Hello all,
I've been looking at the new protocol (defprotocol, deftype,
defrecord, etc) feature in 1.2 and, while I love the core concepts,
it's been bothering me that there's no apparent way to provide
*automatic* default implementations for methods on a protocol.
I know from reading Rich Hickey's discussions on the design rationale
[1] and other threads on providing Scala Trait-like behaviour [2] that
you can choose to mix in a set of defaults when you implement the
protocol, but it seems important to me that a protocol author be able
to provide default implementations to enable protocols to evolve.
Allowing a function body in the defprotocol clause would seem to be
the obvious way, something like:
(defprotocol P
(foo [x] (default-foo x))
(bar-me [x] (bar-me [1 x])
[x y])) ; no default implementation
As I'm sure most of you know, a key problem with "pure" interface
features in environments like CORBA, COM, Java, etc has been that you
can't add new methods to published interfaces: you'd break all
existing implementations. Hence you see a history of evolving
interfaces indicated by names like IFoo, IFoo2, IFooEx, etc. In
contrast, a Scala-like trait system allows you to add a new method
*and* a default implementation (supposing that makes sense), so old
clients still work fine.
The problem, as I see it, with Clojure protocols is that you would
have to rely on clients mixing in a set of defaults in order to
happily extend a "public" protocol.
Am I missing something key here? I realise there are some very
experienced people contributing to Clojure, so am fully expecting to
be told I've missed something obvious ;)
Cheers,
Matthew.
[1] http://groups.google.com/group/clojure/msg/330c230e8dc857a9
[2] http://groups.google.com/group/clojure/msg/53228e04db4799a5
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