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# List

A library for manipulating lists of values. Every value in a list must have the same type.

# Basics

isEmpty : List a -> Bool

Determine if a list is empty.

``````isEmpty [] == True
``````
length : List a -> Int

Determine the length of a list.

``````length [1,2,3] == 3
``````
reverse : List a -> List a

Reverse a list.

``````reverse [1,2,3,4] == [4,3,2,1]
``````
member : a -> List a -> Bool

Figure out whether a list contains a value.

``````member 9 [1,2,3,4] == False
member 4 [1,2,3,4] == True
``````

# Sub-lists

head : List a -> Maybe a

Extract the first element of a list.

``````head [1,2,3] == Just 1
``````
tail : List a -> Maybe (List a)

Extract the rest of the list.

``````tail [1,2,3] == Just [2,3]
tail [] == Nothing
``````
filter : (a -> Bool) -> List a -> List a

Keep only elements that satisfy the predicate.

``````filter isEven [1,2,3,4,5,6] == [2,4,6]
``````
take : Int -> List a -> List a

Take the first n members of a list.

``````take 2 [1,2,3,4] == [1,2]
``````
drop : Int -> List a -> List a

Drop the first n members of a list.

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

# Putting Lists Together

singleton : a -> List a

Create a list with only one element:

``````singleton 1234 == [1234]
singleton "hi" == ["hi"]
``````
repeat : Int -> a -> List a

Create a list with n copies of a value:

``````repeat 3 (0,0) == [(0,0),(0,0),(0,0)]
``````
range : Int -> Int -> List Int

Create a list of numbers, every element increasing by one. You give the lowest and highest number that should be in the list.

``````range 3 6 == [3, 4, 5, 6]
range 3 3 == [3]
range 6 3 == []
``````
append : List a -> List a -> List a

Put two lists together.

``````append [1,1,2] [3,5,8] == [1,1,2,3,5,8]
append ['a','b'] ['c'] == ['a','b','c']
``````

You can also use the `(++)` operator to append lists.

concat : List (List a) -> List a

Concatenate a bunch of lists into a single list:

``````concat [[1,2],[3],[4,5]] == [1,2,3,4,5]
``````
intersperse : a -> List a -> List a

Places the given value between all members of the given list.

``````intersperse "on" ["turtles","turtles","turtles"] == ["turtles","on","turtles","on","turtles"]
``````

# Taking Lists Apart

partition : (a -> Bool) -> List a -> (List a, List a)

Partition a list based on a predicate. The first list contains all values that satisfy the predicate, and the second list contains all the value that do not.

``````partition (\x -> x < 3) [0,1,2,3,4,5] == ([0,1,2], [3,4,5])
partition isEven        [0,1,2,3,4,5] == ([0,2,4], [1,3,5])
``````
unzip : List (a,b) -> (List a, List b)

Decompose a list of tuples into a tuple of lists.

``````unzip [(0, True), (17, False), (1337, True)] == ([0,17,1337], [True,False,True])
``````

# Mapping

map : (a -> b) -> List a -> List b

Apply a function to every element of a list.

``````map sqrt [1,4,9] == [1,2,3]

map not [True,False,True] == [False,True,False]
``````
map2 : (a -> b -> result) -> List a -> List b -> List result

Combine two lists, combining them with the given function. If one list is longer, the extra elements are dropped.

``````map2 (+) [1,2,3] [1,2,3,4] == [2,4,6]

map2 (,) [1,2,3] ['a','b'] == [ (1,'a'), (2,'b') ]

pairs : List a -> List b -> List (a,b)
pairs lefts rights =
map2 (,) lefts rights
``````
map3 : (a -> b -> c -> result) -> List a -> List b -> List c -> List result
map4 : (a -> b -> c -> d -> result) -> List a -> List b -> List c -> List d -> List result
map5 : (a -> b -> c -> d -> e -> result) -> List a -> List b -> List c -> List d -> List e -> List result

If you can think of a legitimate use of `mapN` where `N` is 6 or more, please let us know on the list. The current sentiment is that it is already quite error prone once you get to 4 and possibly should be approached another way.

