bug#36970: 26.2; invalid-read-syntax could print the location of the error

[Lars Ingebrigtsen <larsi@gnus.org>]

ndame <emacsuser@freemail.hu> writes:

> It only says:
>
>     (invalid-read-syntax ". in wrong context")
>
> It could also print the character location or even the line number and
> the position in the line to make it easy to find the error.

Poking around in read1 and friends, this doesn't seem very difficult to
implement -- the errors are signalled from the invalid_syntax function.

The wrinkle is that we may be reading from a string or something else
based on readcharfun.  However, the common case is reading from a
buffer, and we could improve the error message there.

So my idea here would be to change all the calls that are like this:

	      invalid_syntax (". in wrong context");

into

	      invalid_syntax (". in wrong context", readcharfun);

and then, in that function do

  if (BUFFERP (readcharfun))
    make_string_based_on_point ()

Since point is at the problematic element, that could just do a
Fcount_lines/Fcurrent_column to get the line number/column number,
presumably.

Before starting to hack away at this, does anybody see any problems with
this approach?

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no