From: David MacMahon <davidm@...>
Date: 2013-08-03T06:03:06+09:00
Subject: [ruby-core:56344] Re: [CommonRuby - Feature #8723] Array.any? predicate returns true for empty array.

Just for fun...

# true regardless of whether [].all? returns true or false
[].all? { |x| predicate(x) } == [].all? { |x| !predicate(x) }

# true iff enum is empty, but false otherwise
enum.all? { |x| predicate(x) } == enum.all? { |x| !predicate(x) }

# Q: If enum.all? is true, can enum.any? be false?
# A: Yes, if enum is empty.  (This is arguably non-intuituve. IMHO.)
if enum.all? { |x| predicate(x) } && !enum.any? { |x| predicate(x) }
  puts "enum is empty"
end

True statement 1: All of my pets are dogs.
True statement 2: None of my pets are dogs.
Conclusion: I have no pets.

True statement 1: All of my pets are dogs.
True statement 2: All of my pets are cats.
Conclusion: I have no pets.

Dave