# Special Maps

filterMap : (a -> Maybe b) -> List a -> List b

Apply a function that may succeed to all values in the list, but only keep the successes.

``````onlyTeens =
filterMap isTeen [3, 15, 12, 18, 24] == [15, 18]

isTeen : Int -> Maybe Int
isTeen n =
if 13 <= n && n <= 19 then
Just n

else
Nothing
``````
concatMap : (a -> List b) -> List a -> List b

Map a given function onto a list and flatten the resulting lists.

``````concatMap f xs == concat (map f xs)
``````
indexedMap : (Int -> a -> b) -> List a -> List b

Same as `map` but the function is also applied to the index of each element (starting at zero).

``````indexedMap (,) ["Tom","Sue","Bob"] == [ (0,"Tom"), (1,"Sue"), (2,"Bob") ]
``````

# Folding

foldr : (a -> b -> b) -> b -> List a -> b

Reduce a list from the right.

``````foldr (+) 0 [1,2,3] == 6
``````
foldl : (a -> b -> b) -> b -> List a -> b

Reduce a list from the left.

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

# Special Folds

sum : List number -> number

Get the sum of the list elements.

``````sum [1,2,3,4] == 10
``````
product : List number -> number

Get the product of the list elements.

``````product [1,2,3,4] == 24
``````
maximum : List comparable -> Maybe comparable

Find the maximum element in a non-empty list.

``````maximum [1,4,2] == Just 4
maximum []      == Nothing
``````
minimum : List comparable -> Maybe comparable

Find the minimum element in a non-empty list.

``````minimum [3,2,1] == Just 1
minimum []      == Nothing
``````
all : (a -> Bool) -> List a -> Bool

Determine if all elements satisfy the predicate.

``````all isEven [2,4] == True
all isEven [2,3] == False
all isEven [] == True
``````
any : (a -> Bool) -> List a -> Bool

Determine if any elements satisfy the predicate.

``````any isEven [2,3] == True
any isEven [1,3] == False
any isEven [] == False
``````
scanl : (a -> b -> b) -> b -> List a -> List b

Reduce a list from the left, building up all of the intermediate results into a list.

``````scanl (+) 0 [1,2,3,4] == [0,1,3,6,10]
``````

# Sorting

sort : List comparable -> List comparable

Sort values from lowest to highest

``````sort [3,1,5] == [1,3,5]
``````
sortBy : (a -> comparable) -> List a -> List a

Sort values by a derived property.

``````alice = { name="Alice", height=1.62 }
bob   = { name="Bob"  , height=1.85 }
chuck = { name="Chuck", height=1.76 }

sortBy .name   [chuck,alice,bob] == [alice,bob,chuck]
sortBy .height [chuck,alice,bob] == [alice,chuck,bob]

sortBy String.length ["mouse","cat"] == ["cat","mouse"]
``````
sortWith : (a -> a -> Order) -> List a -> List a

Sort values with a custom comparison function.

``````sortWith flippedComparison [1,2,3,4,5] == [5,4,3,2,1]

flippedComparison a b =
case compare a b of
LT -> GT
EQ -> EQ
GT -> LT
``````

