Intro
As an Elixir beginner Keyword Lists are one of the weird features of the language. Coming from other languages, you’d almost always think of map-like structure first. So there’s a gnawing feeling, why keyword list? We’ll start with the unique characteristics of keyword list then try to go into practical use cases that leverage the unique characteristics which are:
- Order
- Duplicates
Order
Keyword List preserve user ordering which could be useful. There are a few practical applications of this that could be useful when you’re developing a project. One application is simpler, in certain situations order does matter and must be enfored. As Jose Valim mentions, Elixir uses this ordering to help warn when the try/catch/after block is in the wrong order.
Another similar application of keyword lists are when ordering of the lists has special meanings. [first, second] might mean something different than [second, first ]. I haven’t run into a strong use case for this so once I do I’ll return back to this.
WARNING: Keyword methods do not promise order
A really important disclaimer is that although the list itself preserves order, the functions in Keyword do not as stated in the docs.
Duplicates Allowed
In the same Elixir Forum comment, Jose Valim points out that since Keyword lists allow for duplicates, this allows us to do something like import Keyword, only: [get: 2, get: 3]. In this case we’re able to specify the function name we’re interested in and since we can “overload” functions in Elixir we can use the arity to distinguish between which function we mean.
Another example of this that I ran into was working with repeated query values in an HTTP GET request. Sometimes when working with APIs you’ll notice a request that works with a list of data. There’s a lot of ways this shows up. Sometimes, since the data you’re requesting is specified in a complex way people use a POST request to retrieve data. This makes the request simpler but then you’re using POST for a reason it wasn’t really meant. Another is to keep it a GET request but the downside is then to specify what we want must be in the query params since GET cannot have a request body. Ignoring which way is better, let’s focus on the get request that looks like the following.
/recipes?ingredient[]=chicken&ingredient[]=paprika
My goal was given a list of ingredients (['chicken', 'paprika']) , how can I create this request? Well in this case Map could not work because the key is the same, ingredient. This highlights a case where Map just plain would not work. So Keyword Lists comes to save us since we place the same key multiple times in a Keyword Lists. Let’s finish out this example.
So I wanted my key to be ingredient[]. We could have kept it simpler but in this case it makes for a good exercise since we’ll briefly learn how to make a complex atom key.
The remaining part is specifying complex atoms. Disclaimer about ETS here. I wanted the key to be ingredient[] which is not a valid atom?
"ingredient[]": "chicken"
Bonus: Pushback
- Try ecto with maps instead of KW… You won’t have much joy…
- Historical