Elixir Protocols vs Clojure Multimethods

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2021-07-17 12:00:05

I am coming to appreciate José Valim’s creation, Elixir, very much. It is fair to say that Rich Hickey set a very high bar with Clojure and Elixir for the most part saunters over it. I think that is a reflection of two very thoughtful language designers at work. But there is some chafing when moving from Clojure to Elixir.

I wrote previously about how Clojure’s metadata feature lets one subtly piggyback data in a way that doesn’t require intermediate code to deal with it, or even know it exists. It creates a ‘functional’ backchannel. Elixir has no equivalent feature.

If your data is a map you can use “special” keys for your metadata. Elixir does this itself with the __struct__ key that it injects into the maps it uses as the implementation of structs. You mostly don’t know it’s there but would have to implement a special case if you ever treated the struct as a map.

However, if the value you want to attach metadata to is, say, an anonymous function then you’re out of luck. In that case, you have to convert your function to a map containing the function and metadata and then change your entire implementation. That could be a chore, or worse.