Fix and improve deriving for indexed data types

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

diff --git a/compiler/typecheck/TcEnv.lhs b/compiler/typecheck/TcEnv.lhs
index e1b9bd3..0972530 100644 (file)
--- a/compiler/typecheck/TcEnv.lhs
+++ b/compiler/typecheck/TcEnv.lhs
@@ -170,10 +170,18 @@ tcLookupFamInst tycon tys
        ; eps <- getEps
        ; let instEnv = (eps_fam_inst_env eps, tcg_fam_inst_env env)
        ; case lookupFamInstEnv instEnv tycon tys of
-          [(subst,fam_inst)] -> return (rep_tc, substTyVars subst (tyConTyVars rep_tc))
+
+          [(subst, fam_inst)] | variable_only_subst -> 
+            return (rep_tc, substTyVars subst (tyConTyVars rep_tc))
                where   -- NB: assumption is that (tyConTyVars rep_tc) is in 
                        --     the domain of the substitution
-                 rep_tc = famInstTyCon fam_inst 
+                 rep_tc              = famInstTyCon fam_inst
+                 subst_domain        = varEnvElts . getTvSubstEnv $ subst
+                 tvs                 = map (Type.getTyVar "tcLookupFamInst") 
+                                           subst_domain
+                 variable_only_subst = all Type.isTyVarTy subst_domain &&
+                                       sizeVarSet (mkVarSet tvs) == length tvs
+                                       -- renaming may have no repetitions
 
           other -> famInstNotFound tycon tys other
        }
@@ -680,7 +688,7 @@ wrongThingErr expected thing name
 famInstNotFound tycon tys what
   = failWithTc (msg <+> quotes (ppr tycon <+> hsep (map pprParendType tys)))
   where
-    msg = case what of
-               [] -> ptext SLIT("No instance for")
-               xs -> ptext SLIT("More than one instance for")
+    msg = ptext $ if length what > 1 
+                 then SLIT("More than one family instance for")
+                 else SLIT("No family instance exactly matching")
 \end{code}