Value Types as map keys in ES7

[Rick Waldron]

On Sunday, April 6, 2014, K. Gadd <kg at luminance.org> wrote:

> I guess the analog for this in traditional JS 'Object' instances is
> that when you use the [] operator, i.e. obj[valueObject], it actually
> does obj[valueObject.toString()], so you can control the 'hashing
> function' in a sense by overriding toString. It seems natural to want
> something equivalent here, preferably not based on strings... (though
> I don't know how people feel about exposing hash internals and saying
> 'your hash key needs to be an integer')
>
> Map would certainly be far more useful if it had support for
> customizable hashing,


Existing hash code related work:

http://wiki.ecmascript.org/doku.php?id=strawman:encapsulated_hashcodes

http://wiki.ecmascript.org/doku.php?id=proposals:hashcodes

There was also discussion when parameterization of the Map and Set
comparator was first added, but custom comparator functions were deferred
until some time in which object hash codes are available:
https://github.com/rwaldron/tc39-notes/blob/master/es6/2013-01/jan-29.md#43-parameterize-the-equivalence-operator-for-mapset

Rick




> otherwise it's just a slightly faster
> replacement for Objects with cleaner semantics. Maybe making it
> possible to get the same value back using two different keys that
> don't compare the same with === is against the purposes here, though.
>
> On Sat, Apr 5, 2014 at 10:21 PM, Benjamin (Inglor) Gruenbaum
> <inglor at gmail.com <javascript:;>> wrote:
> > I'd like to raise an issue with ES7 value objects with maps raised here:
> >
> > http://esdiscuss.org/topic/maps-with-object-keys
> >
> > To save you all time, let me sum things up:
> >
> > ES6 maps don't solve a particular (but common) issue for me - using
> compound
> > objects as keys. I do a lot of statistical analysis and I need keys to be
> > compound tuples or other values. This is currently not possible with ES6
> > since map key equality is checked with `===` so it is always reference
> > equality
> >
> > ES7 introduces value objects - which are really cool anyway. This allows
> for
> > compound keys.which partially solves my issue because value keys are
> > compared structurally with `===`.
> >
> > However, it does not let me index on partial information (described in my
> > last commend on that issues). Since it is impossible to override `===` in
> > value objects from what I understand - It would be impossible for me to
> use
> > partial objects as keys.
> >
> > To illustrate, if for example I have a lot of 10 dim vectors, and I want
> to
> > index them as keys based on their first two dimensions - it is
> impossible at
> > the moment and I have to project them into their 2d counterparts which
> have
> > no further use for me.
> >
> > Is this somehow addressed? I'm unsure where to look for answers. It seems
> > natural to me to allow specifying a hashing function in the `Map`
> > constructor (rather than on every type) to solve this particular (but
> again,
> > rather common in code that does crunching) problem.
> >
> > Thanks,
> > Benjamin
> >
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> > https://mail.mozilla.org/listinfo/es-discuss
> >
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