercalate x . splitOn x === id

splitOn x . intercalate x is the identity on certain lists, but it is tricky to state the precise conditions under which this holds. (For example, it is not enough to say that x does not occur in any elements of the input list. Working out why is left as an exercise for the reader.)

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)

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")

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

The stripPrefix function drops the given prefix from a list. It returns Nothing if the list did not start with the prefix given, or Just the list after the prefix, if it does.

>>> stripPrefix "foo" "foobar"
Just "bar"

>>> stripPrefix "foo" "foo"
Just ""

>>> stripPrefix "foo" "barfoo"
Nothing

>>> stripPrefix "foo" "barfoobaz"
Nothing

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

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

Examples:

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

subsequences :: [a] -> [[a]]

The subsequences function returns the list of all subsequences of the argument.

>>> subsequences "abc"
["","a","b","ab","c","ac","bc","abc"]

tails :: [a] -> [[a]]

The tails function returns all final segments of the argument, longest first. For example,

>>> tails "abc"
["abc","bc","c",""]

Note that tails has the following strictness property: tails _|_ = _|_ : _|_

take :: Int -> [a] -> [a]

take n, applied to a list xs, returns the prefix of xs of length n, or xs itself if n > length xs:

take 5 "Hello World!" == "Hello"
take 3 [1,2,3,4,5] == [1,2,3]
take 3 [1,2] == [1,2]
take 3 [] == []
take (-1) [1,2] == []
take 0 [1,2] == []

It is an instance of the more general genericTake, in which n may be of any integral type.

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)

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

takeWhile, applied to a predicate p and a list xs, returns the longest prefix (possibly empty) of xs of elements that satisfy p:

takeWhile (< 3) [1,2,3,4,1,2,3,4] == [1,2]
takeWhile (< 9) [1,2,3] == [1,2,3]
takeWhile (< 0) [1,2,3] == []

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

A version of takeWhile operating from the end.

takeWhileEnd even [2,3,4,6] == [4,6]

transpose :: [[a]] -> [[a]]

The transpose function transposes the rows and columns of its argument. For example,

>>> transpose [[1,2,3],[4,5,6]]
[[1,4],[2,5],[3,6]]

If some of the rows are shorter than the following rows, their elements are skipped:

>>> transpose [[10,11],[20],[],[30,31,32]]
[[10,20,30],[11,31],[32]]

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

Decompose a list into its head and tail. If the list is empty, returns Nothing. If the list is non-empty, returns Just (x, xs), where x is the head of the list and xs its tail.

Since: base-4.8.0.0

unfoldr :: (b -> Maybe (a, b)) -> b -> [a]

The unfoldr function is a `dual' to foldr: while foldr reduces a list to a summary value, unfoldr builds a list from a seed value. The function takes the element and returns Nothing if it is done producing the list or returns Just (a,b), in which case, a is a prepended to the list and b is used as the next element in a recursive call. For example,

iterate f == unfoldr (\x -> Just (x, f x))

In some cases, unfoldr can undo a foldr operation:

unfoldr f' (foldr f z xs) == xs

if the following holds:

f' (f x y) = Just (x,y)
f' z       = Nothing

A simple use of unfoldr:

>>> unfoldr (\b -> if b == 0 then Nothing else Just (b, b-1)) 10
[10,9,8,7,6,5,4,3,2,1]

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

The union function returns the list union of the two lists. For example,

>>> "dog" `union` "cow"
"dogcw"

Duplicates, and elements of the first list, are removed from the the second list, but if the first list contains duplicates, so will the result. It is a special case of unionBy, which allows the programmer to supply their own equality test.

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

The unionBy function is the non-overloaded version of union.

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)

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

unzip transforms a list of pairs into a list of first components and a list of second components.

unzip3 :: [(a, b, c)] -> ([a], [b], [c])

The unzip3 function takes a list of triples and returns three lists, analogous to unzip.

unzip4 :: [(a, b, c, d)] -> ([a], [b], [c], [d])

The unzip4 function takes a list of quadruples and returns four lists, analogous to unzip.

unzip5 :: [(a, b, c, d, e)] -> ([a], [b], [c], [d], [e])

The unzip5 function takes a list of five-tuples and returns five lists, analogous to unzip.

unzip6 :: [(a, b, c, d, e, f)] -> ([a], [b], [c], [d], [e], [f])

The unzip6 function takes a list of six-tuples and returns six lists, analogous to unzip.

unzip7 :: [(a, b, c, d, e, f, g)] -> ([a], [b], [c], [d], [e], [f], [g])

The unzip7 function takes a list of seven-tuples and returns seven lists, analogous to unzip.

