block_test.rb

def test1
  puts (1..10).to_a.map { |a| a.to_s }.join
end

def test2
  puts((1..10).to_a.map { |a| a.to_s }.join)
end

def test3
  puts (1..10).to_a.map do |a|
    a.to_s
  end.join
end

def test4
  puts((1..10).to_a.map do |a|
    a.to_s
  end.join)
end

test1
# => 12345678910

test2
# => 12345678910

test3
# => #<Enumerator:0x000000018fc158>

test4
# => block_test.rb:12:in `test3': undefined method `join' for nil:NilClass (NoMethodError)
#    from block_test.rb:23:in `<main>'