Why There's No Need For MultiCase Matching in Elm

Published Nov 13, 2017Last updated Nov 21, 2017
Why There's No Need For MultiCase Matching in Elm

As a beginner developer with Elm, I quickly ended up both loving the union types and the _ operator, but found the absence of MultiCase matching a tad bit irritating until... I saw the light!

TL;DR?

Use the following pattern whenever you have some behavior shared by several constructors of your union types and stay away from the _ operator!

avoiding `_`

The problem with single case matching

Let's say I want to create task tracking software. I might end up with this Itemtype to describe my model:

basic model

As you can guess, many operations are going to require the same treatment for most kind of Item: assigning the item to somebody, changing the start or end date, adding/editing a description, etc. However, there will be some that will differ.

How can we deal with that?

Using the underscore matching

When I encountered this problem, my first idea was to use the _ to match all of the kinds of Item that didn't require a specific treatment for any given case ... of.

There are two main problems with that approach: we can't get the properties and it introduces potential BUGS, meaning problems and unexpected behavior that won't be caught by the compiler.

First, let's see how it looks:

using `_`

Let's take back the Item example and pretend that we want to create a new kind of Item: milestone.

Let's say milestones do not have a start date but are just used as deadlines. Say my app is pretty developed and I am doing a lot of things on my Item and use underscore matching a lot. By adding a new kind of Item in my union type, I'd expect the compiler to throw a bunch of errors directly in my face.

However, this will not happen in that case: my new kind Milestone would already be matched by the _, so there won't be anything wrong that will be caught by the compiler. Yet I probably don' t want my Milestone items to be dealt with like any other items, so I would have to look up any _ and make sure that I adapt the code manually.

The replacement

Use let … in! (or make another helper function). It really is as simple as that. Instead of making MultiCase matching, put the common treatment in a function within the let ... in or put it outside if the treatment is consequent and use it afterwards:

the good way

And voilà! It sounds pretty stupid, but I hope it helps!

Here is a small "project" showing this… nothing fancy, only one message and one button.

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