Copyright | (c) The University of Glasgow 2001 |
---|---|

License | BSD-style (see the file libraries/base/LICENSE) |

Maintainer | libraries@haskell.org |

Stability | stable |

Portability | portable |

Safe Haskell | Trustworthy |

Language | Haskell2010 |

Operations on lists.

## Synopsis

- data List a
- (++) :: [a] -> [a] -> [a]
- head :: HasCallStack => [a] -> a
- last :: HasCallStack => [a] -> a
- tail :: HasCallStack => [a] -> [a]
- init :: HasCallStack => [a] -> [a]
- uncons :: [a] -> Maybe (a, [a])
- unsnoc :: [a] -> Maybe ([a], a)
- singleton :: a -> [a]
- null :: Foldable t => t a -> Bool
- length :: Foldable t => t a -> Int
- map :: (a -> b) -> [a] -> [b]
- reverse :: [a] -> [a]
- intersperse :: a -> [a] -> [a]
- intercalate :: [a] -> [[a]] -> [a]
- transpose :: [[a]] -> [[a]]
- subsequences :: [a] -> [[a]]
- permutations :: [a] -> [[a]]
- foldl :: Foldable t => (b -> a -> b) -> b -> t a -> b
- foldl' :: Foldable t => (b -> a -> b) -> b -> t a -> b
- foldl1 :: Foldable t => (a -> a -> a) -> t a -> a
- foldl1' :: HasCallStack => (a -> a -> a) -> [a] -> a
- foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b
- foldr1 :: Foldable t => (a -> a -> a) -> t a -> a
- concat :: Foldable t => t [a] -> [a]
- concatMap :: Foldable t => (a -> [b]) -> t a -> [b]
- and :: Foldable t => t Bool -> Bool
- or :: Foldable t => t Bool -> Bool
- any :: Foldable t => (a -> Bool) -> t a -> Bool
- all :: Foldable t => (a -> Bool) -> t a -> Bool
- sum :: (Foldable t, Num a) => t a -> a
- product :: (Foldable t, Num a) => t a -> a
- maximum :: (Foldable t, Ord a) => t a -> a
- minimum :: (Foldable t, Ord a) => t a -> a
- scanl :: (b -> a -> b) -> b -> [a] -> [b]
- scanl' :: (b -> a -> b) -> b -> [a] -> [b]
- scanl1 :: (a -> a -> a) -> [a] -> [a]
- scanr :: (a -> b -> b) -> b -> [a] -> [b]
- scanr1 :: (a -> a -> a) -> [a] -> [a]
- mapAccumL :: Traversable t => (s -> a -> (s, b)) -> s -> t a -> (s, t b)
- mapAccumR :: Traversable t => (s -> a -> (s, b)) -> s -> t a -> (s, t b)
- iterate :: (a -> a) -> a -> [a]
- iterate' :: (a -> a) -> a -> [a]
- repeat :: a -> [a]
- replicate :: Int -> a -> [a]
- cycle :: HasCallStack => [a] -> [a]
- unfoldr :: (b -> Maybe (a, b)) -> b -> [a]
- take :: Int -> [a] -> [a]
- drop :: Int -> [a] -> [a]
- splitAt :: Int -> [a] -> ([a], [a])
- takeWhile :: (a -> Bool) -> [a] -> [a]
- dropWhile :: (a -> Bool) -> [a] -> [a]
- dropWhileEnd :: (a -> Bool) -> [a] -> [a]
- span :: (a -> Bool) -> [a] -> ([a], [a])
- break :: (a -> Bool) -> [a] -> ([a], [a])
- stripPrefix :: Eq a => [a] -> [a] -> Maybe [a]
- group :: Eq a => [a] -> [[a]]
- inits :: [a] -> [[a]]
- tails :: [a] -> [[a]]
- isPrefixOf :: Eq a => [a] -> [a] -> Bool
- isSuffixOf :: Eq a => [a] -> [a] -> Bool
- isInfixOf :: Eq a => [a] -> [a] -> Bool
- isSubsequenceOf :: Eq a => [a] -> [a] -> Bool
- elem :: (Foldable t, Eq a) => a -> t a -> Bool
- notElem :: (Foldable t, Eq a) => a -> t a -> Bool
- lookup :: Eq a => a -> [(a, b)] -> Maybe b
- find :: Foldable t => (a -> Bool) -> t a -> Maybe a
- filter :: (a -> Bool) -> [a] -> [a]
- partition :: (a -> Bool) -> [a] -> ([a], [a])
- (!?) :: [a] -> Int -> Maybe a
- (!!) :: HasCallStack => [a] -> Int -> a
- elemIndex :: Eq a => a -> [a] -> Maybe Int
- elemIndices :: Eq a => a -> [a] -> [Int]
- findIndex :: (a -> Bool) -> [a] -> Maybe Int
- findIndices :: (a -> Bool) -> [a] -> [Int]
- zip :: [a] -> [b] -> [(a, b)]
- zip3 :: [a] -> [b] -> [c] -> [(a, b, c)]
- zip4 :: [a] -> [b] -> [c] -> [d] -> [(a, b, c, d)]
- zip5 :: [a] -> [b] -> [c] -> [d] -> [e] -> [(a, b, c, d, e)]
- zip6 :: [a] -> [b] -> [c] -> [d] -> [e] -> [f] -> [(a, b, c, d, e, f)]
- zip7 :: [a] -> [b] -> [c] -> [d] -> [e] -> [f] -> [g] -> [(a, b, c, d, e, f, g)]
- zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
- zipWith3 :: (a -> b -> c -> d) -> [a] -> [b] -> [c] -> [d]
- zipWith4 :: (a -> b -> c -> d -> e) -> [a] -> [b] -> [c] -> [d] -> [e]
- zipWith5 :: (a -> b -> c -> d -> e -> f) -> [a] -> [b] -> [c] -> [d] -> [e] -> [f]
- zipWith6 :: (a -> b -> c -> d -> e -> f -> g) -> [a] -> [b] -> [c] -> [d] -> [e] -> [f] -> [g]
- zipWith7 :: (a -> b -> c -> d -> e -> f -> g -> h) -> [a] -> [b] -> [c] -> [d] -> [e] -> [f] -> [g] -> [h]
- unzip :: [(a, b)] -> ([a], [b])
- unzip3 :: [(a, b, c)] -> ([a], [b], [c])
- unzip4 :: [(a, b, c, d)] -> ([a], [b], [c], [d])
- unzip5 :: [(a, b, c, d, e)] -> ([a], [b], [c], [d], [e])
- unzip6 :: [(a, b, c, d, e, f)] -> ([a], [b], [c], [d], [e], [f])
- unzip7 :: [(a, b, c, d, e, f, g)] -> ([a], [b], [c], [d], [e], [f], [g])
- lines :: String -> [String]
- words :: String -> [String]
- unlines :: [String] -> String
- unwords :: [String] -> String
- nub :: Eq a => [a] -> [a]
- delete :: Eq a => a -> [a] -> [a]
- (\\) :: Eq a => [a] -> [a] -> [a]
- union :: Eq a => [a] -> [a] -> [a]
- intersect :: Eq a => [a] -> [a] -> [a]
- sort :: Ord a => [a] -> [a]
- sortOn :: Ord b => (a -> b) -> [a] -> [a]
- insert :: Ord a => a -> [a] -> [a]
- nubBy :: (a -> a -> Bool) -> [a] -> [a]
- deleteBy :: (a -> a -> Bool) -> a -> [a] -> [a]
- deleteFirstsBy :: (a -> a -> Bool) -> [a] -> [a] -> [a]
- unionBy :: (a -> a -> Bool) -> [a] -> [a] -> [a]
- intersectBy :: (a -> a -> Bool) -> [a] -> [a] -> [a]
- groupBy :: (a -> a -> Bool) -> [a] -> [[a]]
- sortBy :: (a -> a -> Ordering) -> [a] -> [a]
- insertBy :: (a -> a -> Ordering) -> a -> [a] -> [a]
- maximumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
- minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a
- genericLength :: Num i => [a] -> i
- genericTake :: Integral i => i -> [a] -> [a]
- genericDrop :: Integral i => i -> [a] -> [a]
- genericSplitAt :: Integral i => i -> [a] -> ([a], [a])
- genericIndex :: Integral i => [a] -> i -> a
- genericReplicate :: Integral i => i -> a -> [a]

# Documentation

The builtin linked list type.

In Haskell, lists are one of the most important data types as they are often used analogous to loops in imperative programming languages. These lists are singly linked, which makes them unsuited for operations that require \(\mathcal{O}(1)\) access. Instead, they are intended to be traversed.

You can use `List a`

or `[a]`

in type signatures:

length :: [a] -> Int

or

length :: List a -> Int

They are fully equivalent, and `List a`

will be normalised to `[a]`

.

