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bug#33870: 27.0.50; xref-goto-xref not configurable

From: Drew Adams
Subject: bug#33870: 27.0.50; xref-goto-xref not configurable
Date: Wed, 30 Jan 2019 13:39:57 -0800 (PST)

> > Example:
> >
> > Whoever designed the Lisp representation of a
> > noncontiguous region trapped us the same way.
> > By using a dotted pair of scalar values, that
> > design pretty much precludes adding other info
> > besides the start and end limits to a region
> > segment.
> You can override region-extract-function with your own
> implementation that can support any shape you want.

1. It's not about the region shape - at all.

2. The point is more general.  Code that invokes
the function that is the value of the variable
does not, in general, know what function that is.

It can only expect, based on the default value of
the function, that for input `bounds' it gets a
cons (START . END).

IOW, any function used as the variable value
really needs to return bounds of the same form,
if it expects to be used in more than an odd,
narrow context.  In general, such a function
will not know or care what context it's used in.

As a result, developers will provide functions
that model the args and return values of the
default function.

The default function was designed with a poor
choice for the value returned by input `bounds'.

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