Re: hysteresis with content in blocks (one strike and you can be out forever)

[Wolfgang Schuster <wolfgang.schuster.lists@gmail.com>]

Sanjoy Mahajan schrieb am 06.02.19 um 16:53:
> 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.

You can set dimensions with the measure system

%\definemeasure [TestValue] [20pt]

\starttext
\measure{TestValue}
\stoptext

and other values with \setvariables

%\setvariables [test] [abc=20]

\starttext

\getvariable{test}{abc}

\getvariabledefault{test}{abc}{10}

\stoptext

Both systems work even with unset values.

Wolfgang
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