Hi everybody,
after playing around with protocols & datatypes, I found them very fun
to use.
Some questions:
Performance
I don't see (with my limited benchmarking) any significant difference
between multifns and protocolfns:
user=> (defprotocol Test (protocol-fn [it] "Protocol-fn"))
Test
user=> (extend Object Test {:protocol-fn (fn [it] nil)})
nil
user=> (defmulti multi-fn type)
#'user/multi-fn
user=> (defmethod multi-fn Object [it] nil)
#<MultiFn clojure.lang.mult...@c8769b>
user=> (defn simple-fn [it] nil)
#'user/simple-fn
user=> (dotimes [_ 10] (time (dotimes [_ 100000] (protocol-fn :it))))
"Elapsed time: 105.532562 msecs"
"Elapsed time: 57.0031 msecs"
"Elapsed time: 33.210602 msecs"
"Elapsed time: 30.47827 msecs"
"Elapsed time: 26.326202 msecs"
"Elapsed time: 27.764654 msecs"
"Elapsed time: 28.381284 msecs"
"Elapsed time: 28.741735 msecs"
"Elapsed time: 28.697525 msecs"
"Elapsed time: 25.894514 msecs"
user=> (dotimes [_ 10] (time (dotimes [_ 100000] (multi-fn :it))))
"Elapsed time: 372.338313 msecs"
"Elapsed time: 73.104641 msecs"
"Elapsed time: 58.832009 msecs"
"Elapsed time: 60.312924 msecs"
"Elapsed time: 58.626328 msecs"
"Elapsed time: 57.005242 msecs"
"Elapsed time: 54.493328 msecs"
"Elapsed time: 56.283221 msecs"
"Elapsed time: 54.575182 msecs"
"Elapsed time: 54.939474 msecs"
user=> (dotimes [_ 10] (time (dotimes [_ 100000] (simple-fn :it))))
"Elapsed time: 28.504607 msecs"
"Elapsed time: 17.564177 msecs"
"Elapsed time: 1.877194 msecs"
"Elapsed time: 2.340661 msecs"
"Elapsed time: 1.581906 msecs"
"Elapsed time: 1.792407 msecs"
"Elapsed time: 1.878591 msecs"
"Elapsed time: 1.919937 msecs"
"Elapsed time: 2.367759 msecs"
"Elapsed time: 1.90555 msecs"
This is what I have been expecting of course (Fn < Protocolfn <
Multifn), but i was thinking that with Protocols dispatching would be
significantly faster than with Multimethods. Am I missing something?
Because they don't seem to provide the speed for implementing the core
abstractions (like (seq <coll>)).
Syntax
With protocols you define the protocols using symbols and extend types
by using the keywordized name
eg.
(defprotocol Test (protocol-fn [it] "Protocol-fn"))
(extend Object Test {:protocol-fn (fn [it] nil)})
i understand that extend is a function and evaluates its arguments,
and that {:protocol-fn (fn [it] nil)} is a real map, but wouldn't it
be possible just to use a {<generic-fn> <new-method-fn>} map instead?
The protocol should know its generic functions, so that wouldn't be
ambiguous.
{protocol-fn (fn [it] nil)}) ; seems a lot clearer to me
Extensibility
I've noticed that extending a protocol-fn redefines it:
(def old-fn protocol-fn) ; from above
(extend Number Test {:protocol-fn (fn [it] :a-number)})
(def new-fn protocol-fn)
(= old-fn new-fn)
false
this worries me, because the semantics are differing from MultiFns and
are less dynamic. Especially coupled with dynamic development this
could lead to some gotchas.
Clojure 1.1.0-alpha-SNAPSHOT
user=> (defprotocol Test (prtcfn [it]))
Test
user=> (extend Object Test {:prtcfn (fn [it] :object)})
nil
user=> (prtcfn (Object.))
:object
user=> (def old-fn prtcfn)
#'user/old-fn
user=> (old-fn (Object.))
:object
user=> (extend Number Test {:prtcfn (fn [it] :number)})
nil
user=> (prtcfn (Object.))
:object
user=> (prtcfn 1)
:number
user=> (def new-fn prtcfn)
#'user/new-fn
user=> (new-fn 1)
:number
user=> (old-fn 1)
:object
When some protocol-fns get bound in a closure this could hurt a lot.
Sorry if this seems like nitpicking, but this is just what I noticed
while experimenting
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