[commit: ghc] master: Make the on-the-fly unifier defer forall/forall unification (0f5c163)

[git at git.haskell.org]

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

On branch  : master
Link       : http://ghc.haskell.org/trac/ghc/changeset/0f5c16370de2af82045d6ae8e64a3096241ceff1/ghc

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

commit 0f5c16370de2af82045d6ae8e64a3096241ceff1
Author: Simon Peyton Jones <simonpj at microsoft.com>
Date:   Fri Nov 21 11:04:28 2014 +0000

    Make the on-the-fly unifier defer forall/forall unification
    
    This has to be done by the full constraint solver anyway, and it's
    rare, so there's really no point in doing it twice.  This change
    just deletes some (tricky) code.


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

0f5c16370de2af82045d6ae8e64a3096241ceff1
 compiler/typecheck/TcUnify.lhs | 26 +-------------------------
 1 file changed, 1 insertion(+), 25 deletions(-)

diff --git a/compiler/typecheck/TcUnify.lhs b/compiler/typecheck/TcUnify.lhs
index 966e564..9e3e68d 100644
--- a/compiler/typecheck/TcUnify.lhs
+++ b/compiler/typecheck/TcUnify.lhs
@@ -759,11 +759,8 @@ uType origin orig_ty1 orig_ty2
       = ASSERT( isDecomposableTyCon tc1 )
         go_app (TyConApp tc1 ts1') t1' s2 t2
 
-    go ty1 ty2
-      | tcIsForAllTy ty1 || tcIsForAllTy ty2
-      = unifySigmaTy origin ty1 ty2
-
         -- Anything else fails
+        -- E.g. unifying for-all types, which is relative unusual
     go ty1 ty2 = uType_defer origin ty1 ty2 -- failWithMisMatch origin
 
     ------------------
@@ -771,27 +768,6 @@ uType origin orig_ty1 orig_ty2
       = do { co_s <- uType origin s1 s2  -- See Note [Unifying AppTy]
            ; co_t <- uType origin t1 t2
            ; return $ mkTcAppCo co_s co_t }
-
-unifySigmaTy :: CtOrigin -> TcType -> TcType -> TcM TcCoercion
-unifySigmaTy origin ty1 ty2
-  = do { let (tvs1, body1) = tcSplitForAllTys ty1
-             (tvs2, body2) = tcSplitForAllTys ty2
-
-       ; defer_or_continue (not (equalLength tvs1 tvs2)) $ do {
-         (subst1, skol_tvs) <- tcInstSkolTyVars tvs1
-                  -- Get location from monad, not from tvs1
-       ; let tys      = mkTyVarTys skol_tvs
-             phi1     = Type.substTy subst1                   body1
-             phi2     = Type.substTy (zipTopTvSubst tvs2 tys) body2
-             skol_info = UnifyForAllSkol skol_tvs phi1
-
-       ; (ev_binds, co) <- checkConstraints skol_info skol_tvs [] $
-                           uType origin phi1 phi2
-
-       ; return (foldr mkTcForAllCo (TcLetCo ev_binds co) skol_tvs) } }
-  where
-    defer_or_continue True  _ = uType_defer origin ty1 ty2
-    defer_or_continue False m = m
 \end{code}
 
 Note [Care with type applications]