#### Usage

Lists are constructed recursively using the right-associative constructor operator (or *cons*)
`(:) :: a -> [a] -> [a]`

, which prepends an element to a list,
and the empty list `[]`

.

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

Lists can also be constructed using list literals
of the form `[x_1, x_2, ..., x_n]`

which are syntactic sugar and, unless `-XOverloadedLists`

is enabled,
are translated into uses of `(:)`

and `[]`

`String`

literals, like `"I 💜 hs"`

, are translated into
Lists of characters, `['I', ' ', '💜', ' ', 'h', 's']`

.

#### Implementation

Internally and in memory, all the above are represented like this, with arrows being pointers to locations in memory.

╭───┬───┬──╮ ╭───┬───┬──╮ ╭───┬───┬──╮ ╭────╮ │(:)│ │ ─┼──>│(:)│ │ ─┼──>│(:)│ │ ─┼──>│ [] │ ╰───┴─┼─┴──╯ ╰───┴─┼─┴──╯ ╰───┴─┼─┴──╯ ╰────╯ v v v 1 2 3

#### Examples

>>> ['H', 'a', 's', 'k', 'e', 'l', 'l'] "Haskell"

>>> 1 : [4, 1, 5, 9] [1,4,1,5,9]

>>> [] : [] : [] [[],[]]

*Since: ghc-prim-0.10.0*

#### Instances

Alternative [] Source # | Combines lists by concatenation, starting from the empty list.
| ||||

Applicative [] Source # |
| ||||

Functor [] Source # |
| ||||

Monad [] Source # |
| ||||

MonadPlus [] Source # | Combines lists by concatenation, starting from the empty list.
| ||||

MonadFail [] Source # |
| ||||

Defined in GHC.Internal.Control.Monad.Fail | |||||

MonadFix [] Source # |
| ||||

Defined in GHC.Internal.Control.Monad.Fix | |||||

Foldable [] Source # |
| ||||

Defined in GHC.Internal.Data.Foldable fold :: Monoid m => [m] -> m Source # foldMap :: Monoid m => (a -> m) -> [a] -> m Source # foldMap' :: Monoid m => (a -> m) -> [a] -> m Source # foldr :: (a -> b -> b) -> b -> [a] -> b Source # foldr' :: (a -> b -> b) -> b -> [a] -> b Source # foldl :: (b -> a -> b) -> b -> [a] -> b Source # foldl' :: (b -> a -> b) -> b -> [a] -> b Source # foldr1 :: (a -> a -> a) -> [a] -> a Source # foldl1 :: (a -> a -> a) -> [a] -> a Source # elem :: Eq a => a -> [a] -> Bool Source # maximum :: Ord a => [a] -> a Source # minimum :: Ord a => [a] -> a Source # | |||||

Traversable [] Source # |
| ||||

Generic1 [] Source # | |||||

Defined in GHC.Internal.Generics
| |||||

Monoid [a] Source # |
| ||||

Semigroup [a] Source # |
| ||||

Data a => Data [a] Source # | For historical reasons, the constructor name used for
| ||||

Defined in GHC.Internal.Data.Data gfoldl :: (forall d b. Data d => c (d -> b) -> d -> c b) -> (forall g. g -> c g) -> [a] -> c [a] Source # gunfold :: (forall b r. Data b => c (b -> r) -> c r) -> (forall r. r -> c r) -> Constr -> c [a] Source # toConstr :: [a] -> Constr Source # dataTypeOf :: [a] -> DataType Source # dataCast1 :: Typeable t => (forall d. Data d => c (t d)) -> Maybe (c [a]) Source # dataCast2 :: Typeable t => (forall d e. (Data d, Data e) => c (t d e)) -> Maybe (c [a]) Source # gmapT :: (forall b. Data b => b -> b) -> [a] -> [a] Source # gmapQl :: (r -> r' -> r) -> r -> (forall d. Data d => d -> r') -> [a] -> r Source # gmapQr :: forall r r'. (r' -> r -> r) -> r -> (forall d. Data d => d -> r') -> [a] -> r Source # gmapQ :: (forall d. Data d => d -> u) -> [a] -> [u] Source # gmapQi :: Int -> (forall d. Data d => d -> u) -> [a] -> u Source # gmapM :: Monad m => (forall d. Data d => d -> m d) -> [a] -> m [a] Source # gmapMp :: MonadPlus m => (forall d. Data d => d -> m d) -> [a] -> m [a] Source # gmapMo :: MonadPlus m => (forall d. Data d => d -> m d) -> [a] -> m [a] Source # | |||||

a ~ Char => IsString [a] Source # |
| ||||

Defined in GHC.Internal.Data.String fromString :: String -> [a] Source # | |||||

Generic [a] Source # | |||||

Defined in GHC.Internal.Generics
| |||||

IsList [a] Source # |
| ||||

Read a => Read [a] Source # |
| ||||

Show a => Show [a] Source # |
| ||||

Eq a => Eq [a] | |||||

Ord a => Ord [a] | |||||

type Rep1 [] Source # |
| ||||

Defined in GHC.Internal.Generics type Rep1 [] = D1 ('MetaData "List" "GHC.Types" "ghc-prim" 'False) (C1 ('MetaCons "[]" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons ":" ('InfixI 'RightAssociative 5) 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) Par1 :*: S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec1 []))) | |||||

type Rep [a] Source # |
| ||||

Defined in GHC.Internal.Generics type Rep [a] = D1 ('MetaData "List" "GHC.Types" "ghc-prim" 'False) (C1 ('MetaCons "[]" 'PrefixI 'False) (U1 :: Type -> Type) :+: C1 ('MetaCons ":" ('InfixI 'RightAssociative 5) 'False) (S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 a) :*: S1 ('MetaSel ('Nothing :: Maybe Symbol) 'NoSourceUnpackedness 'NoSourceStrictness 'DecidedLazy) (Rec0 [a]))) | |||||

type Item [a] Source # | |||||

Defined in GHC.Internal.IsList type Item [a] = a |

# Basic functions

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

`(++)`

appends 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.

#### Performance considerations

This function takes linear time in the number of elements of the
**first** list. Thus it is better to associate repeated
applications of `(++)`

to the right (which is the default behaviour):
`xs ++ (ys ++ zs)`

or simply `xs ++ ys ++ zs`

, but not `(xs ++ ys) ++ zs`

.
For the same reason `concat`

`=`

`foldr`

`(++)`

`[]`

has linear performance, while `foldl`

`(++)`

`[]`

is prone
to quadratic slowdown

#### Examples

`>>>`

[1,2,3,4,5,6]`[1, 2, 3] ++ [4, 5, 6]`

`>>>`

[1,2,3]`[] ++ [1, 2, 3]`

`>>>`

[3,2,1]`[3, 2, 1] ++ []`

head :: HasCallStack => [a] -> a Source #

Warning: This is a partial function, it throws an error on empty lists. Use pattern matching, `uncons`

or `listToMaybe`

instead. Consider refactoring to use Data.List.NonEmpty.

\(\mathcal{O}(1)\). Extract the first element of a list, which must be non-empty.

To disable the warning about partiality put `{-# OPTIONS_GHC -Wno-x-partial -Wno-unrecognised-warning-flags #-}`

at the top of the file. To disable it throughout a package put the same
options into `ghc-options`

section of Cabal file. To disable it in GHCi
put `:set -Wno-x-partial -Wno-unrecognised-warning-flags`

into `~/.ghci`

config file.
See also the migration guide.

##### Examples

`>>>`

1`head [1, 2, 3]`

`>>>`

1`head [1..]`

`>>>`

*** Exception: Prelude.head: empty list`head []`

last :: HasCallStack => [a] -> a Source #

\(\mathcal{O}(n)\). Extract the last element of a list, which must be finite and non-empty.

WARNING: This function is partial. Consider using `unsnoc`

instead.

#### Examples

`>>>`

3`last [1, 2, 3]`

`>>>`

* Hangs forever *`last [1..]`

`>>>`

*** Exception: Prelude.last: empty list`last []`

tail :: HasCallStack => [a] -> [a] Source #

Warning: This is a partial function, it throws an error on empty lists. Replace it with `drop`

1, or use pattern matching or `uncons`

instead. Consider refactoring to use Data.List.NonEmpty.

\(\mathcal{O}(1)\). Extract the elements after the head of a list, which must be non-empty.

To disable the warning about partiality put `{-# OPTIONS_GHC -Wno-x-partial -Wno-unrecognised-warning-flags #-}`

at the top of the file. To disable it throughout a package put the same
options into `ghc-options`

section of Cabal file. To disable it in GHCi
put `:set -Wno-x-partial -Wno-unrecognised-warning-flags`

into `~/.ghci`

config file.
See also the migration guide.

