Re: feature/type-hierarchy 8a63e50036f 1/5: * Define 'cl--type-hierarchy' and compute 'cl--typeof-types' from it

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

>>>>> +    (symbol-with-pos keyword))
>>> Can't we have keyword that is a symbol-with-pos as well?
>> I'm not sure (it depends if we agree that a `symbol-with-pos` is also
>> a `symbol`.   In practice it presumably depends on the value of
>> `symbols-with-pos-enabled` 🙁),
> I *think* (thought at this point?) symbols-with-pos are symbols 🤷.

`symbolp` on a sympos will return t or nil depending on
`symbols-with-pos-enabled`, and similarly functions that accept symbols
will usually accept sympos only if that variable is set :-(

>> but I know that not all `symbol-with-pos`s are `keyword`s.
> How do you think this should be expressed in the DAG?
> I thought this was the correct way but I can indeed be wrong.

It's clearly not right, because a subtype means set-inclusion, which
here means that the current table asserts that all keywords are also
`symbol-with-pos`.

AFAIK `symbol-with-pos` just doesn't have any subtypes.  We could
introduce a new type `keyword-with-pos`, which would be a subtype of
`symbol-with-pos` and `keyword`, but I don't see much need for it.

Our DAG is "incomplete" in the sense that some values don't have a "most
specific type".  "keyword with pos" is indeed such a case.  A list of
the form (lambda ...) is another such case,

While I'm here, I also notice you added `class` and `structure` in
there, but I don't think these can be currently considered as "types"
(or at least, I don't know how they would be defined).

Of course, there are more quirks, like the `subrp` which returns t for
special forms, which aren't functions (that's a FIXME you threw away
when you replaced `cl--typeof-types` with `cl--type-hierarchy`).

> Could you explain why you think `symbols-with-pos-enabled` should
> influence our type hierarchy?

AFAIK types are defined by the set of values they can hold and/or by the
set of operations we can apply to those values.  And the set of
operations we can apply on sympos depends on this variable :-(


        Stefan