Safe Haskell	Trustworthy
Language	Haskell2010

Data.List.Compat

Synopsis

module Data.List
compareLength :: [a] -> Int -> Ordering
inits1 :: [a] -> [NonEmpty a]
tails1 :: [a] -> [NonEmpty a]
data List a
(!?) :: [a] -> Int -> Maybe a
unsnoc :: [a] -> Maybe ([a], a)

Documentation

module Data.List

compareLength :: [a] -> Int -> Ordering Source #

Use compareLength xs n as a safer and faster alternative to compare (length xs) n. Similarly, it's better to write compareLength xs 10 == LT instead of length xs < 10.

While length would force and traverse the entire spine of xs (which could even diverge if xs is infinite), compareLength traverses at most n elements to determine its result.

>>> compareLength [] 0
EQ
>>> compareLength [] 1
LT
>>> compareLength ['a'] 1
EQ
>>> compareLength ['a', 'b'] 1
GT
>>> compareLength [0..] 100
GT
>>> compareLength undefined (-1)
GT
>>> compareLength ('a' : undefined) 0
GT

Since: 4.21.0.0

inits1 :: [a] -> [NonEmpty a] Source #

The inits1 function returns all non-empty initial segments of the argument, shortest first.

Laziness

Expand

Note that inits1 has the following strictness property: inits1 (xs ++ _|_) = inits1 xs ++ _|_

In particular, inits1 _|_ = _|_

Examples

Expand

>>> inits1 "abc"
['a' :| "",'a' :| "b",'a' :| "bc"]

>>> inits1 []
[]

inits1 is productive on infinite lists:

>>> take 3 $ inits1 [1..]
[1 :| [],1 :| [2],1 :| [2,3]]

Since: 4.21.0.0

tails1 :: [a] -> [NonEmpty a] Source #

$\mathcal{O}(n)$. The tails1 function returns all non-empty final segments of the argument, longest first.

Laziness

Expand

Note that tails1 has the following strictness property: tails1 _|_ = _|_

>>> tails1 undefined
*** Exception: Prelude.undefined

>>> drop 1 (tails1 [undefined, 1, 2])
[1 :| [2],2 :| []]

Examples

Expand

>>> tails1 "abc"
['a' :| "bc",'b' :| "c",'c' :| ""]

>>> tails1 [1, 2, 3]
[1 :| [2,3],2 :| [3],3 :| []]

>>> tails1 []
[]

Since: 4.21.0.0

data List a #

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

Examples

Expand

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

Instances details

MonadFail List	Since: base-4.9.0.0
Instance details Defined in Control.Monad.Fail Methods fail :: String -> [a] #
Foldable List	Since: base-2.1
Instance details Defined in Data.Foldable Methods fold :: Monoid m => [m] -> m # foldMap :: Monoid m => (a -> m) -> [a] -> m # foldMap' :: Monoid m => (a -> m) -> [a] -> m # foldr :: (a -> b -> b) -> b -> [a] -> b # foldr' :: (a -> b -> b) -> b -> [a] -> b # foldl :: (b -> a -> b) -> b -> [a] -> b # foldl' :: (b -> a -> b) -> b -> [a] -> b # foldr1 :: (a -> a -> a) -> [a] -> a # foldl1 :: (a -> a -> a) -> [a] -> a # toList :: [a] -> [a] # null :: [a] -> Bool # length :: [a] -> Int # elem :: Eq a => a -> [a] -> Bool # maximum :: Ord a => [a] -> a # minimum :: Ord a => [a] -> a # sum :: Num a => [a] -> a # product :: Num a => [a] -> a #
Traversable List	Since: base-2.1
Instance details Defined in Data.Traversable Methods traverse :: Applicative f => (a -> f b) -> [a] -> f [b] # sequenceA :: Applicative f => [f a] -> f [a] # mapM :: Monad m => (a -> m b) -> [a] -> m [b] # sequence :: Monad m => [m a] -> m [a] #
Alternative List	Combines lists by concatenation, starting from the empty list. Since: base-2.1
Instance details Defined in GHC.Base Methods empty :: [a] # (<\|>) :: [a] -> [a] -> [a] # some :: [a] -> [[a]] # many :: [a] -> [[a]] #
Applicative List	Since: base-2.1
Instance details Defined in GHC.Base Methods pure :: a -> [a] # (<>) :: [a -> b] -> [a] -> [b] # liftA2 :: (a -> b -> c) -> [a] -> [b] -> [c] # (>) :: [a] -> [b] -> [b] # (<*) :: [a] -> [b] -> [a] #
Functor List	Since: base-2.1
Instance details Defined in GHC.Base Methods fmap :: (a -> b) -> [a] -> [b] # (<$) :: a -> [b] -> [a] #
Monad List	Since: base-2.1
Instance details Defined in GHC.Base Methods (>>=) :: [a] -> (a -> [b]) -> [b] # (>>) :: [a] -> [b] -> [b] # return :: a -> [a] #
MonadPlus List	Combines lists by concatenation, starting from the empty list. Since: base-2.1
Instance details Defined in GHC.Base Methods mzero :: [a] # mplus :: [a] -> [a] -> [a] #
a ~ Char => IsString [a]	`(a ~ Char)` context was introduced in `4.9.0.0` Since: base-2.1
Instance details Defined in Data.String Methods fromString :: String -> [a] #
Monoid [a]	Since: base-2.1
Instance details Defined in GHC.Base Methods mempty :: [a] # mappend :: [a] -> [a] -> [a] # mconcat :: [[a]] -> [a] #
Semigroup [a]	Since: base-4.9.0.0
Instance details Defined in GHC.Base Methods (<>) :: [a] -> [a] -> [a] # sconcat :: NonEmpty [a] -> [a] # stimes :: Integral b => b -> [a] -> [a] #
Read a => Read [a]	Since: base-2.1
Instance details Defined in GHC.Read Methods readsPrec :: Int -> ReadS [a] # readList :: ReadS [[a]] # readPrec :: ReadPrec [a] # readListPrec :: ReadPrec [[a]] #
Show a => Show [a]	Since: base-2.1
Instance details Defined in GHC.Show Methods showsPrec :: Int -> [a] -> ShowS # show :: [a] -> String # showList :: [[a]] -> ShowS #
Eq a => Eq [a]
Instance details Defined in GHC.Classes Methods (==) :: [a] -> [a] -> Bool # (/=) :: [a] -> [a] -> Bool #
Ord a => Ord [a]
Instance details Defined in GHC.Classes Methods compare :: [a] -> [a] -> Ordering # (<) :: [a] -> [a] -> Bool # (<=) :: [a] -> [a] -> Bool # (>) :: [a] -> [a] -> Bool # (>=) :: [a] -> [a] -> Bool # max :: [a] -> [a] -> [a] # min :: [a] -> [a] -> [a] #

(!?) :: [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.

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 Just (xs, x), where xs is the initial part of the list and x is its last element.

Since: 4.19.0.0

>>> unsnoc []
Nothing
>>> unsnoc [1]
Just ([],1)
>>> unsnoc [1, 2, 3]
Just ([1,2],3)

Laziness:

>>> fst <$> unsnoc [undefined]
Just []
>>> head . fst <$> unsnoc (1 : undefined)
Just *** Exception: Prelude.undefined
>>> head . fst <$> unsnoc (1 : 2 : undefined)
Just 1

unsnoc is dual to uncons: for a finite list xs

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