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comp.lang.ruby

barenames and case statements

Eric Will

4/7/2009 3:10:00 PM

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 Will
6 Answers

LAMBEAU Bernard

4/7/2009 3:21:00 PM

0

Because the =3D=3D=3D 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=3D=3D=3Dklass, 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

On Tue, Apr 7, 2009 at 5:10 PM, Eric Will <rakaur@malkier.net> wrote:
> 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|
> =A0 =A0case klass
> =A0 =A0when String
> =A0 =A0 =A0 =A0puts "String!"
> =A0 =A0when Array
> =A0 =A0 =A0 =A0puts "Array!"
> =A0 =A0when Hash
> =A0 =A0 =A0 =A0puts "Hash!"
> =A0 =A0else
> =A0 =A0 =A0 =A0puts "I don't know: #{klass}"
> =A0 =A0end
> 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|
> =A0 =A0case klass.to_s
> =A0 =A0when "String"
> =A0 =A0 =A0 =A0puts "String!"
> =A0 =A0when "Array"
> =A0 =A0 =A0 =A0puts "Array!"
> =A0 =A0when "Hash"
> =A0 =A0 =A0 =A0puts "Hash!"
> =A0 =A0else
> =A0 =A0 =A0 =A0puts "I don't know: #{klass}"
> =A0 =A0end
> 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 Will
>
>

Robert Klemme

4/7/2009 3:59:00 PM

0

On 07.04.2009 17:21, LAMBEAU Bernard wrote:
> 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

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.===(c)
String == c
end

klass = String

case klass
when IsString
puts "Ah!"
end


Kind regards

robert

Eric Will

4/7/2009 4:26:00 PM

0

On Tue, Apr 7, 2009 at 11:21 AM, LAMBEAU Bernard <blambeau@gmail.com> 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 Will

Eric Will

4/7/2009 4:44:00 PM

0

On Tue, Apr 7, 2009 at 12:04 PM, Robert Klemme
<shortcutter@googlemail.com> 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
>
> =A0 =A0 =A0 =A0robert

-- Eric Will

botp

4/7/2009 4:52:00 PM

0

On Wed, Apr 8, 2009 at 12:43 AM, Eric Will <rakaur@malkier.net> wrote:
> I think the .to_s works just as well, but thanks.

try also,
case klass.new
when String
...

The Higgs bozo

4/7/2009 6:08:00 PM

0


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...
--
Posted via http://www.ruby-....