For single-method classes use newtypes

[ghc-hetmet.git] / compiler / typecheck / TcCanonical.lhs

diff --git a/compiler/typecheck/TcCanonical.lhs b/compiler/typecheck/TcCanonical.lhs
index b9edd5f..60d1836 100644 (file)
--- a/compiler/typecheck/TcCanonical.lhs
+++ b/compiler/typecheck/TcCanonical.lhs
@@ -247,8 +247,11 @@ canClass fl v cn tys
                             else setDictBind v' (EvCast v (mkSymCoercion dict_co))
                           ; return v' }
 
-       -- Add the superclasses of this one here, See Note [Adding superclasses]
-       ; sc_cts <- newSCWorkFromFlavored v_new fl cn xis
+       -- Add the superclasses of this one here, See Note [Adding superclasses]. 
+       -- But only if we are not simplifying the LHS of a rule. 
+       ; sctx <- getTcSContext
+       ; sc_cts <- if simplEqsOnly sctx then return emptyCCan 
+                   else newSCWorkFromFlavored v_new fl cn xis
 
        ; return (sc_cts `andCCan` ccs `extendCCans` CDictCan { cc_id     = v_new
                                                              , cc_flavor = fl
@@ -317,7 +320,10 @@ happen.
 newSCWorkFromFlavored :: EvVar -> CtFlavor -> Class -> [Xi] -> TcS CanonicalCts
 -- Returns superclasses, see Note [Adding superclasses]
 newSCWorkFromFlavored ev orig_flavor cls xis 
-  = do { let (tyvars, sc_theta, _, _) = classBigSig cls 
+  | isEmptyVarSet (tyVarsOfTypes xis)
+  = return emptyCCan
+  | otherwise
+  = do { let (tyvars, sc_theta, _, _) = classBigSig cls
              sc_theta1 = substTheta (zipTopTvSubst tyvars xis) sc_theta
        ; sc_vars <- zipWithM inst_one sc_theta1 [0..]
        ; mkCanonicals flavor sc_vars }