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Re: Work with Contexts

• To: mathgroup at smc.vnet.net
• Subject: [mg35785] Re: Work with Contexts
• From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
• Date: Wed, 31 Jul 2002 01:33:20 -0400 (EDT)
• Organization: Universitaet Leipzig
• References: <ai06l0$19t$1@smc.vnet.net> <200207290713.DAA05847@smc.vnet.net> <ai5u8n$brs$1@smc.vnet.net>
• Reply-to: kuska at informatik.uni-leipzig.de
• Sender: owner-wri-mathgroup at wolfram.com

Hi,

according to Allan's an my explanation

Mathematica *must* parse the definition of oo`new12[] *before*
it is evaluated. Because you don't like local variables you
write

oo`new12[] := (
Print["Name klasse - Anfang"];
 Print["$Context: ", $Context];
 Print["$ContextPath: ", $ContextPath];
 ToExpression[oo`hfield];
 Print["Context von klasse: ", Context[klasse]];
 oo`resumeContext[];
 Print["Name klasse - Ende"];
 );

and parsing this create a global symbol "klasse" because
the parser see the line

 Print["Context von klasse: ", Context[klasse]];

If you wish to avoid this you can use

oo`new12[] := Module[{UsingObjectsIsDaft`class},
      Print["Name klasse - Anfang"];
      Print["$Context: ", $Context];
      Print["$ContextPath: ", $ContextPath];
      UsingObjectsIsDaft`class = ToExpression[oo`hfield];
      Print["Context von klasse: ", 
        Context[Evaluate[UsingObjectsIsDaft`class]]];
      oo`resumeContext[];
      Print["Name klasse - Ende"];
      ];


