base-4.17.0.0: Basic libraries
Copyright(c) The University of Glasgow 1994-2002
Licensesee libraries/base/LICENSE
Maintainercvs-ghc@haskell.org
Stabilityinternal
Portabilitynon-portable (GHC Extensions)
Safe HaskellTrustworthy
LanguageHaskell2010

GHC.List

Description

The List data type and its operations

Synopsis

Documentation

data List a Source #

The builtin list type, usually written in its non-prefix form [a].

Examples

Expand

Unless the OverloadedLists extension is enabled, list literals are syntatic sugar for repeated applications of : and [].

>>> 1:2:3:4:[] == [1,2,3,4]
True

Similarly, unless the OverloadedStrings extension is enabled, string literals are syntactic sugar for a lists of characters.

>>> ['h','e','l','l','o'] == "hello"
True

Instances

Instances details
MonadFail List Source #

Since: base-4.9.0.0

Instance details

Defined in Control.Monad.Fail

Methods

fail :: String -> [a] Source #

MonadFix List Source #

Since: base-2.1

Instance details

Defined in Control.Monad.Fix

Methods

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

MonadZip List Source #

Since: base-4.3.1.0

Instance details

Defined in Control.Monad.Zip

Methods

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

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

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

Foldable List Source #

Since: base-2.1

Instance details

Defined in Data.Foldable

Methods

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 #

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

null :: [a] -> Bool Source #

length :: [a] -> Int Source #

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

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

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

sum :: Num a => [a] -> a Source #

product :: Num a => [a] -> a Source #

Eq1 List Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftEq :: (a -> b -> Bool) -> [a] -> [b] -> Bool Source #

Ord1 List Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftCompare :: (a -> b -> Ordering) -> [a] -> [b] -> Ordering Source #

Read1 List Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftReadsPrec :: (Int -> ReadS a) -> ReadS [a] -> Int -> ReadS [a] Source #

liftReadList :: (Int -> ReadS a) -> ReadS [a] -> ReadS [[a]] Source #

liftReadPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [a] Source #

liftReadListPrec :: ReadPrec a -> ReadPrec [a] -> ReadPrec [[a]] Source #

Show1 List Source #

Since: base-4.9.0.0

Instance details

Defined in Data.Functor.Classes

Methods

liftShowsPrec :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> Int -> [a] -> ShowS Source #

