Describes an empty or non-empty list, making it more convenient than any Nonempty.

We can require a NotEmpty for example:

toNonempty : ListIs NotEmpty a -> Nonempty a

ListIs is equivalent to a MaybeIs of a non-empty list tuple:

MaybeIs emptyOrNot ( a, List a )

so we can treat it like a normal MaybeIs:

import MaybeIs exposing (MaybeIs(..))


ListIs.empty
--> MaybeIs.nothing


[ ... ]
    |> ListIs.fromList
    |> MaybeIs.map ListIs.head
--: MaybeIs (CanBe nothingTag_ ()) head_


toList : ListIs emptyOrNot_ a -> List a
toList list =
    case list of
        IsJust ( head_, tail_ ) ->
            head_ :: tail_


        IsNothing _ ->
            []

NotEmpty can be used to require a non-empty list as an argument:

head : ListWithHeadType head NotEmpty tailElement_ -> head

Describes an empty or non-empty list where the head type can be different from the tail element type.

Use ListIs if you have matching head and tail element types.

ListWithHeadType is the result of:

• empty
• only
• fromCons
• fromUnConsed
• cons
• mapHead
• map2HeadsAndTails

This is equivalent to a MaybeIs of a ( head, tail ) tuple:

import MaybeIs exposing (MaybeIs(..))


ListIs.empty
--> MaybeIs.nothing


MaybeIs.map ListIs.head
--: ListWithHeadType head (CanBe empty_ yesOrNever) tailElement_
--: -> MaybeIs (CanBe nothingTag_ yesOrNever) head


tail : ListWithHead head_ tailElement -> List tailElement
tail listNotEmpty =
    case listNotEmpty of
        IsJust ( _, tailList ) ->
            tailList


        IsNothing _ ->
            []

A ListIs without elements.

Equivalent to MaybeIs.nothing.

A ListIs with just 1 element.

ListIs.only ":)"
--> ListIs.empty |> ListIs.cons ":)"

Build a notEmpty_ from its head and tail.

Convert a non-empty list tuple ( a, List b ) to a ListWithHeadType a notEmpty_ b.

Equivalent to MaybeIs.just.

Convert a List a to a ListIs (CanBe emptyTag_ ()) a.

[] |> ListIs.fromList
--> ListIs.empty


[ "hello", "emptiness" ] |> ListIs.fromList
--> ListIs.fromCons "hello" [ "emptiness" ]
--: ListIs (CanBe emptyTag_ ()) String

When constructing from known elements, always prefer

ListIs.fromCons "hello" [ "emptiness" ]

The first value in the ListIs.

ListIs.only 3
    |> ListIs.cons 2
    |> ListIs.head
--> 2

Everything after the first value in the ListIs.

ListIs.only 2
    |> ListIs.cons 3
    |> ListIs.append (ListIs.fromCons 1 [ 0 ])
    |> ListIs.tail
--> [ 2, 1, 0 ]

How many element there are.

ListIs.only 3
    |> ListIs.cons 2
    |> ListIs.length
--> 2

Add an element to the front of a list.

ListIs.fromCons 2 [ 3 ] |> ListIs.cons 1
--> ListIs.fromCons 1 [ 2, 3 ]


ListIs.empty |> ListIs.cons 1
--> ListIs.only 1

Glue the elements of a ListIs to the end of a ListIs.

ListIs.fromCons 1 [ 2 ]
    |> ListIs.append
        (ListIs.fromCons 3 [ 4 ])
--> ListIs.fromCons 1 [ 2, 3, 4 ]

Prefer this over appendNotEmpty if the piped ListIs is already known as NotEmpty or if both can be empty.

Glue the elements of a ListIs NotEmpty ... to the end of a ListIs.

ListIs.empty
    |> ListIs.appendNotEmpty
        (ListIs.fromCons 1 [ 2 ])
    |> ListIs.append
        (ListIs.fromCons 3 [ 4, 5 ])
--> ListIs.fromCons 1 [ 2, 3, 4, 5 ]

Prefer append if the piped ListIs is already known as NotEmpty or if both can be empty.