This is also the most general sort function, allowing you to define any other: `sort == sortWith compare`

``````module List exposing
( isEmpty, length, reverse, member
, head, tail, filter, take, drop
, singleton, repeat, range, (::), append, concat, intersperse
, partition, unzip
, map, map2, map3, map4, map5
, filterMap, concatMap, indexedMap
, foldr, foldl
, sum, product, maximum, minimum, all, any, scanl
, sort, sortBy, sortWith
)

{-| A library for manipulating lists of values. Every value in a
list must have the same type.

# Basics
@docs isEmpty, length, reverse, member

# Sub-lists
@docs head, tail, filter, take, drop

# Putting Lists Together
@docs singleton, repeat, range, (::), append, concat, intersperse

# Taking Lists Apart
@docs partition, unzip

# Mapping
@docs map, map2, map3, map4, map5

If you can think of a legitimate use of `mapN` where `N` is 6 or more, please
let us know on [the list](https://groups.google.com/forum/#!forum/elm-discuss).
The current sentiment is that it is already quite error prone once you get to
4 and possibly should be approached another way.

# Special Maps
@docs filterMap, concatMap, indexedMap

# Folding
@docs foldr, foldl

# Special Folds
@docs sum, product, maximum, minimum, all, any, scanl

# Sorting
@docs sort, sortBy, sortWith

-}

import Basics exposing (..)
import Maybe
import Maybe exposing ( Maybe(Just,Nothing) )
import Native.List

{-| Add an element to the front of a list. Pronounced *cons*.

1 :: [2,3] == [1,2,3]
1 :: [] == [1]
-}
(::) : a -> List a -> List a
(::) =
Native.List.cons

infixr 5 ::

{-| Extract the first element of a list.

-}
head : List a -> Maybe a
case list of
x :: xs ->
Just x

[] ->
Nothing

{-| Extract the rest of the list.

tail [1,2,3] == Just [2,3]
tail [] == Nothing
-}
tail : List a -> Maybe (List a)
tail list =
case list of
x :: xs ->
Just xs

[] ->
Nothing

{-| Determine if a list is empty.

isEmpty [] == True
-}
isEmpty : List a -> Bool
isEmpty xs =
case xs of
[] ->
True

_ ->
False

{-| Figure out whether a list contains a value.

member 9 [1,2,3,4] == False
member 4 [1,2,3,4] == True
-}
member : a -> List a -> Bool
member x xs =
any (\a -> a == x) xs

{-| Apply a function to every element of a list.

map sqrt [1,4,9] == [1,2,3]

map not [True,False,True] == [False,True,False]
-}
map : (a -> b) -> List a -> List b
map f xs =
foldr (\x acc -> f x :: acc) [] xs

{-| Same as `map` but the function is also applied to the index of each
element (starting at zero).

indexedMap (,) ["Tom","Sue","Bob"] == [ (0,"Tom"), (1,"Sue"), (2,"Bob") ]
-}
indexedMap : (Int -> a -> b) -> List a -> List b
indexedMap f xs =
map2 f (range 0 (length xs - 1)) xs

{-| Reduce a list from the left.

foldl (::) [] [1,2,3] == [3,2,1]
-}
foldl : (a -> b -> b) -> b -> List a -> b
foldl func acc list =
case list of
[] ->
acc

x :: xs ->
foldl func (func x acc) xs

{-| Reduce a list from the right.

foldr (+) 0 [1,2,3] == 6
-}
foldr : (a -> b -> b) -> b -> List a -> b
foldr =
Native.List.foldr

{-| Reduce a list from the left, building up all of the intermediate results into a list.

scanl (+) 0 [1,2,3,4] == [0,1,3,6,10]
-}
scanl : (a -> b -> b) -> b -> List a -> List b
scanl f b xs =
let
scan1 x accAcc =
case accAcc of
acc :: _ ->
f x acc :: accAcc

[] ->
[] -- impossible
in
reverse (foldl scan1 [b] xs)

{-| Keep only elements that satisfy the predicate.

filter isEven [1,2,3,4,5,6] == [2,4,6]
-}
filter : (a -> Bool) -> List a -> List a
filter pred xs =
let
conditionalCons front back =
if pred front then
front :: back

else
back
in
foldr conditionalCons [] xs

