[erlang-questions] core erlang apply requires variable in function position?

[Mikael Pettersson]

This simple Core Erlang module fails to compile with OTP-20.0.1:

> cat test1.core
module 'test1' ['test1'/2]
    attributes []
'i'/1 =
  fun (_f) -> _f
'test1'/2 =
  fun (_f, _x) ->
    apply apply 'i'/1 (_f) (_x)
end
> erlc test1.core
no_file: Warning: invalid function call

and the .beam produced just contains code to throw a badfun error.

The warning comes from sys_core_fold:expr/3:

expr(#c_apply{anno=Anno,op=Op0,args=As0}=App, _, Sub) ->
    Op1 = expr(Op0, value, Sub),
    As1 = expr_list(As0, value, Sub),
    case Op1 of
	#c_var{} ->
	    App#c_apply{op=Op1,args=As1};
	_ ->
	    add_warning(App, invalid_call),
	    ...

indicating that it's not prepared to handle apply forms with
anything but plain variables in the function position.  This
surprised me since:

1. I'm generating the above code using the cerl module's constructor
   functions which do not mention this restriction, and
2. the module so constructed passes core_lint:module/1, and
3. core_pp:format/1 is also happy with it, and
4. the Core Erlang language specification document doesn't
   restrict apply's function position beyond the general class
   of expressions.

So is this restriction deliberate or unintentional?

For now I work around it by wrapping the apply with a 'let'
binding the function position expression to a temporary.

/Mikael