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Re: Package dependencies

• To: mathgroup at smc.vnet.net
• Subject: [mg45753] Re: [mg45688] Package dependencies
• From: Omega Consulting <info at omegaconsultinggroup.com>
• Date: Fri, 23 Jan 2004 03:15:40 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

On a modern computer, package loading is quick. Unless the package is very 
large (or part of a large set of interdependent packages), DeclarePackage 
is not really necessary. Just use Get or Needs in c.m.

However, if you must load the packages when f is first used, you can create 
a "stub" function. Something like:

BeginPackage["a`"]
f := (
   Clear[f];
   Get["a`"];
   Needs["b`"];
   f
)
EndPackage[]

In[1]:= << c`

In[2]:=
?f
a`f
f := (Clear[f]; << a`; Needs[b`]; f)

So f is defined to load a` and b`, which re-defines f.

In[3]:= f[1]
Out[3]= 1

In[4]:=
?f
a`f
f[0] = 0
f[1] = 1

At 04:07 AM 1/20/2004, Maxim wrote:
>Suppose we have a package a.m defining function f:
>
>BeginPackage["a`"]
>f[0] = 0
>EndPackage[]
>
>Then we create package b.m which introduces additional definitions for
>f:
>
>BeginPackage["b`","a`"]
>f[1] = f[0] + 1
>EndPackage[]
>
>Now after loading b.m we'll have f[0]=0 and f[1]=1. Finally, we want to
>create a package c.m that determines when b.m should be autoloaded. We
>add the following line to c.m:
>
>DeclarePackage["b`","f"]
>
>Unfortunately, it doesn't work that way:
>
>In[1]:=
><<c`
>f[1]
>
>Out[2]=
>1+f[0]
>
>(standalone kernel will also give f::shdw warning). The reason is that
>DeclarePackage creates symbol f (with attribute Stub) in the b` context.
>This implementation decision seems rather questionable: the idea of
>DeclarePackage is to load b.m when f is first used, which is exactly
>what we want here -- but nobody said the symbol f was needed in context
>b`!
>
>Probably this difficulty can be avoided by some creative usage of Remove
>and $NewSymbol, but simply adding Remove["b`f"] to b.m won't solve the
>problem completely.
>
>This is exactly the situation with Geometry\Polytopes.m,
>Graphics\Polyhedra.m and Graphics\Kernel\init.m -- you can use
><<graphics` to declare most of the symbols in all the Graphics\*
>packages, but not Tetrahedron, Cube and others because there are
>definitions for them both in Polytopes.m and Polyhedra.m.
>
>Maxim Rytin
>m.r at prontomail.com

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