Issue #6806 has been updated by Alexey Muranov.


Besides functional programming, IMO this would support [commandquery separation](https://en.wikipedia.org/wiki/Commandquery_separation).

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Feature #6806: Support functional programming: forbid instance/class variables for ModuleName::method_name, allow for ModuleName.method_name
https://bugs.ruby-lang.org/issues/6806#change-47475

* Author: Alexey Muranov
* Status: Open
* Priority: Normal
* Assignee: Yukihiro Matsumoto
* Category: core
* Target version: Next Major
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=begin
What would you say about this proposal?  Is there a better alternative?

I suggest to support functional programming in Ruby by making module methods called with (({ModuleName::method_name})) syntax raise an exception if the method uses instance or class variables (instance variables of the singleton class, of course).
If i understand correctly, currently (({ModuleName::method_name})) and (({ModuleName.method_name})) behave identically, so i propose that they be different:

 module M
   module_function
     def f(x)
       x*x
     end
     def g(x)
       @x ||= x
       @x*@x
     end
 end

 M.f(2) # => 4
 M.g(2) # => 4
 M::f(3) # => 9
 M::g(3) # => Error: instance variable `@x` used in a functional call `M::g`

Current behavior:

 M.f(2) # => 4
 M.g(2) # => 4
 M::f(3) # => 9
 M::g(3) # => 4
=end




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