Custom RSPec matcher for detailed hash compasion and diff

detailed_hash_diff.rb

RSpec::Matchers.define :be_a_hash_like do |expected_hash|
  match do |actual_hash|
    matching_results = actual_hash == expected_hash
    unless matching_results
      system(
        "git --no-pager diff $(echo '#{JSON.pretty_generate(expected_hash)}' | git hash-object -w --stdin) " +
                 "$(echo '#{JSON.pretty_generate(actual_hash)}' | git hash-object -w --stdin) --word-diff",
        out: $stdout,
        err: :out
      )
    end
    matching_results
  end

  failure_message { 'Look at the Diff above! ^^^' }
end

# then use:
expect(actual).to be_a_hash_like(expected)

it produces this nice output:

That's really clever. I didn't know you could trick git diff into being used for general purpose diffing like that!

If anyone is interested, I made a version that:

1. Sorts hashes before comparing so that the diff only shows actual changes between keys and values (and not diffs due to accidental differences in key order, which is almost never significant in Ruby).

2. Uses diff-so-fancy if available. While this doesn't give you a word diff, it gives a really clean line diff (using colors instead of +/- symbols). I wish I could find a way to get a word diff like that. I find the [- -]{+ +} distracting.