For Opa, we used a variant on the technique. We had a rather complex Ast
that was progressively refined through a number of passes.

Each pass could remove or add cases, and we wanted to have type
guarantees that ast nodes did not escape their scope.

To do this, we used something along the lines of

type 'variant Expr =
   | Let of ...
   | ...
   | Variant of 'variant
and 'variant Foo =
   | ...


In which ['variant] was instantiated with a polymorphic variant. This
gave us considerable flexibility and generally avoided the problem of
over-complicated error messages.

You can find the full source code at
https://github.com/MLstate/opalang/blob/master/compiler/opalang/surfaceAst.ml
.

Cheers,
 David