From: "Aaron D. Gifford" <astounding@...>
Date: 2011-01-27T09:10:06+09:00
Subject: Re: Problem: use case stmt on object's class in simple test

user@host:/path$ irb
irb(main):001:0> a = "Foo"
=> "Foo"
irb(main):002:0> a.class === String
=> false
irb(main):003:0> a === String
=> false
irb(main):004:0> a.class === String.class
=> false
irb(main):005:0> a
=> "Foo"
irb(main):006:0> a.inspect
=> "\"Foo\""
irb(main):007:0> String.inspect
=> "String"
irb(main):008:0> case a
irb(main):009:1> when String
irb(main):010:1> puts 'string'
irb(main):011:1> else
irb(main):012:1* puts 'not'
irb(main):013:1> end
string
=> nil
irb(main):014:0> String === a
=> true
irb(main):015:0> a === String
=> false
irb(main):016:0> root@secure:/usr/ports# irb
irb(main):001:0> str = "a string"
=> "a string"
irb(main):002:0> str.class
=> String
irb(main):003:0> String === str.class
=> false
irb(main):004:0> String === str
=> true
irb(main):005:0> case str.class
irb(main):006:1> when String
irb(main):007:1>   puts 'it is a string'
irb(main):008:1> else
irb(main):009:1*   puts 'else executed'
irb(main):010:1> end
else executed
=> nil
irb(main):011:0> case str
irb(main):012:1> when String
irb(main):013:1>   puts 'it is a string'
irb(main):014:1> else
irb(main):015:1*   puts 'else executed'
irb(main):016:1> end
it is a string
=> nil
irb(main):017:0>

>  Why isn't the "when String" statement executed?

I believe case/when uses the when object's === method for matching,
and as you can see from the above irb session, String === "a
string".class evaluates to false.  Hence your code doesn't work like
you expected.

However, if you change to hash[key] instead of hash[key].class, a
direct comparison to the class object should work (note the String ===
"a string" evaluates to true).

Note that "a string" === String evaluates to false even though "a
string" === String evaluates to true.  That way case String when "a
string" won't match but case "a string" when String will.  (Which
totally makes sense in the context of case/when.)

I'm sure I read a great blog post or discussion on this very thing in
the past on the web, one that explains the gory details about exactly
why case/when was designed to work this way.  It currently slips my
mind.

Anyone got a good URL for an article/post like that?

Aaron out.