Re: Why do we need “toclose” (or those angle brackets) at all?

[David Heiko Kolf <david@...>]

Paul Ducklin wrote:
> A <toclose> variable *must have* a __close metamethod or else an error is raised when it goes out of scope. So why can’t the presence of a __close metamethod be the signal to Lua that it is handling a to-be-closed variable?
> 
> The <toclose> flag for a variable should simply be set automatically at runtime when the setmetatable() function is called for the variable.

Lua cannot easily tell the intended scope of a variable without some
extra informations. Consider this case:

  function foo (fname)
    local f, err = io.open(fname, "w")
    if not f then
      print(("Failed to open %s: %s"):format(fname, err))
    else
      bar(f)
    end
  end

Now what does "bar(f)" do to the file? It could just write some stuff in
the file and it should be closed once f in "foo" goes out of scope:

  function bar (f)
    f:write("some stuff")
  end

However, it could also save the file handle somewhere else where it is
expected to be still open at a later point:

  function bar (f)
    activefile = f
  end

How would Lua know there is still a reference to the file? At least
without running the GC every time a variable with a __close metamethod
goes out of scope.

Best regards

David