On Tue, Nov 4, 2008 at 10:00 AM, Christian Vest Hansen
<[EMAIL PROTECTED]> wrote:
>
> On Tue, Nov 4, 2008 at 3:12 PM, Rich Hickey <[EMAIL PROTECTED]> wrote:
>> On Nov 4, 9:00 am, "Christian Vest Hansen" <[EMAIL PROTECTED]>
>> wrote:
>>> "Generally" by custom but not required by contract of the Comparable
>>> interface. And those are all Numbers, right?
>>>
>>> Comparable imposes natural ordering, and the individual implementor is
>>> free to decide how to handle objects of different types; throwing a
>>> ClassCastException only being one of the possible reactions.
>>>
>>
>> True, except the individual implementors of Integer, Long, Double etc
>> have already decided to have them compare only to their exact type, so
>> we're stuck with that fact.
>
> So it does. I did not expect that; thought the standard numeric
> coercion rules applied.
For discussion's sake, here are a couple (lightly tested) functions
that might be helpful in determining the "greatest thing in a
sequence", given an arbitrary comparator or an arbitrary coercion
function. Two coercion-based fns are given -- a simpler one, and a
more efficient one (for expensive coercions).
Best,
Graham
(defn greatest-by
"Return the 'largest' argument, using compare-fn as a comparison function."
[compare-fn & args]
(reduce #(if (pos? (compare-fn %1 %2)) %1 %2) args))
(defn greatest-coerce
"Return the 'largest' argument by first coercing each argument to
some type using coerce-fn, which must return a Comparable
instance."
[coerce-fn & args]
(apply greatest-by #(compare (coerce-fn %1) (coerce-fn %2)) args))
(defn greatest-coerce-2
"Return the 'largest' argument by first coercing each argument to
some type using coerce-fn, which must return a Comparable
instance. This one performs a minimum number of coercions, which
may be important if the coercion function is expensive."
[coerce-fn & args]
(second (reduce (fn [[bc b] a]
(let [ac (coerce-fn a)]
(if (pos? (compare ac bc))
[ac a]
[bc b])))
[(coerce-fn (first args)) (first args)]
(rest args))))
;; example
(let [nums [1 23423424.234234 2.1 4/5 1/10 1/256 2341/2]]
(list (apply greatest-coerce float nums)
(apply greatest-coerce-2 float nums))) ;; both should return same value
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