Re: What are the use cases of in-do-end?

[Richard Hundt <richardhundt@...>]

GrayFace wrote:
> So far I've read mentions of several use cases, but I don't understand 
> how exactly it may be useful in them. Here's what I read about:
> 1) Obvious usecase: something like Deplhi's 'with'. This is not the main 
> use case Lua developers targeted, for sure. Inability to call global 
> functions, a need for additional local variable to call methods makes 
> this incomplete.
> 2) Modules. Maybe it's due to the fact that I didn't work with modules, 
> but I don't see it useful. I don't see a reason for setting current 
> environment like 'module' does in 5.1 or use of in-end of 5.2 due to the 
> same reason: inability to call global functions. Generally, I'd prefer 
> to directly write "function P.f()  end" instead of "function f()  end" 
> given P is the module table.


2) Modules...

This is the one that I'm playing with atm.

Basically using environments to trap setglobal in class and module 
definitions to set up metatables: function definitions become virtual 
methods, "types" become attributes, constants become static members... etc.

It's mainly sugar, but you can get really close to Ruby's class and 
module semantics, and still have it look nice:


in module .Acme do
   in module .Friendly do
      function greet(self, whom) -- instance method (visible on mixin)
         print(self.name.." says: Hello "..whom.."!")
      end
      function static.answer()   -- static method (doesn't mixin)
         return 42
      end
   end
   answer() -- 42 (static methods visible here)
   in class .Person do
      name = String -- instance attribute (type checked)
      function __init(self, name)
         self.name = name -- TypeError if name is not a string
      end
   end
end

in class .Hacker (Acme.Person) do
   import(Acme.Friendly)
   function __init(self, name)
      super.__init(self, name) -- "super" injected into environ
   end
end

local bob = Hacker.new("Roberto")
bob:greet("World") -- "Roberto says: Hello World!"


I've got the above working (by breaking just about every rule: "module" 
is no longer a function, but a magic table, and all primitives have 
metatables, except for nil).

I appreciate it's not very Lua'esque to everything-is-an-object'ify, but 
I think it's pretty cool that you can create new syntax like that with 
just the native language features.

-Rich