Documentation

List foldr but with a base case for the singleton list.

Like union. Duplicates in the first list are not removed. O(nm).

Like mapMaybe.

Like catMaybes.

Concatenate one or more non-empty lists.

Like lefts.

Like rights.

Like partitionEithers.

Like find.

Like unwords.

Like zipWithM.

Like zipWithM.

Build a list with one element.

More precise type for snoc.

Like unzipWith.

Split a list into sublists. Generalisation of the prelude function words. Same as wordsBy and wordsBy, but with the non-emptyness guarantee on the chunks. O(n).

words xs == wordsBy isSpace xs

Update the first element of a non-empty list. O(1).

Update the last element of a non-empty list. O(n).

Return the last element and the rest.

The more general type Null m => [a] -> (List1 a -> m) -> m often causes type inference to fail, as we do not in general have instance Applicative m => Null (m ()).

Last two elements (safe). O(n).

Lift a function on non-empty lists to a function on lists.

This is in essence fmap for Maybe, if we take [a] = Maybe (List1 a).

Breaks a list just after an element satisfying the predicate is found.

>>> breakAfter even [1,3,5,2,4,7,8]
(1 :| [3,5,2],[4,7,8])

Checks if all the elements in the list are equal. Assumes that the Eq instance stands for an equivalence relation. O(n).

Non-efficient, monadic nub. O(n²).

Lossless toList, opposite of nonEmpty.

Safe version of fromList.

groupOn f = groupBy ((==) `on` f) . sortBy (compare `on` f). O(n log n).

More precise type for groupBy'.

A variant of groupBy which applies the predicate to consecutive pairs. O(n).

Group consecutive items that share the same first component.

Group consecutive items that share the same first component.

Focus on the first element of a non-empty list. O(1).

Focus on the last element of a non-empty list. O(n).

Construct a NonEmpty list from a single element.

Since: base-4.15

insert x xs inserts x into the last position in xs where it is still less than or equal to the next element. In particular, if the list is sorted beforehand, the result will also be sorted.

Map a function over a NonEmpty stream.

filter p xs removes any elements from xs that do not satisfy p.

Non-empty (and non-strict) list type.

Since: base-4.9.0.0

The unfoldr function is analogous to Data.List's unfoldr operation.

Sort a NonEmpty on a user-supplied projection of its elements. See sortOn for more detailed information.

Examples

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

>>> sortOn length $ "jim" :| ["creed", "pam", "michael", "dwight", "kevin"]
"jim" :| ["pam","creed","kevin","dwight","michael"]

Performance notes

>>> sortBy (comparing fst) $ (3, 1) :| [(2, 2), (1, 3)]
(1,3) :| [(2,2),(3,1)]

>>> (sortBy . comparing) fst $ (3, 1) :| [(2, 2), (1, 3)]
(1,3) :| [(2,2),(3,1)]

Since: base-4.20.0.0

transpose for NonEmpty, behaves the same as transpose The rows/columns need not be the same length, in which case > transpose . transpose /= id

sortBy for NonEmpty, behaves the same as sortBy

cycle xs returns the infinite repetition of xs:

cycle (1 :| [2,3]) = 1 :| [2,3,1,2,3,...]

The zip function takes two streams and returns a stream of corresponding pairs.

Extract the first element of the stream.

uncons produces the first element of the stream, and a stream of the remaining elements, if any.

Extract the possibly-empty tail of the stream.

Extract the last element of the stream.

Extract everything except the last element of the stream.

Number of elements in NonEmpty list.

scanl is similar to foldl, but returns a stream 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.

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

scanl1 f [x1, x2, ...] == x1 :| [x1 `f` x2, x1 `f` (x2 `f` x3), ...]

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

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

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

iterate f x produces the infinite sequence of repeated applications of f to x.

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

repeat x returns a constant stream, where all elements are equal to x.

takeWhile p xs returns the longest prefix of the stream xs for which the predicate p holds.

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

take n xs returns the first n elements of xs.

drop n xs drops the first n elements off the front of the sequence xs.

splitAt n xs returns a pair consisting of the prefix of xs of length n and the remaining stream immediately following this prefix.

'splitAt' n xs == ('take' n xs, 'drop' n xs)
xs == ys ++ zs where (ys, zs) = 'splitAt' n xs

span p xs returns the longest prefix of xs that satisfies p, together with the remainder of the stream.

