[erlang-questions] New EEP 45 - FUNCTION macro

[Richard A. O'Keefe]

On 30/10/2015, at 2:11 am, Björn Gustavsson <bjorn@REDACTED> wrote:

> There is a new EEP 45 that proposes a new
> FUNCTION macro in the preprocessor.
> 
> http://www.erlang.org/eeps/eep-0045.html
> https://github.com/erlang/eep/blob/master/eeps/eep-0045.md

The problem with ?FUNCTION => {Name,Arity} is that
obviously sensible things like 
	fun ?FUNCTION
don't work.

The problem with ?FUNCTION => Name/Arity would be that
obviously sensible things like
	Name/_Arity = ?FUNCTION
wouldn't work.

Why not cut the Gordian knot?
A __FUNCTION__ macro for C makes sense because the arity of
a C function is not part of its name (but part of its type).
For Erlang, I respectfully suggest that it makes more sense
to have *two* macros:

	?FUNCTION_NAME => the name of the containing function
	?FUNCTION_ARITY => the arity of the containing function

and then people can easily write whatever they want.  For example,

f(G, X) when is_function(G, ?FUNCTION_ARITY) -> ...

is a lot clearer than anything involving ?FUNCTION.

The really good thing about defining ?FUNCTION_NAME and
?FUNCTION_ARITY is that anyone who *wants* ?FUNCTION is
then at liberty to
-define(?FUNCTION, {?FUNCTION_NAME,?FUNCTION_ARITY}).
if that's what they need.

I did notice that there are definitions of ?FUNCTION_NAME
and ?FUNCTION_ARITY in the EEP

  *** BUT THEY DOEs NOT WORK IN FUNCTION HEADS **

Nor for that matter will they work after 'fun'.

This makes the claim that ?FUNCTION_NAME and ?FUNCTION_ARITY
can "easily be defined in terms of FUNCTION" false.
Given the asymmetry, it is obvious which should be taken as
basic and which as derived.

Looking at the text,
  "must be efficient ... slowdown"
appears to mean macro-expansion time.  It would be good to be
explicit about that.

As for FUNCTION_STRING, it's not clear why it's
-define(FUNCTION_STRING, atom_to_list(?FUNCTION_NAME) ++ "/" ++
          integer_to_list(?FUNCTION_ARITY)).
rather than
-define(FUNCTION_STRING,\
    atom_to_list(?MODULE) ++ ":" ++
    atom_to_list(?FUNCTION_NAME) ++ "/" ++
    integer_to_list(?FUNCTION_ARITY)).