Re: [proposal] A statement that can return value (and can used in expr)

[Lorenzo Donati <lorenzodonatibz@...>]

On 13/12/2011 21.39, Xavier Wang wrote:
> 2011/12/14 Luiz Henrique de Figueiredo<lhf@tecgraf.puc-rio.br>:
> > > local a = (function(a, b) print "a statement" return a + b end)(10, 20) + 30
> >
> > Lua 5.1 will create a new closure every time this statement is execute.
> > Lua 5.2 will not: it will resuse the closure.
> >
> > Try the code below in each version:
> >
> > for i=1,3 do
> >         local f = function(a, b) print "a statement" return a + b end
> >         local a = f(10, 20) + 30    + i
> >         print(f,a)
> > end
>
> I mean, if Lua found a anonymous function don't assign to any value,
> then just translate it into a new level with it's separate scope, like
> this:
>
> local a = (function(a, b) return a + b end)(10, 20) + 30
> becomes:
> do
>     local a, b = 10, 20
>     local ret = a  + b
> end
> local a = `ret` + 30
>
> ret is in above scope. and its the only variable that "export" from
> above scope. its will allow write a block that return a value. and can
> be embed it into a expression.
Sometimes I've dreamed the following syntax would be supported:

-- do..end blocks can return values with "local return"s
local a = do
  local a, b = 10, 20
  -- "local return" makes the block to return its value
  -- instead of terminating the enclosing function
  local return a + b
end

But I don't know if this could cause other problems and is orthogonal to 
other features, besides being considered "useless fat" by Lua Team :-).

Now that 5.2 reuses closures maybe it may be made syntactic sugar for:

local a = (function()
  local a, b = 10, 20
  -- "local return" makes the block to return its value
  -- instead of terminating the enclosing function
  return a + b
end)()

but I see problems with this code transformation approach, for example 
what if the block contained a "non-local", i.e. plain, return?

Food for thought.	

Cheers.
-- Lorenzo