To start with, IObj and its .meta method are used with clojure.core/meta to
get the metadata on an object. Since this method can be the same between
refs and immutable data, they all use the same interface.
IMeta is used with withMeta and clojure.core/with-meta to return a new
object with the given metadata. Notice that:
(let [d {}]
(identical? d (with-meta d {:some :meta}))))
; => false
IReference however has alterMeta which is used with
clojure.core/alter-meta, and as the name suggests it alters the metadata on
the object. It's a mutation of the object. This is needed with refs since
returning a new atom once you change the meta wouldn't be much help at all.
So there you have it, use with-meta for immutable objects, and alter-meta
for mutable objects.
Timothy
On Wed, Nov 1, 2017 at 7:20 PM, Didier <didi...@gmail.com> wrote:
> Hey, I was surprised to find that IMeta does not work for (with-meta).
>
> So it seems that withMeta is added to IObj, which extends IMeta. And it
> seems like alterMeta and resetMeta are added in IReference, which also
> extends IMeta.
>
> So now if you want to support meta, you have to implement IReference, IObj
> and IMeta?
>
> Is there a reason for this? Particularly, I find it strange I can't use
> with-meta on a Var, because it only implements IReference, and not IObj. So
> I can use all meta functions on Vars except for with-meta.
>
> Thank you.
>
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