Provides framework for "phased" data. This is a generalization of the "Higher-kinded data" approach where instead of merely annotating a data type with some constructor e.g.

data Foo f = MkFoo
  { bar :: f Bool,
    baz :: f String,
    ...
  }

we use a type family:

data Foo s = MkFoo
  { bar :: Fam1 s,
    baz :: Fam2 s,
  ...
  }

Not only does this allow fields to vary independently, it also means we can avoid clunky wrappers like Identity.

We do this so that we can represent data that "evolves" e.g. we may initially parse a data type as Foo1 (phase 1) but then further process this data type into Foo2 (phase 2). Using this phase approach avoids duplication of constructors and fields.