Re: locally changing global variables

[Jaroslav Hajek]

On Tue, May 18, 2010 at 8:26 AM, John W. Eaton <address@hidden> wrote:
> On 18-May-2010, Jaroslav Hajek wrote:
>
> | Currently, I think not. It would be possible if we had a working
> | onCleanup implementation. Despite we have unwind_protect, which is, in
> | most practical ways, more versatile, onCleanup being an object allows
> | for doing magic, such as injecting a cleanup action to caller scope
> | (which could be the solution here). I'll start a separate thread about
> | this.
>
> Thinking about this a little more, I guess I'd like to do something
> like:
>
>  function retval = my_special_value (val, opt)
>   persistent curr_val = "default_value";
>   make_local = nargin == 2 && strcmp (opt, "local");
>   if (nargin == 1 || nargin == 2)
>     if (nargin == 2)
>       __make_local_variable__ (@my_special_value, curr_val);
>     endif
>     if (nargout > 0)
>       retval = curr_val;
>     endif
>     curr_val = val;
>   elseif (nargin == 0)
>     retval = curr_val;
>   else
>     print_usage ();
>   endif
>  endfunction
>
> Is there no way to have the __make_local_variable__ function call
> above hook into your current mechanism for restoring variable values?

with onCleanup, you could do something like:

 ## Check for existing restore action first. If none, create it.
 if (! evalin ("caller", "exist(\"__my_special_value__restore\", \"var\")"))
   restore = onCleanup (@() my_special_value (curr_val));
   assignin ("caller", "__my_special_value__restore", restore);
endif

and it is even Matlab compatible. It is a small snippet, though kinda
ugly. It would also be possible to create a function that would allow
adding user hooks to the unwind_protect frame (like my recent change
does), but you would still need evalin().

-- 
RNDr. Jaroslav Hajek, PhD
computing expert & GNU Octave developer
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz