Use an existential type, usually called Some in Haskell: https://hackage.haskell.org/package/some-1.0.5/docs/Data-Som... The implementation of this type one has in their head (a GADT) adds a boxing overhead, but the actual implementation in this library uses a newtype.
That way if you have a type Expr a, you can have a list type [Some Expr].
That way if you have a type Expr a, you can have a list type [Some Expr].