On Nov 13, 4:55 am, Alex Osborne <a...@meshy.org> wrote:
> 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.
Yes, the latter (mixin maps) is preferred.
Rich
--
You received this message because you are subscribed to the Google
Groups "Clojure" group.
To post to this group, send email to clojure@googlegroups.com
Note that posts from new members are moderated - please be patient with your
first post.
To unsubscribe from this group, send email to
clojure+unsubscr...@googlegroups.com
For more options, visit this group at
http://groups.google.com/group/clojure?hl=en
