Re: User story: typst multiline math in pandoc markdown and pandoc AST

[Gwern Branwen <gwern-v26ZT+9V8bxeoWH0uzbU5w@public.gmane.org>]

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On Thu, Oct 19, 2023 at 12:11 PM John MacFarlane <fiddlosopher@gmail.com>
wrote:

> Any change to our algebraic data types is a breaking change that would
> require modifications throughout the ecosystem (all writers and readers,
> filters, etc.).  That's why we're very conservative about such changes and
> make them infrequently and only with very strong reasons.
>

This expression-problem topic merits an FAQ entry at this point.

Dehaene's puzzlement here is a common & understandable one, because in many
languages there would be little problem, and it's intuitive: how could
merely tucking in an additional optional completely-backwards-compatible
field be such a big deal? Only to a hardcore static-typing Haskeller, I
think, is it immediately obvious that adding to the AST is a huge deal
which will break *everything*, almost, because the problem is not
backwards-compatibility, but forward-compatibility. It's an instance of the
'expression problem': the cost of static type safety of code operating on
the AST is that they are going to break that static type safety should the
type ever change.

You can tell it's common & understandable because it comes up several times
a year on this mailing list or the bugtracker, and shows no sign of going
away.

I think at this point it'd be good to have a short section somewhere on the
expression problem and why changes to the AST are not going to happen...
Not in https://pandoc.org/MANUAL.html , of course, as only extension
writers will run into this, but perhaps in
https://pandoc.org/using-the-pandoc-api.html ? (To explain why 'Yes, your
extension just ran into a problem where enriching the AST would help, but
that's not going to happen no matter how good it'd be, so don't bother
asking'.)

-- 
gwern
https://gwern.net <https://www.gwern.net>

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