[Proposal] dual alias of the same module should result in a warning

[ku...@goyman.com]

If a module is aliased or imported multipled times by the module, it would be handy to have a warning.

For example:

defmodule Foo do

  alias Ecto.Changeset

  alias Ecto.Changeset

end

should result in a warning "duplicate import".

This is low priority, but as there is one for unused import/alias it would help make the code cleaner.

[José Valim]

We can warn it for aliases no problems and a PR would be welcome.

However, for imports it is a bit more complicated because you can import something multiple times with different parts. Furthermore, maybe a library Bar is doing "import Foo" and you want to repeat "import Foo" for clarity. I think we will have false positives.

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[ku...@goyman.com]

Yeah I agree that import is more tricky. But for me, alias is the main use case. I'll see if I can manage a PR.

[Jon Rowe]

One of the same problems with `import` applies to `alias` too though, if a library has `alias Module` in its `use` macro it would cause this warning if you aliased it yourself to be explicit. Prehaps this is more suited to a static analysis tool like credo which already warns you if your aliases and import declarations are out of order, it could warn you if you have them twice in the same file without hassling those who just like to be explicit in files.

Just my 2¢

Cheers

Jon

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[Aleksei Matiushkin]

Another issue with an alias would be with many different aliases of the same module. E. g. in Ecto if the schema has two distinct associations with the same foreign schema, one might want to improve code readability by

alias User, as: Customer # for customer-aware queries

alias User, as: Seller # for seller-aware queries

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[José Valim]

Said case would not warn. I think we should warn only if you are defining an alias, and it already exists, and it points to the same module.

Elixir also knows if an alias was defined by a macro, so we can skip the warning in such cases too.

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