@a8m any thoughts on this or assistance on how i should fix the tests?

Hey @ashtonian!
Thanks for your contribution, and sorry for the delayed response. I just returned back from a vacation.

any thoughts on this or assistance on how i should fix the tests?

I would be happy to clone this branch and check the test failures. Also, I would like to see integration tests as well (see the rql/integration directory).

I have a different suggestion for how the ValidateFn/ConvertFn will deal with slice of params, and I would like to know what do you think about it.

func (p *parseState) field(f *field, v interface{}) {
  // ...
  fn := f.ValidateFn
  if op == NIN || op == IN {
    fn = validateSlice(fn)
  }
  must(fn(opVal), "invalid datatype for field %q", f.Name)
  // code continues the same
  // ...
}

func validateSlice(fn ValidateFn) ValidateFn {
  return func(v interface{}) error {
     vs, ok := v.([]interface{})
     // handle error.
     for _, v := range vs {
       if err := fn(v); err != nil {
         return err
       }   
     }
   }
}

Just to share - I didn't know that the $nin operator exists in MongoDB, and at the start, I thought to implement the NOT IN operation with the "$not": { "$in": <val> } syntax.

This led me to discover that the $not operator is missing and it should be added.

I was hoping there was a better place to tack it in but didn't see it, looks like that'd be much cleaner ha. I'll update shortly and add some integration tests. Thank you for the response!

closing in favor of new pr