Our app uses a list of bird names.

bird_names = %w(
  redbreast
  crow
  eagle
  hedge-sparrow
  sparrow
  pigeon
  owl
  penguin
)

One day, someone mentions that a couple of the birds – the redbreast and hedge-sparrow – are now more commonly known as the robin and dunnock. Not wanting to confuse our users, we decide we should use the modern vernacular.

Instead of going back and changing the data we can create a new list of bird names – updating any names that have changed with their modern equivalents.

We pair the old with the new names in a hash.

bird_names_and_modern_equivalents = {
  "redbreast" => "robin",
  "hedge-sparrow" => "dunnock"
}

We treat the hash as a proc.

> bird_names.map &bird_names_and_modern_equivalents

The result isn't quite what we want. We have the new names for the redbreast and the dunnock but all the other birds are now nil.

=> ["robin", nil, nil, "dunnock", nil, nil, nil, nil]

We want our list to contain the original names if there are no substitions in our hash.

Just as we can set a default hash value, we can also set a default proc.

Below, if we ask the hash for the value of a key that doesn't exist, it returns the value of the default proc instead. In this case, it returns the name of the bird we asked for unchanged.

bird_names_and_modern_equivalents.default_proc = Proc.new { |_h, k| k }

> bird_names.map &bird_names_and_modern_equivalents
=> ["robin", "crow", "eagle", "dunnock", "sparrow", "pigeon", "owl", "penguin"]