In context of typespecs:

mfa()	{module(), atom(), arity()}

For child_spec() Erlang uses mfargs() to refer to a {M,F,A} tuple (module, function atom, arguments list - Kernel.apply/3, :erlang.apply/3).

Its a good question! The nearest I could find was an erlang type definition in the docs at http://erlang.org/doc/reference_manual/typespec.html

This has recently come up for me as I am working on a library where I am adding support for a “function reference” argument. Previously I was expecting {module, function, args} even though the args component was not really used for anything simply because that format seems to be universal in Elixir. But due to a behavior change I really need to know what arity to expect and I’m sort of at a loss when it comes to best practice!

I am using NimbleOptions and there is no built in support for this. In fact, the documentation has this line which was puzzling to me right off the bat:

:mfa - A named function in the format {module, function, arity} where arity is a list of arguments. For example, {MyModule, :my_fun, [arg1, arg2]}.

(There is also {:fun, arity} but in my use case the module is also variable, edit: and also on closer reading this appears to be for functions of set arity?)

I have to wonder why it seems that in Elixir mfa has come to mean specifically the args variation and I don’t think I’ve seen a mfargs type in the wild…

Did a little code excavation and it turns out that, even in the Erlang repository “mfa” can mean both, but if in the “args” variant, I see a lot more adhoc type specs. It’s probably one of those cases where its interchangeable use has caused it to have multiple meanings. It does help that you can match on the third element for the various cases:

• {m, f, a} when is_integer(a) and a >= 0 - known arity
• {m, f, :_} - unknown arity
• {m, f, a} when is_list(a) - args instead of arity

I think capturing &SomeModule.function/2 is more familiar, but {SomeModule, :function, 2} does make it easier to check if the function exists (vs having to check the capture with Function.info/2).