Re: question on Mathematica package loading.
From: Maxim (ab_def_at_prontomail.com)
Date: 11/01/04
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Date: 1 Nov 2004 01:17:12 -0800
bill_bw_1000@yahoo.com (Bill) wrote in message news:<40daf724.0410301511.59355902@posting.google.com>...
> I have posted this question 3 days ago (oct 27) to the Mathematica
> news group, and as of today oct 30, it still has not shown up, I assume
> it was not allowed on that group.
>
> It might be possible becuase I put the word 'bug?' in the subject.
> So I am removing the word 'bug' from the subject and trying this
> newsgroup.
>
> I am posting the question here in the hope someone can help.
>
> cheers,
> Bill
>
> ----- post -------
>
> I am having hard time understanding why Mathematica is doing
> the following.
>
> To summarize, I create a simple package with one
> function test3[] in it. Load the package, and can call
> the function ok. I removed the 'usage' line, and try to call
> the function again, but it will not work AS expected. But this
> results in a new symbol added by the same name of the function,
> again as expected.
>
> But I then put the 'usage' line back into the package,
> clean the environment again, and reload the package,
> but I still can not access the package function again, it
> is still calling a fake symbol instead.
>
> ----- package file strange.m ----
>
> BeginPackage["strange`"]
> test3::usage="test3" <----- (1)
> Begin["`Private`"]
> test3[x_]:=x^3;
> End[]
> EndPackage[];
>
> ---- now test the package, we see we can call strange`test3[]
>
> <<strange.m
> Names["strange`*"]
> {test3}
> strange`test3[4]
> 64
>
> ----------------------
> Now edit the file strange.m, and remove the line
> marked by (1), so we now get
>
> BeginPackage["strange`"]
>
> Begin["`Private`"]
> test3[x_]:=x^3;
> End[]
> EndPackage[];
>
>
> ---- Now remove all symbols, and reload, we see the
> ---- name test3[] is not visible
>
> Remove["Global`*"]; Remove["strange`*"]; Names["strange`*"]
> {}
>
> Now load the packge again and try to call test3
>
> Get["strange.m"]; Names["strange`*"]
> {}
>
> You see, NO names in package. Now try to call strange`test3[],
> this will cause a new symbol to be created instead as expected:
>
> strange`test3[4]
> test3(4)
>
>
> Ok, now it seems to fix this, all what I have to do is
> to remove all the names in the strange context, and add the
> usage line back to the package, and reload, and then
> it should work. However, it still does not work.
>
> Added line(1) back, SAVED the package to disk again (file->save)
> then did
>
> Remove["Global`*"]; Remove["strange`*"]; Names["strange`*"]
> {}
>
> <<strange.m
> Names["strange`*"]
> {test3}
>
> -- so the name is back. OK. Now I would expect to be able
> -- to call it again and it should work as before. BUT it
> -- does not!
>
> strange`test3[4]
> test3(4) <----- This should come back as 64
>
>
> -- Why has not Mathematica call the correct function again??
> -- checked that there is NO global test3 function anywhere:
>
> Names["Global`test3"]
> {}
>
> -- So what is going on??
Let us see how your first package definition works:
BeginPackage["strange`"]
test3::usage="test3"
Begin["`Private`"]
test3[x_]:=x^3;
End[]
EndPackage[]
First BeginPackage makes strange` the current context; the usage line
isn't necessary per se, but its purpose is to create the symbol test3
in the current context strange` -- that is, it creates the symbol with
the full name strange`test3. Next we enter the context
strange`Private` and evaluate the expression test3[x_]:=x^3. The
parser sees two symbols: test3 and x. It searches for test3 and
successfully finds it, because strange` is on the context search path
($ContextPath) now, which is one of the functions of BeginPackage.
This means that you can access strange`test3 simply as test3.
Therefore, no new symbol strange`Private`test3 is created (but the
parser does create the new symbol strange`Private`x).
On the next step, when you remove the usage line, the symbol
strange`test3 isn't created; instead, when the evaluator reaches the
test3[x_]:=x^3 line, it cannot find the test3 symbol and creates the
new symbol strange`Private`test3. After the End[] statement you cannot
access this symbol directly as test3, because strange`Private` is not
put on the context search path (it's not the task of Begin/End) but
you can still access it by its full name, e.g. as
strange`Private`test3[3].
Finally, when you load the package with the usage line added again,
the symbol strange`test3 will be created once more, but what will
happen when we reach the test3[x_]:=x^3 line? The key point is that
the current context is always searched before the contexts on
$ContextPath. Therefore, the evaluator finds the symbol
strange`Private`test3 (left from the previous execution of the
package) and redefines it! In other words, that line will still look
like
strange`Private`test3[strange`Private`x_]:=strange`Private`x^3
just as during the second run (with the usage line removed) -- the
definition isn't attached to the strange`test3 symbol.
The bottom line is that all three examples work as expected and to get
the desired behaviour you should do Remove[strange`Private`test3].
Maxim Rytin
m.r@inbox.ru
- Next message: Jean-Michel Collard: "Re: Differential Geometry with Maple 9"
- Previous message: Norb: "Differential Geometry with Maple 9"
- Maybe in reply to: David Park: "Re: question on Mathematica package loading."
- Next in thread: Marcus Stollsteimer: "Re: question on Mathematica package loading."
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