Support wild cards in data/type family instances

Handle anonymous wild cards in type or data family instance
declarations like
unnamed type variables. For instance (pun intented):

    type family F (a :: *) (b :: *) :: *
    type instance F Int _ = Int

Is now the same as:

    type family F (a :: *) (b :: *) :: *
    type instance F Int x = Int

Note that unlike wild cards in partial type signatures, no errors (or
warnings
with -XPartialTypeSignatures) are generated for these wild cards, as
there is
nothing interesting to report to the user, i.e. the inferred kind.

Only anonymous wild cards are supported here, named and
extra-constraints wild
card are not.

Test Plan: pass new tests

Reviewers: goldfire, austin, simonpj, bgamari

Reviewed By: simonpj, bgamari

Subscribers: goldfire, thomie

Differential Revision: https://phabricator.haskell.org/D1092

GHC Trac Issues: #3699, #10586

2 files changed, 41 insertions, 3 deletions

diff --git a/compiler/rename/RnSource.hs b/compiler/rename/RnSource.hs
index 8396e84b25..f6a300763f 100644
--- a/compiler/rename/RnSource.hs
+++ b/compiler/rename/RnSource.hs
@@ -550,12 +550,19 @@ rnFamInstDecl doc mb_cls tycon pats payload rnPayload
 
 
        ; let all_fvs = fvs `addOneFV` unLoc tycon'
+             awcs = concatMap collectAnonymousWildCardNames pats'
        ; return (tycon',
                  HsWB { hswb_cts = pats', hswb_kvs = kv_names,
-                        hswb_tvs = tv_names, hswb_wcs = [] },
+                        hswb_tvs = tv_names, hswb_wcs = awcs },
                  payload',
                  all_fvs) }
              -- type instance => use, hence addOneFV
+  where
+    collectAnonymousWildCardNames ty
+      = [ wildCardName wc
+        | L _ wc <- snd (collectWildCards ty)
+        , isAnonWildCard wc ]
+
 
 rnTyFamInstDecl :: Maybe (Name, [Name])
                 -> TyFamInstDecl RdrName
diff --git a/compiler/typecheck/TcTyClsDecls.hs b/compiler/typecheck/TcTyClsDecls.hs
index 2c18d5e701..3750be8d6b 100644
--- a/compiler/typecheck/TcTyClsDecls.hs
+++ b/compiler/typecheck/TcTyClsDecls.hs
@@ -1009,7 +1009,8 @@ tc_fam_ty_pats :: FamTyConShape
 -- (and, if C is poly-kinded, so will its kind parameter).
 
 tc_fam_ty_pats (name, arity, kind)
-               (HsWB { hswb_cts = arg_pats, hswb_kvs = kvars, hswb_tvs = tvars })
+               (HsWB { hswb_cts = arg_pats, hswb_kvs = kvars
+                     , hswb_tvs = tvars, hswb_wcs = wcs })
                kind_checker
   = do { let (fam_kvs, fam_body) = splitForAllTys kind
 
@@ -1029,8 +1030,11 @@ tc_fam_ty_pats (name, arity, kind)
        ; let (arg_kinds, res_kind)
                  = splitKindFunTysN (length arg_pats) $
                    substKiWith fam_kvs fam_arg_kinds fam_body
+             -- Treat (anonymous) wild cards as type variables without a name.
+             -- See Note [Wild cards in family instances]
+             anon_tvs = [L (nameSrcSpan wc) (UserTyVar wc) | wc <- wcs]
              hs_tvs = HsQTvs { hsq_kvs = kvars
-                             , hsq_tvs = userHsTyVarBndrs loc tvars }
+                             , hsq_tvs = anon_tvs ++ userHsTyVarBndrs loc tvars }
 
          -- Kind-check and quantify
          -- See Note [Quantifying over family patterns]
@@ -1114,6 +1118,33 @@ none. The role of the kind signature (a :: Maybe k) is to add a constraint
 that 'a' must have that kind, and to bring 'k' into scope.
 
 
+Note [Wild cards in family instances]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Wild cards can be used in type/data family instance declarations to indicate
+that the name of a type variable doesn't matter. Each wild card will be
+replaced with a new unique type variable. For instance:
+
+    type family F a b :: *
+    type instance F Int _ = Int
+
+is the same as
+
+    type family F a b :: *
+    type instance F Int b = Int
+
+This is implemented as follows: during renaming anonymous wild cards are given
+freshly generated names. These names are collected after renaming
+(rnFamInstDecl) and used to make new type variables during type checking
+(tc_fam_ty_pats). One should not confuse these wild cards with the ones from
+partial type signatures. The latter generate fresh meta-variables whereas the
+former generate fresh skolems.
+
+Named and extra-constraints wild cards are not supported in type/data family
+instance declarations.
+
+Relevant tickets: #3699 and #10586.
+
 ************************************************************************
 *                                                                      *
                Data types