At this moment after reading again this thread, I lost my sense of what false
and true
mean :) I'll now stick with maybe, it's much clearer...
Luc P.
> 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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