#### Examples

`>>>`

[2,3]`tail [1, 2, 3]`

`>>>`

[]`tail [1]`

`>>>`

*** Exception: Prelude.tail: empty list`tail []`

init :: HasCallStack => [a] -> [a] Source #

\(\mathcal{O}(n)\). Return all the elements of a list except the last one. The list must be non-empty.

WARNING: This function is partial. Consider using `unsnoc`

instead.

#### Examples

`>>>`

[1,2]`init [1, 2, 3]`

`>>>`

[]`init [1]`

`>>>`

*** Exception: Prelude.init: empty list`init []`

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

\(\mathcal{O}(1)\). Decompose a list into its `head`

and `tail`

.

- If the list is empty, returns
`Nothing`

. - If the list is non-empty, returns

, where`Just`

(x, xs)`x`

is the`head`

of the list and`xs`

its`tail`

.

#### Examples

`>>>`

Nothing`uncons []`

`>>>`

Just (1,[])`uncons [1]`

`>>>`

Just (1,[2,3])`uncons [1, 2, 3]`

*Since: base-4.8.0.0*

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

\(\mathcal{O}(n)\). Decompose a list into `init`

and `last`

.

- If the list is empty, returns
`Nothing`

. - If the list is non-empty, returns

, where`Just`

(xs, x)`xs`

is the`init`

ial part of the list and`x`

is its`last`

element.

`unsnoc`

is dual to `uncons`

: for a finite list `xs`

unsnoc xs = (\(hd, tl) -> (reverse tl, hd)) <$> uncons (reverse xs)

#### Examples

`>>>`

Nothing`unsnoc []`

`>>>`

Just ([],1)`unsnoc [1]`

`>>>`

Just ([1,2],3)`unsnoc [1, 2, 3]`

#### Laziness

`>>>`

Just []`fst <$> unsnoc [undefined]`

`>>>`

Just *** Exception: Prelude.undefined`head . fst <$> unsnoc (1 : undefined)`

`>>>`

Just 1`head . fst <$> unsnoc (1 : 2 : undefined)`

*Since: base-4.19.0.0*

singleton :: a -> [a] Source #

Construct a list from a single element.

#### Examples

`>>>`

[True]`singleton True`

`>>>`

[[1,2,3]]`singleton [1, 2, 3]`

`>>>`

"c"`singleton 'c'`

*Since: base-4.15.0.0*

null :: Foldable t => t a -> Bool Source #

Test whether the structure is empty. The default implementation is Left-associative and lazy in both the initial element and the accumulator. Thus optimised for structures where the first element can be accessed in constant time. Structures where this is not the case should have a non-default implementation.

#### Examples

Basic usage:

`>>>`

True`null []`

`>>>`

False`null [1]`

`null`

is expected to terminate even for infinite structures.
The default implementation terminates provided the structure
is bounded on the left (there is a leftmost element).

`>>>`

False`null [1..]`

*Since: base-4.8.0.0*

length :: Foldable t => t a -> Int Source #

Returns the size/length of a finite structure as an `Int`

. The
default implementation just counts elements starting with the leftmost.
Instances for structures that can compute the element count faster
than via element-by-element counting, should provide a specialised
implementation.

#### Examples

Basic usage:

`>>>`

0`length []`

`>>>`

3`length ['a', 'b', 'c']`

`>>>`

* Hangs forever *`length [1..]`

*Since: base-4.8.0.0*

# List transformations

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

\(\mathcal{O}(n)\). `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, ...]

this means that `map id == id`

#### Examples

`>>>`

[2,3,4]`map (+1) [1, 2, 3]`

`>>>`

[1,2,3]`map id [1, 2, 3]`

`>>>`

[4,7,10]`map (\n -> 3 * n + 1) [1, 2, 3]`

reverse :: [a] -> [a] Source #

\(\mathcal{O}(n)\). `reverse`

`xs`

returns the elements of `xs`

in reverse order.
`xs`

must be finite.

#### Laziness

`reverse`

is lazy in its elements.

`>>>`

1`head (reverse [undefined, 1])`

`>>>`

*** Exception: Prelude.undefined`reverse (1 : 2 : undefined)`

#### Examples

`>>>`

[]`reverse []`

`>>>`

[42]`reverse [42]`

`>>>`

[7,5,2]`reverse [2,5,7]`

`>>>`

* Hangs forever *`reverse [1..]`

intersperse :: a -> [a] -> [a] Source #

\(\mathcal{O}(n)\). The `intersperse`

function takes an element and a list
and `intersperses' that element between the elements of the list.

#### Laziness

`intersperse`

has the following properties

`>>>`

"a"`take 1 (intersperse undefined ('a' : undefined))`

`>>>`

"a*** Exception: Prelude.undefined`take 2 (intersperse ',' ('a' : undefined))`

#### Examples

`>>>`

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

`>>>`

[3,1,4,1,5]`intersperse 1 [3, 4, 5]`

intercalate :: [a] -> [[a]] -> [a] Source #

`intercalate`

`xs xss`

is equivalent to `(`

.
It inserts the list `concat`

(`intersperse`

xs xss))`xs`

in between the lists in `xss`

and concatenates the
result.

#### Laziness

`intercalate`

has the following properties:

`>>>`

"Lorem"`take 5 (intercalate undefined ("Lorem" : undefined))`

`>>>`

"Lorem*** Exception: Prelude.undefined`take 6 (intercalate ", " ("Lorem" : undefined))`

#### Examples

`>>>`

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

`>>>`

[2,3,0,1,4,5,6,0,1]`intercalate [0, 1] [[2, 3], [4, 5, 6], []]`

`>>>`

[1,2,3]`intercalate [1, 2, 3] [[], []]`

transpose :: [[a]] -> [[a]] Source #

The `transpose`

function transposes the rows and columns of its argument.

#### Laziness

`transpose`

is lazy in its elements

`>>>`

["ab"]`take 1 (transpose ['a' : undefined, 'b' : undefined])`

#### Examples

`>>>`

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

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

`>>>`

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

For this reason the outer list must be finite; otherwise `transpose`

hangs:

`>>>`

* Hangs forever *`transpose (repeat [])`

subsequences :: [a] -> [[a]] Source #

The `subsequences`

function returns the list of all subsequences of the argument.

#### Laziness

`subsequences`

does not look ahead unless it must:

`>>>`

[[]]`take 1 (subsequences undefined)`

`>>>`

["","a"]`take 2 (subsequences ('a' : undefined))`

#### Examples

`>>>`

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

This function is productive on infinite inputs:

`>>>`

["","a","b","ab","c","ac","bc","abc"]`take 8 $ subsequences ['a'..]`

permutations :: [a] -> [[a]] Source #

The `permutations`

function returns the list of all permutations of the argument.

Note that the order of permutations is not lexicographic. It satisfies the following property:

map (take n) (take (product [1..n]) (permutations ([1..n] ++ undefined))) == permutations [1..n]

#### Laziness

The `permutations`

function is maximally lazy:
for each `n`

, the value of

starts with those permutations
that permute `permutations`

xs

and keep `take`

n xs

.`drop`

n xs

#### Examples

`>>>`

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

`>>>`

[[1,2],[2,1]]`permutations [1, 2]`

`>>>`

[[]]`permutations []`

This function is productive on infinite inputs:

`>>>`

["abc","bac","cba","bca","cab","acb"]`take 6 $ map (take 3) $ permutations ['a'..]`

# Reducing lists (folds)

foldl :: Foldable t => (b -> a -> b) -> b -> t a -> b Source #

Left-associative fold of a structure, lazy in the accumulator. This is rarely what you want, but can work well for structures with efficient right-to-left sequencing and an operator that is lazy in its left argument.

In the case of lists, `foldl`

, when applied to a binary operator, a
starting value (typically the left-identity of the operator), and a
list, reduces the list using the binary operator, from left to right:

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

Note that to produce the outermost application of the operator the
entire input list must be traversed. Like all left-associative folds,
`foldl`

will diverge if given an infinite list.

If you want an efficient strict left-fold, you probably want to use
`foldl'`

instead of `foldl`

. The reason for this is that the latter
does not force the *inner* results (e.g. `z `f` x1`

in the above
example) before applying them to the operator (e.g. to `(`f` x2)`

).
This results in a thunk chain *O(n)* elements long, which then must be
evaluated from the outside-in.

For a general `Foldable`

structure this should be semantically identical
to:

foldl f z =`foldl`

f z .`toList`

#### Examples

The first example is a strict fold, which in practice is best performed
with `foldl'`

.

`>>>`

52`foldl (+) 42 [1,2,3,4]`

Though the result below is lazy, the input is reversed before prepending it to the initial accumulator, so corecursion begins only after traversing the entire input string.

