Revised signature of tcLookupFamInst and lookupFamInstEnv

- This changes the signature of FamInstEnv.lookupFamInstEnv and
  FamInstEnv.lookupFamInstEnvUnify in a manner similar to SPJ's
  previous patch for InstEnv.llokupInstEnv
- tcLookupFamInst now permits the lookup of instances that are more
  general than the type instance requested.

5 files changed, 68 insertions, 25 deletions

diff --git a/compiler/simplCore/LiberateCase.lhs b/compiler/simplCore/LiberateCase.lhs
index a7b613dbb5..9f03adfe1d 100644
--- a/compiler/simplCore/LiberateCase.lhs
+++ b/compiler/simplCore/LiberateCase.lhs
@@ -274,10 +274,9 @@ mkCase :: LibCaseEnv -> CoreExpr -> Id -> Type -> [CoreAlt] -> CoreExpr
 -- See Note [Indexed data types]
 mkCase env scrut bndr ty [(DEFAULT,_,rhs)]
   | Just (tycon, tys)   <- splitTyConApp_maybe (idType bndr)
-  , [(subst, fam_inst)] <- lookupFamInstEnv (lc_fams env) tycon tys
+  , [(fam_inst, rep_tys)] <- lookupFamInstEnv (lc_fams env) tycon tys
   = let 
 	rep_tc     = famInstTyCon fam_inst
-	rep_tys    = map (substTyVar subst) (tyConTyVars rep_tc)
 	bndr'      = setIdType bndr (mkTyConApp rep_tc rep_tys)
 	Just co_tc = tyConFamilyCoercion_maybe rep_tc
 	co	   = mkTyConApp co_tc rep_tys
diff --git a/compiler/typecheck/FamInst.lhs b/compiler/typecheck/FamInst.lhs
index fd98fe919a..f85f6b9266 100644
--- a/compiler/typecheck/FamInst.lhs
+++ b/compiler/typecheck/FamInst.lhs
@@ -180,7 +180,7 @@ checkForConflicts inst_envs famInst
 
        ; let { matches   = lookupFamInstEnvUnify inst_envs fam tys'
 	     ; conflicts = [ conflictingFamInst
-			   | match@(_, conflictingFamInst) <- matches
+			   | match@(conflictingFamInst, _) <- matches
 			   , conflicting fam tys' tycon match 
 			   ]
 	     }
diff --git a/compiler/typecheck/TcDeriv.lhs b/compiler/typecheck/TcDeriv.lhs
index 1a9a8813e5..4e1a065797 100644
--- a/compiler/typecheck/TcDeriv.lhs
+++ b/compiler/typecheck/TcDeriv.lhs
@@ -47,6 +47,8 @@ import Util
 import ListSetOps
 import Outputable
 import Bag
+
+import Monad (unless)
 \end{code}
 
 %************************************************************************
@@ -395,7 +397,7 @@ mkEqnHelp orig tvs cls cls_tys tc_app
 	      full_tc_args = tc_args ++ mkTyVarTys extra_tvs
 	      full_tvs = tvs ++ extra_tvs
 		
-	; (rep_tc, rep_tc_args) <- tcLookupFamInst tycon full_tc_args
+	; (rep_tc, rep_tc_args) <- tcLookupFamInstExact tycon full_tc_args
 
 	; gla_exts <- doptM Opt_GlasgowExts
 	; overlap_flag <- getOverlapFlag
@@ -415,6 +417,27 @@ mkEqnHelp orig tvs cls cls_tys tc_app
 baleOut err = addErrTc err >> returnM (Nothing, Nothing) 
 \end{code}
 
+Auxiliary lookup wrapper which requires that looked up family instances are
+not type instances.
+
+\begin{code}
+tcLookupFamInstExact :: TyCon -> [Type] -> TcM (TyCon, [Type])
+tcLookupFamInstExact tycon tys
+  = do { result@(rep_tycon, rep_tys) <- tcLookupFamInst tycon tys
+       ; let { tvs		      = map (Type.getTyVar 
+                                               "TcDeriv.tcLookupFamInstExact") 
+					    tys
+	     ; variable_only_subst = all Type.isTyVarTy rep_tys &&
+				     sizeVarSet (mkVarSet tvs) == length tvs
+					-- renaming may have no repetitions
+             }
+       ; unless variable_only_subst $
+           famInstNotFound tycon tys [result]
+       ; return result
+       }
+       
+\end{code}
+
 
