GNU bug report logs - #69739
30.0.50; `type-of` is not precise enough

Previous Next

Full log

View this message in rfc822 format

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: 69739 <at> debbugs.gnu.org
Cc: monnier <at> iro.umontreal.ca
Subject: bug#69739: 30.0.50; `type-of` is not precise enough
Date: Mon, 11 Mar 2024 19:19:40 -0400

Package: Emacs
Version: 30.0.50


`type-of` is supposed to return "the" type of its argument.  Given that
ELisp has a notion of subtyping, "the" type is expected to mean "the
most precise type".  This is used in `cl-generic` to decide which method
to apply, so it's important that it returns precise information.

Currently, there `type-of` fails to return precise enough information in
a few cases:

- When the argument is nil, it returns `symbol`.  The problem here is
  that `symbol` is not a subtype of `list`, whereas nil is a list.

- When the argument is a special form, a C primitive, or
  a native-compiled function it returns `subr`.  Currently our
  type hierarchy says that `subr` is a subtype of `compiled-function`
  (and hence of `function`), but a special form is *not* a function
  (it fails the `functionp` test and can't be `funcall`ed).

Currently `cl-generic` works around the first point above by using
(if FOO (type-of FOO) 'null) instead of calling `type-of` directly.

Suggestion:

    I suggest we change `type-of` to return `null` for `nil`,
    `special-form` for subrs that are special forms, `subr-primitive`
    for C primitives, and `subr-native-elisp` for native-compiled subrs.

There are a few other cases where we could improve the precision, tho
they are less important because they don't cause problems w.r.t
subtyping like the above does.

Further improvements could include:

- Return `boolean` for `t`.  This would be nice otherwise (with the
  above suggestion) `cl-generic` can dispatch on "nil is a boolean"
  but not on "t is a boolean".
- Return `keyword` for symbols that are keywords.
- Return `fixnum` or `bignum` rather than just `integer`.
  Probably not worth the trouble.
- We could go crazy and return `keyword-with-pos` for `symbols-with-pos`
  that are keywords.

Of these further improvements, only the first (return `boolean` for `t`)
seems worth the trouble.

Still, any change as suggested here would be an incompatible change, so
there's risk it'll break some code out there (`type-of` is not used very
often, but it *is* used).  Another option is to introduce a new function
which does the same as `type-of` but with changes like the ones above.
(we could even decide to give it a `cl-generic-` prefix to discourage
its use elsewhere so we can be more free to change its return value in
the future).


        Stefan

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.