On 23 May 2013 18:30, atkaaz <atk...@gmail.com> wrote:
> when you say the word "false" I'm assuming you're referring to "false?" the
> function (not "false" the boolean value), otherwise I don't understand
I mean false-the-Boolean-value.
To rephrase the point I was making previously, "(false x) is a truthy
value for any x in []" is a true sentence, indeed trivially so because
[] is empty. Thus (every? false []) returning true totally makes
sense.
Of course, in Haskell any such expression would be ill-typed, and that
makes sense too -- in the context of that language.
Opinions on the desired behaviour aside, in Clojure, implementing a
changed contract of every? whereby it would be required to throw when
passed something which cannot be treated as a predicate in the first
argument would be problematic -- checking that something is a function
is easy, but checking that a function has a useful unary overload is
not, in general. Well, you could simply try and call it with an
arbitrary object, but every? should be able to accept predicates which
it only makes sense to call on objects of a particular type,
predicates which have side effects, predicates which are expensive to
evaluate etc., so it's not an acceptable solution here.
And going back to opinions again, note also that in any case the
result would still be to throw an exception at runtime. The current
behaviour is also to throw at runtime, except with empty collections.
So, assuming non-empty collections are the norm, we get an exception
either way -- would having the exception come from every? rather than
the attempt to call the not-really-a-predicate object be of much help
in debugging?
Cheers,
M.
>
> so like: "What matters is that false? returns truthy values when called with
> any members of []"
> makes sense to me.
>
> So all I was saying above is that it should throw when [] is empty just as
> it does when [] is not empty, but it doesn't throw when empty because it's
> never called (by "it" i mean "false" not "false?")
>
> => (type false)
> java.lang.Boolean
> => (type false?)
> clojure.core$false_QMARK_
> => (fn? false)
> false
> => (fn? false?)
> true
>
> But really, if you were not talking about "false?" then I don't get it (??)
>
>
> On Thu, May 23, 2013 at 4:48 PM, Michał Marczyk <michal.marc...@gmail.com>
> wrote:
>>
>> 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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