liftShowList :: (Int -> a -> ShowS) -> ([a] -> ShowS) -> [[a]] -> ShowS Source #

Traversable List Source #

Since: base-2.1

Instance details

Defined in Data.Traversable

Methods

traverse :: Applicative f => (a -> f b) -> [a] -> f [b] Source #

sequenceA :: Applicative f => [f a] -> f [a] Source #

mapM :: Monad m => (a -> m b) -> [a] -> m [b] Source #

sequence :: Monad m => [m a] -> m [a] Source #

Alternative List Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

empty :: [a] Source #

(<|>) :: [a] -> [a] -> [a] Source #

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

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

Applicative List Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

pure :: a -> [a] Source #

(<*>) :: [a -> b] -> [a] -> [b] Source #

liftA2 :: (a -> b -> c) -> [a] -> [b] -> [c] Source #

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

(<*) :: [a] -> [b] -> [a] Source #

Functor List Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

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

(<$) :: a -> [b] -> [a] Source #

Monad List Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

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

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

return :: a -> [a] Source #

MonadPlus List Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

mzero :: [a] Source #

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

Generic1 List Source # 
Instance details

Defined in GHC.Generics

Associated Types

type Rep1 List :: k -> Type Source #

Methods

from1 :: forall (a :: k). [a] -> Rep1 List a Source #

to1 :: forall (a :: k). Rep1 List a -> [a] Source #

Data a => Data [a] Source #

Since: base-4.0.0.0

Instance details

Defined in Data.Data

Methods

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 #

(a ~ Char) context was introduced in 4.9.0.0

Since: base-2.1

Instance details

Defined in Data.String

Methods

fromString :: String -> [a] Source #

Monoid [a] Source #

Since: base-2.1

Instance details

Defined in GHC.Base

Methods

mempty :: [a] Source #

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

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

Semigroup [a] Source #

Since: base-4.9.0.0

Instance details

Defined in GHC.Base

Methods

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

sconcat :: NonEmpty [a] -> [a] Source #

stimes :: Integral b => b -> [a] -> [a] Source #

Generic [a] Source # 
Instance details

Defined in GHC.Generics

Associated Types

type Rep [a] :: Type -> Type Source #

Methods

from :: [a] -> Rep [a] x Source #

to :: Rep [a] x -> [a] Source #

IsList [a] Source #

Since: base-4.7.0.0

Instance details

Defined in GHC.IsList

Associated Types

type Item [a] Source #

Methods

fromList :: [Item [a]] -> [a] Source #

fromListN :: Int -> [Item [a]] -> [a] Source #

toList :: [a] -> [Item [a]] Source #

Read a => Read [a] Source #

Since: base-2.1

Instance details

Defined in GHC.Read

Show a => Show [a] Source #

Since: base-2.1

Instance details

Defined in GHC.Show

Methods

showsPrec :: Int -> [a] -> ShowS Source #

show :: [a] -> String Source #

showList :: [[a]] -> ShowS Source #

IsChar c => PrintfArg [c] Source #

Since: base-2.1

Instance details

Defined in Text.Printf

IsChar c => PrintfType [c] Source #

Since: base-2.1

Instance details

Defined in Text.Printf

Methods

spr :: String -> [UPrintf] -> [c]

Eq a => Eq [a] 
Instance details

Defined in GHC.Classes

Methods

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

(/=) :: [a] -> [a] -> Bool Source #

Ord a => Ord [a] 
Instance details

Defined in GHC.Classes

Methods

compare :: [a] -> [a] -> Ordering Source #

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

(<=) :: [a] -> [a] -> Bool Source #

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

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

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

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

type Rep1 List Source #

Since: base-4.6.0.0

Instance details

Defined in GHC.Generics

type Rep [a] Source #

Since: base-4.6.0.0

Instance details

Defined in GHC.Generics

type Item [a] Source # 
Instance details

Defined in GHC.IsList

type Item [a] = a

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

foldr, 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)...)

foldr' :: (a -> b -> b) -> b -> [a] -> b Source #

foldr' is a variant of foldr that begins list reduction from the last element and evaluates the accumulator strictly as it unwinds the stack back to the beginning of the list. The input list must be finite, otherwise foldr' runs out of space (diverges).

Note that if the function that combines the accumulated value with each element is strict in the accumulator, other than a possible improvement in the constant factor, you get the same \(\mathcal{O}(n)\) space cost as with just foldr.

If you want a strict right fold in constant space, you need a structure that supports faster than \(\mathcal{O}(n)\) access to the right-most element, such as Seq from the containers package.

Use of this function is a hint that the [] structure may be a poor fit for the task at hand. If the order in which the elements are combined is not important, use foldl' instead.

>>> foldr' (+) [1..4]  -- Use foldl' instead!
10
>>> foldr' (&&) [True, False, True, True] -- Use foldr instead!
False
>>> foldr' (||) [False, False, True, True] -- Use foldr instead!
True

foldr1 :: HasCallStack => (a -> a -> a) -> [a] -> a Source #

foldr1 is a variant of foldr that has no starting value argument, and thus must be applied to non-empty lists. Note that unlike foldr, the accumulated value must be of the same type as the list elements.

>>> foldr1 (+) [1..4]
10
>>> foldr1 (+) []
*** Exception: Prelude.foldr1: empty list
>>> foldr1 (-) [1..4]
-2
>>> foldr1 (&&) [True, False, True, True]
False
>>> foldr1 (||) [False, False, True, True]
True
>>> force $ foldr1 (+) [1..]
*** Exception: stack overflow

foldl :: forall a b. (b -> a -> b) -> b -> [a] -> b Source #

foldl, 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

The list must be finite.

>>> foldl (+) 0 [1..4]
10
>>> foldl (+) 42 []
42
>>> foldl (-) 100 [1..4]
90
>>> foldl (\reversedString nextChar -> nextChar : reversedString) "foo" ['a', 'b', 'c', 'd']
"dcbafoo"
>>> foldl (+) 0 [1..]
* Hangs forever *

foldl' :: forall a b. (b -> a -> b) -> b -> [a] -> b Source #

A strict version of foldl.