`>>>`

"hgfeabcd"`foldl (\acc c -> c : acc) "abcd" "efgh"`

A left fold of a structure that is infinite on the right cannot terminate, even when for any finite input the fold just returns the initial accumulator:

`>>>`

* Hangs forever *`foldl (\a _ -> a) 0 $ repeat 1`

WARNING: When it comes to lists, you always want to use either `foldl'`

or `foldr`

instead.

foldl' :: Foldable t => (b -> a -> b) -> b -> t a -> b Source #

Left-associative fold of a structure but with strict application of the operator.

This ensures that each step of the fold is forced to Weak Head Normal
Form before being applied, avoiding the collection of thunks that would
otherwise occur. This is often what you want to strictly reduce a
finite structure to a single strict result (e.g. `sum`

).

For a general `Foldable`

structure this should be semantically identical
to,

foldl' f z =`foldl'`

f z .`toList`

*Since: base-4.6.0.0*

foldl1 :: Foldable t => (a -> a -> a) -> t a -> a Source #

A variant of `foldl`

that has no base case,
and thus may only be applied to non-empty structures.

This function is non-total and will raise a runtime exception if the structure happens to be empty.

`foldl1`

f =`foldl1`

f .`toList`

#### Examples

Basic usage:

`>>>`

10`foldl1 (+) [1..4]`

`>>>`

*** Exception: Prelude.foldl1: empty list`foldl1 (+) []`

`>>>`

*** Exception: foldl1: empty structure`foldl1 (+) Nothing`

`>>>`

-8`foldl1 (-) [1..4]`

`>>>`

False`foldl1 (&&) [True, False, True, True]`

`>>>`

True`foldl1 (||) [False, False, True, True]`

`>>>`

* Hangs forever *`foldl1 (+) [1..]`

foldl1' :: HasCallStack => (a -> a -> a) -> [a] -> a Source #

A strict version of `foldl1`

.

foldr :: Foldable t => (a -> b -> b) -> b -> t a -> b Source #

Right-associative fold of a structure, lazy in the accumulator.

In the case of lists, `foldr`

, when applied to a binary operator, a
starting value (typically the right-identity of the operator), and a
list, reduces the list using the binary operator, from right to left:

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

Note that since the head of the resulting expression is produced by an
application of the operator to the first element of the list, given an
operator lazy in its right argument, `foldr`

can produce a terminating
expression from an unbounded list.

For a general `Foldable`

structure this should be semantically identical
to,

foldr f z =`foldr`

f z .`toList`

#### Examples

Basic usage:

`>>>`

True`foldr (||) False [False, True, False]`

`>>>`

False`foldr (||) False []`

`>>>`

"foodcba"`foldr (\c acc -> acc ++ [c]) "foo" ['a', 'b', 'c', 'd']`

##### Infinite structures

⚠️ Applying `foldr`

to infinite structures usually doesn't terminate.

It may still terminate under one of the following conditions:

- the folding function is short-circuiting
- the folding function is lazy on its second argument

###### Short-circuiting

`(`

short-circuits on `||`

)`True`

values, so the following terminates
because there is a `True`

value finitely far from the left side:

`>>>`

True`foldr (||) False (True : repeat False)`

But the following doesn't terminate:

`>>>`

* Hangs forever *`foldr (||) False (repeat False ++ [True])`

###### Laziness in the second argument

Applying `foldr`

to infinite structures terminates when the operator is
lazy in its second argument (the initial accumulator is never used in
this case, and so could be left `undefined`

, but `[]`

is more clear):

`>>>`

[1,4,7,10,13]`take 5 $ foldr (\i acc -> i : fmap (+3) acc) [] (repeat 1)`

foldr1 :: Foldable t => (a -> a -> a) -> t a -> a Source #

A variant of `foldr`

that has no base case,
and thus may only be applied to non-empty structures.

This function is non-total and will raise a runtime exception if the structure happens to be empty.

#### Examples

Basic usage:

`>>>`

10`foldr1 (+) [1..4]`

`>>>`

Exception: Prelude.foldr1: empty list`foldr1 (+) []`

`>>>`

*** Exception: foldr1: empty structure`foldr1 (+) Nothing`

`>>>`

-2`foldr1 (-) [1..4]`

`>>>`

False`foldr1 (&&) [True, False, True, True]`

`>>>`

True`foldr1 (||) [False, False, True, True]`

`>>>`

* Hangs forever *`foldr1 (+) [1..]`

## Special folds

concat :: Foldable t => t [a] -> [a] Source #

The concatenation of all the elements of a container of lists.

#### Examples

Basic usage:

`>>>`

[1,2,3]`concat (Just [1, 2, 3])`

`>>>`

[]`concat (Left 42)`

`>>>`

[1,2,3,4,5,6]`concat [[1, 2, 3], [4, 5], [6], []]`

concatMap :: Foldable t => (a -> [b]) -> t a -> [b] Source #

Map a function over all the elements of a container and concatenate the resulting lists.

#### Examples

Basic usage:

`>>>`

[1,2,3,10,11,12,100,101,102,1000,1001,1002]`concatMap (take 3) [[1..], [10..], [100..], [1000..]]`

`>>>`

[1,2,3]`concatMap (take 3) (Just [1..])`

and :: Foldable t => t Bool -> Bool Source #

`and`

returns the conjunction of a container of Bools. For the
result to be `True`

, the container must be finite; `False`

, however,
results from a `False`

value finitely far from the left end.

#### Examples

Basic usage:

`>>>`

True`and []`

`>>>`

True`and [True]`

`>>>`

False`and [False]`

`>>>`

False`and [True, True, False]`

`>>>`

False`and (False : repeat True) -- Infinite list [False,True,True,True,...`

`>>>`

* Hangs forever *`and (repeat True)`

or :: Foldable t => t Bool -> Bool Source #

`or`

returns the disjunction of a container of Bools. For the
result to be `False`

, the container must be finite; `True`

, however,
results from a `True`

value finitely far from the left end.

#### Examples

Basic usage:

`>>>`

False`or []`

`>>>`

True`or [True]`

`>>>`

False`or [False]`

`>>>`

True`or [True, True, False]`

`>>>`

True`or (True : repeat False) -- Infinite list [True,False,False,False,...`

`>>>`

* Hangs forever *`or (repeat False)`

any :: Foldable t => (a -> Bool) -> t a -> Bool Source #

Determines whether any element of the structure satisfies the predicate.

#### Examples

Basic usage:

`>>>`

False`any (> 3) []`

`>>>`

False`any (> 3) [1,2]`

`>>>`

True`any (> 3) [1,2,3,4,5]`

`>>>`

True`any (> 3) [1..]`

`>>>`

* Hangs forever *`any (> 3) [0, -1..]`

all :: Foldable t => (a -> Bool) -> t a -> Bool Source #

Determines whether all elements of the structure satisfy the predicate.

#### Examples

Basic usage:

`>>>`

True`all (> 3) []`

`>>>`

False`all (> 3) [1,2]`

`>>>`

False`all (> 3) [1,2,3,4,5]`

`>>>`

False`all (> 3) [1..]`

`>>>`

* Hangs forever *`all (> 3) [4..]`

sum :: (Foldable t, Num a) => t a -> a Source #

The `sum`

function computes the sum of the numbers of a structure.

#### Examples

Basic usage:

`>>>`

0`sum []`

`>>>`

42`sum [42]`

`>>>`

55`sum [1..10]`

`>>>`

7.8`sum [4.1, 2.0, 1.7]`

`>>>`

* Hangs forever *`sum [1..]`

*Since: base-4.8.0.0*

product :: (Foldable t, Num a) => t a -> a Source #

The `product`

function computes the product of the numbers of a
structure.

#### Examples

Basic usage:

`>>>`

1`product []`

`>>>`

42`product [42]`

`>>>`

3628800`product [1..10]`

`>>>`

13.939999999999998`product [4.1, 2.0, 1.7]`

`>>>`

* Hangs forever *`product [1..]`

*Since: base-4.8.0.0*

maximum :: (Foldable t, Ord a) => t a -> a Source #

The largest element of a non-empty structure.

This function is non-total and will raise a runtime exception if the structure happens to be empty. A structure that supports random access and maintains its elements in order should provide a specialised implementation to return the maximum in faster than linear time.

#### Examples

Basic usage:

`>>>`

10`maximum [1..10]`

`>>>`

*** Exception: Prelude.maximum: empty list`maximum []`

`>>>`

*** Exception: maximum: empty structure`maximum Nothing`

WARNING: This function is partial for possibly-empty structures like lists.

*Since: base-4.8.0.0*

minimum :: (Foldable t, Ord a) => t a -> a Source #

The least element of a non-empty structure.

This function is non-total and will raise a runtime exception if the structure happens to be empty. A structure that supports random access and maintains its elements in order should provide a specialised implementation to return the minimum in faster than linear time.