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

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

zip takes two lists and returns a list of corresponding pairs. If one input list is short, excess elements of the longer list are discarded.

zip is right-lazy:

zip [] _|_ = []

zip3 :: [a] -> [b] -> [c] -> [(a, b, c)]

zip3 takes three lists and returns a list of triples, analogous to zip.

zip4 :: [a] -> [b] -> [c] -> [d] -> [(a, b, c, d)]

The zip4 function takes four lists and returns a list of quadruples, analogous to zip.

zip5 :: [a] -> [b] -> [c] -> [d] -> [e] -> [(a, b, c, d, e)]

The zip5 function takes five lists and returns a list of five-tuples, analogous to zip.

zip6 :: [a] -> [b] -> [c] -> [d] -> [e] -> [f] -> [(a, b, c, d, e, f)]

The zip6 function takes six lists and returns a list of six-tuples, analogous to zip.

zip7 :: [a] -> [b] -> [c] -> [d] -> [e] -> [f] -> [g] -> [(a, b, c, d, e, f, g)]

The zip7 function takes seven lists and returns a list of seven-tuples, analogous to zip.

zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]

zipWith generalises zip by zipping with the function given as the first argument, instead of a tupling function. For example, zipWith (+) is applied to two lists to produce the list of corresponding sums.

zipWith is right-lazy:

zipWith f [] _|_ = []

zipWith3 :: (a -> b -> c -> d) -> [a] -> [b] -> [c] -> [d]

The zipWith3 function takes a function which combines three elements, as well as three lists and returns a list of their point-wise combination, analogous to zipWith.

zipWith4 :: (a -> b -> c -> d -> e) -> [a] -> [b] -> [c] -> [d] -> [e]

The zipWith4 function takes a function which combines four elements, as well as four lists and returns a list of their point-wise combination, analogous to zipWith.

zipWith5 :: (a -> b -> c -> d -> e -> f) -> [a] -> [b] -> [c] -> [d] -> [e] -> [f]

The zipWith5 function takes a function which combines five elements, as well as five lists and returns a list of their point-wise combination, analogous to zipWith.

zipWith6 :: (a -> b -> c -> d -> e -> f -> g) -> [a] -> [b] -> [c] -> [d] -> [e] -> [f] -> [g]

The zipWith6 function takes a function which combines six elements, as well as six lists and returns a list of their point-wise combination, analogous to zipWith.

zipWith7 :: (a -> b -> c -> d -> e -> f -> g -> h) -> [a] -> [b] -> [c] -> [d] -> [e] -> [f] -> [g] -> [h]

The zipWith7 function takes a function which combines seven elements, as well as seven lists and returns a list of their point-wise combination, analogous to zipWith.

String

type String = [Char]

A String is a list of characters. String constants in Haskell are values of type String.

words :: String -> [String]

words breaks a string up into a list of words, which were delimited by white space.

>>> words "Lorem ipsum\ndolor"
["Lorem","ipsum","dolor"]

unwords :: [String] -> String

unwords is an inverse operation to words. It joins words with separating spaces.

>>> unwords ["Lorem", "ipsum", "dolor"]
"Lorem ipsum dolor"

lines :: String -> [String]

lines breaks a string up into a list of strings at newline characters. The resulting strings do not contain newlines.

Note that after splitting the string at newline characters, the last part of the string is considered a line even if it doesn't end with a newline. For example,

>>> lines ""
[]

>>> lines "\n"
[""]

>>> lines "one"
["one"]

>>> lines "one\n"
["one"]

>>> lines "one\n\n"
["one",""]

>>> lines "one\ntwo"
["one","two"]

>>> lines "one\ntwo\n"
["one","two"]

Thus lines s contains at least as many elements as newlines in s.

unlines :: [String] -> String

unlines is an inverse operation to lines. It joins lines, after appending a terminating newline to each.

>>> unlines ["Hello", "World", "!"]
"Hello\nWorld\n!\n"

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

Remove spaces from either side of a string. A combination of trimEnd and trimStart.

trim      "  hello   " == "hello"
trimStart "  hello   " == "hello   "
trimEnd   "  hello   " == "  hello"
\s -> trim s == trimEnd (trimStart s)

trimStart :: String -> String

Remove spaces from the start of a string, see trim.

trimEnd :: String -> String

Remove spaces from the end of a string, see trim.

showString :: String -> ShowS

utility function converting a String to a show function that simply prepends the string unchanged.

lexDigits :: ReadS String

Reads a non-empty string of decimal digits.

class IsString a where

Class for string-like datastructures; used by the overloaded string extension (-XOverloadedStrings in GHC).

