[latex3-commits] [git/LaTeX3-latex3-latex3] master: Tweak some brace tricks to ensure braces remain always balanced (dd6d03e)

[Bruno Le Floch]

Repository : https://github.com/latex3/latex3
On branch  : master
Link       : https://github.com/latex3/latex3/commit/dd6d03e86bca9f387874227eb334db25fe4ab646

>---------------------------------------------------------------

commit dd6d03e86bca9f387874227eb334db25fe4ab646
Author: Bruno Le Floch <bruno at le-floch.fr>
Date:   Thu Feb 28 09:41:15 2019 +0100

    Tweak some brace tricks to ensure braces remain always balanced


>---------------------------------------------------------------

dd6d03e86bca9f387874227eb334db25fe4ab646
 l3kernel/l3candidates.dtx |    2 +-
 l3kernel/l3expan.dtx      |    5 +++++
 l3kernel/l3tl.dtx         |   21 +++++++++++++--------
 l3kernel/source3body.tex  |    2 +-
 4 files changed, 20 insertions(+), 10 deletions(-)

diff --git a/l3kernel/l3candidates.dtx b/l3kernel/l3candidates.dtx
index 01e8ceb..aca1af3 100644
--- a/l3kernel/l3candidates.dtx
+++ b/l3kernel/l3candidates.dtx
@@ -2477,8 +2477,8 @@
         \prg_return_false:
       }
       {
-        \if_false: { \fi:
         \group_begin:
+          \if_false: { \fi:
           \int_set_eq:NN \tex_tracingnesting:D \c_zero_int
           \exp_args:No \tex_everyeof:D { \c_@@_marker_tl }
           #2 \scan_stop:
diff --git a/l3kernel/l3expan.dtx b/l3kernel/l3expan.dtx
index c93e326..e15fefa 100644
--- a/l3kernel/l3expan.dtx
+++ b/l3kernel/l3expan.dtx
@@ -1790,6 +1790,11 @@
 %   stream, followed by the result of the expansion.  There are many
 %   special cases: spaces, brace groups, \tn{noexpand}, \tn{unexpanded},
 %   \tn{the}, \tn{primitive}.
+%   While we use brace tricks \cs{if_false:} |{| \cs{fi:}, the expansion
+%   of this function is always triggered by \cs{exp:w} so brace balance
+%   is eventually restored after that is hit with a single step of
+%   expansion.  Otherwise we could not nest \texttt{e}-type expansions
+%   within each other.
 %    \begin{macrocode}
     \cs_new:Npn \@@_e:nn #1
       {
diff --git a/l3kernel/l3tl.dtx b/l3kernel/l3tl.dtx
index 1cecc62..c4465ad 100644
--- a/l3kernel/l3tl.dtx
+++ b/l3kernel/l3tl.dtx
@@ -1581,6 +1581,10 @@
 %
 %   The two \cs{if_false:} \ldots{} \cs{fi:} are there to prevent
 %   alignment tabs to cause a change of tabular cell while rescanning.
+%   We put the \enquote{opening} one after \cs{group_begin:} so that if
+%   one accidentally \texttt{f}-expands \cs{tl_set_rescan:Nnn} braces
+%   remain balanced.  This is essential in \texttt{e}-type arguments
+%   when \tn{expanded} is not available.
 %    \begin{macrocode}
 \cs_new_protected:Npn \tl_set_rescan:Nnn
   { \@@_set_rescan:NNnn \tl_set:No }
@@ -1594,8 +1598,8 @@
   }
 \cs_new_protected:Npn \@@_set_rescan:NNnn #1#2#3#4
   {
-    \if_false: { \fi:
     \group_begin:
+      \if_false: { \fi:
       \int_set_eq:NN \tex_tracingnesting:D \c_zero_int
       \exp_args:No \tex_everyeof:D { \c_@@_rescan_marker_tl }
       \int_compare:nNnT \tex_endlinechar:D = { 32 }
@@ -2136,9 +2140,12 @@
 %   groups makes the test work also for empty arguments.
 %   The \cs{if_false:} constructions are a faster way to do
 %   \cs{group_align_safe_begin:} and \cs{group_align_safe_end:}.
+%   The \cs{scan_stop:} ensures that \texttt{f}-expanding
+%   \cs{tl_if_in:nn} does not lead to unbalanced braces.
 %    \begin{macrocode}
 \prg_new_protected_conditional:Npnn \tl_if_in:nn #1#2 { T  , F , TF }
   {
+    \scan_stop:
     \if_false: { \fi:
     \cs_set:Npn \@@_tmp:w ##1 #2 { }
     \tl_if_empty:oTF { \@@_tmp:w #1 {} {} #2 }
@@ -2985,13 +2992,11 @@
 % \end{macro}
 %
 % \begin{macro}[EXP,pTF]{\tl_if_head_is_group:n}
-%   Pass the first token of |#1| through \cs{token_to_str:N},
-%   then check for the brace balance. The extra \texttt{?}
-%   caters for an empty argument.\footnote{Bruno: this could
-%     be made faster, but we don't: if we hope to ever have
-%     an \texttt{e}-type argument, we need all brace \enquote{tricks}
-%     to happen in one step of expansion, keeping the token
-%     list brace balanced at all times.}
+%   Pass the first token of |#1| through \cs{token_to_str:N}, then check
+%   for the brace balance.  The extra \texttt{?} caters for an empty
+%   argument.  This could be made faster, but we need all brace tricks
+%   to happen in one step of expansion, keeping the token list brace
+%   balanced at all times.
 %    \begin{macrocode}
 \prg_new_conditional:Npnn \tl_if_head_is_group:n #1 { p , T , F , TF }
   {
diff --git a/l3kernel/source3body.tex b/l3kernel/source3body.tex
index cbe37a8..aa01477 100644
--- a/l3kernel/source3body.tex
+++ b/l3kernel/source3body.tex
@@ -321,7 +321,7 @@ different types of argument:
 \paragraph{Fully expandable functions}
 \hypertarget{expstar}{Some functions are fully expandable},
 which allows them to be used within
-an \texttt{x}-type or \texttt{e}-type argument (in plain \TeX{} terms, inside an \cs{edef}),
+an \texttt{x}-type or \texttt{e}-type argument (in plain \TeX{} terms, inside an \tn{edef} or \tn{expanded}),
 as well as within an \texttt{f}-type argument.
 These fully expandable functions are indicated in the documentation by
 a star: