On Thursday, November 8, 2018 at 10:44:34 AM UTC-6, tbc++ wrote:
>
> The instance based polymorphism is a bit wonky in some cases. Can we get
> some sort of spec that tells us what the rules are for resolution?
>
>From the top of the thread: "Protocol implementations are checked first for
direct definitions (defrecord, deftype, reify), then metadata definitions,
then external extensions (extend, extend-type, extend-protocol)." Was that
unclear in some way? I think it explains everything you're seeing.
We do plan to update the protocols reference page for final release, but
waiting on that.
We are still doing performance eval on this too - still might change a bit
more.
> See these cases where it breaks in some rather strange ways.
>
> Clojure 1.10.0-beta5
> user=> (defprotocol ILevel (level [this]))
> ILevel
> user=> (extend-protocol ILevel
> clojure.lang.IPersistentVector
> (level [this] "interface"))
> nil
> user=> (extend-protocol ILevel
> Object
> (level [this] "object"))
> nil
> user=> (level [])
> "interface"
> user=> (level :key)
> "object"
> user=> (level (with-meta 'foo {`level (fn [this] "instance")}))
> "instance"
> user=> (level (with-meta [] {`level (fn [this] "instance")}))
> "instance"
> user=> (defrecord MyType [] ILevel (level [this] "type"))
> user.MyType
> ;; Fails to override
> user=> (level (with-meta (->MyType) {`level (fn [this] "instance")}))
> "type"
> user=> (defrecord MyType3 [])
> user.MyType3
> user=> (extend-protocol ILevel
> MyType3
> (level [this] "type"))
> nil
> user=> (level (->MyType3))
> "type"
> ;; Overrides
> user=> (level (with-meta (->MyType3) {`level (fn [this] "instance")}))
> "instance"
> user=>
>
> So it looks like if someone uses inline protocol extension that class
> cannot participate in instance level polymorphism. If however they use
> extend on the type after it's defined then the behavior changes and
> instance polymorphism is supported.
>
>
>
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