{-| Apply a function that may succeed to all values in the list, but only keep
the successes.

onlyTeens =
filterMap isTeen [3, 15, 12, 18, 24] == [15, 18]

isTeen : Int -> Maybe Int
isTeen n =
if 13 <= n && n <= 19 then
Just n

else
Nothing
-}
filterMap : (a -> Maybe b) -> List a -> List b
filterMap f xs =
foldr (maybeCons f) [] xs

maybeCons : (a -> Maybe b) -> a -> List b -> List b
maybeCons f mx xs =
case f mx of
Just x ->
x :: xs

Nothing ->
xs

{-| Determine the length of a list.

length [1,2,3] == 3
-}
length : List a -> Int
length xs =
foldl (\_ i -> i + 1) 0 xs

{-| Reverse a list.

reverse [1,2,3,4] == [4,3,2,1]
-}
reverse : List a -> List a
reverse list =
foldl (::) [] list

{-| Determine if all elements satisfy the predicate.

all isEven [2,4] == True
all isEven [2,3] == False
all isEven [] == True
-}
all : (a -> Bool) -> List a -> Bool
all isOkay list =
not (any (not << isOkay) list)

{-| Determine if any elements satisfy the predicate.

any isEven [2,3] == True
any isEven [1,3] == False
any isEven [] == False
-}
any : (a -> Bool) -> List a -> Bool
any isOkay list =
case list of
[] ->
False

x :: xs ->
-- note: (isOkay x || any isOkay xs) would not get TCO
if isOkay x then
True

else
any isOkay xs

{-| Put two lists together.

append [1,1,2] [3,5,8] == [1,1,2,3,5,8]
append ['a','b'] ['c'] == ['a','b','c']

You can also use [the `(++)` operator](Basics#++) to append lists.
-}
append : List a -> List a -> List a
append xs ys =
case ys of
[] ->
xs

_ ->
foldr (::) ys xs

{-| Concatenate a bunch of lists into a single list:

concat [[1,2],[3],[4,5]] == [1,2,3,4,5]
-}
concat : List (List a) -> List a
concat lists =
foldr append [] lists

{-| Map a given function onto a list and flatten the resulting lists.

concatMap f xs == concat (map f xs)
-}
concatMap : (a -> List b) -> List a -> List b
concatMap f list =
concat (map f list)

{-| Get the sum of the list elements.

sum [1,2,3,4] == 10
-}
sum : List number -> number
sum numbers =
foldl (+) 0 numbers

{-| Get the product of the list elements.

product [1,2,3,4] == 24
-}
product : List number -> number
product numbers =
foldl (*) 1 numbers

{-| Find the maximum element in a non-empty list.

maximum [1,4,2] == Just 4
maximum []      == Nothing
-}
maximum : List comparable -> Maybe comparable
maximum list =
case list of
x :: xs ->
Just (foldl max x xs)

_ ->
Nothing

{-| Find the minimum element in a non-empty list.

minimum [3,2,1] == Just 1
minimum []      == Nothing
-}
minimum : List comparable -> Maybe comparable
minimum list =
case list of
x :: xs ->
Just (foldl min x xs)

_ ->
Nothing

{-| Partition a list based on a predicate. The first list contains all values
that satisfy the predicate, and the second list contains all the value that do
not.

partition (\x -> x < 3) [0,1,2,3,4,5] == ([0,1,2], [3,4,5])
partition isEven        [0,1,2,3,4,5] == ([0,2,4], [1,3,5])
-}
partition : (a -> Bool) -> List a -> (List a, List a)
partition pred list =
let
step x (trues, falses) =
if pred x then
(x :: trues, falses)

else
(trues, x :: falses)
in
foldr step ([],[]) list

{-| Combine two lists, combining them with the given function.
If one list is longer, the extra elements are dropped.

map2 (+) [1,2,3] [1,2,3,4] == [2,4,6]

map2 (,) [1,2,3] ['a','b'] == [ (1,'a'), (2,'b') ]

pairs : List a -> List b -> List (a,b)
pairs lefts rights =
map2 (,) lefts rights
-}
map2 : (a -> b -> result) -> List a -> List b -> List result
map2 =
Native.List.map2

{-|-}
map3 : (a -> b -> c -> result) -> List a -> List b -> List c -> List result
map3 =
Native.List.map3

{-|-}
