From: Ralph Shnelvar <ralphs@...32.com>
Date: 2010-08-21T08:13:22+09:00
Subject: Re: case and class

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Colin,

Friday, August 20, 2010, 4:29:37 PM, you wrote:

CB> On Fri, Aug 20, 2010 at 10:51 PM, Ralph Shnelvar <ralphs@dos32.com> wrote:

>> Why does "YYY" get printed out here?

>> - - -

>> class XXX
>> end

>> x = XXX.new

>> case x.class
>> when XXX
>>  puts "XXX"
>> else
>>  puts "YYY"   # surprise!
>> end

CB> It's because of the definition of "===" which is used in the case statement.
CB> Does the following help?

CB> class XXX
CB> end

CB> x = XXX.new

CB> case x.class
CB> when XXX then puts "XXX"
CB> else puts "YYY" # surprise!
end #=>> YYY

CB> case x
CB> when XXX then puts "XXX 2"
CB> else puts "YYY 2"
end #=>> XXX 2

CB> case x.class
CB> when Class then puts "XXX 3"
CB> else puts "YYY 3"
end #=>> XXX 3


Colin, thank you.   Very clear.

Now it's my turn to complain about the unexpected behavior.

Having read your explanation and Dave Thomas' *Programming Ruby 1.9* book about the use of === in comparisons in case statements ...

Is it he fact that *case* uses == (two ='s) for non-Class *when's* and uses === (three ='s) for *when's* that are Classes?

It just seems to me to be an unnecessary surprise to do things differently for objects of class Class.

Indeed
  x.class == XXX  # => true

X.class _is_ XXX .... and yet the *when* does not pick it up because === is being used for comparison.

Anyway, thank for your lucid and well-written response.
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