On Tue, Jan 29, 2019 at 4:23 PM Rostislav Svoboda <
rostislav.svob...@gmail.com> wrote:
> Hi Alex,
>
> > Sets are treated as predicate functions which are valid when they
> produce a logically true value.
>
> So then my question boils down to:
> Why is then (boolean (quote nil)) => false and (boolean (quote
> anything)) => true?
>
nil, false, and true are the only three special "symbols" that the Clojure
reader reads as something other than a symbol. They are read as
respectively nil and boolean false and true. nil's "type" is nil (there is
no type to represent nil). false and true's type is java.lang.Boolean.
Moreover, the false and true values are exactly the static constants
java.lang.Boolean/FALSE and java.lang.Boolean/TRUE.
quote's purpose is to read (with the Clojure reader, converting from string
to value) but NOT evaluate. For these three values, they all evaluate to
themselves, so quote has no effect.
For something like (type (quote anything)), (quote anything) will read the
string "anything", and construct the symbol anything, with type
clojure.lang.Symbol. The quote prevents evaluation.
Skipping some important details, every value in Clojure evaluates to itself
except for the two special cases of symbols and lists. Symbols evaluate by
looking up their associated value in the current namespace. Lists evaluates
its elements, then invokes the first element as a function while passing
the rest of the elements as arguments.
So, no bug here. Everything is going as planned. :)
> And this boils down to:
> Why a type of quoted symbol (type (quote nil)) is nil and not
> clojure.lang.Symbol as is it the case for (type (quote anything))?
> Here the source says:
> (defn type
> "Returns the :type metadata of x, or its Class if none"
> {:added "1.0"
> :static true}
> [x]
> (or (get (meta x) :type) (class x)))
>
> And if I put few println's here, I see that in both cases (type (quote
> nil)), (type (quote anything)) the input arg x is nil, IOW the
> information "the arg x has a type of quoted symbol" is lost here. IOW
> the clojure.core/type eagerly reports the type of e.g.:
> (type (quote true)) => java.lang.Boolean
> (type (quote "foo")) => java.lang.String
> (type (quote nil)) => nil
> (type (quote 1)) => java.lang.Long
> etc. when I expect clojure.lang.Symbol, as for:
> (type (quote anything)) => clojure.lang.Symbol
>
> Hmm... *if* this type-hiding is a bug then I guess it's crawling too
> deeply to fix it, right? :)
> Anyway, thank you for your response, Alex!
>
> Bost
>
>
> Le mar. 29 janv. 2019 à 21:40, Alex Miller <a...@puredanger.com> a écrit :
> >
> > Sets are treated as predicate functions which are valid when they
> produce a logically true value. Sets with logically false values nil or
> false will fail this check. This is not a spec thing, but a general thing
> to be aware of in Clojure when using sets as functions.
> >
> > If you want nils, use (s/gen nil?).
> > If you want falses, use (s/gen false?).
> > If you want either, the simplest thing is probably (s/gen (s/nilable
> false?)), but keep in mind that the s/nilable generator is constructed to
> only produce nils 10% of the time, so you'll get 10% nils, 90% falses.
> >
> > You could also use the more cumbersome (s/gen (s/nonconforming (s/or :n
> nil? :f false?))) which should give you about 50/50 mix.
> >
> >
> > On Tuesday, January 29, 2019 at 2:23:50 PM UTC-6, Bost wrote:
> >>
> >> Could anybody explain please why I can't get a sample of falses or
> >> quoted nils / falses here?
> >>
> >> foo.core> (clojure-version)
> >> "1.10.0"
> >> foo.core> (require '[clojure.spec.gen.alpha :as gen]
> >> '[clojure.spec.alpha :as s])
> >> nil
> >> foo.core> (gen/sample (s/gen #{'nil}))
> >> Error printing return value (ExceptionInfo) at
> >> clojure.test.check.generators/such-that-helper (generators.cljc:320).
> >> Couldn't satisfy such-that predicate after 100 tries.
> >>
> >>
> >> Other interesting and/or relevant cases are:
> >>
> >> foo.core> (gen/sample (s/gen #{'n}))
> >> (n n n n n n n n n n)
> >> foo.core> (gen/sample (s/gen #{true}))
> >> (true true true true true true true true true true)
> >> foo.core> (gen/sample (s/gen #{'true}))
> >> (true true true true true true true true true true)
> >> foo.core> (gen/sample (s/gen #{false}))
> >> Error printing return value (ExceptionInfo) at
> >> clojure.test.check.generators/such-that-helper (generators.cljc:320).
> >> Couldn't satisfy such-that predicate after 100 tries.
> >> foo.core> (gen/sample (s/gen #{'false}))
> >> Error printing return value (ExceptionInfo) at
> >> clojure.test.check.generators/such-that-helper (generators.cljc:320).
> >> Couldn't satisfy such-that predicate after 100 tries.
> >> foo.core> (gen/sample (s/gen #{nil}))
> >> Error printing return value (ExceptionInfo) at
> >> clojure.test.check.generators/such-that-helper (generators.cljc:320).
> >> Couldn't satisfy such-that predicate after 100 tries.
> >>
> >>
> >> Thanx
> >
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