Glue together a bunch of lists.

ListIs.fromCons
    (ListIs.fromCons 0 [ 1 ])
    [ ListIs.fromCons 10 [ 11 ]
    , ListIs.empty
    , ListIs.fromCons 20 [ 21, 22 ]
    ]
    |> ListIs.concat
--> ListIs.fromCons 0 [ 1, 10, 11, 20, 21, 22 ]

For this to return a ListIs notEmpty, there must be a non-empty first list.

Keep elements that satisfy the test.

ListIs.fromCons 1 [ 2, 5, -3, 10 ]
    |> ListIs.when (\x -> x < 5)
--> ListIs.fromCons 1 [ 2, -3 ]
--: ListIs (CanBe emptyTag_ ()) number_

Keep all just values and drop all Nothings.

ListIs.fromCons Nothing [ Nothing ]
    |> ListIs.whenJust
--> ListIs.empty


ListIs.fromCons (just 1) [ Nothing, just 3 ]
    |> ListIs.whenJust
--> ListIs.fromCons 1 [ 3 ]
--: ListIs NotEmpty number

As you can see, if the head is just a value, the result is NotEmpty.

Apply a function to every element.

ListIs.fromCons 1 [ 4, 9 ]
    |> ListIs.map negate
--> ListIs.fromCons -1 [ -4, -9 ]

Apply a function to the head only.

ListIs.fromCons 1 [ 4, 9 ]
    |> ListIs.mapHead negate
--> ListIs.fromCons -1 [ 4, 9 ]

Apply a function to every element of its tail.

ListIs.fromCons 1 [ 4, 9 ]
    |> ListIs.mapTail negate
--> ListIs.fromCons 1 [ -4, -9 ]

Reduce a List in a direction.

import LinearDirection exposing (LinearDirection(..))


ListIs.fromCons 'l' [ 'i', 'v', 'e' ]
    |> ListIs.foldFrom "" LastToFirst String.cons
--> "live"


ListIs.fromCons 'l' [ 'i', 'v', 'e' ]
    |> ListIs.foldFrom "" FirstToLast String.cons
--> "evil"

A fold in a direction where the initial result is the first value in the ListIs.

import LinearDirection exposing (LinearDirection(..))


ListIs.fromCons 234 [ 345, 543 ]
    |> ListIs.fold FirstToLast max
--> 543

Convert the ListIs to a List.

ListIs.fromCons 1 [ 7 ]
    |> ListIs.toList
--> [ 1, 7 ]

Convert a NotEmpty to a non-empty list tuple ( a, List a ).

Equivalent to MaybeIs.value.

Combine 2 ListIss with a given function. If one list is longer, its extra elements are dropped.

ListIs.map2 (+)
    (ListIs.fromCons 1 [ 2, 3 ])
    (ListIs.fromCons 4 [ 5, 6, 7 ])
--> ListIs.fromCons 5 [ 7, 9 ]


ListIs.map2 Tuple.pair
    (ListIs.fromCons 1 [ 2, 3 ])
    ListIs.empty
--> ListIs.empty

For ListWithHeadType head ... tailElement where head and tailElement have a different type, there's map2HeadsAndTails.

Combine the head and tail elements of 2 ListIss using given functions. If one list is longer, its extra elements are dropped.

ListIs.map2HeadsAndTails Tuple.pair (+)
    (ListIs.fromCons "hey" [ 0, 1 ])
    (ListIs.fromCons "there" [ 1, 6, 7 ])
--> ListIs.fromCons ( "hey", "there" ) [ 1, 7 ]


ListIs.map2HeadsAndTails Tuple.pair (+)
    (ListIs.fromCons 1 [ 2, 3 ])
    ListIs.empty
--> ListIs.empty

For matching head and tailElement types, there's map2.