Dear Joseph,

On Fri, Jul 17, 2015 at 09:45:54AM +0100, Joseph Wright wrote:
>Hello all,
>
>expl3 current provides the idea of expandable boolean expressions in the
>form
>
>    \bool_if:nTF
>      {
>        ( <test-1> && <test-2> ) || <test-3> ...
>      }
>
>with lazy evaluation. However, as detailed in
>https://github.com/latex3/svn-mirror/issues/217 there is an issue with
>this: as parentheses are part of the syntax, any that are sent through
>to the tests from 'above' can cause an issue. For example
>
>    \cs_new:Npn \foo:N #1
>      {
>        \bool_if:nTF
>          {
>            \token_if_eq_charcode_p:NN + #1 ||
>            \token_if_eq_charcode_p:NN - #1
>          }
>
>will fail if #1 is ")". This situation is clearly not ideal and we want
>to find the best way to address it with minimal impact on real-world use
>of \bool_if:n(TF).
>
>It seems that any fix requires some change of documented behaviour. In
>particular, we note that the problem can be dealt with if we drop lazy
>evaluation. However, that might impact on current use cases. An obvious
>issue is that something like the code currently in the expl3 driver
>selection
>
>    \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