Re: Proposal: stack traces with line numbers

[Helmut Eller <eller.helmut@gmail.com>]

On Mon, Oct 16 2017, John Williams wrote:

> What I had in mind was a single global hashtable, because that way
> it's easy to make it look as if the source refs are physically part of
> the annotated cons cells, and users of the API don't need to be aware
> that a supplementary data structure even exists. But of course using a
> global hashtable with strong keys would create a huge space leak in
> the reader.
>
> Is there any particular disadvantage to using weak keys?

The question is probably more if a global hashtable is a good idea.

I think an interface to read, like

(let* ((read-with-symbol-positions t)
       (read-symbol-positions-list '())
       (read-cons-position-table (make-hash-table :key 'eq))
       (form (read ...))
    ... do stuff with form ...)

would be fairly clean. Actually, it's quite hard to imagine a different
solution :-).  So users of read will probably have the choice anyway
whether to bind read-cons-position-table (or whatever the name will be)
to a fresh hashtable or reuse a global table.

> The file name would be a single string object shared by every ref in a
> given file (or nil when there is no file), so we'd only be saving a
> few words per source ref (one for the string itself, plus one or two
> saved by using a cons cell instead of a two-element vector.)

For the interface to read (the macro expander/compiler is a different
story) I would only record character positions.  Certainly easier to
handle for the garbage collector than a vector.  But it's not my call to
make.

Helmut