Using a class object in case statement

The problem with using something like this:

case a.to_s
when "String" then ...
when "Fixnum" then ...
end

is that it completely misses subclasses so you can get something that is a String but is missed by your first branch. Also, name would be a better choice than to_s since semantically, you're testing the class's name rather than its string representation; the result may be the same but case a.name would be clearer.

If you want to use a case and deal with subclassing then you could use Module#<= like this:

case
when a <= String then ...
when a <= Fixnum then ...
end

Yes, you have to repeat a in each when but that's just how case works.

I wouldn't use to_s, because "String".to_s would be "String", so maybe I'd do

case
when a == String then ...
when a == Fixnum then ...
end

or

a = String

case [a]
when [String] then puts "String"
when [Array] then puts "Array"
end