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Re: Returning rules from functions inside a package context

  • To: mathgroup at smc.vnet.net
  • Subject: [mg82457] Re: Returning rules from functions inside a package context
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Sat, 20 Oct 2007 06:02:32 -0400 (EDT)
  • References: <ff9rjl$4mv$1@smc.vnet.net>

Art wrote:
> The following question has been asked and answered in a variety of
> ways in this forum, but I am new to Mathematica and am still not sure what is
> best practice after doing Trace of various trivial statements for the
> past few hours.
> 
> For example, I would like f[] below to return {a->1.}, not
> {A`Private`a->1.}.

Don't return 'a' at all!  You cannot know whether users of your package 
will have 'a' defined or not.  So it is a very *bad* idea to create 
Global` symbols in your package.

Use

f[] := a /. FindFit[{{0,0}, {1,1}}, a x, {a}, x]

instead.  This way 'x' and 'a' will stay inside A`Private` and they 
won't interfere with the user's own symbols.

Note that if you're defining such a function outside a package, in a 
Global context, (and you cannot be sure that 'x' and 'a' won't be 
defined later), then the definition should be

f[] := Module[{a,x}, a /. FindFit[{{0,0}, {1,1}}, a x, {a}, x]]

to isolate 'a' and 'x'.  The environment inside a package can be 
controlled, so Module[] is not strictly necessary there.

> 
> BeginPackage["A`"]
> f::usage = ""
> Begin["`Private`"]
> 
> f[] := FindFit[{{0, 0}, {1, 1}}, a x, {a}, x]
> 
> End[]
> 
> EndPackage[]
> 
> The use of Rule[] in different scoping constructs is confusing.
> 
> Thanks.
> 
> 


-- 
Szabolcs


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