If it's not been mentioned, this tutorial regarding Type Families and the kinds of restrictions you mention might provide a few pointers: https://www.fpcomplete.com/school/to-infinity-and-beyond/pick-of-the-week/ty... In general, I've found key concepts to hold in mind are 'exclusion of bad programs' mentioned in the paper Fun With Types: http://research.microsoft.com/en-us/um/people/simonpj/papers/assoc-types/fun... I'm sorry if any of this has been mentioned previous. I asked a related question yesterday about the relative merits of creating type restrictions using using newtypes (unions), which I agree seem unwieldy, and using multi-parameter-type-classes (and/or type families) to enable exclusion-of-bad-programms via instances. I'd be interested to follow along and see what you discover. Simon Yarde