Passing in options: Maps vs. Keyword lists

Have you seen how, for example, System.cmd/3 does it?

https://github.com/elixir-lang/elixir/blob/87710cd49521fc396f3f04fa8615f05e7f57ecc0/lib/elixir/lib/system.ex#L578-L603

There are situations where the latter two properties are very handy. Ecto’s query syntax and Elixir’s OptionParser are situations where you need both. The import special form also leverages the last property (keys can be given more than once). I am pretty sure there are more examples but those are three from the top of my mind.

One of the reasons I also like to use keywords list for options is because we have a well defined dichotomy where keyword lists are used for incomplete sets (you do not need to pass all keys) while maps expect all keys to be there. This allows me to use the assertive map.key syntax when using maps. It doesn’t need to be an OR choice though. I agree pattern matching on maps is really handy and for such cases I would likely accept a keyword list but convert it to a map with defaults:

map = Enum.into(user_options, %{key: "default", other_key: false, ...})

This way the API is consistent with Elixir and you can still leverage the map properties when you don’t care about duplicates nor ordering.

This will sound terrible, but sometimes the biggest advantage of doing something is just because everybody else is doing it that way.

In particular, if you are supporting a public library going against the flow is going to lead to an endless stream of questions about keywords not working as options. You’d think programmers would be slightly better than most at reading documentation, but sadly programmers are just people. People as a rule don’t read documentation[1].

While this may seem like a little thing, it’s annoyances like this that sap your energy for supporting open software. It’s one thing to be paid to read man pages to people, it’s a whole 'nuther thing to do it for free in your spare time. Matz’s “principle of least surprise” that drove the design of Ruby is a valuable lesson.

Compared to Elixir, Ruby had terrible documentation for a large part of it’s early history, yet it thrived despite that. One of the reasons for this is that you’d just try the obvious thing and 90% of the time, it would just work.

[1]- Actually this is not true, there’s just always a significant percentage that don’t and seem much a much larger percentage than they really are.

Yeah I agree. Phoenix and Elixir both use Keyword Lists so I do as well.

Thank you all for your answers! I think I’ll play with converting keyword lists to maps for internal use, that seems like a good solution Also thanks for arguing for using the Elixir standard against what I might prefer and the way System.cmd/3 deals with the problem - you’ve all been very helpful (as always!)

The Erlang Team has changed the default in favor of maps. Not sure if Elixir will follow.

bit late but thanks for passing this along, very interesting and good to know!

In his talk at the virtual DevsForUkraine conference, José Valim addressed the question.

TLDL; he still favors lists

But when we want him to reconsider, all we need to say is: José, I think there is a performance issue with using lists for config. He will then benchmark it extensively, re-read every line involved, find 4 unrelated bugs, write fixes, push them to master and finally answer. All within 15 minutes.

Thanks for summarizing it!

I was actually the one asking José that question there, thanks to this thread

Link to the moment in the Q&A for future reference: https://www.youtube.com/watch?v=2Vhd5fw59UY&t=1116s

As a bit of input here, I don’t think performance is or will be an issue. Even erlang maps are internally represented as lists until there is a map size of 32 iirc. Reason being, hashing is expensive and so for small maps a linear search is actually faster. Since I don’t hope many people pass along more than 30 (I hope more than 5) options, performance should be the same/comparable.