[NTG-context] hysteresis with content in blocks (one strike and you can be out forever)

Sanjoy Mahajan sanjoy at olin.edu
Wed Feb 6 16:53:39 CET 2019


Apologies for the cutesy subtitle.  In plainer English, the information
saved in the .tuc file can make it so that a valid ConTeXt file doesn't
compile merely because of what it contained when it was compiled before.

Here is a MIE (minimal illustrative example), on Linux amd64, 2019.01.28 beta:

1. Make the following file:

========== file: testhysteresis.tex ===============
\defineblock[foo]

\starttext

\def\abc{20}
\beginfoo
Use \abc
\endfoo

\useblocks[foo]

\stoptext
========== stop: testhysteresis.tex ===============

2. Compile it with "context --nonstopmode testhysteresis.tex" (which
   will work fine).

3. Modify it so that it doesn't define or use \abc (and so that it is
   still valid):

========== file: testhysteresis.tex ===============
\defineblock[foo]

\starttext

\beginfoo
Use
\endfoo

\useblocks[foo]

\stoptext
========== stop: testhysteresis.tex ===============

4. Without deleting testhysteresis.tuc, recompile with "context
   --nonstopmode testhysteresis.tex".  This run fails with the fatal error:

     tex error       > tex error on line 1 in file virtual://block.foo.1: ! Undefined control sequence

     l.1 Use \abc

   The cause seems to be that testhysteresis.tuc contains (from the run
   it step 2 above):

     utilitydata.structures.blocks.collected={
      {
       ["data"]="Use \\abc",
       ["index"]=1,
       ["metadata"]=3,
       ["references"]={
        ["section"]=0,
       },
      },
     }

5. Delete testhysteresis.tuc.

6. Now "context --nonstopmode testhysteresis.tex" works fine.

One workaround is always to delete the .tuc file before running context.
But that approach seems like overkill (although in my scripts that I use
for running expeirments with blocks etc., I take this approach), and
maybe there is a simpler way.

Best,
-Sanjoy


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