Hello all (especially Joseph, who might have to deal with this),

Based on the discussion at http://tex.stackexchange.com/questions/179180
there seems to be a problem with the definition of \__keys_bool_set:Nn.
Namely, it is not obvious that the following code should fail

    \keys_define:nn { mymodule }
      {
        key-1            .choice: ,
        key-1 / choice-a .bool_set:N = \l_mymodule_choice_a_bool,
        key-1 / choice-b .bool_set:N = \l_mymodule_choice_b_bool
      }
    \keys_set:nn { mymodule } { key-1 = choice-a }

and yet it hangs because \__keys_bool_set:Nn actually creates a choice
key with the choices 'true' and 'false' under the hood.

I feel it would be more natural if the code above behaved in the same
way as the following:

    \keys_define:nn { mymodule }
      {
        key-2 .bool_set:N = \l_mymodule_choice_a_bool
      }
    \keys_set:nn { mymodule } { key-2 }

In other words, the code that now hangs should just be setting the
boolean to 'true'. If \__keys_bool_set:Nn and \__keys_bool_set_inverse:Nn
were redefined along the lines of the code below, we'd get that result.

    \cs_new_protected:Npn \__keys_bool_set:Nn #1#2
      {
        \bool_if_exist:NF #1 { \bool_new:N #1 }
        \__keys_cmd_set:nn { \l_keys_path_tl }
          {
            \str_case_x:nnF { \l_keys_value_tl }
              {
                { true }  { \use:c { bool_ #2 set_true:N } #1 }
                { false } { \use:c { bool_ #2 set_false:N } #1 }
              }
              {
                \cs_if_exist:cTF
                  { \c__keys_code_root_tl \l_keys_path_tl / \l_keys_value_tl
}
                  { \use:c { bool_ #2 set_true:N } #1 }
                  {
                    \__msg_kernel_error:nnx { kernel } { boolean-values-only
}
                      { \l_keys_key_tl }
                  }
              }
          }
        \__keys_default_set:n { true }
      }
    \cs_new_protected:Npn \__keys_bool_set_inverse:Nn #1#2
      {
        \bool_if_exist:NF #1 { \bool_new:N #1 }
        \__keys_cmd_set:nn { \l_keys_path_tl }
          {
            \str_case_x:nnF { \l_keys_value_tl }
              {
                { true }  { \use:c { bool_ #2 set_false:N } #1 }
                { false } { \use:c { bool_ #2 set_true:N } #1 }
              }
              {
                \cs_if_exist:cTF
                  { \c__keys_code_root_tl \l_keys_path_tl / \l_keys_value_tl
}
                  { \use:c { bool_ #2 set_false:N } #1 }
                  {
                    \__msg_kernel_error:nnx { kernel } { boolean-values-only
}
                      { \l_keys_key_tl }
                  }
              }
          }
        \__keys_default_set:n { true }
      }

Finally, and a lower priority: it might also be useful to add the
properties .bool_set_true:N and .bool_set_false:N as well. For example,
they could work like this:

    \cs_new_protected:Npn \__keys_bool_set_true:Nn #1#2
      {
        \bool_if_exist:NF #1 { \bool_new:N #1 }
        \__keys_cmd_set:nn { \l_keys_path_tl }
          { \use:c { bool_ #2 set_true:N } #1 }
        \__keys_value_requirement:n { forbidden }
      }
    \cs_generate_variant:Nn \__keys_bool_set_true:Nn { c }
    \cs_new_protected:Npn \__keys_bool_set_false:Nn #1#2
      {
        \bool_if_exist:NF #1 { \bool_new:N #1 }
        \__keys_cmd_set:nn { \l_keys_path_tl }
          { \use:c { bool_ #2 set_false:N } #1 }
        \__keys_value_requirement:n { forbidden }
      }
    \cs_generate_variant:Nn \__keys_bool_set_false:Nn { c }
    \cs_new_protected:cpn { \c__keys_props_root_tl .bool_set_true:N } #1
      { \__keys_bool_set_true:Nn #1 { } }
    \cs_new_protected:cpn { \c__keys_props_root_tl .bool_set_true:c } #1
      { \__keys_bool_set_true:cn {#1} { } }
    \cs_new_protected:cpn { \c__keys_props_root_tl .bool_gset_true:N } #1
      { \__keys_bool_set_true:Nn #1 { g } }
    \cs_new_protected:cpn { \c__keys_props_root_tl .bool_gset_true:c } #1
      { \__keys_bool_set_true:cn {#1} { g } }
    \cs_new_protected:cpn { \c__keys_props_root_tl .bool_set_false:N } #1
      { \__keys_bool_set_false:Nn #1 { } }
    \cs_new_protected:cpn { \c__keys_props_root_tl .bool_set_false:c } #1
      { \__keys_bool_set_false:cn {#1} { } }
    \cs_new_protected:cpn { \c__keys_props_root_tl .bool_gset_false:N } #1
      { \__keys_bool_set_false:Nn #1 { g } }
    \cs_new_protected:cpn { \c__keys_props_root_tl .bool_gset_false:c } #1
      { \__keys_bool_set_false:cn {#1} { g } }

All the best,

Jura Pintar