fetch-warns-about-superseding-block-argument.md

Fetch Warns About Superseding Block Argument

Ruby's #fetch can be used in a couple ways beyond just grabbing the value out of a hash.

If you include a second argument in the #fetch call, that will be treated as a default value to fallback to when the first argument key doesn't appear in the hash.

If you instead specify a block argument, that block will be executed when the key is missing.

What happens when you specify both a second argument and a block argument?

data = { taco: 'bell' }

data.fetch(:burrito, 'house') do
  puts 'the block gets executed'
  'shack'
end

warning: block supersedes default value argument
the block gets executed
=> 'shack'

The block argument wins. The second argument is ignored. And Ruby warns you that, "block supersedes default value argument".