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

foldl1 is a variant of foldl that has no starting value argument, and thus must be applied to non-empty lists. Note that unlike foldl, the accumulated value must be of the same type as the list elements.

>>> foldl1 (+) [1..4]
10
>>> foldl1 (+) []
*** Exception: Prelude.foldl1: empty list
>>> foldl1 (-) [1..4]
-8
>>> foldl1 (&&) [True, False, True, True]
False
>>> foldl1 (||) [False, False, True, True]
True
>>> foldl1 (+) [1..]
* Hangs forever *

null :: [a] -> Bool Source #

\(\mathcal{O}(1)\). Test whether a list is empty.

>>> null []
True
>>> null [1]
False
>>> null [1..]
False

length :: [a] -> Int Source #

\(\mathcal{O}(n)\). length returns the length of a finite list as an Int. It is an instance of the more general genericLength, the result type of which may be any kind of number.

>>> length []
0
>>> length ['a', 'b', 'c']
3
>>> length [1..]
* Hangs forever *

elem :: Eq a => a -> [a] -> Bool infix 4 Source #

elem is the list membership predicate, usually written in infix form, e.g., x `elem` xs. For the result to be False, the list must be finite; True, however, results from an element equal to x found at a finite index of a finite or infinite list.

>>> 3 `elem` []
False
>>> 3 `elem` [1,2]
False
>>> 3 `elem` [1,2,3,4,5]
True
>>> 3 `elem` [1..]
True
>>> 3 `elem` [4..]
* Hangs forever *

notElem :: Eq a => a -> [a] -> Bool infix 4 Source #

notElem is the negation of elem.

>>> 3 `notElem` []
True
>>> 3 `notElem` [1,2]
True
>>> 3 `notElem` [1,2,3,4,5]
False
>>> 3 `notElem` [1..]
False
>>> 3 `notElem` [4..]
* Hangs forever *

maximum :: (Ord a, HasCallStack) => [a] -> a Source #

maximum returns the maximum value from a list, which must be non-empty, finite, and of an ordered type. It is a special case of maximumBy, which allows the programmer to supply their own comparison function.

>>> maximum []
*** Exception: Prelude.maximum: empty list
>>> maximum [42]
42
>>> maximum [55, -12, 7, 0, -89]
55
>>> maximum [1..]
* Hangs forever *

minimum :: (Ord a, HasCallStack) => [a] -> a Source #

minimum returns the minimum value from a list, which must be non-empty, finite, and of an ordered type. It is a special case of minimumBy, which allows the programmer to supply their own comparison function.

>>> minimum []
*** Exception: Prelude.minimum: empty list
>>> minimum [42]
42
>>> minimum [55, -12, 7, 0, -89]
-89
>>> minimum [1..]
* Hangs forever *

sum :: Num a => [a] -> a Source #

The sum function computes the sum of a finite list of numbers.

>>> sum []
0
>>> sum [42]
42
>>> sum [1..10]
55
>>> sum [4.1, 2.0, 1.7]
7.8
>>> sum [1..]
* Hangs forever *

product :: Num a => [a] -> a Source #

The product function computes the product of a finite list of numbers.

>>> product []
1
>>> product [42]
42
>>> product [1..10]
3628800
>>> product [4.1, 2.0, 1.7]
13.939999999999998
>>> product [1..]
* Hangs forever *

and :: [Bool] -> Bool Source #

and returns the conjunction of a Boolean list. For the result to be True, the list must be finite; False, however, results from a False value at a finite index of a finite or infinite list.

>>> and []
True
>>> and [True]
True
>>> and [False]
False
>>> and [True, True, False]
False
>>> and (False : repeat True) -- Infinite list [False,True,True,True,True,True,True...
False
>>> and (repeat True)
* Hangs forever *

or :: [Bool] -> Bool Source #

or returns the disjunction of a Boolean list. For the result to be False, the list must be finite; True, however, results from a True value at a finite index of a finite or infinite list.

