Re: Contexts and Block
- To: mathgroup at smc.vnet.net
- Subject: [mg108794] Re: Contexts and Block
- From: Albert Retey <awnl at gmx-topmail.de>
- Date: Wed, 31 Mar 2010 06:51:19 -0500 (EST)
- References: <hov81p$93a$1@smc.vnet.net>
Am 31.03.2010 12:27, schrieb Chris Degnen:
> I have some code which reads in rules using variable names that
> already exist, e.g. a and b. I used Block to achieve the replacement
> instructions, but was surprised to have to use BeginPackage and Begin
> again to get the context right. Can anybody comment on what's going
> on here?
I think everything behaves as expected. The function definition of
TestFunction is made in context "Test`Private`" so the symbols a and b
used there are actually Test`Private`a and Test`Private`b. When you use
the function, it runs in context Global` , so when you read the file
with Get the new symbols are created as Global`a and Global`b so that
the patterns in TestFunction don't match. Only if you do set the Context
to Test`Private` before reading the file, the symbols a and b will be in
the context Test`Private` and the patterns will match.
> BeginPackage["Test`"];
> TestFunction::usage = "Testing contexts";
> Begin["`Private`"];
> TestFunction[] := Module[{},
>
> a = 1;
> b = 1;
>
> BeginPackage["Test`"];
> Begin["`Private`"];
>
> Block[{a, b},
> x = Get["x.dat"];
> a2 = a /. x;
> b2 = b /. x;];
>
> End[];
> EndPackage[];
>
> Return[{a2, b2}]];
>
> End[];
> EndPackage[];
>
> x = {a -> "one", b -> "two"};
> x >> x.dat;
> TestFunction[]
>
here are some suggestions for your code: a Module with an empty variable
list does nothing, so there is no reason to use it. Also the Return at
the end is not necessary. Finally I don't see any good reason to use
symbols in your replacement rules. If you use strings instead you will
avoid any context issues. In some cases this might also be more
efficient, after all a symbol is something much more complicated than
just a string. The following is basically doing the same thing with much
less effort:
BeginPackage["Test`"];
TestFunction::usage = "Testing contexts";
Begin["`Private`"];
TestFunction[] := ({"a", "b"} /. Get["x.dat"]);
End[];
EndPackage[];
x = {"a" -> "one", "b" -> "two"};
x >> x.dat;
TestFunction[]