Whether (false 1) or (false true) is truthy is irrelevant. What
matters is that false returns truthy values when called with any
members of [], which is of course the case, as [] has no members. (For
it not to be the case, there would have to exist an x in [] for which
(false x) were not truthy -- clearly there is no such x.)
This is the same story as with quantification restricted to the empty set:
\forall x \in \emptyset . \phi(x)
is true regardless of what \phi is, and intimately related to how
implication works in classical logic (since the above is shorthand for
a formula involving implication):
x -> y
is true when x is false, regardless of what value y takes. (It's also
true when y is true, regardless of what value x takes; this, however,
is not relevant here.)
Cheers,
M.
On 23 May 2013 06:31, atkaaz <atk...@gmail.com> wrote:
> Well, seems to me more like this:
> if [] is empty then return true
> otherwise check (pred everyx in coll)
> however this allows for any pred especially(in this case) invalid preds:
> `false` is not a function/pred
> => (false 1)
> ClassCastException java.lang.Boolean cannot be cast to clojure.lang.IFn
> cgws.notcore/eval2542 (NO_SOURCE_FILE:1)
> => (false true)
> ClassCastException java.lang.Boolean cannot be cast to clojure.lang.IFn
> cgws.notcore/eval2564 (NO_SOURCE_FILE:1)
>
> doesn't seem truthy to me
>
> Thanks.
>
>
> On Thu, May 23, 2013 at 3:08 AM, Michał Marczyk <michal.marc...@gmail.com>
> wrote:
>>
>> On 22 May 2013 18:34, atkaaz <atk...@gmail.com> wrote:
>> > I think the exception is thrown because you basically called (every?
>> > false
>> > coll) however on my clojure version I cannot reproduce it oh wait there
>> > we
>> > go, some bug here with empty collection (maybe someone can pick it up):
>> > => (every? false [1 2 3])
>> > ClassCastException java.lang.Boolean cannot be cast to clojure.lang.IFn
>> > clojure.core/every? (core.clj:2423)
>> > => (every? false [])
>> > true
>> >
>> > => *clojure-version*
>> > {:interim true, :major 1, :minor 6, :incremental 0, :qualifier "master"}
>>
>> (every? false []) should return true if and only if (false x) is
>> truthy for every x in [], which is certainly the case.
>>
>> Cheers,
>> Michał
>>
>>
>> >
>> >
>> >
>> >
>> >
>> > On Wed, May 22, 2013 at 7:17 PM, Peter Mancini <peter.manc...@gmail.com>
>> > wrote:
>> >>
>> >> So I did some coding and came up with this but it is broken;
>> >>
>> >> (= java.lang.Boolean (type false)) ;;evaluates to true
>> >>
>> >> (defn all-true?
>> >> [coll]
>> >> (every? (cond (= java.lang.Boolean (type identity)) identity :else
>> >> false) coll)) ;;compiles
>> >>
>> >> (all-true? '(true true true)) ;; throws java.lang.ClassCastException:
>> >> java.lang.Boolean cannot be cast to clojure.lang.IFn
>> >> (all-true? '(true true false))
>> >> (all-true? '(true true 3))
>> >>
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