Ruby blocks and required parentheses

ruby_block.rb

def group(symbol, &block)
  block.call
end

symbol = :cheese

# Simple argument-and-block form:
group symbol do
  puts "hello"
end # => "hello"

# Which you'd think could be flattened into this:
group symbol { puts "hello" } # => undefined method `symbol'

# But really you need:
group(symbol) { puts "hello" } # => "hello"