Using Kernel.match?/2 to check if variable contains anonymous function AST increases size by 5 in Credo.Check.Refactor.ABCSize

[Eiji7]

Environment

• Credo version (mix credo -v):

$ mix credo -v
1.7.10-ref.main.1d74cf6+uncommittedchanges

• Erlang/Elixir version (elixir -v):

$ elixir -v
Erlang/OTP 27 [erts-15.1] [source] [64-bit] [smp:32:32] [ds:32:32:10] [async-threads:1] [jit:ns]

Elixir 1.17.3 (compiled with Erlang/OTP 27)

• Operating system:

Gentoo Linux using desktop/plasma profile for version 23.0 and with kernel version 6.6.41.

What were you trying to do?

Replaced if condition by true for debug ABCSize check

if match?({:fn, _meta, _args}, ast) do
  # …
end

Expected outcome

No idea how much points are expected, but 5 is definitely too much as just 6 lines with such a simple match? call would reach a limit.

The ABC size describes a metric based on assignments, branches and conditions.

1. Since ast is assigned above if condition I guess it should not count, right?
2. Branches are dot calls (like: mod.fun(…), right? Maybe it counts as +1 because of AST expansion to Kernel.match?(…) form …
3. Kernel.match?/2 is not a condition (if, unless, for, try, case, cond, and, or, &&, ||)

If that's correct then it should count as +1 for size. Not sure from where 5 comes - it does not match even if we count skipped patterns (_some_name) and ast with match? call would give at most 4 points. If you agree it would be awesome to count is as just +1 size, but as above I may be wrong here …

Actual outcome

Such a change decreases size by 5:

# after
if true do
  # …
end

[Eiji7]

Update: I forgot that I can use with/1 special form. It's not only a working solution, but also better one as match?/2 is a macro which expands to case/2 statement. @rrrene I wonder if ABCSize check is expanding macros and therefore counting their size too? Otherwise I have no idea what's wrong. 🤔

Anyway, we need 2 things:

1. Check if macros are expanded which may result in increased ABCSize
2. There should be a test which verifies that match?/2 and with have same ABCSize

if match?({:fn, _meta, _args}, ast) do
  # …
end

# vs

with {:fn, _meta, _args} <- ast do
  # …
end

Hope that helps!

[rrrene]

I wonder if ABCSize check is expanding macros

I don't think so, especially since Credo is only doing static analysis and does not compile the code.

You seem to have a good argument, but I will have to look into this more closely.

Side note: ABC size is one of the more controversial checks (see docs). I doubt that it makes much sense in the context of Elixir. The reason why it exists is that it was part of the very early protoypes of Credo.

[Eiji7]

@rrrene Yes, I understand but in 99% of cases it's working very well. There are just few standalone edge cases where you have few extra size points, but you do not see from where they came. I'm currently working on a lot of meta-programming code and checks like that are very helpful. Instead of ignoring such low priority credo warnings I started to look for an alternative implementation ways and from that I have learned many good things, so I believe that even such check is very useful. Thanks for your time for maintaining it and whole credo project. ❤️