[FIXED] Why do I sometimes need to prefix proc with & when passing it as an arg?

Issue

I'm learning Ruby and have been told that, to quote Codecademy:

When passing a proc to a method, an & is used to convert the proc into a block.

Hence the following works:

cube = Proc.new do |num| num ** 3 end
puts [1, 2, 3].map!(&cube) #errors without &

But if I pass a proc to a user-defined function, it works only without the & and errors with it.

foo = Proc.new do puts 'foo' end
def bar(cb)
  cb.call
end
bar(foo) #bar(&foo) results in error

So how should I understand the whole with-& and without-& difference? Is it to do with inbuilt methods (map(), in my case) vs. user-defined methods?

Solution

If I pass a proc to a user-defined function, it works only without the & and errors with it.

That's because in your example cb is a positional argument. To declared it as a block argument, you need to prefix it with &:

def bar(&cb)
  cb.call
end

foo = Proc.new { puts 'foo' }
bar(&foo) 

Or simply:

def bar
  yield
end

foo = Proc.new { puts 'foo' }
bar(&foo)

Having an actual block argument also allows you to skip the proc creation and call it as:

bar { puts 'foo' }

# or

bar do
  puts 'foo'
end

So how should I understand the whole with-& and without-& difference?

There are 3 kinds of method arguments in Ruby:

• positional arguments, e.g. foo(1, 2, 3)
• keyword arguments, e.g. foo(a: 1, b: 2, c: 3)
• block arguments, e.g. foo { puts "hello" }

Since procs are objects, you can pass them as positional or keyword arguments, just like any other object.

But if you want a proc to become a block argument, you need & to convert it.

Answered By - Stefan

Answer Checked By - Senaida (PHPFixing Volunteer)