>>> or []
False
>>> or [True]
True
>>> or [False]
False
>>> or [True, True, False]
True
>>> or (True : repeat False) -- Infinite list [True,False,False,False,False,False,False...
True
>>> or (repeat False)
* Hangs forever *

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

Applied to a predicate and a list, any determines if any element of the list satisfies the predicate. For the result to be False, the list must be finite; True, however, results from a True value for the predicate applied to an element at a finite index of a finite or infinite list.

>>> any (> 3) []
False
>>> any (> 3) [1,2]
False
>>> any (> 3) [1,2,3,4,5]
True
>>> any (> 3) [1..]
True
>>> any (> 3) [0, -1..]
* Hangs forever *

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

Applied to a predicate and a list, all determines if all elements of the list satisfy the predicate. For the result to be True, the list must be finite; False, however, results from a False value for the predicate applied to an element at a finite index of a finite or infinite list.

>>> all (> 3) []
True
>>> all (> 3) [1,2]
False
>>> all (> 3) [1,2,3,4,5]
False
>>> all (> 3) [1..]
False
>>> all (> 3) [4..]
* Hangs forever *

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

A strict version of foldl1.

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

Concatenate a list of lists.

>>> concat []
[]
>>> concat [[42]]
[42]
>>> concat [[1,2,3], [4,5], [6], []]
[1,2,3,4,5,6]

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

Map a function returning a list over a list and concatenate the results. concatMap can be seen as the composition of concat and map.

concatMap f xs == (concat . map f) xs
>>> concatMap (\i -> [-i,i]) []
[]
>>> concatMap (\i -> [-i,i]) [1,2,3]
[-1,1,-2,2,-3,3]

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, ...]
>>> map (+1) [1, 2, 3]
[2,3,4]

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

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.

WARNING: This function takes linear time in the number of elements of the first list.

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

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

\(\mathcal{O}(n)\). lookup key assocs looks up a key in an association list.

>>> lookup 2 []
Nothing
>>> lookup 2 [(1, "first")]
Nothing
>>> lookup 2 [(1, "first"), (2, "second"), (3, "third")]
Just "second"

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

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

>>> head [1, 2, 3]
1
>>> head [1..]
1
>>> head []
*** Exception: Prelude.head: empty list

WARNING: This function is partial. You can use case-matching, uncons or listToMaybe instead.

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

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

>>> last [1, 2, 3]
3
>>> last [1..]
* Hangs forever *
>>> last []
*** Exception: Prelude.last: empty list

WARNING: This function is partial. You can use reverse with case-matching, uncons or listToMaybe instead.

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

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

>>> tail [1, 2, 3]
[2,3]
>>> tail [1]
[]
>>> tail []
*** Exception: Prelude.tail: empty list

WARNING: This function is partial. You can use case-matching or uncons instead.

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.

>>> init [1, 2, 3]
[1,2]
>>> init [1]
[]
>>> init []
*** Exception: Prelude.init: empty list

WARNING: This function is partial. You can use reverse with case-matching or uncons instead.

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 Just (x, xs), where x is the head of the list and xs its tail.
>>> uncons []
Nothing
>>> uncons [1]
Just (1,[])
>>> uncons [1, 2, 3]
Just (1,[2,3])

Since: base-4.8.0.0

(!!) :: 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.

>>> ['a', 'b', 'c'] !! 0
'a'
>>> ['a', 'b', 'c'] !! 2
'c'
>>> ['a', 'b', 'c'] !! 3
*** Exception: Prelude.!!: index too large
>>> ['a', 'b', 'c'] !! (-1)
*** Exception: Prelude.!!: negative index

WARNING: This function is partial. You can use <atMay https://hackage.haskell.org/package/safe-0.3.19/docs/Safe.html#v:atMay> instead.

