MathGroup Archive 2007

[Date Index] [Thread Index] [Author Index]

Search the Archive

Second argument of BeginPackage, revisited

  • To: mathgroup at
  • Subject: [mg78295] Second argument of BeginPackage, revisited
  • From: "Andrew Moylan" <andrew.j.moylan at>
  • Date: Wed, 27 Jun 2007 05:42:01 -0400 (EDT)

Unfortunately (in my opinion), BeginPackage["a`", {"b`"}] does not *quite*
call Needs[] on its second argument(s).
Suppose b.m looks like this:
BeginPackage["b`", {"PhysicalConstants`"}]
(* some code *)
Then calling Needs["b`"] in a new kernel results in "PhysicalConstants`"
being on the list of contexts, so that e.g. SpeedOfLight works as expected.
But calling BeginPackage["a`", {"b`"}] instead does *not* put
PhysicalConstants on the list of contexts during the definition of the
package, so that e.g. SpeedOfLight results in the creation of a new, useless
symbol called a`SpeedOfLight.
Finally, for maximum confusion (in my opinion), once EndPackage[] is called
to finish loading the "a`" package, "PhysicalConstants`" *is* placed on the
list of contexts, causing shadowing between a`SpeedOfLight and
Effectively, BeginPackage appears to
1. Call Needs[] on its second arguments, then
2. Temporarily remove from the context path any extra contexts those
packages added to the context path, then finally
3. Put all those temporarily removed packages back onto the context path
once EndPackage[] is called.
Can anyone help me understand why this behaviour is useful?

  • Prev by Date: Re: Maximize with Integer constraints
  • Next by Date: Re: search for an operator in an expression
  • Previous by thread: Is "Find Currently Evaluating Cell" really "Find Topmost Cell in Evaluation Queue"?
  • Next by thread: Re: Second argument of BeginPackage, revisited