On 8/11/2019 6:09 PM, Henri Menke wrote:
Dear devs,
There seems to be a bug in the MetaPost integration of ConTeXt. The MWE
below should produce three different labels “dummy foo bar” but instead
produces “bar bar bar”. The same example works correctly in plain
MetaPost. Originally reported on
https://tex.stackexchange.com/questions/503773/strange-behaviour-of-the-btex-etex-construct
Cheers, Henri
---
\starttext
\startMPpage
def drawtest(expr i) = %i is not used here
draw btex dummy etex shifted (0,0);
draw btex foo etex shifted (1.5cm,0);
draw btex bar etex shifted (3cm,0);
enddef;
drawtest(5);
\stopMPpage
\stoptext
It's not a really bug but a side effect. When metapost parses btex ..
etex it immediately replaces its content. In mpost the program that is
hardcoded as 'call out to tex and convert the dvi result into a
picture'. This parsing is not 'delayed' by wrapping in macro.
So in this case, what happens is that where normally there is some
interplay between context and the library, this kind of gets messed up.
Using btex inside a macro body is just a bad idea.
def drawtest(expr i) = %i is not used here
draw textext("dummy") shifted (0,0);
draw textext("foo") shifted (1.5cm,0);
draw textext("bar") shifted (3cm,0);
enddef;
works fine because it is not replaced in the macro body when parsed.
In context lmtx I will give a warning when "btex .. etex" is used inside
a macro definition. Actually btex/etex is just supported in context as a
kind of 'service' (we could just drop it).
Anyway, you can advice not to use "btex .. etex" but textext instead. As
with all things tex (and mp) with some hasty programming much can be
solved but in this case it's not worth the effort and for sure it will
bring up other side effects.
(In a similar fashion it's not a good idea to have global textexts
because there are local stored for them but in that case one can enforce
a persistent textext if needed.)
Hans
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