• eir@cis.upenn.edu's avatar
    Add kind equalities to GHC. · 67465497
    eir@cis.upenn.edu authored
    This implements the ideas originally put forward in
    "System FC with Explicit Kind Equality" (ICFP'13).
    There are several noteworthy changes with this patch:
     * We now have casts in types. These change the kind
       of a type. See new constructor `CastTy`.
     * All types and all constructors can be promoted.
       This includes GADT constructors. GADT pattern matches
       take place in type family equations. In Core,
       types can now be applied to coercions via the
       `CoercionTy` constructor.
     * Coercions can now be heterogeneous, relating types
       of different kinds. A coercion proving `t1 :: k1 ~ t2 :: k2`
       proves both that `t1` and `t2` are the same and also that
       `k1` and `k2` are the same.
     * The `Coercion` type has been significantly enhanced.
       The documentation in `docs/core-spec/core-spec.pdf` reflects
       the new reality.
     * The type of `*` is now `*`. No more `BOX`.
     * Users can write explicit kind variables in their code,
       anywhere they can write type variables. ...