#### Examples

Basic usage:

`>>>`

1`minimum [1..10]`

`>>>`

*** Exception: Prelude.minimum: empty list`minimum []`

`>>>`

*** Exception: minimum: empty structure`minimum Nothing`

WARNING: This function is partial for possibly-empty structures like lists.

*Since: base-4.8.0.0*

# Building lists

## Scans

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

\(\mathcal{O}(n)\). `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

#### Examples

`>>>`

[0,1,3,6,10]`scanl (+) 0 [1..4]`

`>>>`

[42]`scanl (+) 42 []`

`>>>`

[100,99,97,94,90]`scanl (-) 100 [1..4]`

`>>>`

["foo","afoo","bafoo","cbafoo","dcbafoo"]`scanl (\reversedString nextChar -> nextChar : reversedString) "foo" ['a', 'b', 'c', 'd']`

`>>>`

[0,1,3,6,10,15,21,28,36,45]`take 10 (scanl (+) 0 [1..])`

`>>>`

"a"`take 1 (scanl undefined 'a' undefined)`

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

\(\mathcal{O}(n)\). `scanl1`

is a variant of `scanl`

that has no starting
value argument:

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

#### Examples

`>>>`

[1,3,6,10]`scanl1 (+) [1..4]`

`>>>`

[]`scanl1 (+) []`

`>>>`

[1,-1,-4,-8]`scanl1 (-) [1..4]`

`>>>`

[True,False,False,False]`scanl1 (&&) [True, False, True, True]`

`>>>`

[False,False,True,True]`scanl1 (||) [False, False, True, True]`

`>>>`

[1,3,6,10,15,21,28,36,45,55]`take 10 (scanl1 (+) [1..])`

`>>>`

"a"`take 1 (scanl1 undefined ('a' : undefined))`

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

\(\mathcal{O}(n)\). `scanr`

is the right-to-left dual of `scanl`

. Note that the order of parameters on the accumulating function are reversed compared to `scanl`

.
Also note that

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

#### Examples

`>>>`

[10,9,7,4,0]`scanr (+) 0 [1..4]`

`>>>`

[42]`scanr (+) 42 []`

`>>>`

[98,-97,99,-96,100]`scanr (-) 100 [1..4]`

`>>>`

["abcdfoo","bcdfoo","cdfoo","dfoo","foo"]`scanr (\nextChar reversedString -> nextChar : reversedString) "foo" ['a', 'b', 'c', 'd']`

`>>>`

*** Exception: stack overflow`force $ scanr (+) 0 [1..]`

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

\(\mathcal{O}(n)\). `scanr1`

is a variant of `scanr`

that has no starting
value argument.

#### Examples

`>>>`

[10,9,7,4]`scanr1 (+) [1..4]`

`>>>`

[]`scanr1 (+) []`

`>>>`

[-2,3,-1,4]`scanr1 (-) [1..4]`

`>>>`

[False,False,True,True]`scanr1 (&&) [True, False, True, True]`

`>>>`

[True,True,False,False]`scanr1 (||) [True, True, False, False]`

`>>>`

*** Exception: stack overflow`force $ scanr1 (+) [1..]`

## Accumulating maps

mapAccumL :: Traversable t => (s -> a -> (s, b)) -> s -> t a -> (s, t b) Source #

The `mapAccumL`

function behaves like a combination of `fmap`

and `foldl`

; it applies a function to each element of a structure,
passing an accumulating parameter from left to right, and returning
a final value of this accumulator together with the new structure.

#### Examples

Basic usage:

`>>>`

(55,[0,1,3,6,10,15,21,28,36,45])`mapAccumL (\a b -> (a + b, a)) 0 [1..10]`

`>>>`

("012345",["0","01","012","0123","01234"])`mapAccumL (\a b -> (a <> show b, a)) "0" [1..5]`

mapAccumR :: Traversable t => (s -> a -> (s, b)) -> s -> t a -> (s, t b) Source #

The `mapAccumR`

function behaves like a combination of `fmap`

and `foldr`

; it applies a function to each element of a structure,
passing an accumulating parameter from right to left, and returning
a final value of this accumulator together with the new structure.

#### Examples

Basic usage:

`>>>`

(55,[54,52,49,45,40,34,27,19,10,0])`mapAccumR (\a b -> (a + b, a)) 0 [1..10]`

`>>>`

("054321",["05432","0543","054","05","0"])`mapAccumR (\a b -> (a <> show b, a)) "0" [1..5]`

## Infinite lists

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

`iterate`

`f x`

returns an infinite list of repeated applications
of `f`

to `x`

:

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

#### Laziness

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.

`>>>`

[42]`take 1 $ iterate undefined 42`

#### Examples

`>>>`

[True,False,True,False,True,False,True,False,True,False]`take 10 $ iterate not True`

`>>>`

[42,45,48,51,54,57,60,63,66,69]`take 10 $ iterate (+3) 42`

`iterate id == `

:`repeat`

`>>>`

[1,1,1,1,1,1,1,1,1,1]`take 10 $ iterate id 1`

`repeat`

`x`

is an infinite list, with `x`

the value of every element.

#### Examples

`>>>`

[17,17,17,17,17,17,17,17,17, 17]`take 10 $ repeat 17`

`>>>`

[*** Exception: Prelude.undefined`repeat undefined`

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

`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.

#### Examples

`>>>`

[]`replicate 0 True`

`>>>`

[]`replicate (-1) True`

`>>>`

[True,True,True,True]`replicate 4 True`

cycle :: HasCallStack => [a] -> [a] Source #

`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.

#### Examples

`>>>`

*** Exception: Prelude.cycle: empty list`cycle []`

`>>>`

[42,42,42,42,42,42,42,42,42,42]`take 10 (cycle [42])`

`>>>`

[2,5,7,2,5,7,2,5,7,2]`take 10 (cycle [2, 5, 7])`

`>>>`

[42]`take 1 (cycle (42 : undefined))`

## Unfolding

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

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

#### Laziness

`>>>`

"a"`take 1 (unfoldr (\x -> Just (x, undefined)) 'a')`

#### Examples

`>>>`

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

`>>>`

[0,1,1,2,3,5,8,13,21,54]`take 10 $ unfoldr (\(x, y) -> Just (x, (y, x + y))) (0, 1)`

# Sublists

## Extracting sublists

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

`take`

`n`

, applied to a list `xs`

, returns the prefix of `xs`

of length `n`

, or `xs`

itself if `n >= `

.`length`

xs

It is an instance of the more general `genericTake`

,
in which `n`

may be of any integral type.

#### Laziness

`>>>`

[]`take 0 undefined`

`>>>`

[1,2]`take 2 (1 : 2 : undefined)`

#### Examples

`>>>`

"Hello"`take 5 "Hello World!"`

`>>>`

[1,2,3]`take 3 [1,2,3,4,5]`

`>>>`

[1,2]`take 3 [1,2]`

`>>>`

[]`take 3 []`

`>>>`

[]`take (-1) [1,2]`

`>>>`

[]`take 0 [1,2]`

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

`drop`

`n xs`

returns the suffix of `xs`

after the first `n`

elements, or `[]`

if `n >= `

.`length`

xs

It is an instance of the more general `genericDrop`

,
in which `n`

may be of any integral type.

#### Examples

`>>>`

"World!"`drop 6 "Hello World!"`

`>>>`

[4,5]`drop 3 [1,2,3,4,5]`

`>>>`

[]`drop 3 [1,2]`

`>>>`

[]`drop 3 []`

`>>>`

[1,2]`drop (-1) [1,2]`

`>>>`

[1,2]`drop 0 [1,2]`

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

`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`

is an instance of the more general `genericSplitAt`

,
in which `n`

may be of any integral type.

#### Laziness

It is equivalent to `(`

unless `take`

n xs, `drop`

n xs)`n`

is `_|_`

:
`splitAt _|_ xs = _|_`

, not `(_|_, _|_)`

).

The first component of the tuple is produced lazily:

`>>>`

[]`fst (splitAt 0 undefined)`

`>>>`

[1]`take 1 (fst (splitAt 10 (1 : undefined)))`

#### Examples

`>>>`

("Hello ","World!")`splitAt 6 "Hello World!"`

`>>>`

([1,2,3],[4,5])`splitAt 3 [1,2,3,4,5]`

`>>>`

([1],[2,3])`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]`

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

`takeWhile`

, applied to a predicate `p`

and a list `xs`

, returns the
longest prefix (possibly empty) of `xs`

of elements that satisfy `p`

.

#### Laziness

`>>>`

*** Exception: Prelude.undefined`takeWhile (const False) undefined`

`>>>`

[]`takeWhile (const False) (undefined : undefined)`

`>>>`

[1]`take 1 (takeWhile (const True) (1 : undefined))`

#### Examples

`>>>`

[1,2]`takeWhile (< 3) [1,2,3,4,1,2,3,4]`

`>>>`

[1,2,3]`takeWhile (< 9) [1,2,3]`

`>>>`

[]`takeWhile (< 0) [1,2,3]`

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

The `dropWhileEnd`

function drops the largest suffix of a list
in which the given predicate holds for all elements.

#### Laziness

This function is lazy in spine, but strict in elements,
which makes it different from `reverse`

`.`

`dropWhile`

`p`

`.`

`reverse`

,
which is strict in spine, but lazy in elements. For instance:

`>>>`

[1]`take 1 (dropWhileEnd (< 0) (1 : undefined))`

`>>>`

*** Exception: Prelude.undefined`take 1 (reverse $ dropWhile (< 0) $ reverse (1 : undefined))`

but on the other hand

`>>>`

*** Exception: Prelude.undefined`last (dropWhileEnd (< 0) [undefined, 1])`

`>>>`

1`last (reverse $ dropWhile (< 0) $ reverse [undefined, 1])`

#### Examples

`>>>`

"foo"`dropWhileEnd isSpace "foo\n"`

`>>>`

"foo bar"`dropWhileEnd isSpace "foo bar"`

`>>>`

[1,2,3,4,5,6,7,8,9,10]`dropWhileEnd (> 10) [1..20]`

*Since: base-4.5.0.0*

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

`span`

, applied to a predicate `p`

and a list `xs`

, returns a tuple where
first element is the longest prefix (possibly empty) of `xs`

of elements that
satisfy `p`

and second element is the remainder of the list:

`span`

`p xs`

is equivalent to `(`

, even if `takeWhile`

p xs, `dropWhile`

p xs)`p`

is `_|_`

.

#### Laziness

`>>>`

([],[])`span undefined []`

`>>>`

*** Exception: Prelude.undefined`fst (span (const False) undefined)`

`>>>`

[]`fst (span (const False) (undefined : undefined))`

`>>>`

[1]`take 1 (fst (span (const True) (1 : undefined)))`

`span`

produces the first component of the tuple lazily:

`>>>`

[1,2,3,4,5,6,7,8,9,10]`take 10 (fst (span (const True) [1..]))`

#### Examples

`>>>`

([1,2],[3,4,1,2,3,4])`span (< 3) [1,2,3,4,1,2,3,4]`

`>>>`

([1,2,3],[])`span (< 9) [1,2,3]`

`>>>`

([],[1,2,3])`span (< 0) [1,2,3]`

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

`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`

`p`

is equivalent to

and consequently to `span`

(`not`

. p)`(`

,
even if `takeWhile`

(`not`

. p) xs, `dropWhile`

(`not`

. p) xs)`p`

is `_|_`

.

#### Laziness

`>>>`

([],[])`break undefined []`

`>>>`

*** Exception: Prelude.undefined`fst (break (const True) undefined)`

`>>>`

[]`fst (break (const True) (undefined : undefined))`

`>>>`

[1]`take 1 (fst (break (const False) (1 : undefined)))`

`break`

produces the first component of the tuple lazily:

`>>>`

[1,2,3,4,5,6,7,8,9,10]`take 10 (fst (break (const False) [1..]))`

#### Examples

`>>>`

([1,2,3],[4,1,2,3,4])`break (> 3) [1,2,3,4,1,2,3,4]`

`>>>`

([],[1,2,3])`break (< 9) [1,2,3]`

`>>>`

([1,2,3],[])`break (> 9) [1,2,3]`

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

\(\mathcal{O}(\min(m,n))\). 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.

##### Examples

`>>>`

Just "bar"`stripPrefix "foo" "foobar"`

`>>>`

Just ""`stripPrefix "foo" "foo"`

`>>>`

Nothing`stripPrefix "foo" "barfoo"`

`>>>`

Nothing`stripPrefix "foo" "barfoobaz"`

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

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 is non-empty, all elements are equal to the
first one, and consecutive equal elements of the input end up in the
same element of the output list.

`group`

is a special case of `groupBy`

, which allows the programmer to supply
their own equality test.

It's often preferable to use `Data.List.NonEmpty.`

`group`

,
which provides type-level guarantees of non-emptiness of inner lists.
A common idiom to squash repeating elements `map`

`head`

`.`

`group`

is better served by
`map`

`Data.List.NonEmpty.`

`head`

`.`

`Data.List.NonEmpty.`

`group`

because it avoids partial functions.

#### Examples

`>>>`

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

`>>>`

[[1,1,1],[2,2],[3],[4],[5,5]]`group [1, 1, 1, 2, 2, 3, 4, 5, 5]`

inits :: [a] -> [[a]] Source #

The `inits`

function returns all initial segments of the argument,
shortest first.

`inits`

is semantically equivalent to

,
but under the hood uses a queue to amortize costs of `map`

`reverse`

. `scanl`

(`flip`

(:)) []`reverse`

.

#### Laziness

Note that `inits`

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

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

#### Examples

`>>>`

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

`>>>`

[[]]`inits []`

inits is productive on infinite lists:

`>>>`

[[],[1],[1,2],[1,2,3],[1,2,3,4]]`take 5 $ inits [1..]`

tails :: [a] -> [[a]] Source #

\(\mathcal{O}(n)\). The `tails`

function returns all final segments of the
argument, longest first.

#### Laziness

Note that `tails`

has the following strictness property:
`tails _|_ = _|_ : _|_`

`>>>`

[*** Exception: Prelude.undefined`tails undefined`

`>>>`

[[1, 2], [2], []]`drop 1 (tails [undefined, 1, 2])`

#### Examples

`>>>`

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

`>>>`

[[1,2,3],[2,3],[3],[]]`tails [1, 2, 3]`

`>>>`

[[]]`tails []`

## Predicates

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

\(\mathcal{O}(\min(m,n))\). The `isPrefixOf`

function takes two lists and
returns `True`

iff the first list is a prefix of the second.

#### Examples

`>>>`

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

`>>>`

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

For the result to be `True`

, the first list must be finite;
`False`

, however, results from any mismatch:

`>>>`

False`[0..] `isPrefixOf` [1..]`

`>>>`

False`[0..] `isPrefixOf` [0..99]`

`>>>`

True`[0..99] `isPrefixOf` [0..]`

`>>>`

* Hangs forever *`[0..] `isPrefixOf` [0..]`

`isPrefixOf`

shortcuts when the first argument is empty:

`>>>`

True`isPrefixOf [] undefined`

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

The `isSuffixOf`

function takes two lists and returns `True`

iff
the first list is a suffix of the second.

#### Examples

`>>>`

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

`>>>`

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

The second list must be finite; however the first list may be infinite:

`>>>`

False`[0..] `isSuffixOf` [0..99]`

`>>>`

* Hangs forever *`[0..99] `isSuffixOf` [0..]`

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

The `isInfixOf`

function takes two lists and returns `True`

iff the first list is contained, wholly and intact,
anywhere within the second.

#### Examples

`>>>`

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

`>>>`

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

For the result to be `True`

, the first list must be finite;
for the result to be `False`

, the second list must be finite:

`>>>`

True`[20..50] `isInfixOf` [0..]`

`>>>`

False`[0..] `isInfixOf` [20..50]`

`>>>`

* Hangs forever *`[0..] `isInfixOf` [0..]`

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

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.

is equivalent to `isSubsequenceOf`

x y`x ``

.`elem`

` (`subsequences`

y)

Note: `isSubsequenceOf`

is often used in infix form.

#### Examples

`>>>`

True`"GHC" `isSubsequenceOf` "The Glorious Haskell Compiler"`

`>>>`

True`['a','d'..'z'] `isSubsequenceOf` ['a'..'z']`

`>>>`

False`[1..10] `isSubsequenceOf` [10,9..0]`

For the result to be `True`

, the first list must be finite;
for the result to be `False`

, the second list must be finite:

`>>>`

True`[0,2..10] `isSubsequenceOf` [0..]`

`>>>`

False`[0..] `isSubsequenceOf` [0,2..10]`

`>>>`

* Hangs forever*`[0,2..] `isSubsequenceOf` [0..]`

*Since: base-4.8.0.0*

# Searching lists

## Searching by equality

elem :: (Foldable t, Eq a) => a -> t a -> Bool infix 4 Source #

Does the element occur in the structure?

Note: `elem`

is often used in infix form.

#### Examples

Basic usage:

`>>>`

False`3 `elem` []`

`>>>`

False`3 `elem` [1,2]`

`>>>`

True`3 `elem` [1,2,3,4,5]`

For infinite structures, the default implementation of `elem`

terminates if the sought-after value exists at a finite distance
from the left side of the structure:

`>>>`

True`3 `elem` [1..]`

`>>>`

* Hangs forever *`3 `elem` ([4..] ++ [3])`

*Since: base-4.8.0.0*

notElem :: (Foldable t, Eq a) => a -> t a -> Bool infix 4 Source #

`notElem`

is the negation of `elem`

.

