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bug#27193: 25.2; tmm should use completing-read-default

From: Ryan Thompson
Subject: bug#27193: 25.2; tmm should use completing-read-default
Date: Fri, 02 Jun 2017 17:37:18 +0000

On Fri, Jun 2, 2017 at 11:26 AM Drew Adams <address@hidden> wrote:
I don't understand that argument.  (But to be clear, I don't
really care much about `tmm'.)
Fair enough. My post certainly assumes quite a bit familiarity with tmm. I'm happy to elaborate below.
I don't see that that is an argument for _preventing_ the
use of a `completing-read-function' by tmm.  I see it
only as a statement that it might not be helpful to use
some particular values of `completing-read-function' with

If there is a general problem then consider adding a
comment in `tmm.el' (or perhaps put it in some doc
string) that says what you are really saying: Some
values of `completing-read-function' might not be
helpful with `tmm', and if you find that is the case,
consider binding that variable to nil.

But now that I've taken a quick look (just now) at the
use by tmm of `completing-read', I don't see that there
is a problem to be fixed in `tmm.el'.  I don't see that
its use of `completing-read' is particularly exotic or
problematic.  This is it:

 (concat gl-str " (up/down to change, PgUp to menu): ")
 (tmm--completion-table (reverse tmm-km-list)) nil t nil
 (cons 'tmm--history (- (* 2 history-len) index-of-default)))

I don't see anything at all unusual about that.

And the collection function, `tmm--completion-table',
is likewise pretty ordinary, I think:

(defun tmm--completion-table (items)
  (lambda (string pred action)
    (if (eq action 'metadata)
        '(metadata (display-sort-function . identity))
      (complete-with-action action items string pred))))

You say:

> tmm already pretty much relies on the assumption that
> completing-read is actually calling completing-read-default.

I don't see any evidence of that.

tmm's weirdness is not in the actual call to completing-read. That completing-read call is wrapped by "(minibuffer-with-setup-hook #'tmm-add-prompt ...)", which in turn calls tmm-define-keys, which sets up a bunch of non-standard key bindings, which have the effect of implementing a menu system with single-keypress activation of menu items, rather than completion of a substring with string matching. The result is not even recognizable as completing-read. The use of completing-read is merely an implementation detail in a system that is *not* actually doing completion of any kind. So pluggable completion backends don't make any sense for tmm, and I can't imagine any other value of completing-read-function that would make sense for tmm besides completing-read-default. Looking at the "git blame" output for tmm, that call to completing-read has definitely not been updated since completing-read-function was introduced except for minor bugfixes, so it makes sense that tmm would be expecting one and only one implementation of completing-read.

This kind of argument could (inappropriately, IMO) be
applied to any number of completely normal uses of

I see no reason to impose a dichotomy of either a
`completing-read-function' similar to yours or else
`completing-read-default'.  There are likely other
benign values of the variable, besides just

I'm not trying to set a general precedent here. tmm is the only code that I'm aware of that uses completing-read in this way.

It sounds like (and no, I haven't looked into it;
it just sounds like it) you have some particular
`completing-read-function' values in mind, which
you have found are incompatible with tmm's use of

The alternative completing-read-function providers that I am aware of are of are ido-ubiquitous (written by me), ivy, and helm. I'll also include icicles, even though uses some other mechanism besides modifying completing-read-function. ido-ubiquitous and ivy both have code to deactivate themselves when completing-read is called by tmm because otherwise their completion systems would break it, while icicles and helm simply break tmm when enabled. Thus, to my knowledge there is currently no other completing-read-function that doesn't break tmm (except for those that make exceptions specifically for tmm).

If so, that's not an argument for preventing the use of
other values of `completing-read-function' with tmm.
(Clearly the value `completing-read-default' is fine,
for instance.)  That's not an argument for tmm to do
something to prevent all use of `completing-read-function'.

Instead, it's an argument for the code that defines and
uses a particular `completing-read-function' to take
care of the contexts where it makes sense, and to stop
itself from being used in other contexts, where it might
cause trouble.

Only that code knows the kinds of context where its own
`completing-read-function' makes sense and where it does
not.  Code such as tmm should not try to guess what kinds
of trouble different values of `completing-read-function'
might cause.

I don't think tmm should throw up its hands and say, "Gee,
there might be some values of `completing-read-function'
that are troublesome, so let's just prevent all use of
that variable."  That doesn't make sense, to me.

Based on my explanation above, that is precisely what I think tmm should do: avoid using completing-read-function entirely. To look at it another way, tmm was originally written to use completing-read as an implementation detail, and later the function that used to be called completing-read was renamed to completing-read-default, but tmm was never updated to use the new name. This patch rectifies that.
If you want additional suggestions, maybe describe just
what the problem is that your completion function causes
for tmm.  It's hard to offer suggestions if you only
state that it is incompatible, without going into any
detail.  (Not that you must ask for input about this.
But if you would like some then giving more detail might

Please use your own judgment (as I said, I don't really
care much about `tmm'), but a priori this sounds like

It sounds a bit like trying to bend Emacs to fit your
`completing-read-function'.  I can understand such a
motivation, believe me; I don't ascribe a bad intention
to you.  A guess is that you are not sure what to do,
to prevent inappropriate application of your value of
`completing-read-function' in this or that context.

If you think it causes trouble in some contexts, or it
is not able to handle some contexts properly, then
I'd suggest you consider walling it off from those use
cases.  It might take some time to discover which
contexts it causes trouble for, but that's OK - you
could add them as you discover them.  Tmm sounds like
a start. 

The right approach, IMO, is to teach your code when to
use its `completing-read-function' and when not to use
it.  Put differently, consider teaching your
`completing-read-function' when and where to hold back
and just punt to the default behavior.

This is exactly how ido-ubiquitous and ivy both currently work: they essentially have a blacklist of callers for which they revert to standard completion. tmm is on the blacklist for both packages. Certainly, for any alternative completion system there will be cases where it needs to fall back to standard completion. In my view, the completion system should be able to determine purely based on the calling context (i.e. its arguments and any relevant dynamically-bound variables) whether or not it needs to punt. Making this decision based on the name of the caller instead of the context to make this decision is admitting that not enough context was provided. I view it as a workaround, not a desirable design pattern, and someday in the future I hope to be able to remove the blacklist from ido-ubiquitous.

In the case of tmm, the best heuristic I can think of would be to inspect the key bindings of all the letters and numbers. If any of them are locally rebound in the minibuffer to something other than self-insert-command, then punt. However, this doesn't work in practice because the bindings happen in minibuffer-setup-hook, so putting a check at the front of this hook is too early, and putting it at the end is too late because (in the case of ido-ubiquitous) an error is triggered before reaching the end of the hook. This was partly my motivation for suggesting the change in tmm rather than working around it in ido-ubiquitous: because the blacklist approach is the only way for ido-ubiquitous to fix it.

It's obvious that it is possible for someone to create
a `completing-read-function' that causes trouble here
or there.  But such trouble is up to the causer to fix
or tame.

For the reasons described above, I would be very surprised if there was *any* alternative completion system that didn't break tmm.

The approach of preventing code like `tmm.el' from
letting other code use `completing-read-function' does
not look like it heads in the right direction.  But
mine is just one opinion.  I ask only that you think
some more about this.
As mentioned above, tmm is the only code I'm aware of that does anything like this, and I don't see this as a general approach to be applied anywhere else. 

Hopefully the above clarifies my rationale in proposing this change.

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