Qqwy
The issue with the static access operator (the dot)
I would like to spark a discussion about the static access operator: ..
For whom does not know: it is used in Elixir to access fields of a struct or map, where you want to be sure (/are sure) that a given (atom) key exists.
It is arguably more easy to write out foo.bar instead of foo[:bar] or binding it in a pattern-match, but I have found that relying on . too quickly/often is a recipe for tightly-coupled code, in my opinion.
Namely, changing the internals of a %Foo{} struct becomes impossible when you (or, even worse, users of your library) rely on being able to type foo.bar. If you want to be able to change the internals of a data structure, you should instead have written a function like Foo.bar(foo). Besides being more explicit (and also slightly longer to type), this allows you to alter the way bar is obtained from within the structure at a later time.
I have run into this issue multiple times now while developing libraries, so I want to ask you all about your opinion about this subject: Do you rely on using . a lot? Do you agree that . should be used with care? Are there alternatives? How to you mitigate this issue? Or is it, in your opinion, not an (important) issue at all?
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ericmj
Typespecs to the rescue. Declare your types as opaque and users should know they shouldn’t depend on the structure. If they use dialyzer they will even get errors when matching on the structure or using the dot field access.
benwilson512
I’m a bit confused, what is “direct field access” here, foo.bar ? If that’s what you mean it isn’t really a function call, IIRC it translates to basically:
case foo do
%{bar: val} -> val
_ -> apply(foo, :bar, []) # or an error handling clause, can't recall
end
foo[:bar] However is a function call Access.get which you can see easily from quoting it:
iex(1)> quote do: foo[:bar]
{{:., [], [Access, :get]}, [], [{:foo, [], Elixir}, :bar]}
I don’t believe any fancy inlining happens there.
benwilson512
But this isn’t the case. Nothing at all happens at compile time to ensure that the key does or does not exist in the map, nor even that foo is a map.
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