Define tyConRolesRepresentational and use it

tyConRolesRepresentational is just a version of tyConRolesX, but
specialised for a Representational argument. Saves a bit of extra
argument passing and pattern matching, and tyConRolesX was often
called when we knew the argument role was Representational.

Rather to my surprise this made the compiler allocate 5% less
for tests T9872{b,c,d}.  At least I think it's this commit.
Good thing, regardless.

3 files changed, 13 insertions, 10 deletions

diff --git a/compiler/typecheck/TcFlatten.hs b/compiler/typecheck/TcFlatten.hs
index 76a339da3f..169232ed56 100644
--- a/compiler/typecheck/TcFlatten.hs
+++ b/compiler/typecheck/TcFlatten.hs
@@ -1035,7 +1035,7 @@ flatten_ty_con_app tc tys
        ; let role = eqRelRole eq_rel
        ; (xis, cos) <- case eq_rel of
                          NomEq  -> flatten_many_nom tys
-                         ReprEq -> flatten_many (tyConRolesX role tc) tys
+                         ReprEq -> flatten_many (tyConRolesRepresentational tc) tys
        ; return (mkTyConApp tc xis, mkTyConAppCo role tc cos) }
 
 {-
diff --git a/compiler/types/Coercion.hs b/compiler/types/Coercion.hs
index c8e48c0b7a..2989bce41e 100644
--- a/compiler/types/Coercion.hs
+++ b/compiler/types/Coercion.hs
@@ -53,7 +53,7 @@ module Coercion (
         splitAppCo_maybe,
         splitForAllCo_maybe,
 
-        nthRole, tyConRolesX, setNominalRole_maybe,
+        nthRole, tyConRolesX, tyConRolesRepresentational, setNominalRole_maybe,
 
         pickLR,
 
@@ -609,7 +609,7 @@ mkAppCo (TyConAppCo r tc args) arg
   = case r of
       Nominal          -> TyConAppCo Nominal tc (args ++ [arg])
       Representational -> TyConAppCo Representational tc (args ++ [arg'])
-        where new_role = (tyConRolesX Representational tc) !! (length args)
+        where new_role = (tyConRolesRepresentational tc) !! (length args)
               arg'     = downgradeRole new_role Nominal arg
       Phantom          -> TyConAppCo Phantom tc (args ++ [toPhantomCo arg])
 mkAppCo co arg = AppCo co  arg
@@ -670,13 +670,13 @@ mkTransAppCo r1 co1 ty1a ty1b r2 co2 ty2a ty2b r3
       , nextRole ty1b == r2
       = (mkAppCo co1_repr (mkNomReflCo ty2a)) `mkTransCo`
         (mkTyConAppCo Representational tc1b
-           (zipWith mkReflCo (tyConRolesX Representational tc1b) tys1b
+           (zipWith mkReflCo (tyConRolesRepresentational tc1b) tys1b
             ++ [co2]))
 
       | Just (tc1a, tys1a) <- splitTyConApp_maybe ty1a
       , nextRole ty1a == r2
       = (mkTyConAppCo Representational tc1a
-           (zipWith mkReflCo (tyConRolesX Representational tc1a) tys1a
+           (zipWith mkReflCo (tyConRolesRepresentational tc1a) tys1a
             ++ [co2]))
         `mkTransCo`
         (mkAppCo co1_repr (mkNomReflCo ty2b))
@@ -1053,20 +1053,23 @@ toPhantomCo co
 -- Convert args to a TyConAppCo Nominal to the same TyConAppCo Representational
 applyRoles :: TyCon -> [Coercion] -> [Coercion]
 applyRoles tc cos
-  = zipWith (\r -> downgradeRole r Nominal) (tyConRolesX Representational tc) cos
+  = zipWith (\r -> downgradeRole r Nominal) (tyConRolesRepresentational tc) cos
 
 -- the Role parameter is the Role of the TyConAppCo
 -- defined here because this is intimiately concerned with the implementation
 -- of TyConAppCo
 tyConRolesX :: Role -> TyCon -> [Role]
-tyConRolesX Representational tc = tyConRoles tc ++ repeat Nominal
+tyConRolesX Representational tc = tyConRolesRepresentational tc
 tyConRolesX role             _  = repeat role
 
+tyConRolesRepresentational :: TyCon -> [Role]
+tyConRolesRepresentational tc = tyConRoles tc ++ repeat Nominal
+
 nthRole :: Role -> TyCon -> Int -> Role
 nthRole Nominal _ _ = Nominal
 nthRole Phantom _ _ = Phantom
 nthRole Representational tc n
-  = (tyConRolesX Representational tc) `getNth` n
+  = (tyConRolesRepresentational tc) `getNth` n
 
 ltRole :: Role -> Role -> Bool
 -- Is one role "less" than another?
diff --git a/compiler/types/OptCoercion.hs b/compiler/types/OptCoercion.hs
index fc6da629ac..210fc22e7c 100644
--- a/compiler/types/OptCoercion.hs
+++ b/compiler/types/OptCoercion.hs
@@ -180,7 +180,7 @@ opt_co4 env sym rep r g@(TyConAppCo _r tc cos)
       (True, Nominal) ->
         mkTyConAppCo Representational tc
                      (zipWith3 (opt_co3 env sym)
-                               (map Just (tyConRolesX Representational tc))
+                               (map Just (tyConRolesRepresentational tc))
                                (repeat Nominal)
                                cos)
       (False, Nominal) ->
@@ -189,7 +189,7 @@ opt_co4 env sym rep r g@(TyConAppCo _r tc cos)
                       -- must use opt_co2 here, because some roles may be P
                       -- See Note [Optimising coercion optimisation]
         mkTyConAppCo r tc (zipWith (opt_co2 env sym)
-                                   (tyConRolesX r tc)  -- the current roles
+                                   (tyConRolesRepresentational tc)  -- the current roles
                                    cos)
       (_, Phantom) -> pprPanic "opt_co4 sees a phantom!" (ppr g)