Copyright | (c) The University of Glasgow 1994-2002 |
---|---|
License | see libraries/base/LICENSE |
Maintainer | cvs-ghc@haskell.org |
Stability | internal |
Portability | non-portable (GHC Extensions) |
Safe Haskell | Trustworthy |
Language | Haskell2010 |
The List data type and its operations
Synopsis
- data List a
- foldr :: (a -> b -> b) -> b -> [a] -> b
- foldr' :: (a -> b -> b) -> b -> [a] -> b
- foldr1 :: HasCallStack => (a -> a -> a) -> [a] -> a
- foldl :: forall a b. (b -> a -> b) -> b -> [a] -> b
- foldl' :: forall a b. (b -> a -> b) -> b -> [a] -> b
- foldl1 :: HasCallStack => (a -> a -> a) -> [a] -> a
- null :: [a] -> Bool
- length :: [a] -> Int
- elem :: Eq a => a -> [a] -> Bool
- notElem :: Eq a => a -> [a] -> Bool
- maximum :: (Ord a, HasCallStack) => [a] -> a
- minimum :: (Ord a, HasCallStack) => [a] -> a
- sum :: Num a => [a] -> a
- product :: Num a => [a] -> a
- and :: [Bool] -> Bool
- or :: [Bool] -> Bool
- any :: (a -> Bool) -> [a] -> Bool
- all :: (a -> Bool) -> [a] -> Bool
- foldl1' :: HasCallStack => (a -> a -> a) -> [a] -> a
- concat :: [[a]] -> [a]
- concatMap :: (a -> [b]) -> [a] -> [b]
- map :: (a -> b) -> [a] -> [b]
- (++) :: [a] -> [a] -> [a]
- filter :: (a -> Bool) -> [a] -> [a]
- lookup :: Eq a => a -> [(a, b)] -> Maybe b
- head :: HasCallStack => [a] -> a
- last :: HasCallStack => [a] -> a
- tail :: HasCallStack => [a] -> [a]
- init :: HasCallStack => [a] -> [a]
- uncons :: [a] -> Maybe (a, [a])
- (!?) :: [a] -> Int -> Maybe a
- (!!) :: HasCallStack => [a] -> Int -> a
- scanl :: (b -> a -> b) -> b -> [a] -> [b]
- scanl1 :: (a -> a -> a) -> [a] -> [a]
- scanl' :: (b -> a -> b) -> b -> [a] -> [b]
- scanr :: (a -> b -> b) -> b -> [a] -> [b]
- scanr1 :: (a -> a -> a) -> [a] -> [a]
- iterate :: (a -> a) -> a -> [a]
- iterate' :: (a -> a) -> a -> [a]
- repeat :: a -> [a]
- replicate :: Int -> a -> [a]
- cycle :: HasCallStack => [a] -> [a]
- take :: Int -> [a] -> [a]
- drop :: Int -> [a] -> [a]
- splitAt :: Int -> [a] -> ([a], [a])
- takeWhile :: (a -> Bool) -> [a] -> [a]
- dropWhile :: (a -> Bool) -> [a] -> [a]
- span :: (a -> Bool) -> [a] -> ([a], [a])
- break :: (a -> Bool) -> [a] -> ([a], [a])
- reverse :: [a] -> [a]
- zip :: [a] -> [b] -> [(a, b)]
- zip3 :: [a] -> [b] -> [c] -> [(a, b, c)]
- zipWith :: (a -> b -> c) -> [a] -> [b] -> [c]
- zipWith3 :: (a -> b -> c -> d) -> [a] -> [b] -> [c] -> [d]
- unzip :: [(a, b)] -> ([a], [b])
- unzip3 :: [(a, b, c)] -> ([a], [b], [c])
- errorEmptyList :: HasCallStack => String -> a
- augment :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a] -> [a]
- build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
Documentation
The builtin list type, usually written in its non-prefix form [a]
.
Examples
Unless the OverloadedLists extension is enabled, list literals are
syntactic 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
MonadFail List Source # | Since: base-4.9.0.0 |
Defined in Control.Monad.Fail | |
MonadFix List Source # | Since: base-2.1 |
Defined in Control.Monad.Fix | |
MonadZip List Source # | Since: base-4.3.1.0 |
Foldable List Source # | Since: base-2.1 |
Defined in 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 # | |
Eq1 List Source # | Since: base-4.9.0.0 |
Ord1 List Source # | Since: base-4.9.0.0 |
Defined in Data.Functor.Classes liftCompare :: (a -> b -> Ordering) -> [a] -> [b] -> Ordering Source # | |
Read1 List Source # | Since: base-4.9.0.0 |
Defined in Data.Functor.Classes | |
Show1 List Source # | Since: base-4.9.0.0 |
Traversable List Source # | Since: base-2.1 |
Alternative List Source # | Combines lists by concatenation, starting from the empty list. Since: base-2.1 |
Applicative List Source # | Since: base-2.1 |
Functor List Source # | Since: base-2.1 |
Monad List Source # | Since: base-2.1 |
MonadPlus List Source # | Combines lists by concatenation, starting from the empty list. Since: base-2.1 |
Generic1 List Source # | |
Data a => Data [a] Source # | Since: base-4.0.0.0 |
Defined in 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 # |
Since: base-2.1 |
Defined in Data.String fromString :: String -> [a] Source # | |
Monoid [a] Source # | Since: base-2.1 |
Semigroup [a] Source # | Since: base-4.9.0.0 |
Generic [a] Source # | |
IsList [a] Source # | Since: base-4.7.0.0 |
Read a => Read [a] Source # | Since: base-2.1 |
Show a => Show [a] Source # | Since: base-2.1 |
IsChar c => PrintfArg [c] Source # | Since: base-2.1 |
Defined in Text.Printf formatArg :: [c] -> FieldFormatter Source # parseFormat :: [c] -> ModifierParser Source # | |
IsChar c => PrintfType [c] Source # | Since: base-2.1 |
Defined in Text.Printf | |
Eq a => Eq [a] | |
Ord a => Ord [a] | |
type Rep1 List Source # | Since: base-4.6.0.0 |
Defined in GHC.Generics type Rep1 List = 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 List))) | |
type Rep [a] Source # | Since: base-4.6.0.0 |
Defined in GHC.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.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 *
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 *
\(\mathcal{O}(1)\). Test whether a list is empty.
>>>
null []
True>>>
null [1]
False>>>
null [1..]
False
\(\mathcal{O}(n)\). 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 *
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
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]
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]
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 #
(!?) :: [a] -> Int -> Maybe a infixl 9 Source #
List index (subscript) operator, starting from 0. Returns Nothing
if the index is out of bounds
>>>
['a', 'b', 'c'] !? 0
Just 'a'>>>
['a', 'b', 'c'] !? 2
Just 'c'>>>
['a', 'b', 'c'] !? 3
Nothing>>>
['a', 'b', 'c'] !? (-1)
Nothing
This is the total variant of the partial !!
operator.
WARNING: This function takes linear time in the index.
(!!) :: 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, 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.
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 *
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...
repeat
x
is an infinite list, with x
the value of every element.
>>>
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>>>
cycle [42]
[42,42,42,42,42,42,42,42,42,42...>>>
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 (
when take
n xs, drop
n xs)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]
[]
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])
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],[])
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.
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,
is applied to two lists to produce the list of
corresponding sums:zipWith
(+)
>>>
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")
errorEmptyList :: HasCallStack => String -> a Source #