Ruby erb templates with yield

Question

I can't understand why this code works fine

def func
  ERB.new('<%= yield %>').result(binding)
end
func { 123 } # => it prints 123 as expected

but this one doesn't work and raises an exception

ERB.new('<%= yield %>').result(binding) { 123 } # => LocalJumpError: no block given (yield)

Any thoughts?

Answer 1

This problem is independent of ERB and is because of the way yield works. Yield expects to be called within the message body and expects a block to yield it. Let's take this example

# This is equivalent to 
# def func
# ERB.new('<%= yield %>').result(binding)
# end

def test_print
  yield
end

If we call the method without a block

irb(main):038:0> test_print
LocalJumpError: no block given (yield)
    from (irb):36:in `test_print'
    from (irb):38
    from /Users/agupta/.rvm/rubies/ruby-2.4.0/bin/irb:11:in `<main>'
irb(main):039:0>

If we call the method with block

irb(main):039:0> test_print { "hello world" }
=> "hello world"
irb(main):040:0>

In the latter case

ERB.new('<%= yield %>').result(binding) { 123 } 

Your block is not being passed as yield is outside the message body and you cannot do

irb(main):042:0> yield.tap { "hello world" }
LocalJumpError: no block given (yield)
    from (irb):42
    from /Users/agupta/.rvm/rubies/ruby-2.4.0/bin/irb:11:in `<main>'
irb(main):043:0>

Answer 2

You should pass a block to the method context, where is binding called, e.g.:

def foo
  binding
end

ERB.new('<%= yield %>').result(foo { 123 })
#=> "123"

Note, that you cannot use yield outside of a method body. ERB#result just executes ruby code in context of passed binding, so binding in your should be inside the method anyway, because of yield.