Re: Recommendation for CAPF setup when you don't know completion string in advance

[Stefan Monnier <monnier@iro.umontreal.ca>]

> (lambda (string pred action)
>   (when (no-result-yet-cached-or-cached-string-is-substring-of-string)
>     (if (eq action 'metadata) `(metadata …)
>       (unless last-prefix
> 	;; talk to ipython and save the completion info
> 	)
>       (if (eq (car-safe action) 'boundaries) ; munge the boundaries
> 	  `(boundaries from . to) ; using saved data from ipython
> 	(complete-with-action action completion-alist string pred)))))
[...]
> and iPython correctly figures out to complete just a portion of that
>  string; say ran -> range.  I tell CAPF about these boundaries (in
>  this case, 9 . 0).  If, however, I later call complete-with-action
>  (which just calls try-completion, all-completions, etc.) with the
>  entire line string (“for in ran”), together with a completion-alist
>  that contains iPython’s sub-string completions (like “range”), it
>  thinks there is no completion.  If instead I just peel off the part
>  of the full line that needs completing (“ran”) and pass that to
>  complete-with-action as STRING, it recognizes that there's a good
>  completion now, but then _replaces the entire line_ with the result.
>
> How do I get
> complete-with-action/try-completion/test-completion/all-completions
> etc. to respect my boundaries?

There's `completion-table-with-context`.
Note that it comes with the following:

    ;; TODO: add `suffix' maybe?

so you probably won't be able to use it as-is and you'll need to write
your function instead.  But hopefully it should be enough to get
you started.  It shouldn't be terribly hard.

I think one problem you may encounter is that in the default UI,
selecting an entry from *Completions* (e.g. with a middle click) will
probably throw away the "suffix" (i.e. the text after the end of the
boundary).
If so, I suggest you `M-x report-emacs-bug` (tho maybe I did fix this
one already; but there might be a few other such corner cases where we
throw away the text after the end of the boundary).


        Stefan