Add a Comparable helper

[mvdan]

I was just thinking that most of the types one might want to diff can already be compared for equality, as per the language spec.

So, we could add func Comparable(a, b interface{}) DiffWriteable. It would ensure that both params are of the same type, that both are slice types, and that both element types are comparable.

For example, this would be useful with integers, arrays, custom struct types, et cetera.

I'd still keep Strings, because for the foreseeable future, avoiding interface parameters will make the code faster and easier.

Potentially, we could also use the Equal method as defined in https://github.com/google/go-cmp.

[josharian]

I have had a similar idea. Quoting from diff.go:

// TODO: consider adding an "it just works" test helper that accepts two slices (via interface{}),
// diffs them using Strings or Bytes or Slices (using reflect.DeepEqual) as appropriate,
// and calls t.Errorf with a generated diff if they're not equal.

My only hesitation is that it expand the API, and the API already feels kinda large. Let's leave this one on the back burner for a bit until someone has a concrete need for it (I don't yet), since it is easy to add and hard to remove.

[mvdan]

Sure!