[commit: ghc] master: Small refactoring in Coercion (3bf910d)

[git at git.haskell.org]

Repository : ssh://git@git.haskell.org/ghc

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/3bf910d0ff7f82b4c316dcc08c143dbb65362366/ghc

>---------------------------------------------------------------

commit 3bf910d0ff7f82b4c316dcc08c143dbb65362366
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Tue Jan 2 17:11:55 2018 +0000

    Small refactoring in Coercion
    
    * Kill unused mkHomoPhantomCo
    * Refactor downgradeRole_maybe to be more perspicuous
    * Don't export toPhantomCo (not used externally)


>---------------------------------------------------------------

3bf910d0ff7f82b4c316dcc08c143dbb65362366
 compiler/types/Coercion.hs | 29 +++++++++++++----------------
 1 file changed, 13 insertions(+), 16 deletions(-)

diff --git a/compiler/types/Coercion.hs b/compiler/types/Coercion.hs
index 2a94755..3f83b09 100644
--- a/compiler/types/Coercion.hs
+++ b/compiler/types/Coercion.hs
@@ -32,7 +32,7 @@ module Coercion (
         mkNthCo, mkNthCoRole, mkLRCo,
         mkInstCo, mkAppCo, mkAppCos, mkTyConAppCo, mkFunCo, mkFunCos,
         mkForAllCo, mkForAllCos, mkHomoForAllCos, mkHomoForAllCos_NoRefl,
-        mkPhantomCo, mkHomoPhantomCo, toPhantomCo,
+        mkPhantomCo,
         mkUnsafeCo, mkHoleCo, mkUnivCo, mkSubCo,
         mkAxiomInstCo, mkProofIrrelCo,
         downgradeRole, maybeSubCo, mkAxiomRuleCo,
@@ -909,8 +909,9 @@ mkKindCo co
   | otherwise
   = KindCo co
 
--- input coercion is Nominal; see also Note [Role twiddling functions]
 mkSubCo :: Coercion -> Coercion
+-- Input coercion is Nominal, result is Representational
+-- see also Note [Role twiddling functions]
 mkSubCo (Refl Nominal ty) = Refl Representational ty
 mkSubCo (TyConAppCo Nominal tc cos)
   = TyConAppCo Representational tc (applyRoles tc cos)
@@ -927,12 +928,16 @@ downgradeRole_maybe :: Role   -- ^ desired role
                     -> Coercion -> Maybe Coercion
 -- In (downgradeRole_maybe dr cr co) it's a precondition that
 --                                   cr = coercionRole co
-downgradeRole_maybe Representational Nominal co = Just (mkSubCo co)
-downgradeRole_maybe Nominal Representational _  = Nothing
-downgradeRole_maybe Phantom Phantom          co = Just co
-downgradeRole_maybe Phantom _                co = Just (toPhantomCo co)
-downgradeRole_maybe _ Phantom                _  = Nothing
-downgradeRole_maybe _ _                      co = Just co
+
+downgradeRole_maybe Nominal          Nominal          co = Just co
+downgradeRole_maybe Nominal          _                _  = Nothing
+
+downgradeRole_maybe Representational Nominal          co = Just (mkSubCo co)
+downgradeRole_maybe Representational Representational co = Just co
+downgradeRole_maybe Representational Phantom          _  = Nothing
+
+downgradeRole_maybe Phantom          Phantom          co = Just co
+downgradeRole_maybe Phantom          _                co = Just (toPhantomCo co)
 
 -- | Like 'downgradeRole_maybe', but panics if the change isn't a downgrade.
 -- See Note [Role twiddling functions]
@@ -1019,14 +1024,6 @@ mkPhantomCo :: Coercion -> Type -> Type -> Coercion
 mkPhantomCo h t1 t2
   = mkUnivCo (PhantomProv h) Phantom t1 t2
 
--- | Make a phantom coercion between two types of the same kind.
-mkHomoPhantomCo :: Type -> Type -> Coercion
-mkHomoPhantomCo t1 t2
-  = ASSERT( k1 `eqType` typeKind t2 )
-    mkPhantomCo (mkNomReflCo k1) t1 t2
-  where
-    k1 = typeKind t1
-
 -- takes any coercion and turns it into a Phantom coercion
 toPhantomCo :: Coercion -> Coercion
 toPhantomCo co