Ruby `when' keyword does not use == in case statement. What does it use?
Case comparisons use ===
rather than ==
. For many objects the behaviour of ===
and ==
is the same, see Numeric
and String
:
5 == 5 #=> true5 === 5 #=> true"hello" == "hello" #=> true"hello" === "hello" #=> true
But for other kinds of object ===
can mean many things, entirely depending on the receiver.
For the case of classes, ===
tests whether an object is an instance of that class:
Class === Class.new #=> true.
For Range it checks whether an object falls in that range:
(5..10) === 6 #=> true
For Procs, ===
actually invokes that Proc
:
multiple_of_10 = proc { |n| (n % 10) == 0 }multiple_of_10 === 20 #=> true (equivalent to multiple_of_10.call(20))
For other objects, check their definition of ===
to uncover their behaviour. It's not always obvious, but they usually make some kind of sense..
Here is an example putting it all together:
case numberwhen 1 puts "One"when 2..9 puts "Between two and nine"when multiple_of_10 puts "A multiple of ten"when String puts "Not a number"end
See this link for more info: http://www.aimred.com/news/developers/2008/08/14/unlocking_the_power_of_case_equality_proc/
In case statement , the comparison is done using ===
operator.
So your code is translated to following:
case xwhen User === x puts "Constant"when "User" === x puts "string"else puts "nothing"end
Different class define ===
in different way:
The Class
class define ===
so that it tests whether the righthand operand (x
)is an instance of the class named by the lefthand operand (User
). So , It is not surprise that User === x
will be evaluated as false
. Instead, User === u
(u = User.new) is true
.
irb(main):001:0> class Userirb(main):002:1> end=> nilirb(main):003:0> u = User.new=> #<User:0xb7a90cd8>irb(main):004:0> User === u.class=> falseirb(main):005:0> User === u=> true