[Proposal] Mix flag to tune deprecations warning

[Thomas Depierre]

Hi,

I want to say upfront that i know this kind of proposal happened before, and i know the stance is "if you disable warnings, why have warnings at all" if i remember correctly. Which i support.

That said, i think there is now an opportunity for a new discussion around a particular kind of tuning for warnings.

Proposal

Tag all elixir deprecation with @deprecated and @since (hard ?)

Offer a flag to restrict deprecation warnings to only warnings before a given version. It could be a flag to mix for xref like: 

--only-deprecated-before 1.6

This would only generate warnings for deprecations that are anterior to deprecation in 1.6+

Why ?

1. The compile flag --warnings-as-errors do make sense to use, among other reasons due to warnings like this one:

 function Foo.bar/0 is undefined (module Foo is not available)

because they do tend to be real errors in the code most of the time.

 (I know there have been some discussions for elixir 2.0)

2. Elixir is deprecating a lot recently on each versions. Which is great, but make for a really noisy output on CI, especially when some dependencies want to keep compatibility with 1.4+ ( a good example is Phoenix recently). But it also happens a lot if you want to compile with 1.6 locally your library to get better error message and all, but you target 1.4+ for the user of your library.

Now that we have the @deprecated and the @since attributes, there is a window to offer the possibility to filter these warnings for deprecations.

It would benefit CI noisiness and make it easier to support library maintainer that want to bump up their versions support without going full bleeding edge. Yes they can compile with the old version and will probably keep doing it, but when newer versions also provides niceties for development...

Why not ?

Some of my own counter arguments :

Hard to do. And useless before 1.6 as we do not have @deprecated and @since before that.

Small value added, you should develop and compile with the version you target.

Deps warning are already ignored for `--warnings-as-errors`

Hiding warnings is a bad pattern, as people will not fix their stuff.

This is not doable right now without the change coming with 1.7, or would need more changes. (no idea if true here)

Would love to hear your thoughts on it. I would happily take a swing at trying to work on that if the team want to go with it.

Thomas Depierre

[José Valim]

The issue is that @since does not mean when a function has been deprecated but when it was added. So the information you are requesting is not available right now (although it could be made available).

But honestly, the best answer would be to reduce the amount of deprecations.

We are very aware of the amount of deprecations Elixir emits and it has been our goal to reduce them. v1.6 already had less deprecations than v1.5 and we hope we will continue this trend with v1.7. The issue is that changes to our deprecation policy takes a while to have an effect because they are long term.

José Valim

www.plataformatec.com.br

Founder and 

Director of R&D

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[Andrea Leopardi]

I also think it's worth noting that libraries don't need to wait before they can depend on a specific version of Elixir before fixing all the deprecation warnings of the versions before that version. You can do something like we do in Gettext for example (https://github.com/elixir-lang/gettext/blob/9fcb138f949185079921f7a26ae1c342587e2235/lib/gettext/po/parser.ex#L176-L178) in order to support multiple Elixir versions without emitting warnings during compilation. The gist is:

  # TODO: remove once we depend on Elixir 1.5 and on.

  chunk_every = if function_exported?(Enum, :chunk_every, 2), do: :chunk_every, else: :chunk

  defp enum_chunk_every(enum, n), do: apply(Enum, unquote(chunk_every), [enum, n])

or something along those lines.

Andrea

Andrea Leopardi

an.le...@gmail.com

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