Question

def show a, &b
yield a
end

show 1 {|x| puts x}

but without &b in the definition, the code works too, so I want to know under what circumstance, the &b is required?

Answer 1

A block is normally an anonymous argument to a method. In most cases, you execute the block right there in the method, using yield. There are two cases where yield is not enough:

• You want to pass block to an another method.

• You want to convert the block to a Proc.

In both of these cases you need to refer your block via name. To do that, you attach one more parameter (last arguament always) and use & as prefix to that parameter.

For first case,

def math(a, b) 
  yield(a, b)
end

def teach_math(a, b, &operation) 
  puts "Let's do the math:"
  puts math(a, b, &operation)
end


teach_math(2, 3) {|x, y| x * y}

# Output
# Let's do the math: 
# 6

---

For second case, (when you need to convert to Proc)

def my_method(&the_proc) 
  the_proc
end

p = my_method {|name| "Hello, #{name}!" } 
puts p.class
puts p.call("Bill")

# Output
# Proc
# Hello, Bill!