From: Ralph Shnelvar <ralphs@...32.com>
Date: 2010-08-21T13:06:10+09:00
Subject: Re: case and class

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Colin,


Friday, August 20, 2010, 8:05:36 PM, you wrote:

CB> On Sat, Aug 21, 2010 at 12:13 AM, Ralph Shnelvar <ralphs@dos32.com> wrote:


>> 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?

CB> No (I think the following is right, but I'd welcome any corrections) - case
CB> always uses "===", it's just that the meaning of "===" may be different to
CB> "==".

CB> 1. case always uses "===" for comparisons; in
CB>   case x
CB>   when y then
CB> the test is y === x

CB> http://ruby-doc.org/docs/ProgrammingRuby/html/tut_expressions.html

CB> Common comparison operators, Operator Meaning
CB> == Test for equal value.
CB> === Used to test equality within a when clause of a case statement.

CB> case operates by comparing the target (the expression after the keyword
CB> case) with each of the comparison expressions after the when keywords. This
CB> test is done using comparison === target. As long as a class defines
CB> meaningful semantics for === (and all the built-in classes do), objects of
CB> that class can be used in case expressions.
CB> ...
CB> Ruby classes are instances of class Class, which defines === as a test to
CB> see if the argument is an instance of the class or one of its superclasses.

CB> 2. http://www.ruby-doc.org/core/classes/Object.html#M000345
obj === other =>> true or false
CB> Case Equality�For class Object, effectively the same as calling #==, but
CB> typically overridden by descendents to provide meaningful semantics in case
CB> statements.

CB> 3. http://www.ruby-doc.org/core/classes/Module.html#M001666
CB> (remembering that Class is a subclass of Module, which I forget when I
CB> initially couldn't find the "===" method in Class)
mod === obj =>> true or false
CB> Case Equality�Returns true if anObject is an instance of mod or one of mod�s
CB> descendents. Of limited use for modules, but can be used in case statements
CB> to classify objects by class.

CB> Putting that all together, and using the String class as an example:
CB> x = "Karel Capek"
CB> y = "Karel Capek"
p x.object_id #=>> 1880688
p y.object_id #=>> 1880656, so x & y are different objects
p x == y #=>> true
p x === y #=>> true, so for String objects "===" is the same as "=="
CB>             # at least for comparing String objects with String objects

p x == String #=>> false
p x === String #=>> false

CB> p x.class == String #=> true
CB> p String == String #=> true
CB> p x.class === String #=> false
CB> p String === String #=> false

p String == x #=>> false
p String === x #=>> true; this is what is being used in the case statement
CB>                  # case x; when String then true; else false; end
CB> p String == x.class #=> true
CB> p String === x.class #=> false; same as String === String

What a great answer.   And, again, clear.

I can, reluctantly, see the use of === in an if statement.

To me,
  case x
  when XXX then ...
  end

would be MUCH clearer as
  case x.class
  when XXX.arrayOfDecendants
  end

Of course, the code above is illegal ... but would have been clearer ... at least to me.
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