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The |+| combinator allows the combination of two constraints: c |+| d applies both c and d constraints. Some times you want to have a constraint that checks that a property holds (e.g. there is a registered user with this id), and only if it is the case you want to perform an additional check (e.g. user's name is “Chuck Norris”). The |+| combinator can not express such things. I think you need some kind of Kleisli combinator.