Why are parenthesis sometimes required in Ruby?

Question

I recently ran into an oddity while looking at some Ruby code from the Rails docs.

Ruby lets you pass arguments like these examples:

redirect_to post_url         


        

Answer 1

Curly braces serve double-duty in Ruby. They can delimit a hash, but they can also delimit a block. In this case, I believe your second example is getting parsed as:

redirect_to do
  action: 'atom' 
end, alert: "Something serious happened"

Therefore your action: 'atom', which is not valid as an independent expression, gets parsed as such.

The parentheses serve to disambiguate the {...} as a hash.