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Proposal: stack traces with line numbers

From: John Williams
Subject: Proposal: stack traces with line numbers
Date: Sat, 14 Oct 2017 17:17:47 -0700

Elisp is a fun language to work in, for the most part, but one thing I
find very irritating compared to other languages is that there's no
way to get a stack trace with line numbers. I'm wondering if others
feel the same way and would be open to accepting a change to add
better support for line numbers. Here's my plan:

1. Revise the reader to attach source references (i.e. filename, line
number, and column number) to forms as they are read.
2. Update the byte compiler to preserve source references in compiled code.
3. Update the debugger to display source references in backtraces
whenever possible.
4. Add a simple API for users to retrieve a stack trace suitable for
writing to logs, etc. (There's already a stack trace API, but the
information you can get from it isn't all that useful.)
5. Possibly add some facilities for macro authors to control the
source refs in macro expansions. I'm not sure about that part because
I believe most macros will propagate source information in a
reasonable way simply by virtue of embedding their arguments in the
expansions they generate.

I already have a working proof of concept for the first part. What it
does is attach a vector of (file name, line number, column number) to
the head of each list as it is read. The information is "attached"
using cons cells as keys in a weak-key hash table. I also added a
little function to fetch data from the hash table so the
representation is abstracted a little bit.

Here's my rationale for the engineering decisions I've made so far:

- I'm using a hash table because the other alternatives I looked at
involved changing the representation of (some) cons cells, which
doesn't sound so bad until you start looking at all the
performance-critical code paths that would need to change, and all the
parts of Emacs (e.g. the garbage collector) where the low-level
representation of cons cells is handled as a special case.

- I'm storing the information in vectors because it seems like a
reasonably efficient use of memory. Certainly better than a list. It
would be easy enough to encode all the relevant information in a
string, but then the reader would be spending time building strings
that will need to be decoded later, and I'm not sure it would help
anyway, because each string would be unique, whereas with a vector,
the same string object can be used for every reference in a file.
Adding a new primitive type would also be an option, but it hardly
seems worth the complexity to save a couple of words per source ref
when 99% of them will probably only be retained long enough to
byte-compile the code.

- I'm saving line and column numbers rather that just byte/character
offsets, because that's what developers need, and if it wasn't saved
in that format, displaying a stack trace would involve opening the
original source code to compute that information from the file
contents. If I dropped the column numbers I could store a source ref
in a cons cell rather than a vector, but it seems like a shame to
throw away that kind of information when it's so easy to collect. (I
could even pack the line and column number into a single integer,
since I don't think it would be a big deal if there was an overflow
for an incredibly large file, or a file with very long lines, but
again, that seems like unnecessary complexity to me.)

- I'm only attaching information to lists because only lists can be
function calls, and attaching information to things like symbols would
be problematic because every occurrence of a given symbol is
represented by the same Lisp object. Of course some lists aren't
function calls, but attaching a source ref to every list is a lot
simpler and more reliable than trying to guess which lists are
ultimately going to become function calls.

- I'm only attaching information to the head of each list purely as a
memory-saving measure. I can't think of scenario where you'd need a
source reference for a list without having its head available, except
maybe in the expansion of a macro that disassembles its arguments and
puts them back together in a new list. If it's an issue in practice, I
think a better solution would be for the macro expander to propagate
source refs to every cons cell in a macro argument at the point where
macro expansion takes place.


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