Question

I’m puzzled over the following code:

Proc.new do |a|
    a.something "test"

    puts a.something
    puts "hello"
end

It doesn’t throw any errors when it runs. However nothing is printed for either puts statement. I’m curious about the a.something “assignment”. Perhaps this is a method call w/ parens omitted. What is happening in the above code?

Answer 1

Proc.new ...             # create a new proc

Proc.new{ |a| ... }      # a new proc that takes a single param and names it "a"

Proc.new do |a| ... end  # same thing, different syntax

Proc.new do |a|
  a.something "test"     # invoke "something" method on "a", passing a string
  puts a.something       # invoke the "something" method on "a" with no params
                         # and then output the result as a string (call to_s)
  puts "hello"           # output a string
end

Since the last expression in the proc is puts, which always returns nil, the return value of the proc if it is ever invoked will be nil.

irb(main):001:0> do_it = Proc.new{ |a| a.say_hi; 42 }
#=> #<Proc:0x2d756f0@(irb):1>

irb(main):002:0> class Person
irb(main):003:1>   def say_hi
irb(main):004:2>     puts "hi!"
irb(main):005:2>   end
irb(main):006:1> end

irb(main):007:0> bob = Person.new
#=> #<Person:0x2c1c168>

irb(main):008:0> do_it.call(bob)  # invoke the proc, passing in bob
hi!
#=> 42                            # return value of the proc is 42

irb(main):009:0> do_it[bob]       # alternative syntax for invocation
hi!
#=> 42