Error When Creating Package

Question

I am trying to make my own package based on the code from this Question. The following is the code from that page:

BeginPackage["SelectData`"];
select::usage = "select[data,where,condition]";
Begin["`Private`"]

    ClearAll[select, where];
    SetAttributes[where, HoldAll];
    select[table : {colNames_List, rows__List}, w : (where[condition_] | None) : None,
         cols : (columns[varNames__] | All) : All] :=
      With[{namingRules = Dispatch[Thread[colNames -> Thread[Slot[Range[Length[colNames]]]]]]}, 
        With[{cl = {varNames} /. namingRules /. Verbatim[Slot][n_] :> n},
          If[cols === All, #, #[[All, cl]]] &@
            If[w === None,
               {rows},
               (* else *)
               With[{selF = Apply[Function, Hold[condition] /. namingRules]},
                  Select[{rows}, selF @@ # &]
               ]
            ]
        ]
      ]; 

End[]
EndPackage[]

Then, on my notebook, use the following data as input:

table = List[{"ID", "Variable 1", "Variable 2"}, {"Alpha", 1, 
   0}, {"Beta", 1, 1}, {"Alpha", 0, 0}]

Call the package, Needs["SelectData`"]

Evaluating, select[table, columns["Variable 1"]]

the output is as follows:

select[{{"ID", "Variable 1", "Variable 2"}, {"Alpha", 1, 0}, {"Beta", 
   1, 1}, {"Alpha", 0, 0}}, columns["Variable 1"]]

I think I have done something wrong when creating my package, but I can't find it. Could anyone give me suggestions?

Answer 1

There are two issues:

1. The function columns needs to be a public function, instead a private one. This is because you have tried to access it from outside. Thus you should also add a line before begin of Private

   columns::usage = "something"

   Similarly, as @LeonidShifrin and @rasher have mentioned, if you also want to access where from outside the package, you should also add

   where::usage = "something"

2. (Minor, but related to a comment) You shouldn't ClearAll[select] after the usage information is defined. Otherwise, the usage is no longer kept. That's why ?select gives the code directly. If you want to clear select, move it to before select::usage. Also, you don't have to clear where, because it is a private variable. There shouldn't be naming conflict unless by purpose.