Re: locally changing Options
- To: mathgroup at smc.vnet.net
- Subject: [mg108337] Re: locally changing Options
- From: Raffy <adraffy at gmail.com>
- Date: Sun, 14 Mar 2010 05:14:12 -0500 (EST)
- References: <hnakgk$5oa$1@smc.vnet.net> <hndb35$dee$1@smc.vnet.net>
On Mar 13, 4:59 am, Raffy <adra... at gmail.com> wrote:
> On Mar 12, 4:12 am, Raffy <adra... at gmail.com> wrote:
>
>
>
>
>
> > On Mar 11, 3:34 am, hemmecke <hemme... at gmail.com> wrote:
>
> > > In a function I'd like to temporarily say
>
> > > foo[x_]:=Module[{...},
> > > ...
> > > SetOptions[RowReduce, Method->OneStepRowReduction];
> > > ...
> > > ]
>
> > > without affecting the global setting for RowReduce when I call foo.
>
> > > My experiments wrapping the SetOptions with a Block structure were not
> > > fruitful so far.
> > > The only thing I can think of is something of the form
>
> > > method = Options[RowReduce, Method];
> > > SetOptions[RowReduce, Method->OneStepRowReduction];
> > > ... do something ...
> > > SetOptions[RowReduce,method]
>
> > > But that looks ugly.
>
> > > Any suggestions with built-in Mathematica features to achieve this kind=
> o=
> > f
> > > locality.
>
> > > Ralf
>
> > Block[
> > {RowReduce = RowReduce[#1, Method -> "OneStepRowReduction", ##2] &},
> > (* your code here *)
> > ]
>
> A few people emailed me today, noting that my solution above, while
> simple, doesn't work and results in infinite recursion. I apologize
> for not testing my solution.
>
> Anyway, after thinking about it, I felt that the simple fix would be:
> Block[{RowReduce=System`RowReduce[#1,Method->...,##2]&}, ...]
>
> However, this also results in recursion. Something seems odd: how is
> the blocked RowReduce scoping the fully qualified symbol
> System`RowReduce? (Looking at the Trace, it appears that it is
> dropping the System` context prior to evaluation...)
>
> This works: Block[{Print=System`Print},Print[1]];
> This doesn't: Block[{Print=System`Print[##] &}, Print[1]];
>
> Really odd behavior can be found using the following:
> Let $ContextPath = { ..., "Temp`", ..., "System`", ... }
> Block[{Temp`Print=System`Print[1, ##] &}, Print[2]] will print "12"
> and also cause Print to become shadowed.
>
> I guess the problem boils down to: Why does System`Print simplify to
> Print?
To make my point a bit more clear:
Imagine your $ContextPath does not contain "Temp`" and then let
Temp`f[x___] := {x};
Now, if I evaluate the following code, I get the expected result:
Block[{f=Temp`f[1, ##]&},f[2]] === {1, 2}
Notice: because Temp` is not on the $ContextPath, the fully qualified
symbol is not being truncated from Temp`f to f.
Instead, lets define System`f[x___] := {x};
The default notebook $ContextPath is similar to {..., "System`",
"Global`"};
The same code above, Block[{f=System`f[1,##]&},f[2]], will now become
recursive.
For some reason, it's REMOVING the System` context on System`f during
evaluation, which is causing it to get scoped by Block since
obviously, Block[{f=f[1,##]&},f[1]], will be recursive.