reify associated types when reifying typeclasses

As reported in Trac #10891, Template Haskell's `reify` was not generating Decls
for associated types. This patch fixes that.

Note that even though `reifyTyCon` function used in this patch returns some
type instances, I'm ignoring that.

Here's an example of how associated types are encoded with this patch:

(Simplified representation)

    class C a where
      type F a :: *

    -->

    OpenTypeFamilyD "F" ["a"]

With default type instances:

    class C a where
      type F a :: *
      type F a = a

    -->

    OpenTypeFamilyD "F" ["a"]
    TySynInstD "F" (TySynEqn [VarT "a"] "a")

Reviewed By: goldfire

Differential Revision: https://phabricator.haskell.org/D1254

GHC Trac Issues: #10891

5 files changed, 80 insertions, 2 deletions

diff --git a/compiler/typecheck/TcSplice.hs b/compiler/typecheck/TcSplice.hs
index 2a21705c77..a07d80b9a3 100644
--- a/compiler/typecheck/TcSplice.hs
+++ b/compiler/typecheck/TcSplice.hs
@@ -1202,12 +1202,13 @@ reifyClass cls
   = do  { cxt <- reifyCxt theta
         ; inst_envs <- tcGetInstEnvs
         ; insts <- reifyClassInstances cls (InstEnv.classInstances inst_envs cls)
+        ; assocTys <- concatMapM reifyAT ats
         ; ops <- concatMapM reify_op op_stuff
         ; tvs' <- reifyTyVars tvs
-        ; let dec = TH.ClassD cxt (reifyName cls) tvs' fds' ops
+        ; let dec = TH.ClassD cxt (reifyName cls) tvs' fds' (assocTys ++ ops)
         ; return (TH.ClassI dec insts) }
   where
-    (tvs, fds, theta, _, _, op_stuff) = classExtraBigSig cls
+    (tvs, fds, theta, _, ats, op_stuff) = classExtraBigSig cls
     fds' = map reifyFunDep fds
     reify_op (op, def_meth)
       = do { ty <- reifyType (idType op)
@@ -1219,6 +1220,29 @@ reifyClass cls
                      ; return [TH.SigD nm' ty, TH.DefaultSigD nm' gdm_ty] }
                 _ -> return [TH.SigD nm' ty] }
 
+    reifyAT :: ClassATItem -> TcM [TH.Dec]
+    reifyAT (ATI tycon def) = do
+      tycon' <- reifyTyCon tycon
+      case tycon' of
+        TH.FamilyI dec _ -> do
+          let (tyName, tyArgs) = tfNames dec
+          (dec :) <$> maybe (return [])
+                            (fmap (:[]) . reifyDefImpl tyName tyArgs)
+                            def
+        _ -> pprPanic "reifyAT" (text (show tycon'))
+
+    reifyDefImpl :: TH.Name -> [TH.Name] -> Type -> TcM TH.Dec
+    reifyDefImpl n args ty =
+      TH.TySynInstD n . TH.TySynEqn (map TH.VarT args) <$> reifyType ty
+
+    tfNames :: TH.Dec -> (TH.Name, [TH.Name])
+    tfNames (TH.OpenTypeFamilyD   n args _ _)   = (n, map bndrName args)
+    tfNames d = pprPanic "tfNames" (text (show d))
+
+    bndrName :: TH.TyVarBndr -> TH.Name
+    bndrName (TH.PlainTV n)    = n
+    bndrName (TH.KindedTV n _) = n
+
 ------------------------------
 -- | Annotate (with TH.SigT) a type if the first parameter is True
 -- and if the type contains a free variable.
diff --git a/testsuite/tests/th/T10891.hs b/testsuite/tests/th/T10891.hs
new file mode 100644
index 0000000000..d91caf94f6
--- /dev/null
+++ b/testsuite/tests/th/T10891.hs
@@ -0,0 +1,39 @@
+{-# LANGUAGE TypeFamilies #-}
+
+module T10891 where
+
+import Language.Haskell.TH
+import System.IO
+
+class C a where
+  f :: a -> Int
+
+class C' a where
+  type F a :: *
+  type F a = a
+  f' :: a -> Int
+
+class C'' a where
+  data Fd a :: *
+
+instance C' Int where
+  type F Int = Bool
+  f' = id
+
+instance C'' Int where
+  data Fd Int = B Bool | C Char
+
+$(return [])
+
+test :: ()
+test =
+  $(let
+      display :: Name -> Q ()
+      display q = do
+        i <- reify q
+        runIO (hPutStrLn stderr (pprint i) >> hFlush stderr)
+    in do
+      display ''C
+      display ''C'
+      display ''C''
+      [| () |])
diff --git a/testsuite/tests/th/T10891.stderr b/testsuite/tests/th/T10891.stderr
new file mode 100644
index 0000000000..874f4f0890
--- /dev/null
+++ b/testsuite/tests/th/T10891.stderr
@@ -0,0 +1,12 @@
+class T10891.C (a_0 :: *)
+    where T10891.f :: forall (a_0 :: *) . T10891.C a_0 =>
+                                          a_0 -> GHC.Types.Int
+class T10891.C' (a_0 :: *)
+    where type T10891.F (a_0 :: *) :: *
+          type T10891.F a_0 = a_0
+          T10891.f' :: forall (a_0 :: *) . T10891.C' a_0 =>
+                                           a_0 -> GHC.Types.Int
+instance T10891.C' GHC.Types.Int
+class T10891.C'' (a_0 :: *)
+    where data T10891.Fd (a_0 :: *) :: *
+instance T10891.C'' GHC.Types.Int
diff --git a/testsuite/tests/th/TH_reifyDecl1.stderr b/testsuite/tests/th/TH_reifyDecl1.stderr
index 503f5331f0..e65558774a 100644
--- a/testsuite/tests/th/TH_reifyDecl1.stderr
+++ b/testsuite/tests/th/TH_reifyDecl1.stderr
@@ -20,6 +20,8 @@ class TH_reifyDecl1.C2 (a_0 :: *)
                                                   a_0 -> GHC.Types.Int
 instance TH_reifyDecl1.C2 GHC.Types.Int
 class TH_reifyDecl1.C3 (a_0 :: *)
+    where type TH_reifyDecl1.AT1 (a_0 :: *) :: *
+          data TH_reifyDecl1.AT2 (a_0 :: *) :: *
 instance TH_reifyDecl1.C3 GHC.Types.Int
 type family TH_reifyDecl1.AT1 (a_0 :: *) :: *
 type instance TH_reifyDecl1.AT1 GHC.Types.Int = GHC.Types.Bool
diff --git a/testsuite/tests/th/all.T b/testsuite/tests/th/all.T
index f72cc30f81..9d4736c10f 100644
--- a/testsuite/tests/th/all.T
+++ b/testsuite/tests/th/all.T
@@ -359,3 +359,4 @@ test('T6018th', normal, compile_fail, ['-v0'])
 test('TH_namePackage', normal, compile_and_run, ['-v0'])
 test('T10811', normal, compile, ['-v0'])
 test('T10810', normal, compile, ['-v0'])
+test('T10891', normal, compile, ['-v0'])