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