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MathGroup Archive 2007

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Re: Returning rules from functions inside a package context

  • To: mathgroup at smc.vnet.net
  • Subject: [mg82439] Re: Returning rules from functions inside a package context
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Sat, 20 Oct 2007 05:53:19 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • 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.}.
> 
> 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.

I do not know whether the following technique could qualify as the best 
practice, still, I would suggest to define the variable "a" as 
"Global`a", so we force its definition to reside in the *Global* context 
(symbols located in the *Global* context are referred by Mathematica 
just by their names). For instance, we could write the package TestRule.m as

BeginPackage["TestRule`"]
f::usage = "How to return a nice rule from a package."
Begin["`Private`"]
f[] := FindFit[{{0, 0}, {1, 1}}, Global`a x, {Global`a}, x]
End[]
EndPackage[]

Then we can check that the rule returned by the function f[] is of the 
desired form.

In[1]:= << TestRule`

In[2]:= f[]

Out[2]= {a -> 1.}

Regards,
-- 
Jean-Marc


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