Elm Christmas

Peeking Inside Lists

A 3 minute read written by
Aksel Wester
08.12.2020

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Working with lists in Elm is nice, but getting a sense of what’s inside a list can be tricky. In this article we will look at a techinque that allows us to get access to the elements inside a list.

We will start by looking at a somewhat finicky example. Let's say we have a list of text notes in our app, that we want to display. If there are multiple notes in the list, we would like to show all the elements in their minimized form. If there is only one note in the list, we would like to show a maximized view of that note, since showing a list with only one element is a waste in that case. And lastly, if the list is empty, we would like to display a message saying that the list is empty.

To meet those requirements, we could write something like this:

view : List Note -> Html a
view notes =
    if List.length notes == 1 then
        case List.first notes of
            Just first ->
                viewNote first

            Nothing ->
                -- This should never happen
                text ""

    else if List.length notes == 0 then
        viewEmptyList

    else
        -- There is more than one element in the list
        viewMinimizedNotes notes

Now, this function is fine. But it could be a lot simpler! The main problem is in the case where we have only one text note. When we have entered the first branch of the if-statement, we know that there is exactly one element in the list, but we still have to use List.first to get access to that element. And since the compiler doesn't know that there is a first element in the list, we still have to account for the Nothing-case.

Wouldn't it be nice if we could get access to the elements inside the list at the same time as we got information about the structure of the list? Well sure, and there is actually a way to do this in Elm.

Pattern Matching on Lists

Before we get to the solution, let's take a short detour through list making. In Elm, we can create lists in multiple ways. We can use square brackets to create them: [ 1, 2, 3 ]. We can use functions, like List.singleton 1 (which results in the list [ 1 ]). We can also use the :: operator from the List module. The :: operator works by adding an element to the start of a list, like this:

1 :: [ 2, 3 ]
-- Results in: [ 1, 2, 3 ]

1 :: []
-- Results in: [ 1 ]

a = [ 3, 4 ]
1 :: 2 :: a
-- Results in: [ 1, 2, 3, 4 ]

Now, the :: operator is used for creating lists, but it can also be used to pattern match on lists in case-statements! Let's take a look at an example:

case list of
    first :: rest ->
        "There is at least one element in the list, and the first element is: " ++ first

    [] ->
        "The list is empty"

As we can see from the example, pattern matching on lists gives us information about the shape of the list at the same time that we get access to the elements in the list!

Let's use this technique to rewrite our view function from before:

view : List Note -> Html a
view notes =
    case notes of
        onlyNote :: [] ->
            viewNote onlyNote

        [] ->
            viewEmptyList

        _ ->
            viewMinimizedNotes notes

Doesn't this look way better?

There are many more things you can do with pattern matching on lists, which we don't have time to get into today. But I suggest that you experiment to see what is possible! You could for instance get access to multiple elements (first :: second :: rest), or reverse the list (with List.reverse) and get access to the elements from the back. The possibilities aren't quite endless, but there are at least a long list of them.

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Bekk