'span' p xs == ('takeWhile' p xs, 'dropWhile' p xs)
xs == ys ++ zs where (ys, zs) = 'span' p xs

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

reverse a finite NonEmpty stream.

xs !! n returns the element of the stream xs at index n. Note that the head of the stream has index 0.

Beware: a negative or out-of-bounds index will cause an error.

The zipWith function generalizes zip. Rather than tupling the elements, the elements are combined using the function passed as the first argument.

Compute n-ary logic exclusive OR operation on NonEmpty list.

The isPrefixOf function returns True if the first argument is a prefix of the second.

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 inequality test.

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

'intersperse x xs' alternates elements of the list with copies of x.

intersperse 0 (1 :| [2,3]) == 1 :| [0,2,0,3]

The partition function takes a predicate p and a stream xs, and returns a pair of lists. The first list corresponds to the elements of xs for which p holds; the second corresponds to the elements of xs for which p does not hold.

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

The group function takes a stream and returns a list of streams such that flattening the resulting list is equal to the argument. Moreover, each stream in the resulting list contains only equal elements, and consecutive equal elements of the input end up in the same stream of the output list. For example, in list notation:

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

groupBy operates like group, but uses the provided equality predicate instead of ==.

The inits function takes a stream xs and returns all the finite prefixes of xs, starting with the shortest. The result is NonEmpty because the result always contains the empty list as the first element.

inits [1,2,3] == [] :| [[1], [1,2], [1,2,3]]
inits [1] == [] :| [[1]]
inits [] == [] :| []

The tails function takes a stream xs and returns all the suffixes of xs, starting with the longest. The result is NonEmpty because the result always contains the empty list as the last element.

tails [1,2,3] == [1,2,3] :| [[2,3], [3], []]
tails [1] == [1] :| [[]]
tails [] == [] :| []

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

Since: base-4.20.0.0

Sort a stream.

sortWith for NonEmpty, behaves the same as:

sortBy . comparing

groupWith operates like group, but uses the provided projection when comparing for equality

unfold produces a new stream by repeatedly applying the unfolding function to the seed value to produce an element of type b and a new seed value. When the unfolding function returns Nothing instead of a new seed value, the stream ends.

nonEmpty efficiently turns a normal list into a NonEmpty stream, producing Nothing if the input is empty.

Prepend an element to the stream.

Synonym for <|.

The inits1 function takes a NonEmpty stream xs and returns all the NonEmpty finite prefixes of xs, starting with the shortest.

inits1 (1 :| [2,3]) == (1 :| []) :| [1 :| [2], 1 :| [2,3]]
inits1 (1 :| []) == (1 :| []) :| []

Since: base-4.18

The tails1 function takes a NonEmpty stream xs and returns all the non-empty suffixes of xs, starting with the longest.

tails1 (1 :| [2,3]) == (1 :| [2,3]) :| [2 :| [3], 3 :| []]
tails1 (1 :| []) == (1 :| []) :| []

Since: base-4.18

some1 x sequences x one or more times.

groupAllWith operates like groupWith, but sorts the list first so that each equivalence class has, at most, one list in the output

group1 operates like group, but uses the knowledge that its input is non-empty to produce guaranteed non-empty output.

groupBy1 is to group1 as groupBy is to group.

groupWith1 is to group1 as groupWith is to group

groupAllWith1 is to groupWith1 as groupAllWith is to groupWith

permutations1 operates like permutations, but uses the knowledge that its input is non-empty to produce output where every element is non-empty.

permutations1 = fmap fromList . permutations . toList

Since: base-4.20.0.0

A monomorphic version of <> for NonEmpty.

>>> append (1 :| []) (2 :| [3])
1 :| [2,3]

Since: base-4.16

Attach a list at the end of a NonEmpty.

>>> appendList (1 :| [2,3]) []
1 :| [2,3]

>>> appendList (1 :| [2,3]) [4,5]
1 :| [2,3,4,5]

Since: base-4.16

Attach a list at the beginning of a NonEmpty.

>>> prependList [] (1 :| [2,3])
1 :| [2,3]

>>> prependList [negate 1, 0] (1 :| [2, 3])
-1 :| [0,1,2,3]

Since: base-4.16

The IsList class and its methods are intended to be used in conjunction with the OverloadedLists extension.

Since: base-4.7.0.0

The Item type function returns the type of items of the structure l.

Generalization of Data.List.unzip.

Examples

>>> unzip (Just ("Hello", "World"))
(Just "Hello",Just "World")

>>> unzip [("I", "love"), ("really", "haskell")]
(["I","really"],["love","haskell"])

Since: base-4.19.0.0