December 01, 2019

How to use List.singleton in elm.

Introduction

I wrote about some basic tips in functional programming a while ago in this article (Basic Functional Programming Tips using elm).

One of the topic was composing functions with >> << to do point free programming.

And in this article I want to introduce about one specific function in the List module which is List.singleton.

What is List.singleton

First, let's see the type definition.

singleton : a -> List a

What it simply does is it creates a list with only one element.

singleton 1234 == [1234]
singleton "hi" == ["hi"]

It's very simple and straight forward, but when I first look at this function, I thought why would any one use this, while you can just define a List on the fly like [1234] and ["hi"].

But beauty of this is, that List.singleton is a function.

Meaning you can compose this with other functions 😤

How to use

Let's take a took at some real life examples.

If you have a List of String, and you want to map the elements and display it as a Html.text. Probably the most simplest approach will be the following.

import Html exposing (Html, div, text)

fruits : List String
fruits = ["Apple", "Banana", "Peach", "Mango"]

view : Model -> Html msg
view model =
    div [] (fruits |> List.map (\(fruit) -> div [] [text fruit]))

-- div [] [ div [] [text "Apple"], div [] [text "Banana"] ...]

This is totally valid, but you can re-write using List.singleton.

import Html exposing (Html, div, text)

fruits : List String
fruits = ["Apple", "Banana", "Peach", "Mango"]

view : Model -> Html msg
view model =
    div [] (fruits |> List.map (div [] << List.singleton << text))

Very concise and clean 🤩

You can also use this technique in any kind of map functions.

Maybe.map

If you want to display the text depending on the Maybe value.

header : Maybe String
header = Just "foobar"

header
  |> Maybe.map (div [] << List.singleton << text)
  |> Maybe.withDefault (Html.text "")

Tuple.mapSecond

If you want to use other Component module from your own modules.

You will call the init function of the Component, then wrapping the Cmd using Cmd.map, with the module's own Msg.

The common approach will be something like this.

--- Component.elm, Some random component

init : (Model, Cmd Msg)
init = (model, cmd)

--- Main.elm

init : (Model, Cmd Msg)
init =
    let
        (model, cmd) = Component.init
    in
    ({component = model, something = 100} , Cmd.batch [myCmd, cmd |> Cmd.map ComponentMsg])

You can do something like this. Though this might be a overkill, and lose some scalability.

--- Main.elm

init : (Model, Cmd Msg)
init =
    Component.init
        |> Tuple.mapFirst (\m -> {component = m, something = 100})
        |> Tuple.mapSecond
             (Cmd.batch << (++) [ myCmd ] << List.singleton << Cmd.map ComponentMsg)

Summary

List.singleton is a very simple function, but it has lots of potentials for making your code much nicer to read.

When I first figured this out, I looked though all the code that I wrote in elm, and refactored using the techniques.

Hope you enjoyed reading this article.

Written by...
Kazushi Kawamura
Software Engineer based in Tokyo.
2019 © Kazushi Kawamura.