Block forwarding

To forward captured blocks, you use a block argument, prefixing an expression with &:

def capture(&block)
  block
end

def invoke(&block)
  block.call
end

proc = capture { puts "Hello" }
invoke(&proc) # prints "Hello"

In the above example, invoke receives a block. We can't pass proc directly to it because invoke doesn't receive regular arguments, just a block argument. We use & to specify that we really want to pass proc as the block argument. Otherwise:

invoke(proc) # Error: wrong number of arguments for 'invoke' (1 for 0)

You can actually pass a proc to a method that yields:

def capture(&block)
  block
end

def twice(&)
  yield
  yield
end

proc = capture { puts "Hello" }
twice &proc

The above is simply rewritten to:

proc = capture { puts "Hello" }
twice do
  proc.call
end

Or, combining the & and -> syntaxes:

twice &-> { puts "Hello" }

Or:

def say_hello
  puts "Hello"
end

twice &->say_hello

Forwarding non-captured blocks

To forward non-captured blocks, you must use yield:

def foo(&)
  yield 1
end

def wrap_foo(&)
  puts "Before foo"
  foo do |x|
    yield x
  end
  puts "After foo"
end

wrap_foo do |i|
  puts i
end

# Output:
# Before foo
# 1
# After foo

You can also use the &block syntax to forward blocks, but then you have to at least specify the input types, and the generated code will involve closures and will be slower:

def foo(&)
  yield 1
end

def wrap_foo(&block : Int32 -> _)
  puts "Before foo"
  foo(&block)
  puts "After foo"
end

wrap_foo do |i|
  puts i
end

# Output:
# Before foo
# 1
# After foo

Try to avoid forwarding blocks like this if doing yield is enough. There's also the issue that break and next are not allowed inside captured blocks, so the following won't work when using &block forwarding:

foo_forward do |i|
  break # error
end

In short, avoid &block forwarding when yield is involved.