Add support for choosing differs

[dchelimsky]

We've had a few issues and pull requests dealing with diffing, but I think we need a more holistic solution. There are a growing number of diffing solutions out there now, each of which are going to appeal more to some people than others, so I think we should have a reasonable default behavior, but then expose an API for choosing your own differ. Something like:

RSpec.configure do |c|
  c.diff {|actual,expected| MyPreferredDefaultDiffer.diff(actual, expected)} # default
  c.diff(Hash)   {|expected,actual| MyPreferredHashDiffer.diff(expected, actual)}
  c.diff(Array)  {|expected,actual| MyPreferredArrayDiffer.diff(expected, actual)}
  c.diff(String) {|expected,actual| MyPreferredStringDiffer.diff(expected, actual)}
  # etc
end

The form with a single argument would invoke the supplied block if both expected and actual values match the argument, otherwise the default would be used.

[dchelimsky]

See #79 for some background info on this.

[myronmarston]

I like this API. For maximum flexibility, I think this should use the === operator to match the argument to expected and actual, rather than something like is_a?.

[dchelimsky]

Agreed.

[dchelimsky]

I'd like to have solid defaults as well. The question is whether to make them hard dependencies (as we do now with diff-lcs) or make it more like if defined?(SomeDiffer), then use it, otherwise ..... Thoughts on that?

[myronmarston]

I think most rspec users (including myself) don't have strong opinions about what differ they want to use, but they do want a decent option that immediately works when they gem install rspec. I've also never heard complaints about the diff-lcs dependency.

Thus, I think it's good to keep diff-lcs (or something similar) as a hard dependency.

[playupchris]

As (another) rspec user I want something that will (predictably) work out of the box.

The issue I was trying to solve with http://github.com/playup/diff_matcher in the first place, was that rspec doesn't have a JSON matcher (or a decent hash matcher that can be used to to perform that match).

diff_matcher ended up (by accident) growing into more than just a hash matcher, but it isn't a replacement for diff-lcs.

And it actually behaves differently to the match_array built in to rspec.

[1,2,3].should   =~ [2,3,1]

(This would pass in match_array, but fail in diff_matcher; which IMHO is the correct behaviour, but to maintain backward compatibility would mean that diff_matcher can't be used as a replacement for match_array.)

We're happy with the diff_matcher gem at our company and use it in rspec with our own custom matcher to match JSON.

Using something like the rspec configuration API above could be a nice compromise, but since JSON is actually a String we'd still need our own custom matcher to match JSON anyway.

[dchelimsky]

@playupchris:

• we can still add be_json_matching (JSON.parse(actual).should eq(expected))
• we can use diff_matcher for hashes and arrays, but still use diff-lcs for strings and other objects
• [1,2,3].should =~ [2,3,1] would still work the way it does now (actual.sort.should eq(expected.sort))