Benefits of using `Hash#fetch` over `Hash#[]`

Question

I am not sure in what situation I would want to use Hash#fetch over Hash#[]. Is there a common scenario in where it would be of good use?

Answer 1

Three main uses:

1. When the value is mandatory, i.e. there is no default:

   options.fetch(:repeat).times{...}
   

   You get a nice error message too:

   key not found: :repeat
   

2. When the value can be nil or false and the default is something else:

   if (doit = options.fetch(:repeat, 1))
     doit.times{...}
   else
     # options[:repeat] is set to nil or false, do something else maybe
   end
   

3. When you don't want to use the default/default_proc of a hash:

   options = Hash.new(42)
   options[:foo] || :default # => 42
   options.fetch(:foo, :default) # => :default