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Re: [groff] Heirloom troff invoke macro from string

From: Mircea Hossu
Subject: Re: [groff] Heirloom troff invoke macro from string
Date: Mon, 07 Jan 2019 10:01:40 -0800

Thank you, Tadziu, it is an interesting trick and it does work in the
test case posted earlier. Unfortunately, in a more realistic document,
I did hit those "unexpected results", e.g. when the string was used
inside a macro argument (thank you for the warning).

The final solution I picked was to customize troff source code, adding
a new request and a new escape sequence:

1) .scripting command: start scripting subprocess
2) \K'expression': evaluate expression in scripting subprocess

This is similar to .pso request (execute subprocess and insert output
as text), but has the following advantages:

1) The subprocess is always alive and can be called efficiently
   thousands of times in the same document.
2) \K escape sequence can be used in macros or in strings.
3) The subprocess can maintain state, if more complex things are needed.

An example:

.do xflag 3
.scripting python    \" use your favorite scripting language
.ps 14p
.vs 24p
.nr x 0 1
.ds s \R'x \K'0 if \\nx==3 else \\nx''\\n+x

\*s \*s \*s \*s \*s \*s \*s \*s \*s

Output: 1 2 3 1 2 3 1 2 3

This is not meant to replace the normal troff language, but only to
address those rare problems which are impossible or overly complex to
express in troff. (Of course, it could be misused, like everything

If anyone else is interested, I can share all my work, which is
implemented in the Heirloom troff source tree. Any other suggestions
are very much appreciated.

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