"--" should have explicit parens in the formatter

[sschuldenzucker]

Elixir and Erlang/OTP versions

Erlang/OTP 26 [erts-14.2.5] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [jit] [dtrace]

Elixir 1.17.0 (compiled with Erlang/OTP 26)

Operating system

MacOS Sonoma 14.5

Current behavior

The "--" operator that 'subtracts' lists from each other has the highly unintuitive property that it's right-associative. Specifically:

iex(1)> [1,2,3,4] -- [1,2] -- [3]
[3, 4]
iex(2)> ([1,2,3,4] -- [1,2]) -- [3]
[4]

Line (1) is parsed as [1,2,3,4] -- ([1,2] -- [3]) = [1,2,3,4] -- [1,2]. This is documented correctly but it seems extremely counter-intuitive: it seems to me that everyone parses line (1) as "take [1,2,3,4], then remove 1 and 2, then remove 3. It's also inconsistent with the behavior of "-" (subtraction for numbers)

Expected behavior

"--" should be left-associative so that the two lines above are equivalent.

[sabiwara]

This is mostly due to consistency with ++ (which has to be right-associative for performance reasons) and consistency with erlang which made the same decision (see this thread).

Changing it would be a breaking change so I'm not sure we can go ahead.

I wonder if there are other options to mitigate this and avoid the surprise, such as:

• warn in such cases and ask to add parentheses to make the intended order explicit
• have the formatter add parentheses in such cases so x -- y -- z becomes x -- (y -- z) (making it explicit what will happen)

[josevalim]

Yes, we cannot change the precedence and warning would be complicated. We could make the parens explicit in the formatter though.