map4 : (a -> b -> c -> d -> result) -> List a -> List b -> List c -> List d -> List result
map4 =
Native.List.map4

{-|-}
map5 : (a -> b -> c -> d -> e -> result) -> List a -> List b -> List c -> List d -> List e -> List result
map5 =
Native.List.map5

{-| Decompose a list of tuples into a tuple of lists.

unzip [(0, True), (17, False), (1337, True)] == ([0,17,1337], [True,False,True])
-}
unzip : List (a,b) -> (List a, List b)
unzip pairs =
let
step (x,y) (xs,ys) =
(x :: xs, y :: ys)
in
foldr step ([], []) pairs

{-| Places the given value between all members of the given list.

intersperse "on" ["turtles","turtles","turtles"] == ["turtles","on","turtles","on","turtles"]
-}
intersperse : a -> List a -> List a
intersperse sep xs =
case xs of
[] ->
[]

hd :: tl ->
let
step x rest =
sep :: x :: rest

spersed =
foldr step [] tl
in
hd :: spersed

{-| Take the first *n* members of a list.

take 2 [1,2,3,4] == [1,2]
-}
take : Int -> List a -> List a
take n list =
takeFast 0 n list

takeFast : Int -> Int -> List a -> List a
takeFast ctr n list =
if n <= 0 then
[]
else
case ( n, list ) of
( _, [] ) ->
list

( 1, x :: _ ) ->
[ x ]

( 2, x :: y :: _ ) ->
[ x, y ]

( 3, x :: y :: z :: _ ) ->
[ x, y, z ]

( _, x :: y :: z :: w :: tl ) ->
if ctr > 1000 then
x :: y :: z :: w :: takeTailRec (n - 4) tl
else
x :: y :: z :: w :: takeFast (ctr + 1) (n - 4) tl

_ ->
list

takeTailRec : Int -> List a -> List a
takeTailRec n list =
reverse (takeReverse n list [])

takeReverse : Int -> List a -> List a -> List a
takeReverse n list taken =
if n <= 0 then
taken
else
case list of
[] ->
taken

x :: xs ->
takeReverse (n - 1) xs (x :: taken)

{-| Drop the first *n* members of a list.

drop 2 [1,2,3,4] == [3,4]
-}
drop : Int -> List a -> List a
drop n list =
if n <= 0 then
list

else
case list of
[] ->
list

x :: xs ->
drop (n-1) xs

{-| Create a list with only one element:

singleton 1234 == [1234]
singleton "hi" == ["hi"]
-}
singleton : a -> List a
singleton value =
[value]

{-| Create a list with *n* copies of a value:

repeat 3 (0,0) == [(0,0),(0,0),(0,0)]
-}
repeat : Int -> a -> List a
repeat n value =
repeatHelp [] n value

repeatHelp : List a -> Int -> a -> List a
repeatHelp result n value =
if n <= 0 then
result

else
repeatHelp (value :: result) (n-1) value

{-| Create a list of numbers, every element increasing by one.
You give the lowest and highest number that should be in the list.

range 3 6 == [3, 4, 5, 6]
range 3 3 == [3]
range 6 3 == []
-}
range : Int -> Int -> List Int
range lo hi =
rangeHelp lo hi []

rangeHelp : Int -> Int -> List Int -> List Int
rangeHelp lo hi list =
if lo <= hi then
rangeHelp lo (hi - 1) (hi :: list)

else
list

{-| Sort values from lowest to highest

sort [3,1,5] == [1,3,5]
-}
sort : List comparable -> List comparable
sort xs =
sortBy identity xs

{-| Sort values by a derived property.

alice = { name="Alice", height=1.62 }
bob   = { name="Bob"  , height=1.85 }
chuck = { name="Chuck", height=1.76 }

sortBy .name   [chuck,alice,bob] == [alice,bob,chuck]
sortBy .height [chuck,alice,bob] == [alice,chuck,bob]

sortBy String.length ["mouse","cat"] == ["cat","mouse"]
-}
sortBy : (a -> comparable) ->  List a -> List a
sortBy =
Native.List.sortBy

{-| Sort values with a custom comparison function.

sortWith flippedComparison [1,2,3,4,5] == [5,4,3,2,1]

flippedComparison a b =
case compare a b of
LT -> GT
EQ -> EQ
GT -> LT

This is also the most general sort function, allowing you
to define any other: `sort == sortWith compare`
-}
sortWith : (a -> a -> Order) ->  List a -> List a
sortWith =
Native.List.sortWith
```
```