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I'm trying to write a module that implements algorithms on another module that may be implemented in various ways. So my thinking was to write the first module as module type Base = sig type t val f : t -> t end Then I write a second module that is parametrised over a module compatible with Base : module type BasedOnBase = functor (B : Base) -> sig type b val g : B.t -> b end Now I'm trying to write a module that is parametrised over a module that is compatible with BasedOnBase and this is where I'm stuck. My naive approach doesn't work, I've tried (* won't compile *) module type Alg = functor (BoB : BasedOnBase) -> sig val h : BoB.b -> bool end as well as (* won't compile *) module type Alg = functor (BoB : functor (B : Base) -> BasedOnBase) -> sig val h : BoB.b -> bool end but both attempts cause this error: [...] Error: Unbound type constructor BoB.b So I'm obviously missing something here but I can't seem to be able to get my head around the problem. How would I go about achieving what I want, probably in a completely different way?

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