Barenames and case statements


#1

I’ve encountered this in the past and thought I was messing something
up, but it’s happening still.

Can anyone tell me why this:

puts “ruby-#{RUBY_VERSION} [#{RUBY_PLATFORM}]”

[String, Array, Hash].each do |klass|
case klass
when String
puts “String!”
when Array
puts “Array!”
when Hash
puts “Hash!”
else
puts “I don’t know: #{klass}”
end
end

Produces this:

ruby-1.8.7 [x86_64-linux]
I don’t know: String
I don’t know: Array
I don’t know: Hash

ruby-1.9.1 [x86_64-linux]
I don’t know: String
I don’t know: Array
I don’t know: Hash

Am I missing something here? In order to get the expected result I
have to do this:

[String, Array, Hash].each do |klass|
case klass.to_s
when “String”
puts “String!”
when “Array”
puts “Array!”
when “Hash”
puts “Hash!”
else
puts “I don’t know: #{klass}”
end
end

This works of course, but why doesn’t the former? I find it hard to
believe this has been overlooked. What am I missing?

– Eric W.


#2

Because the === operator of the Class class (used in case statements)
checks for ‘is an instance of’.

So, the following:

case klass
when String
puts “String!”
end

is to be interpreted as String===klass, that is to,
klass.is_a?(String), which is false. But the following is true:

case klass
when Class
puts ‘It’s a class of course’
end

Hope it helps
blambeau


#3

On 07.04.2009 17:21, LAMBEAU Bernard wrote:

is to be interpreted as String===klass, that is to,
klass.is_a?(String), which is false. But the following is true:

case klass
when Class
puts ‘It’s a class of course’
end

A solution would be to use the other form of “case”:

[String, Array, Hash].each do |klass|
case
when klass == String
puts “String!”
when klass == Array
puts “Array!”
when klass == Hash
puts “Hash!”
else
puts “I don’t know: #{klass}”
end
end

Or encapsulate tests in a special instance:

IsString = Object.new
def IsString.===©
String == c
end

klass = String

case klass
when IsString
puts “Ah!”
end

Kind regards

robert


#4

On Tue, Apr 7, 2009 at 11:21 AM, LAMBEAU Bernard removed_email_address@domain.invalid
wrote:

Because the === operator of the Class class (used in case statements)
checks for ‘is an instance of’.

I understand. Definitely not what I’d expect. So my .to_s is the only
way to reasonably accomplish what I need to do? Overriding these
things would probably not be a good idea.

Hope it helps
blambeau

Thanks,

– Eric W.


#5

On Wed, Apr 8, 2009 at 12:43 AM, Eric W. removed_email_address@domain.invalid wrote:

I think the .to_s works just as well, but thanks.

try also,
case klass.new
when String


#6

On Tue, Apr 7, 2009 at 12:04 PM, Robert K.
removed_email_address@domain.invalid wrote:

A solution would be to use the other form of “case”:
Or encapsulate tests in a special instance:

I think the .to_s works just as well, but thanks.

Kind regards

   robert

– Eric W.


#7

class Object
def singleton_class
class << self
self
end
end
end

[String, Array, Hash].each do |klass|
case klass
when String.singleton_class
puts “String!”
when Array.singleton_class
puts “Array!”
when Hash.singleton_class
puts “Hash!”
else
puts “I don’t know: #{klass}”
end
end

Hrm it would be nice to have some kind of voting system for feature
requests
http://redmine.ruby-lang.org/issues/show/1082