Joseph,

On Fri, Jul 17, 2015 at 12:09:27PM +0100, Joseph Wright wrote:
>On 17/07/2015 10:59, Alexander Grahn wrote:
>>>    \bool_if:nTF
>>>      {
>>>           \cs_if_exist_p:N  \pdftex_pdfoutput:D
>>>        && \int_compare_p:nNn \pdftex_pdfoutput:D > \c_zero
>>>      }
>>>      { pdfmode }
>>>      { dvips }
>>>
>>> will fail as the second test will not be skipped.  We note though that
>>> this case is likely unusual: in general expl3 variables should be
>>> declared before use so should. A quick sweep over TL2015 shows the
>>> following packages would have to be updated following any change:
>>>
>>> - media9
>>>  -ocgx2
>>>
>>> (Other packages do use \cs_if_exist_p:(N|c) but not in a way which
>>> relies on lazy evaluation.)
>>>
>>> Are there other obvious issues if we switch from lazy evaluation? If the
>>> change is to be made we intend to work with package authors in advance
>>> to make sure that users are not impacted.
>> 
>> thank you  for pointing out this. Could you please give me specific
>> advice on the changes I will have to do?
>> 
>> Kind regards,
>> Alexander
>
>Hi Alexander,
>
>As I said, at present we've not changed anything precisely because we
>are not sure whether dropping lazy evaluation is acceptable. (It's a
>tricky balance: having an issue with ")" tokens is also not ideal.) The
>reason I've raised on LaTeX-L is we want to be sure the proposed change
>doesn't cause significant issues either for end-users or for programmers.
>
>Assuming we do make a change and evaluate all parts of an expression
>then doing
>
>    \bool_if:nT
>      {
>            \cs_if_exist_p:N <some-macro>
>        &&  \some_test:N <some-macro>
>      }
>
>won't work if the second test only works if <some-macro> is defined. In
>your code you've got three places I can see where \cs_if_exist_p:N or
>\cs_if_exist_p:c is used as the first test in an expression, so the
>second one will fail. The options are then to rewrite as a set of nested
>TF versions
>
>    \cs_if_exist:NTF  \foo
>      { }
>      { }
>
>or to use a bit of imagination to avoid that. Assuming the latter is
>wanted, in media9 you could replace
>
>    \bool_if:nTF
>      {
>           !\cs_if_exist_p:c {#1}
>        || !\str_if_eq_x_p:nn {\tl_use:c{#1} } {#2}
>      }
>
>by
>
>    \bool_if:nTF
>      {
>        !\cs_if_exist:cTF {#1}
>          {
>            \str_if_eq_x_p:nn { \tl_use:c {#1} } {#2}
>          }
>          { \c_false_bool }
>      }
>
>and replace
>
>    \bool_if:nT
>      {
>             \cs_if_exist_p:N \fxocg@insert@OC
>         && !\tl_if_blank_p:V \fxocg@insert@OC
>      }
>
>by
>
>    \bool_if:nT
>      {
>        ! \cs_if_exist:NTF \fxocg@insert@OC
>          { \tl_if_blank_p:V \fxocg@insert@OC }
>          { \c_true_bool }
>      }
>
>In ocgx2.sty there is
>
>    \bool_if:nT
>      {
>           !\cs_if_exist_p:c { OCGpdfobj#3 }
>        || !\str_if_eq_x_p:nn { \use:c { OCGpdfobj#3 } } { \use:c {
>ocgxii@#3 } }
>      }
>
>which could be redone as for the first case in media9. However,
>reviewing the code I realise that as you have \use:c here you'll
>actually get away with it (the second part will evaluate even if the
>csname is undefined). If you do want a 'safe' change then
>
>    \bool_if:nTF
>      {
>        ! \cs_if_exist:cTF { OCGpdfobj#3 }
>          {
>            \str_if_eq_x_p:nn { \use:c { OCGpdfobj#3 } } { \use:c {
>ocgxii@#3 } }
>          }
>          { \c_false_bool }
>      }
>
>(BTW, I assume ocgxii@#3 is known to be defined!)
>
>I hope this makes sense.
>--
>Joseph Wright
>

Thank you for the detailed instructions! I will keep a copy of this
message in case the suggested changes become necessary.

Alexander