Can procs be used with case statements in Ruby 2.0?
I remember something about procs being allowed in case statements in Ruby 2.0, but I can't google it.
I tried checking Ruby 2.0.0 NEWS and How to write a switch statement in Ruby. I also visited http://ruby-doc.org , but the link it had for keywords was for Ruby 1.9, not Ruby 2.0.
Are procs allowed in case statements?
Yes.
2.0.0p0 :001> lamb = ->(x){ x%2==1 }
#=> #<Proc:0x007fdd6a97dd90@(irb):1 (lambda)>
2.0.0p0 :002> case 3; when lamb then p(:yay); end
:yay
#=> :yay
2.0.0p0 :003> lamb === 3
#=> true
2.0.0p0 :007> lamb === 2
#=> false
However, this is no different than 1.9.1 since Proc#=== was defined back then. Since ruby-docs seems to have a problem showing this method, to be clear the documentation says that proc === obj :
Invokes the block with obj as the proc's parameter like #call . It is to allow a proc object to be a target of when clause in a case statement.
For the Ruby beginner, then when clause in Ruby's case statements takes the value in the clause and calls the === method on it, passing in the argument to the case statement. So, for example, this code…
case "cats"
when /^cat/ then puts("line starts with cat!")
when /^dog/ then puts("line starts with dog!")
end
…runs /^cat/ === "cats" to decide if it's a match; the RegExp class defines the === method to perform regex matching. Thus, you can use your own object in a when clause as long as you define === for it.
Moddable = Struct.new(:n) do
def ===(numeric)
numeric % n == 0
end
end
mod4 = Moddable.new(4)
mod3 = Moddable.new(3)
12.times do |i|
case i
when mod4
puts "#{i} is a multiple of 4!"
when mod3
puts "#{i} is a multiple of 3!"
end
end
#=> 0 is a multiple of 4!
#=> 3 is a multiple of 3!
#=> 4 is a multiple of 4!
#=> 6 is a multiple of 3!
#=> 8 is a multiple of 4!
#=> 9 is a multiple of 3!