Provide commands with the flafter functionality

[user227621]

Brief outline of the enhancement

LaTeX provides the flafter package, which prevents floats from being placed before their call-out. As this is implemented as a package, this feature cannot be activated for just parts of a document, as it cannot be deactivated or reactivated later.

As far as I understand, the flafter package only changes a single line in \@addtocurcol (from \@addtotoporbot to \@addtobot):

\def \@addtocurcol {%
   \@insertfalse
   \@setfloattypecounts
   \ifnum \@fpstype=8
   \else
     \ifnum \@fpstype=24
     \else
       \@flsettextmin
       \advance \@textmin \@textfloatsheight
       \@reqcolroom \@pageht
       \ifdim \@textmin>\@reqcolroom
         \@reqcolroom \@textmin
       \fi
       \advance \@reqcolroom \ht\@currbox
       \ifdim \@colroom>\@reqcolroom
         \@flsetnum \@colnum
         \ifnum \@colnum>\z@
           \@bitor\@currtype\@deferlist
          \@testwrongwidth\@currbox
           \if@test
           \else
             \@bitor\@currtype\@botlist
             \if@test
               \@addtobot
             \else
               \ifodd \count\@currbox
                 \advance \@reqcolroom \intextsep
                 \ifdim \@colroom>\@reqcolroom
                   \global \advance \@colnum \m@ne
                   \global \advance \@textfloatsheight \ht\@currbox
                   \global \advance \@textfloatsheight 2\intextsep
                   \@cons \@midlist \@currbox
                   \if@nobreak
                     \nobreak
                     \@nobreakfalse
                     \everypar{}%
                   \else
                     \addpenalty \interlinepenalty
                   \fi
                   \vskip \intextsep
                   \box\@currbox
                   \penalty\interlinepenalty
                   \vskip\intextsep
                   \ifnum\outputpenalty <-\@Mii \vskip -\parskip\fi
                   \outputpenalty \z@
                   \@inserttrue
                 \fi
               \fi
               \if@insert
               \else
                 \@addtotoporbot % <--------------------------------------------
               \fi
             \fi
           \fi
         \fi
       \fi
     \fi
   \fi
   \if@insert
   \else
     \@resethfps
     \@cons\@deferlist\@currbox
   \fi
}%

Would it be possible to provide two commands that switch on and switch off the restriction to place floats after thier call-out?

You could e. g. change the definition of \@addtocurcol like this

...
               \else
                 \@addtocurcol@action % <--------------------------------------------
               \fi
...

and provide two commands that execute \let\@addtocurcol@action\@addtotoporbot and \let\@addtocurcol@action\@addtobot. (The flafter package could of course still be provided, it would then only have to execute the latter command.)

Minimal example showing the desired new behaviour

\documentclass{article}

\makeatletter

\def \@addtocurcol {%
   \@insertfalse
   \@setfloattypecounts
   \ifnum \@fpstype=8
   \else
     \ifnum \@fpstype=24
     \else
       \@flsettextmin
       \advance \@textmin \@textfloatsheight
       \@reqcolroom \@pageht
       \ifdim \@textmin>\@reqcolroom
         \@reqcolroom \@textmin
       \fi
       \advance \@reqcolroom \ht\@currbox
       \ifdim \@colroom>\@reqcolroom
         \@flsetnum \@colnum
         \ifnum \@colnum>\z@
           \@bitor\@currtype\@deferlist
          \@testwrongwidth\@currbox
           \if@test
           \else
             \@bitor\@currtype\@botlist
             \if@test
               \@addtobot
             \else
               \ifodd \count\@currbox
                 \advance \@reqcolroom \intextsep
                 \ifdim \@colroom>\@reqcolroom
                   \global \advance \@colnum \m@ne
                   \global \advance \@textfloatsheight \ht\@currbox
                   \global \advance \@textfloatsheight 2\intextsep
                   \@cons \@midlist \@currbox
                   \if@nobreak
                     \nobreak
                     \@nobreakfalse
                     \everypar{}%
                   \else
                     \addpenalty \interlinepenalty
                   \fi
                   \vskip \intextsep
                   \box\@currbox
                   \penalty\interlinepenalty
                   \vskip\intextsep
                   \ifnum\outputpenalty <-\@Mii \vskip -\parskip\fi
                   \outputpenalty \z@
                   \@inserttrue
                 \fi
               \fi
               \if@insert
               \else
                 \@addtocurcol@action % <---------------------------------------
               \fi
             \fi
           \fi
         \fi
       \fi
     \fi
   \fi
   \if@insert
   \else
     \@resethfps
     \@cons\@deferlist\@currbox
   \fi
}%

\let\@addtocurcol@action\@addtotoporbot

\newcommand\off{\let\@addtocurcol@action\@addtotoporbot}
\newcommand\on{\let\@addtocurcol@action\@addtobot}
% use more elaborate command names in a real implementation, of course

\makeatother

\begin{document}

\on % prevent floats from being placed before their call-out

Text.

\begin{table}
\caption{...}
\end{table}

\newpage

\off % lift the restriction

Text.

\begin{table}
\caption{...}
\end{table}

\end{document}

[FrankMittelbach]

This is a sensible extension but it will have to wait until we start putting hooks and sockets into the output routine and adjust it to fully support tagging.