#### Examples

Basic usage:

`>>>`

True`3 `notElem` []`

`>>>`

True`3 `notElem` [1,2]`

`>>>`

False`3 `notElem` [1,2,3,4,5]`

For infinite structures, `notElem`

terminates if the value exists at a
finite distance from the left side of the structure:

`>>>`

False`3 `notElem` [1..]`

`>>>`

* Hangs forever *`3 `notElem` ([4..] ++ [3])`

## Searching with a predicate

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

\(\mathcal{O}(n)\). `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]

#### Examples

`>>>`

[1,3]`filter odd [1, 2, 3]`

`>>>`

["Hello","World"]`filter (\l -> length l > 3) ["Hello", ", ", "World", "!"]`

`>>>`

[1,2,4,2,1]`filter (/= 3) [1, 2, 3, 4, 3, 2, 1]`

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

The `partition`

function takes a predicate and 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)

#### Examples

`>>>`

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

`>>>`

([2,4,6,8,10],[1,3,5,7,9])`partition even [1..10]`

`>>>`

([1,2,3,4],[5,6,7,8,9,10])`partition (< 5) [1..10]`

# Indexing lists

These functions treat a list `xs`

as an indexed collection,
with indices ranging from 0 to

.`length`

xs - 1

(!?) :: [a] -> Int -> Maybe a infixl 9 Source #

List index (subscript) operator, starting from 0. Returns `Nothing`

if the index is out of bounds

This is the total variant of the partial `!!`

operator.

WARNING: This function takes linear time in the index.

#### Examples

`>>>`

Just 'a'`['a', 'b', 'c'] !? 0`

`>>>`

Just 'c'`['a', 'b', 'c'] !? 2`

`>>>`

Nothing`['a', 'b', 'c'] !? 3`

`>>>`

Nothing`['a', 'b', 'c'] !? (-1)`

(!!) :: HasCallStack => [a] -> Int -> a infixl 9 Source #

List index (subscript) operator, starting from 0.
It is an instance of the more general `genericIndex`

,
which takes an index of any integral type.

WARNING: This function is partial, and should only be used if you are
sure that the indexing will not fail. Otherwise, use `!?`

.

WARNING: This function takes linear time in the index.

#### Examples

`>>>`

'a'`['a', 'b', 'c'] !! 0`

`>>>`

'c'`['a', 'b', 'c'] !! 2`

`>>>`

*** Exception: Prelude.!!: index too large`['a', 'b', 'c'] !! 3`

`>>>`

*** Exception: Prelude.!!: negative index`['a', 'b', 'c'] !! (-1)`

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

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.
For the result to be `Nothing`

, the list must be finite.

#### Examples

`>>>`

Just 4`elemIndex 4 [0..]`

`>>>`

Nothing`elemIndex 'o' "haskell"`

`>>>`

* hangs forever *`elemIndex 0 [1..]`

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

The `elemIndices`

function extends `elemIndex`

, by returning the
indices of all elements equal to the query element, in ascending order.

#### Examples

`>>>`

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

`>>>`

[0,3]`elemIndices 1 [1, 2, 3, 1, 2, 3]`

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

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.
For the result to be `Nothing`

, the list must be finite.

#### Examples

`>>>`

Just 5`findIndex isSpace "Hello World!"`

`>>>`

Nothing`findIndex odd [0, 2, 4, 6]`

`>>>`

Just 1`findIndex even [1..]`

`>>>`

* hangs forever *`findIndex odd [0, 2 ..]`

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

The `findIndices`

function extends `findIndex`

, by returning the
indices of all elements satisfying the predicate, in ascending order.

#### Examples

`>>>`

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

`>>>`

[1,3]`findIndices (\l -> length l > 3) ["a", "bcde", "fgh", "ijklmnop"]`

# Zipping and unzipping lists

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

\(\mathcal{O}(\min(m,n))\). `zip`

takes two lists and returns a list of
corresponding pairs.

`zip`

is right-lazy:

`>>>`

[]`zip [] undefined`

`>>>`

*** Exception: Prelude.undefined ...`zip undefined []`

`zip`

is capable of list fusion, but it is restricted to its
first list argument and its resulting list.

#### Examples

`>>>`

[(1,'a'),(2,'b'),(3,'c')]`zip [1, 2, 3] ['a', 'b', 'c']`

If one input list is shorter than the other, excess elements of the longer list are discarded, even if one of the lists is infinite:

`>>>`

[(1,'a')]`zip [1] ['a', 'b']`

`>>>`

[(1,'a')]`zip [1, 2] ['a']`

`>>>`

[]`zip [] [1..]`

`>>>`

[]`zip [1..] []`

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

\(\mathcal{O}(\min(m,n))\). `zipWith`

generalises `zip`

by zipping with the
function given as the first argument, instead of a tupling function.

zipWith (,) xs ys == zip xs ys zipWith f [x1,x2,x3..] [y1,y2,y3..] == [f x1 y1, f x2 y2, f x3 y3..]

`zipWith`

is right-lazy:

`>>>`

`let f = undefined`

`>>>`

[]`zipWith f [] undefined`

`zipWith`

is capable of list fusion, but it is restricted to its
first list argument and its resulting list.

#### Examples

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

\(\mathcal{O}(\min(l,m,n))\). The `zipWith3`

function takes a function which combines three
elements, as well as three lists and returns a list of the function applied
to corresponding elements, analogous to `zipWith`

.
It is capable of list fusion, but it is restricted to its
first list argument and its resulting list.

zipWith3 (,,) xs ys zs == zip3 xs ys zs zipWith3 f [x1,x2,x3..] [y1,y2,y3..] [z1,z2,z3..] == [f x1 y1 z1, f x2 y2 z2, f x3 y3 z3..]

#### Examples

`>>>`

["1ax","2by","3cz"]`zipWith3 (\x y z -> [x, y, z]) "123" "abc" "xyz"`

`>>>`

[11,18,27]`zipWith3 (\x y z -> (x * y) + z) [1, 2, 3] [4, 5, 6] [7, 8, 9]`

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

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

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

`unzip`

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

#### Examples

`>>>`

([],[])`unzip []`

`>>>`

([1,2],"ab")`unzip [(1, 'a'), (2, 'b')]`

# Special lists

## Functions on strings

lines :: String -> [String] Source #

Splits the argument into a list of *lines* stripped of their terminating
`\n`

characters. The `\n`

terminator is optional in a final non-empty
line of the argument string.

When the argument string is empty, or ends in a `\n`

character, it can be
recovered by passing the result of `lines`

to the `unlines`

function.
Otherwise, `unlines`

appends the missing terminating `\n`

. This makes
`unlines . lines`

*idempotent*:

(unlines . lines) . (unlines . lines) = (unlines . lines)

#### Examples

`>>>`

[]`lines "" -- empty input contains no lines`

`>>>`

[""]`lines "\n" -- single empty line`

`>>>`

["one"]`lines "one" -- single unterminated line`

`>>>`

["one"]`lines "one\n" -- single non-empty line`

`>>>`

["one",""]`lines "one\n\n" -- second line is empty`

`>>>`

["one","two"]`lines "one\ntwo" -- second line is unterminated`

`>>>`

["one","two"]`lines "one\ntwo\n" -- two non-empty lines`

## "Set" operations

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

\(\mathcal{O}(n^2)\). 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.

If there exists `instance Ord a`

, it's faster to use `nubOrd`

from the `containers`

package
(link to the latest online documentation),
which takes only \(\mathcal{O}(n \log d)\) time where `d`

is the number of
distinct elements in the list.

Another approach to speed up `nub`

is to use
`map`

`Data.List.NonEmpty.`

`head`

. `Data.List.NonEmpty.`

`group`

. `sort`

,
which takes \(\mathcal{O}(n \log n)\) time, requires `instance Ord a`

and doesn't
preserve the order.

#### Examples

`>>>`

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

`>>>`

"helo, wrd!"`nub "hello, world!"`

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

\(\mathcal{O}(n)\). `delete`

`x`

removes the first occurrence of `x`

from
its list argument.

It is a special case of `deleteBy`

, which allows the programmer to
supply their own equality test.

#### Examples

`>>>`

"bnana"`delete 'a' "banana"`

`>>>`

["haskell","is","awesome"]`delete "not" ["haskell", "is", "not", "awesome"]`

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

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`

.

It is a special case of `deleteFirstsBy`

, which allows the programmer
to supply their own equality test.

#### Examples

`>>>`

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

The second list must be finite, but the first may be infinite.

`>>>`

[0,1,5,6,7]`take 5 ([0..] \\ [2..4])`

`>>>`

* Hangs forever *`take 5 ([0..] \\ [2..])`

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

The `union`

function returns the list union of the two lists.
It is a special case of `unionBy`

, which allows the programmer to supply
their own equality test.

#### Examples

`>>>`

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

If equal elements are present in both lists, an element from the first list will be used. If the second list contains equal elements, only the first one will be retained:

`>>>`

`import Data.Semigroup(Arg(..))`

`>>>`

[Arg () "dog"]`union [Arg () "dog"] [Arg () "cow"]`

`>>>`

[Arg () "dog"]`union [] [Arg () "dog", Arg () "cow"]`

However if the first list contains duplicates, so will the result:

`>>>`

"cootduk"`"coot" `union` "duck"`

`>>>`

"duckot"`"duck" `union` "coot"`

`union`

is productive even if both arguments are infinite.

`>>>`

[0,2,4,6,8,10,12..`[0, 2 ..] `union` [1, 3 ..]`

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

The `intersect`

function takes the list intersection of two lists.
It is a special case of `intersectBy`

, which allows the programmer to
supply their own equality test.

##### Examples

`>>>`

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

If equal elements are present in both lists, an element from the first list will be used, and all duplicates from the second list quashed:

`>>>`

`import Data.Semigroup`

`>>>`

[Arg () "dog"]`intersect [Arg () "dog"] [Arg () "cow", Arg () "cat"]`

However if the first list contains duplicates, so will the result.

`>>>`

"oo"`"coot" `intersect` "heron"`

`>>>`

"o"`"heron" `intersect` "coot"`

If the second list is infinite, `intersect`

either hangs
or returns its first argument in full. Otherwise if the first list
is infinite, `intersect`

might be productive:

`>>>`

[100,101,102,103...`intersect [100..] [0..]`

`>>>`

* Hangs forever *`intersect [0] [1..]`

`>>>`

* Hangs forever *`intersect [1..] [0]`

`>>>`

[2,2,2,2...`intersect (cycle [1..3]) [2]`

## Ordered lists

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

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 lowest to highest, keeping duplicates in the order they appeared in the input.

The argument must be finite.

#### Examples

`>>>`

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

`>>>`

"aehklls"`sort "haskell"`

`>>>`

`import Data.Semigroup(Arg(..))`

`>>>`

[Arg ":)" 0,Arg ":)" 1,Arg ":3" 0,Arg ":D" 0,Arg ":D" 1]`sort [Arg ":)" 0, Arg ":D" 0, Arg ":)" 1, Arg ":3" 0, Arg ":D" 1]`

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

Sort a list by comparing the results of a key function applied to each
element.

is equivalent to `sortOn`

f

, but has the
performance advantage of only evaluating `sortBy`

(`comparing`

f)`f`

once for each element in the
input list. This is called the decorate-sort-undecorate paradigm, or
Schwartzian transform.

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

The argument must be finite.

#### Examples

`>>>`

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

`>>>`

["jim","pam","creed","kevin","dwight","michael"]`sortOn length ["jim", "creed", "pam", "michael", "dwight", "kevin"]`

#### Performance notes

This function minimises the projections performed, by materialising the projections in an intermediate list.

For trivial projections, you should prefer using `sortBy`

with
`comparing`

, for example:

`>>>`

[(1,3),(2,2),(3,1)]`sortBy (comparing fst) [(3, 1), (2, 2), (1, 3)]`

Or, for the exact same API as `sortOn`

, you can use `sortBy . comparing`:

`>>>`

[(1,3),(2,2),(3,1)]`(sortBy . comparing) fst [(3, 1), (2, 2), (1, 3)]`

*Since: base-4.8.0.0*

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

\(\mathcal{O}(n)\). 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.

#### Examples

`>>>`

[-1,1,2,3]`insert (-1) [1, 2, 3]`

`>>>`

"abcdefg"`insert 'd' "abcefg"`

`>>>`

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

# Generalized functions

## The "`By`

" operations

By convention, overloaded functions have a non-overloaded
counterpart whose name is suffixed with ``By`

'.

It is often convenient to use these functions together with
`on`

, for instance

.`sortBy`

(`compare`

``on``

`fst`

)

### User-supplied equality (replacing an `Eq`

context)

The predicate is assumed to define an equivalence.

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

The `nubBy`

function behaves just like `nub`

, except it uses a
user-supplied equality predicate instead of the overloaded `(==)`

function.

#### Examples

`>>>`

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

`>>>`

[2,2,2]`nubBy (/=) [2, 7, 1, 8, 2, 8, 1, 8, 2, 8]`

`>>>`

[1,2,3,5,5]`nubBy (>) [1, 2, 3, 2, 1, 5, 4, 5, 3, 2]`

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

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. This is the non-overloaded version of `(\\)`

.

(\\) == deleteFirstsBy (==)

The second list must be finite, but the first may be infinite.

#### Examples

`>>>`

[4,5,6,7,8,9,10]`deleteFirstsBy (>) [1..10] [3, 4, 5]`

`>>>`

[4,5,6,7,8,9,10]`deleteFirstsBy (/=) [1..10] [1, 3, 5]`

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

The `intersectBy`

function is the non-overloaded version of `intersect`

.
It is productive for infinite arguments only if the first one
is a subset of the second.

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

The `groupBy`

function is the non-overloaded version of `group`

.

When a supplied relation is not transitive, it is important to remember that equality is checked against the first element in the group, not against the nearest neighbour:

`>>>`

[[0,1,2,3,4],[5,6,7,8,9],[10,11,12,13,14],[15,16,17,18,19]]`groupBy (\a b -> b - a < 5) [0..19]`

It's often preferable to use `Data.List.NonEmpty.`

`groupBy`

,
which provides type-level guarantees of non-emptiness of inner lists.

#### Examples

`>>>`

[[1],[1],[1,2,3],[1,4,4,5]]`groupBy (/=) [1, 1, 1, 2, 3, 1, 4, 4, 5]`

`>>>`

[[1],[3],[5,1,4,2],[6,5,4]]`groupBy (>) [1, 3, 5, 1, 4, 2, 6, 5, 4]`

`>>>`

[[True,False],[True,False,False,False],[True]]`groupBy (const not) [True, False, True, False, False, False, True]`

### User-supplied comparison (replacing an `Ord`

context)

The function is assumed to define a total ordering.

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

The `sortBy`

function is the non-overloaded version of `sort`

.
The argument must be finite.

The supplied comparison relation is supposed to be reflexive and antisymmetric,
otherwise, e. g., for `_ _ -> GT`

, the ordered list simply does not exist.
The relation is also expected to be transitive: if it is not then `sortBy`

might fail to find an ordered permutation, even if it exists.

#### Examples

`>>>`

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

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

\(\mathcal{O}(n)\). The non-overloaded version of `insert`

.

#### Examples

`>>>`

[[1],[1,2],[1,2,3],[1,2,3,4]]`insertBy (\x y -> compare (length x) (length y)) [1, 2] [[1], [1, 2, 3], [1, 2, 3, 4]]`

maximumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a Source #

The largest element of a non-empty structure with respect to the given comparison function.

#### Examples

Basic usage:

`>>>`

"Longest"`maximumBy (compare `on` length) ["Hello", "World", "!", "Longest", "bar"]`

WARNING: This function is partial for possibly-empty structures like lists.

minimumBy :: Foldable t => (a -> a -> Ordering) -> t a -> a Source #

The least element of a non-empty structure with respect to the given comparison function.

#### Examples

Basic usage:

`>>>`

"!"`minimumBy (compare `on` length) ["Hello", "World", "!", "Longest", "bar"]`

WARNING: This function is partial for possibly-empty structures like lists.

## The "`generic`

" operations

The prefix ``generic`

' indicates an overloaded function that
is a generalized version of a Prelude function.

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

\(\mathcal{O}(n)\). 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`

.

#### Examples

`>>>`

3`genericLength [1, 2, 3] :: Int`

`>>>`

3.0`genericLength [1, 2, 3] :: Float`

Users should take care to pick a return type that is wide enough to contain
the full length of the list. If the width is insufficient, the overflow
behaviour will depend on the `(+)`

implementation in the selected `Num`

instance. The following example overflows because the actual list length
of 200 lies outside of the `Int8`

range of `-128..127`

.

`>>>`

-56`genericLength [1..200] :: Int8`

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

The `genericTake`

function is an overloaded version of `take`

, which
accepts any `Integral`

value as the number of elements to take.

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

The `genericDrop`

function is an overloaded version of `drop`

, which
accepts any `Integral`

value as the number of elements to drop.

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

The `genericSplitAt`

function is an overloaded version of `splitAt`

, which
accepts any `Integral`

value as the position at which to split.

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

The `genericIndex`

function is an overloaded version of `!!`

, which
accepts any `Integral`

value as the index.

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

The `genericReplicate`

function is an overloaded version of `replicate`

,
which accepts any `Integral`

value as the number of repetitions to make.