Personally I really don’t like such a default behaviour. In my opinion you should support all assert-like macros based on the official documentation.

use, import, assign or require

Should be:

use, import, alias or require

Don’t use anonymous functions in pipelines (L3),

I guess you can easily consider it as outdated. I would recommend to rewrite it in this way:

# Bad
sentence
|> String.split(~r/\s/)
|> (fn words -> [@sentence_start | words] end).()
|> Enum.join(" ")

# Good
sentence
|> String.split(~r/\s/)
|> then(&[@sentence_start | &1])
|> Enum.join(" ")

Enforce predicate functions to end with a question mark (N3),

That should be easy as long as you require the @spec, simply remove prefix is_ and add ? suffix if such character is not already at the end of the function name.

Avoid superfluous comments (C2).

This is handled by the Elixir formatter. It moves the comment after the expression to above said expression.

Put the expression being tested by comparison on the left side (U1)

I would not implement this. I’m not sure if it’s too far. However it should be easy to implement assuming “standardised” describe block naming:

describe "foo/1" do
  test ":default works as same as in foo/0" do
    # Bad
    assert MyApp.foo() == MyApp.foo(:default)
    # Good
    assert MyApp.foo(:default) == MyApp.foo()
  end
end

You guys have been debating the comment rule but in fairness to the OP his docs explicitly marked that particular rule as being impossible to implement because it’s subjective.

I think what the rule is getting at is that new programmers can sometimes fall into a habit of over-commenting every line, even what it’s obvious what the code does. Everyone posting in this thread is probably experienced enough to have learned not to do that long ago

This looks great. Does it work alongside :styler or do they diverge in places? I am on the phone so lazily asking without reading docs.

I don’t think I want my formatter renaming my functions.

Sure it moves them. It certainly doesn’t detect if it is superfluous and delete it. Probably out of scope for a formatter.

Styler has its own style, while Green aims to implement the existing popular style guides.

It automates formatting that is currently enforced by hand.