Regards
  Jens


Hermann Schmitt wrote:
> 
> Hello Allan,
> I have a new program:
> 
> oo`hfield = "klasse1";
> oo`newContext[ctxp_] := (
> 
>   $ContextPath = Join[ctxp, $ContextPath];
>       BeginPackage[ctxp[[1]]];
>    If[Length[ctxp]>1,
>        $ContextPath = Join[ctxp, Rest[$ContextPath]];
>     ]
>     )
> oo`resumeContext[] := (
>         EndPackage[];
>        $ContextPath = Drop[$ContextPath, 1];
> )
> oo`new11[x_] := (
> Print["Anfang new11"];
> klassnam = x;
> tsx = ToString[klassnam];
> oo`dat = "D:\\Math_OO\\classes\\" <> tsx <> ".cl";
> ctx = klassnam <> "`";
> oo`newContext[{ctx}];
> )
> oo`new12[] := (
> Print["Name klasse - Anfang"];
>  Print["$Context: ", $Context];
>  Print["$ContextPath: ", $ContextPath];
>  ToExpression[oo`hfield];
>  Print["Context von klasse: ", Context[klasse]];
>  oo`resumeContext[];
>  Print["Name klasse - Ende"];
>  );
>   Print["Anfang Programm:"];
>   Print["$Context: ", $Context];
>  Print["$ContextPath: ", $ContextPath];
>  oo`new11["Rechteck"];
>   Print["nach new11:"];
>   Print["$Context: ", $Context];
>  Print["$ContextPath: ", $ContextPath];
>  oo`hfield = "klasse";
>  oo`new12[];
>    Print["nach new12:"];
>    Print["$Context: ", $Context];
>  Print["$ContextPath: ", $ContextPath];
> 
> and the results:
> 
> Anfang Programm:
> $Context: Global`
> $ContextPath: {Global`, System`}
> Anfang new11
> nach new11:
> $Context: Rechteck`
> $ContextPath: {Rechteck`, System`}
> Name klasse - Anfang
> $Context: Rechteck`
> $ContextPath: {Rechteck`, System`}
> klasse
> klasse::shdw: Symbol klasse appears in multiple contexts {Rechteck`,
> Global`}
>     ; definitions in context Rechteck`
>      may shadow or be shadowed by other definitions.Context von klasse:
> Global`
> Name klasse - Ende
> nach new12:
> $Context: Global`
> $ContextPath: {Global`, System`}
> In[19]:=
> 
> According your theory, I think, "klasse1" should be defined in Global` and
> "klasse" only in Rechteck But this is not the case. Therefore - excuse me -
> I doubt, that your theory is correct.
> 
> Hermann Schmitt
> 
> ----- Original Message -----
> From: "Allan Hayes" <hay at haystack.demon.co.uk>
To: mathgroup at smc.vnet.net
> Subject: [mg35785]  Re: Work with Contexts
> 
> > Hermann,
> >
> > > My question is:
> > > Why is the variable "klasse" also defined in the context Global?
> >
> > klasse is created in the context "Global`" when the assignment
> >
> > oo`new12[] := (Print["Name klasse - Begin"];
> >     Print["$Context: ", $Context];
> >     Print["$ContextPath: ", $ContextPath];
> >     ToExpression["klasse"];
> >     Print["Context of klasse: ", Context[klasse]];
> >     oo`resumeContext[];
> >     Print["Name klasse - End"];)
> >
> > is converted into internal form (with contexted symbols) prior to
> > evaluation.
> >
> > This is before
> >     oo`new11["Rechteck"];
> > is evaluated
> >
> > --
> > Allan
> >
> > ---------------------
> > Allan Hayes
> > Mathematica Training and Consulting
> > Leicester UK
> > www.haystack.demon.co.uk
> > hay at haystack.demon.co.uk
> > Voice: +44 (0)116 271 4198
> > Fax: +44 (0)870 164 0565
> >
> >
> > "Hermann Schmitt" <schmitther at netcologne.de> wrote in message
> > news:ai06l0$19t$1 at smc.vnet.net...
> > > Hello,
> > > I have the following program:
> > >
> > > oo`newContext[ctxp_] := (
> > >     BeginPackage[ctxp[[1]]];
> > >     )
> > > oo`resumeContext[] := (
> > >         EndPackage[];
> > >        $ContextPath = Drop[$ContextPath, 1];
> > >      )
> > > oo`new11[x_] := (
> > > Print["Begin new11"];
> > > klassnam = x;
> > > ctx = klassnam <> "`";
> > > oo`newContext[{ctx}];
> > > );
> > > oo`new12[] := (
> > > Print["Name klasse - Begin"];
> > >  Print["$Context: ", $Context];
> > >  Print["$ContextPath: ", $ContextPath];
> > >  ToExpression["klasse"];
> > > Print["Context of klasse: ", Context[klasse]];
> > >  oo`resumeContext[];
> > >  Print["Name klasse - End"];
> > >  );
> > >   Print["Begin Programm:"];
> > >   Print["$Context: ", $Context];
> > >  Print["$ContextPath: ", $ContextPath];
> > >  oo`new11["Rechteck"];
> > >  oo`new12[];
> > >    Print["after new12:"];
> > >    Print["$Context: ", $Context];
> > >  Print["$ContextPath: ", $ContextPath];
> > >
> > > I get the following results:
> > >
> > > Begin Programm:
> > > $Context: Global`
> > > $ContextPath: {Global`, System`}
> > > Begin new11
> > > Name klasse - Begin
> > > $Context: Rechteck`
> > > $ContextPath: {Rechteck`, System`}
> > > klasse
> > > klasse::shdw: Symbol klasse appears in multiple contexts {Rechteck`,
> > > Global`}
> > >     ; definitions in context Rechteck`
> > >      may shadow or be shadowed by other definitions.Context of klasse:
> > > Global`
> > > Name klasse - End
> > > after new12:
> > > $Context: Global`
> > > $ContextPath: {Global`, System`}
> > > In[14]:=
> > >
> > > My question is:
> > > Why is the variable "klasse" also defined in the context Global?
> > >
> > > Hermann Schmitt
> > >
> > >
> > >
> >
> >
> >

• References:
  • Re: Work with Contexts
    • From: "Allan Hayes" <hay@haystack.demon.co.uk>