Here’s a simple example of recursive types interacting badly with subtyping: T={ foo: T -> A} U={ foo: U -> B} Consider T <: U, therefore (T -> A) <: (U -> B) Which implies: U <: T So T <: U but also U <: T, which is true iff A <: B and B <: A.

In my case, the subtyping relation is polymorphic subsumption: T is subsumed by U iff U is “more polymorphic”, intuitively, it can be instantiated to all types that T can.

This situation arises in rather simple code, involving polymorphic vs. non-polymorphic row fields. For example, A is a row with a polymorphic method, whereas B is a row with a monomorphic (but compatible) method, such as: A = { method: forall a. a -> () } B = { method: String -> () } In this case subsumption (the form of subtyping in play) fails.

One way around this is to avoid subsumption issues altogether by keeping things rank-1, and not using higher-rank row fields. Unfortunately, throwing away polymorphic methods is very bad: consider a non-polymorphic array.map (in JS).

A slightly better workaround is to push the foralls all the way out, keeping all types (including row types) rank-1. Every time an object method is accessed via the property syntax obj.method, we end up instantiating the object’s row type, and get a “fresh” type for the method. We get practically polymorphic methods. That’s the approach I’m investigating for infernu.