Improve comments on coreView/tcView, and combine coreExpandTyCon/tcExpandTyCon

This is minor stuff triggered by Trac #10103.

* Fix outdated comments on tcView/coreView (we should really combine
  them with a new name, but I'll leave that slightly-disruptive change
  for now)

* Combine tcExpandTyCon_maybe and coreExpandTyCon_maybe (which were identical)
  into expandSynTyCon_maybe

* A few more comment fixups

compiler/typecheck/TcDeriv.hs

@@ -1623,7 +1623,7 @@ mkNewTypeEqn dflags overlap_mode tvs
         -- We generate the instance
         --      instance Monad (ST s) => Monad (T s) where
 
-        nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
+        nt_eta_arity = newTyConEtadArity rep_tycon
                 -- For newtype T a b = MkT (S a a b), the TyCon machinery already
                 -- eta-reduces the representation type, so we know that
                 --      T a ~ S a a

compiler/typecheck/TcFlatten.hs

@@ -796,7 +796,7 @@ flatten_one fmode (TyConApp tc tys)
 
   -- Expand type synonyms that mention type families
   -- on the RHS; see Note [Flattening synonyms]
-  | Just (tenv, rhs, tys') <- tcExpandTyCon_maybe tc tys
+  | Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc tys
   , let expanded_ty = mkAppTys (substTy (mkTopTvSubst tenv) rhs) tys'
   = case fe_mode fmode of
       FM_FlattenAll | anyNameEnv isTypeFamilyTyCon (tyConsOfType rhs)

compiler/typecheck/TcTyDecls.hs

@@ -409,7 +409,7 @@ calcRecFlags boot_details is_boot mrole_env tyclss
         --     for vanilla-ness of data constructors; and that depends
         --     on empty existential type variables; and that is figured
         --     out by tcResultType; which uses tcMatchTy; which uses
-        --     coreView; which calls coreExpandTyCon_maybe; which uses
+        --     coreView; which calls expandSynTyCon_maybe; which uses
         --     the recursiveness of the TyCon.  Result... a black hole.
         -- YUK YUK YUK
 

compiler/types/Coercion.hs

@@ -981,7 +981,7 @@ mkAppCos co1 cos = foldl mkAppCo co1 cos
 mkTyConAppCo :: Role -> TyCon -> [Coercion] -> Coercion
 mkTyConAppCo r tc cos
                -- Expand type synonyms
-  | Just (tv_co_prs, rhs_ty, leftover_cos) <- tcExpandTyCon_maybe tc cos
+  | Just (tv_co_prs, rhs_ty, leftover_cos) <- expandSynTyCon_maybe tc cos
   = mkAppCos (liftCoSubst r tv_co_prs rhs_ty) leftover_cos
 
   | Just tys <- traverse isReflCo_maybe cos

compiler/types/FamInstEnv.hs

@@ -913,7 +913,7 @@ normaliseTcApp :: FamInstEnvs -> Role -> TyCon -> [Type] -> (Coercion, Type)
 -- See comments on normaliseType for the arguments of this function
 normaliseTcApp env role tc tys
   | isTypeSynonymTyCon tc
-  , Just (tenv, rhs, ntys') <- tcExpandTyCon_maybe tc ntys
+  , Just (tenv, rhs, ntys') <- expandSynTyCon_maybe tc ntys
   , (co2, ninst_rhs) <- normaliseType env role (Type.substTy (mkTopTvSubst tenv) rhs)
   = if isReflCo co2 then (args_co,                 mkTyConApp tc ntys)
                     else (args_co `mkTransCo` co2, mkAppTys ninst_rhs ntys')

compiler/types/TyCon.hs

@@ -76,7 +76,7 @@ module TyCon(
         tupleTyConBoxity, tupleTyConSort, tupleTyConArity,
 
         -- ** Manipulating TyCons
-        tcExpandTyCon_maybe, coreExpandTyCon_maybe,
+        expandSynTyCon_maybe,
         makeTyConAbstract,
         newTyConCo, newTyConCo_maybe,
         pprPromotionQuote,
@@ -829,8 +829,7 @@ which we need to make the derived instance for Monad Parser.
 Well, yes.  But to see that easily we eta-reduce the RHS type of
 Parser, in this case to ([], Froogle), so that even unsaturated applications
 of Parser will work right.  This eta reduction is done when the type
-constructor is built, and cached in NewTyCon.  The cached field is
-only used in coreExpandTyCon_maybe.
+constructor is built, and cached in NewTyCon.
 
 Here's an example that I think showed up in practice
 Source code:
@@ -845,14 +844,7 @@ Source code:
 
 After desugaring, and discarding the data constructors for the newtypes,
 we get:
-        w2 :: Foo T
-        w2 = w1
-And now Lint complains unless Foo T == Foo [], and that requires T==[]
-
-This point carries over to the newtype coercion, because we need to
-say
         w2 = w1 `cast` Foo CoT
-
 so the coercion tycon CoT must have
         kind:    T ~ []
  and    arity:   0
@@ -1477,7 +1469,7 @@ tyConCType_maybe _ = Nothing
 -----------------------------------------------
 -}
 
-tcExpandTyCon_maybe, coreExpandTyCon_maybe
+expandSynTyCon_maybe
         :: TyCon
         -> [tyco]                 -- ^ Arguments to 'TyCon'
         -> Maybe ([(TyVar,tyco)],
@@ -1487,32 +1479,18 @@ tcExpandTyCon_maybe, coreExpandTyCon_maybe
                                   -- and any arguments remaining from the
                                   -- application
 
--- ^ Used to create the view the /typechecker/ has on 'TyCon's.
--- We expand (closed) synonyms only, cf. 'coreExpandTyCon_maybe'
-tcExpandTyCon_maybe (SynonymTyCon { tyConTyVars = tvs
-                                  , synTcRhs    = rhs }) tys
-   = expand tvs rhs tys
-tcExpandTyCon_maybe _ _ = Nothing
-
----------------
-
--- ^ Used to create the view /Core/ has on 'TyCon's. We expand
--- not only closed synonyms like 'tcExpandTyCon_maybe',
--- but also non-recursive @newtype@s
-coreExpandTyCon_maybe tycon tys = tcExpandTyCon_maybe tycon tys
-
-
-----------------
-expand  :: [TyVar] -> Type                 -- Template
-        -> [a]                             -- Args
-        -> Maybe ([(TyVar,a)], Type, [a])  -- Expansion
-expand tvs rhs tys
+-- ^ Expand a type synonym application, if any
+expandSynTyCon_maybe tc tys
+  | SynonymTyCon { tyConTyVars = tvs, synTcRhs = rhs } <- tc
+  , let n_tvs = length tvs
   = case n_tvs `compare` length tys of
         LT -> Just (tvs `zip` tys, rhs, drop n_tvs tys)
         EQ -> Just (tvs `zip` tys, rhs, [])
         GT -> Nothing
-   where
-     n_tvs = length tvs
+   | otherwise
+   = Nothing
+
+----------------
 
 -- | As 'tyConDataCons_maybe', but returns the empty list of constructors if no
 -- constructors could be found

compiler/types/Type.hs

@@ -245,29 +245,25 @@ infixr 3 `mkFunTy`      -- Associates to the right
 
 {-# INLINE coreView #-}
 coreView :: Type -> Maybe Type
--- ^ In Core, we \"look through\" non-recursive newtypes and 'PredTypes': this
--- function tries to obtain a different view of the supplied type given this
---
--- Strips off the /top layer only/ of a type to give
--- its underlying representation type.
+-- ^ This function Strips off the /top layer only/ of a type synonym
+-- application (if any) its underlying representation type.
 -- Returns Nothing if there is nothing to look through.
 --
 -- By being non-recursive and inlined, this case analysis gets efficiently
 -- joined onto the case analysis that the caller is already doing
-coreView (TyConApp tc tys) | Just (tenv, rhs, tys') <- coreExpandTyCon_maybe tc tys
+coreView (TyConApp tc tys) | Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc tys
               = Just (mkAppTys (substTy (mkTopTvSubst tenv) rhs) tys')
                -- Its important to use mkAppTys, rather than (foldl AppTy),
                -- because the function part might well return a
                -- partially-applied type constructor; indeed, usually will!
-coreView _                 = Nothing
+coreView _ = Nothing
 
 -----------------------------------------------
 {-# INLINE tcView #-}
 tcView :: Type -> Maybe Type
--- ^ Similar to 'coreView', but for the type checker, which just looks through synonyms
-tcView (TyConApp tc tys) | Just (tenv, rhs, tys') <- tcExpandTyCon_maybe tc tys
-                         = Just (mkAppTys (substTy (mkTopTvSubst tenv) rhs) tys')
-tcView _                 = Nothing
+-- ^ Historical only; 'tcView' and 'coreView' used to differ, but don't any more
+tcView = coreView
+  -- ToDo: get rid of tcView altogether
   -- You might think that tcView belows in TcType rather than Type, but unfortunately
   -- it is needed by Unify, which is turn imported by Coercion (for MatchEnv and matchList).
   -- So we will leave it here to avoid module loops.
@@ -281,7 +277,7 @@ expandTypeSynonyms ty
   = go ty
   where
     go (TyConApp tc tys)
-      | Just (tenv, rhs, tys') <- tcExpandTyCon_maybe tc tys
+      | Just (tenv, rhs, tys') <- expandSynTyCon_maybe tc tys
       = go (mkAppTys (substTy (mkTopTvSubst tenv) rhs) tys')
       | otherwise
       = TyConApp tc (map go tys)