bug#28753: 25.3; Functions to get alist from hash table and vice versa

[Philipp Stephani <p.stephani2@gmail.com>]

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Michael Heerdegen <michael_heerdegen@web.de> schrieb am Di., 7. Nov. 2017
um 14:29 Uhr:

> Noam Postavsky <npostavs@users.sourceforge.net> writes:
>
> > Drew Adams <drew.adams@oracle.com> writes:
> >
> > > Any news on this?  There is no general, abstract
> > > solution proposed, so far, to meet the needs met
> > > by the specific alist <-> hash-table code I sent.
> >
> > Did you have any comments for my proposal in #29?
>
> That's clever.
>
> OTOH I'm not convinced that `map-into' is a good abstraction.  Every
> goal type might have an individual set of useful parameters, especially
> when we want to add support for other map types in the future.  Our
> choice now to unite those in one conversion interface function might
> result in quite a mess later.
>

I don't think a unified conversion interface is that important. The various
structures used for mappings are just too different. For example, alists
and plists aren't real types, they are only defined by their usage. Hash
tables, on the other hand, are real types, with a per-object comparison
function, a non-nil empty value, etc. These two kinds of objects are just
too different to treat uniformly. Also, in most cases it is statically
known which kinds of objects are involved, so a generic function that
dynamically selects on the type of object isn't that useful. How about
adding some simple conversion functions to subr.el such as
(defun alist-to-hashtable (alist &rest keys) ...)
(defun hashtable-to-alist (hashtable) ...)

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