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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