Re: Down with Upvalues

[Edgar Toernig <froese@...>]

"John D. Ramsdell" wrote:
> 
> Your example show code that demonstrates nested scoping, but the
> following code shows that Lua variables lack nested scope.
> 
> 1.     function addn(x)
> 2.       function sum(y)
> 3.         return x+y      -- illegal reference to x
> 4.       end
> 5.       return sum
> 6.     end
> 7.     print((addn(3))(1))
> 
> If the variable x had nested scope, the scope of variable x would be
> delimited by the function/end keyword pair.  It would start at line 2,
> and continue until line 5.  With nested scoping, the reference to
> varible x on line 3 would be legal.

You mix scope with accessibility.  The scope of x starts at line 1
and ends at line 6.  You get a compile time error just because Lua
has nested lexical scope.  When seeing the use of the variable x in
line 3 the compiler decides (based on nested lexical scope rules)
that the declaration of x in line 1 has to be taken.  But because
this access is not allowed in current implementation it issues an
error message.  If Lua hadn't nested scope the compiler would most
likely have chosen some other declaration of x (i.e. global x or an
implied declaration of x local to sum) and would not have issued an
error message.

Over and out, ET.