LaTeX3: l3keys Multiple-Choices Keys and Startup Value for Predefined Choice Integer Variable

Question

In the interface3 documentation it can be read (what follows is not a literal quotation) that the variable \l_keys_choice_int is available to be used as an index register for a multiple-choices key with its values starting up from 0. Nonetheless, according to my experience with the code below, it is as though the starting value is actually 1.

This is my class, in a file insightfully called myclass.cls:

\RequirePackage{l3keys2e,xparse}
\ProvidesExplClass
    {myclass}
    {2013/08/25}
    {1.0}
    {myclass}

\bool_new:N  \g_myclass_enum_bool
\bool_new:N  \g_myclass_enumalt_bool

\NewDocumentCommand \SetTBool { m }
 {
    \bool_gset_true:c
     { g_myclass_\tl_trim_spaces:n {#1} _bool}
 }

\NewDocumentCommand \TypeBool { m }
 { \bool_if:cTF { g_myclass_#1_bool } { T } { F } }

\keys_define:nn { myclass }
 {
   alter .choice_code:n =
    {   
       \AtBeginDocument{Option ~ Index: ~ \int_use:N \l_keys_choice_int\par}    
       \int_compare:nNnTF { \l_keys_choice_int } =  { 1 }
        { \SetTBool { enum } }
        { \SetTBool { enumalt } }
    },
   alter .generate_choices:n = { enum, enumalt }
 }

\ProcessKeysOptions { myclass }

\LoadClass{memoir}

Then I have a document file by the original name mydoc.tex reading thus:

\documentclass[alter = enum]{myclass}

\begin{document}
\TypeBool{enum}\par
\TypeBool{enumalt}
\end{document}

What I get in my typeset document is 1 T F, whereas according to the documentation I should get 0 T F. Moreover, if I set the class option alter = enumalt, I get 2 F T. Am I getting something wrong?

Answer 1

I suspect you have on older copy of interface3: we changed generally from index-from-zero to index-from-one a little while ago now. Thus your observation is correct: the index for \l_keys_choice_int begins at one.