extend mutates some state (the protocol definition), so what is happen here is comp is returning a new function built from the value of the rest-serialize var (the protocol function before the extend changes it) and the value of the deref var.
I have not verified this, but I suspect if you use (fn [x]
(rest-serialize (deref x))) instead of the comp version, you will get
the behaviour you expected
On 12/10/13, 11:50 AM, Michael Blume wrote:
> I have a protocol RestSerializable to represent data that can be serialized
> to json from a REST interface.
>
> (defprotocol RestSerializable
> (rest-serialize [this]
> "Convert to something Cheshire can JSONify"))
>
> By default, things are left alone
>
> (extend Object
> RestSerializable
> {:rest-serialize identity})
>
> But I want atoms and the like to be transparent when serialized
>
> (extend clojure.lang.IDeref
> RestSerializable
> {:rest-serialize (comp rest-serialize deref)})
>
> I would expect this to mean that, say,
>
> (-> 42 atom atom atom atom rest-serialize)
>
> would just unwrap to 42, but in fact it only unwraps the outer atom and
> leaves the inner ones alone.
>
> Here's where it gets weird though. If I just evaluate the extend
> clojure.lang.IDeref form again, rest-serialize suddenly gains the ability
> to unwrap *two* layers of atoms. Eval it again and it can unwrap *three*.
> Kinda feels like the exercises in Little Schemer when they're building up
> to the Y Combinator.
>
> Here's my REPL session demonstrating this (not shown, but I've verified
> this behavior is the same in Clojure 1.5.1 and Clojure 1.6.0-alpha3)
>
> $ lein repl
> nREPL server started on port 46049 on host 127.0.0.1
> REPL-y 0.2.1
> Clojure 1.5.1
> Docs: (doc function-name-here)
> (find-doc "part-of-name-here")
> Source: (source function-name-here)
> Javadoc: (javadoc java-object-or-class-here)
> Exit: Control+D or (exit) or (quit)
> Results: Stored in vars *1, *2, *3, an exception in *e
>
> user=> (defprotocol RestSerializable
> #_=> (rest-serialize [this]
> #_=> "Convert to something Cheshire can JSONify"))
> RestSerializable
> user=>
>
> user=> (extend Object
> #_=> RestSerializable
> #_=> {:rest-serialize identity})
> nil
> user=>
>
> user=> (extend clojure.lang.IDeref
> #_=> RestSerializable
> #_=> {:rest-serialize (comp rest-serialize deref)})
> nil
> user=> (-> 7 atom atom atom rest-serialize)
> #<Atom@56153406: #<Atom@a0aa211: 7>>
> user=> (-> 7 atom atom atom rest-serialize)
> #<Atom@3c19a5b0: #<Atom@37cce4a3: 7>>
> user=> (-> 7 atom atom atom rest-serialize)
> #<Atom@9f6e629: #<Atom@308092db: 7>>
> user=> (extend clojure.lang.IDeref
> #_=> RestSerializable
> #_=> {:rest-serialize (comp rest-serialize deref)})
> nil
> user=> (-> 7 atom atom atom rest-serialize)
> #<Atom@7fb48906: 7>
> user=> (-> 7 atom atom atom rest-serialize)
> #<Atom@41e224a5: 7>
> user=> (extend clojure.lang.IDeref
> #_=> RestSerializable
> #_=> {:rest-serialize (comp rest-serialize deref)})
> nil
> user=> (-> 7 atom atom atom rest-serialize)
> 7
> user=> (quit)
> Bye for now!
>
--
And what is good, Phaedrus,
And what is not good—
Need we ask anyone to tell us these things?
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