Minimal complete definition

fromString

Methods

fromString :: String -> a

Instances

IsString ByteString

Defined in Data.ByteString.Internal

Methods

fromString :: String -> ByteString

IsString ByteString

Defined in Data.ByteString.Lazy.Internal

Methods

fromString :: String -> ByteString

IsString Builder

Defined in Data.Text.Internal.Builder

Methods

fromString :: String -> Builder

IsString Value

Defined in Data.Aeson.Types.Internal

Methods

fromString :: String -> Value

IsString ShortByteString

Defined in Data.ByteString.Short.Internal

Methods

fromString :: String -> ShortByteString

IsString PropertyName

Defined in Hedgehog.Internal.Property

Methods

fromString :: String -> PropertyName

IsString GroupName

Defined in Hedgehog.Internal.Property

Methods

fromString :: String -> GroupName

IsString Doc

Defined in Text.PrettyPrint.HughesPJ

Methods

fromString :: String -> Doc

IsString ShortText

Note: Surrogate pairs ([U+D800 .. U+DFFF]) in string literals are replaced by U+FFFD.

This matches the behaviour of IsString instance for Text.

Defined in Data.Text.Short.Internal

Methods

fromString :: String -> ShortText

a ~ Char => IsString [a]

(a ~ Char) context was introduced in 4.9.0.0

Since: base-2.1

Defined in Data.String

Methods

fromString :: String -> [a]

IsString a => IsString (Identity a)

Defined in Data.String

Methods

fromString :: String -> Identity a

(IsString s, FoldCase s) => IsString (CI s)

Defined in Data.CaseInsensitive.Internal

Methods

fromString :: String -> CI s

a ~ Char => IsString (Seq a)

Since: containers-0.5.7

Defined in Data.Sequence.Internal

Methods

fromString :: String -> Seq a

a ~ Char => IsString (DList a)

Defined in Data.DList

Methods

fromString :: String -> DList a

(IsString a, Hashable a) => IsString (Hashed a)

Defined in Data.Hashable.Class

Methods

fromString :: String -> Hashed a

IsString (Doc a)

Defined in Text.PrettyPrint.Annotated.HughesPJ

Methods

fromString :: String -> Doc a

IsString (Doc ann)

>>> pretty ("hello\nworld")
hello
world

This instance uses the Pretty Text instance, and uses the same newline to line conversion.

Defined in Data.Text.Prettyprint.Doc.Internal

Methods

fromString :: String -> Doc ann

(streamType ~ STInput, res ~ ()) => IsString (StreamSpec streamType res)

This instance uses byteStringInput to convert a raw string into a stream of input for a child process.

Since: typed-process-0.1.0.0

Defined in System.Process.Typed

Methods

fromString :: String -> StreamSpec streamType res

IsString a => IsString (Const a b)

Since: base-4.9.0.0

Defined in Data.String

Methods

fromString :: String -> Const a b

IsString a => IsString (Tagged s a)

Defined in Data.Tagged

Methods

fromString :: String -> Tagged s a

(stdin ~ (), stdout ~ (), stderr ~ ()) => IsString (ProcessConfig stdin stdout stderr)

Defined in System.Process.Typed

Methods

fromString :: String -> ProcessConfig stdin stdout stderr

(IsString t, Eq t, a ~ t) => IsString (Prod r e t a)

String literals can be interpreted as Terminals that match that string.

>>> :set -XOverloadedStrings
>>> import Data.Text (Text)
>>> let determiner = "the" <|> "a" <|> "an" :: Prod r e Text Text

Defined in Text.Earley.Grammar

Methods

fromString :: String -> Prod r e t a

(a ~ Tokens s, IsString a, Eq a, Stream s, Ord e) => IsString (ParsecT e s m a)

Since: megaparsec-6.3.0

Defined in Text.Megaparsec.Internal

Methods

fromString :: String -> ParsecT e s m a

Optics

prefixed :: Eq a => [a] -> Prism' [a] [a]

A Prism stripping a prefix from a list when used as a Traversal, or prepending that prefix when run backwards:

>>> "preview" ^? prefixed "pre"
Just "view"

>>> "review" ^? prefixed "pre"
Nothing

>>> prefixed "pre" # "amble"
"preamble"

suffixed :: Eq a => [a] -> Prism' [a] [a]

A Prism stripping a suffix from a list when used as a Traversal, or appending that suffix when run backwards:

>>> "review" ^? suffixed "view"
Just "re"

>>> "review" ^? suffixed "tire"
Nothing

>>> suffixed ".o" # "hello"
"hello.o"