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
>>> scanl (+) 0 [1..4]
[0,1,3,6,10]
>>> scanl (+) 42 []
[42]
>>> scanl (-) 100 [1..4]
[100,99,97,94,90]
>>> scanl (\reversedString nextChar -> nextChar : reversedString) "foo" ['a', 'b', 'c', 'd']
["foo","afoo","bafoo","cbafoo","dcbafoo"]
>>> scanl (+) 0 [1..]
* Hangs forever *

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, ...]
>>> scanl1 (+) [1..4]
[1,3,6,10]
>>> scanl1 (+) []
[]
>>> scanl1 (-) [1..4]
[1,-1,-4,-8]
>>> scanl1 (&&) [True, False, True, True]
[True,False,False,False]
>>> scanl1 (||) [False, False, True, True]
[False,False,True,True]
>>> scanl1 (+) [1..]
* Hangs forever *

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

\(\mathcal{O}(n)\). A strict version of scanl.

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.
>>> scanr (+) 0 [1..4]
[10,9,7,4,0]
>>> scanr (+) 42 []
[42]
>>> scanr (-) 100 [1..4]
[98,-97,99,-96,100]
>>> scanr (\nextChar reversedString -> nextChar : reversedString) "foo" ['a', 'b', 'c', 'd']
["abcdfoo","bcdfoo","cdfoo","dfoo","foo"]
>>> force $ scanr (+) 0 [1..]
*** Exception: stack overflow

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

\(\mathcal{O}(n)\). scanr1 is a variant of scanr that has no starting value argument.

>>> scanr1 (+) [1..4]
[10,9,7,4]
>>> scanr1 (+) []
[]
>>> scanr1 (-) [1..4]
[-2,3,-1,4]
>>> scanr1 (&&) [True, False, True, True]
[False,False,True,True]
>>> scanr1 (||) [True, True, False, False]
[True,True,False,False]
>>> force $ scanr1 (+) [1..]
*** Exception: stack overflow

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), ...]

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.

>>> take 10 $ iterate not True
[True,False,True,False...
>>> take 10 $ iterate (+3) 42
[42,45,48,51,54,57,60,63...

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

iterate' is the strict version of iterate.

It forces the result of each application of the function to weak head normal form (WHNF) before proceeding.

repeat :: a -> [a] Source #

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

>>> take 20 $ repeat 17
[17,17,17,17,17,17,17,17,17...

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.

>>> replicate 0 True
[]
>>> replicate (-1) True
[]
>>> replicate 4 True
[True,True,True,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.

>>> cycle []
*** Exception: Prelude.cycle: empty list
>>> take 20 $ cycle [42]
[42,42,42,42,42,42,42,42,42,42...
>>> take 20 $ cycle [2, 5, 7]
[2,5,7,2,5,7,2,5,7,2,5,7...

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.

>>> 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.

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

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.

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 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.

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.

>>> 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]
[]

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

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]

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

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)

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 (> 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).

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

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

>>> reverse []
[]
>>> reverse [42]
[42]
>>> reverse [2,5,7]
[7,5,2]
>>> reverse [1..]
* Hangs forever *

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

\(\mathcal{O}(\min(m,n))\). zip takes two lists and returns a list of corresponding pairs.

>>> zip [1, 2] ['a', 'b']
[(1,'a'),(2,'b')]

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:

>>> zip [1] ['a', 'b']
[(1,'a')]
>>> zip [1, 2] ['a']
[(1,'a')]
>>> zip [] [1..]
[]
>>> zip [1..] []
[]

zip is right-lazy:

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

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

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

zip3 takes three lists and returns a list of triples, analogous to zip. It is capable of list fusion, but it is restricted to its first list argument and its resulting list.

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..]

For example, zipWith (+) is applied to two lists to produce the list of corresponding sums:

>>> zipWith (+) [1, 2, 3] [4, 5, 6]
[5,7,9]

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.

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

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..]

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

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

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

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

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

>>> unzip3 []
([],[],[])
>>> unzip3 [(1, 'a', True), (2, 'b', False)]
([1,2],"ab",[True,False])

GHC List fusion

augment :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a] -> [a] Source #

A list producer that can be fused with foldr. This function is merely

   augment g xs = g (:) xs

but GHC's simplifier will transform an expression of the form foldr k z (augment g xs), which may arise after inlining, to g k (foldr k z xs), which avoids producing an intermediate list.

build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a] Source #

A list producer that can be fused with foldr. This function is merely

   build g = g (:) []

but GHC's simplifier will transform an expression of the form foldr k z (build g), which may arise after inlining, to g k z, which avoids producing an intermediate list.