As belisarius points out, your question as it stands is a bit v5-centric. The problem, however, still exists in current versions. As an example.
As belisarius points out, your question as it stands is a bit v5-centric. The problem, however, still exists in current versions. As an example Needs"Combinatorica`" ToCycles{3, 4, 1, 2} works fine, while (after restarting the kernel), Needs"Combinatorica`"; ToCycles{3, 4, 1, 2} fails with an error that "ToCycles::shdw: Symbol ToCycles appears in multiple contexts {Combinatorica`,Global`}; definitions in context Combinatorica` may shadow or be shadowed by other definitions."
In Mathematica terms, the reason the one-liner doesn't work is that Mathematica tries to resolve all symbols in the line before evaluating Needs (this was a surprise to me). This resolves ToCycles to Global`ToCycles (thus entering this symbol in the symbol table), before Needs gets a chance to load the definition of Combinatorica`ToCycles and add Combinatorica to the $ContextPath. To make the one-liner work, you must use the full name of ToCyles: Needs"Combinatorica`"; Combinatorica`ToCycles{3, 4, 1, 2} To understand the error, you need to know that all Symbols in Mathematica have a full name of the form context`name.
A context is similar to a namespace in many other languages. Now, if a symbol (such as ToCycles) is referenced without a context, Mathematica will look through the contexts currently in $ContextPath and see if the symbol is defined in any of those contexts. If not, the symbol is resolved in the current context, $Context which is Global in normal use.
When you load a package, the symbols of that package are defined in a package context (e.g. Combinatorica), and when the package is fully loaded this context is added to the $ContextPath so that you can access the symbols by their short name. Now, you can see what the error means: Since the Combinatorica has not yet been loaded when the symbols are resolved, ToCycles resolves to Global`ToCycles. After the package loads, Mathematica helpfully checks that all short names are unique, and finds in this case that the short name ToCycles is now defined in two contexts on $ContextPath one thus "shadowing" the other.
To refer to a specific of these symbols, you must use the full name, e.g. Combinatorica`ToCycles.To resolve a shadow conflict, simply Remove the unwanted symbol: RemoveGlobal`ToCycles Don't know how readable this was, but hope it helps a bit...
Many thanks! – Qiang Li Jan 12 at 2:19 Well explained! It is a bit surprising that the naive 1-liner doesn't work.
– Simon Jan 12 at 2:19 1 @Quang Li: It has to be a "backtick" between Combinatorica and ToCycles, but it took me a while to figure out how to enter a literal backtick in my answer. Should be ok now? Also, I added the correct one-liner before I even saw your comment :) – Janus Jan 12 at 2:26 @Simon: Thanks!
It surprised me as well. Always thought that semicolons and line breaks were completely interchangeable... – Janus Jan 12 at 2:28 +10 Never happened to me ... just because I always use Needs in a line on its own ... Good to know why! – belisarius Jan 12 at 3:21.
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