---
title: "Climbing trees"
url: "https://www.bekk.christmas/post/2020/06/climbing-trees"
authors: ["Ragnhild Aalvik"]
published: "2020-12-06"
updated: "2024-11-07T13:05:28Z"
language: "en-US"
type: "article"
tags: ["Elm"]
---

# Climbing trees

Custom types are powerful data structures that might seem somewhat complicated to work with, at least if they are nested. Let's take a look at how we can deal with nested custom types in a simple way! 🍊

Custom types in Elm are used when defining data types that can have different possible _variants_. We can think of this data structure as a tree, with each branch representing one variant of the data type. Each variant, or branch, can itself contain a new custom type, resulting in a nested tree structure. We call this a nested custom type.

Let's look at an example. Say we have a, very simplified, custom type `Fruit`:

```elm
type Fruit
	= Apple
	| Pear
	| Citrus CitrusFruit

type CitrusFruit
	= Orange
	| Lemon
	| Lime
```

The `Fruit` type contains different variants of a fruit, with one of them being `Citrus`. `Citrus` itself contains another custom type, namely `CitrusFruit`, which makes `Fruit` an example of a nested custom type.

What if we want to write a function that returns a png-image of a given fruit? Then we would have to destructure the fruit using a `case..of` expression. This could be done like this:

```elm
image : Fruit -> String
image fruit =
    case fruit of
        Apple ->
            "apple.png"

        Pear ->
            "pear.png"

        Citrus citrus ->
            case citrus of
                Orange ->
                    "orange.png"

                Lemon ->
                    "lemon.png"

                Lime ->
                    "lime.png"
```

As you can see, this way of unwrapping the `Fruit` requires two `case..of` expressions. The resulting code is not very readable and can, in fact, be written in a much simpler way! We can simply destructure the whole `Fruit` in one `case..of` expression like this:

```elm
imageImproved : Fruit -> String
imageImproved fruit =
    case fruit of
        Apple ->
            "apple.png"

        Pear ->
            "pear.png"

        Citrus Orange ->
            "orange.png"

        Citrus Lemon ->
            "lemon.png"

        Citrus Lime ->
            "lime.png"
```

See how much easier this function is to scan? 🧐 This little "trick" is something people new to Elm often don't know about, which is a pity as it makes the code both easier to write and more readable. It is especially helpful in the `update` function if you have a nested `Msg`. Try it out the next time you climb trees!

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