Difference between block and &block in Ruby

Question

Why sometimes I should use block and other times &block inside functions that accept blocks?

Answer 1

If you don't set the & before block, Ruby won't recognize it's relationship to the "block" you pass to the function. Here some examples.

def f(x, block); end
f(3) { 2+2 }    # gives an error, because "block" is a
                # regular second argument (which is missing)

def g(x, &block); end
g(3) { 2+2 }    # legal

def h(x); end
h(3) { 2+2 }    # legal

For later use in a function:

def x(&block)   # x is a 0 param function
  y(block)      # y is a 1 param function (taking one "Proc")
  z(&block)     # z is a 0 param function (like x) with the block x received
end

So, if you call z(&block) it's (nearly!!) the same as calling z { yield }: You just pass the block to the next function.