Mark Engelberg wrote:
> Protocols:
>
> I don't understand whether there's any way to provide a partial
> implementation or default implementation of a given
> protocol/interface, and I believe this to be an important issue.
>
> For example, a protocol for < and > that provides a default
> implementation of > in terms of < and a default implementation of < in
> terms of >, so that you only need to implement one and you get the
> other for free.
>
How about this?
(defprotocol MyComparable
(comparinate [x y] "Returns a negative integer if x < y, zero if they
are equal and a positive integer if x > y.")
(equal-to [x y] "True if x is equal to y.")
(less-than [x y] "True if x is smaller than y.")
(greater-than [x y] "True if x is greater than y."))
(defn mixin-comparable [type compare-fn]
(extend type
MyComparable
{:comparinate compare-fn
:equal-to (fn [x y] (zero? (comparinate x y)))
:less-than (fn [x y] (neg? (comparinate x y)))
:greater-than (fn [x y] (neg? (comparinate x y)))}))
(mixin-comparable Integer -)
(mixin-comparable String #(- (count %1) (count %2)))
(less-than 8 2) ; => false
(less-than "x" "xxxx") ; => true
> I'm also thinking about the relationship in Clojure's source between
> ISeq and ASeq. ASeq provides the partial, default implementation of
> more in terms of next, for example. How does this kind of thing look
> with the new protocol system?
See above. But another way would just be to define ASeq as a map and
then merge with it:
(def aseq-impl {:more (fn [obj] (if-let [s (next obj)] s '() ))
:count (fn [obj] ...)})
(extend MyList
ISeq
(merge
aseq-impl
{:next (fn [lst] ...)
:count (fn [lst] ...) ; overrides the count from aseq-impl
:cons (fn [lst x] ...)}))
Code is data. :-)
I don't know whether doing things this way is a good idea or not, but
protocols are new: lets experiment and find out what works and what doesn't.
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