Visible dependent quantification

This implements GHC proposal 35
(https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0035-forall-arrow.rst)
by adding the ability to write kinds with
visible dependent quantification (VDQ).

Most of the work for supporting VDQ was actually done _before_ this
patch. That is, GHC has been able to reason about kinds with VDQ for
some time, but it lacked the ability to let programmers directly
write these kinds in the source syntax. This patch is primarly about
exposing this ability, by:

* Changing `HsForAllTy` to add an additional field of type
  `ForallVisFlag` to distinguish between invisible `forall`s (i.e,
  with dots) and visible `forall`s (i.e., with arrows)
* Changing `Parser.y` accordingly

The rest of the patch mostly concerns adding validity checking to
ensure that VDQ is never used in the type of a term (as permitting
this would require full-spectrum dependent types). This is
accomplished by:

* Adding a `vdqAllowed` predicate to `TcValidity`.
* Introducing `splitLHsSigmaTyInvis`, a variant of `splitLHsSigmaTy`
  that only splits invisible `forall`s. This function is used in
  certain places (e.g., in instance declarations) to ensure that GHC
  doesn't try to split visible `forall`s (e.g., if it tried splitting
  `instance forall a -> Show (Blah a)`, then GHC would mistakenly
  allow that declaration!)

This also updates Template Haskell by introducing a new `ForallVisT`
constructor to `Type`.

Fixes #16326. Also fixes #15658 by documenting this feature in the
users' guide.

1 files changed, 9 insertions, 6 deletions

diff --git a/compiler/rename/RnTypes.hs b/compiler/rename/RnTypes.hs
index 499fd74bd9..b84bbe3bae 100644
--- a/compiler/rename/RnTypes.hs
+++ b/compiler/rename/RnTypes.hs
@@ -170,11 +170,13 @@ rnWcBody ctxt nwc_rdrs hs_ty
 
     rn_ty :: RnTyKiEnv -> HsType GhcPs -> RnM (HsType GhcRn, FreeVars)
     -- A lot of faff just to allow the extra-constraints wildcard to appear
-    rn_ty env hs_ty@(HsForAllTy { hst_bndrs = tvs, hst_body = hs_body })
+    rn_ty env hs_ty@(HsForAllTy { hst_fvf = fvf, hst_bndrs = tvs
+                                , hst_body = hs_body })
       = bindLHsTyVarBndrs (rtke_ctxt env) (Just $ inTypeDoc hs_ty) Nothing tvs $ \ tvs' ->
         do { (hs_body', fvs) <- rn_lty env hs_body
-           ; return (HsForAllTy { hst_xforall = noExt, hst_bndrs = tvs'
-                                , hst_body = hs_body' }, fvs) }
+           ; return (HsForAllTy { hst_fvf = fvf, hst_xforall = noExt
+                                , hst_bndrs = tvs', hst_body = hs_body' }
+                    , fvs) }
 
     rn_ty env (HsQualTy { hst_ctxt = dL->L cx hs_ctxt
                         , hst_body = hs_ty })
@@ -479,13 +481,14 @@ rnLHsTyKi env (dL->L loc ty)
 
 rnHsTyKi :: RnTyKiEnv -> HsType GhcPs -> RnM (HsType GhcRn, FreeVars)
 
-rnHsTyKi env ty@(HsForAllTy { hst_bndrs = tyvars, hst_body  = tau })
+rnHsTyKi env ty@(HsForAllTy { hst_fvf = fvf, hst_bndrs = tyvars
+                            , hst_body = tau })
   = do { checkPolyKinds env ty
        ; bindLHsTyVarBndrs (rtke_ctxt env) (Just $ inTypeDoc ty)
                            Nothing tyvars $ \ tyvars' ->
     do { (tau',  fvs) <- rnLHsTyKi env tau
-       ; return ( HsForAllTy { hst_xforall = noExt, hst_bndrs = tyvars'
-                             , hst_body =  tau' }
+       ; return ( HsForAllTy { hst_fvf = fvf, hst_xforall = noExt
+                             , hst_bndrs = tyvars' , hst_body =  tau' }
                 , fvs) } }
 
 rnHsTyKi env ty@(HsQualTy { hst_ctxt = lctxt, hst_body = tau })