 %************************************************************************
 %*									*
@@ -980,7 +1003,7 @@ genInst spec
 
           -- In case of a family instance, we need to use the representation
           -- tycon (after all, it has the data constructors)
-        ; (tycon, _) <- tcLookupFamInst visible_tycon tyArgs
+        ; (tycon, _) <- tcLookupFamInstExact visible_tycon tyArgs
 	; let (meth_binds, aux_binds) = genDerivBinds clas fix_env tycon
 
 	-- Bring the right type variables into 
diff --git a/compiler/typecheck/TcEnv.lhs b/compiler/typecheck/TcEnv.lhs
index 787616aa3e..0f9bf231c8 100644
--- a/compiler/typecheck/TcEnv.lhs
+++ b/compiler/typecheck/TcEnv.lhs
@@ -42,7 +42,10 @@ module TcEnv(
 	topIdLvl, 
 
 	-- New Ids
-	newLocalName, newDFunName, newFamInstTyConName
+	newLocalName, newDFunName, newFamInstTyConName,
+
+        -- Errors
+        famInstNotFound
   ) where
 
 #include "HsVersions.h"
@@ -159,7 +162,21 @@ tcLookupLocatedTyCon :: Located Name -> TcM TyCon
 tcLookupLocatedTyCon = addLocM tcLookupTyCon
 
 -- Look up the representation tycon of a family instance.
--- Return the rep tycon and the corresponding rep args
+--
+-- The match must be unique - ie, match exactly one instance - but the 
+-- type arguments used for matching may be more specific than those of 
+-- the family instance declaration.
+--
+-- Return the instance tycon and its type instance.  For example, if we have
+--
+--  tcLookupFamInst 'T' '[Int]' yields (':R42T', 'Int')
+--
+-- then we have a coercion (ie, type instance of family instance coercion)
+--
+--  :Co:R42T Int :: T [Int] ~ :R42T Int
+--
+-- which implies that :R42T was declared as 'data instance T [a]'.
+--
 tcLookupFamInst :: TyCon -> [Type] -> TcM (TyCon, [Type])
 tcLookupFamInst tycon tys
   | not (isOpenTyCon tycon)
@@ -169,20 +186,8 @@ 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)] | 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
-		  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
+	   [(fam_inst, rep_tys)] -> return (famInstTyCon fam_inst, rep_tys)
+	   other                 -> famInstNotFound tycon tys other
        }
 \end{code}
 
diff --git a/compiler/types/FamInstEnv.lhs b/compiler/types/FamInstEnv.lhs
index 481c680a7f..b8c82f8623 100644
--- a/compiler/types/FamInstEnv.lhs
+++ b/compiler/types/FamInstEnv.lhs
@@ -198,10 +198,24 @@ Multiple matches are only possible in case of type families (not data
 families), and then, it doesn't matter which match we choose (as the
 instances are guaranteed confluent).
 
+We return the matching family instances and the type instance at which it
+matches.  For example, if we lookup 'T [Int]' and have a family instance
+
+  data instance T [a] = ..
+
+desugared to
+
+  data :R42T a = ..
+  coe :Co:R42T a :: T [a] ~ :R42T a
+
+we return the matching instance '(FamInst{.., fi_tycon = :R42T}, Int)'.
+
 \begin{code}
+type FamInstMatch = (FamInst, [Type])           -- Matching type instance
+
 lookupFamInstEnv :: FamInstEnvs
 	         -> TyCon -> [Type]		-- What we are looking for
-	         -> [(TvSubst, FamInst)] 	-- Successful matches
+	         -> [FamInstMatch] 	        -- Successful matches
 lookupFamInstEnv (pkg_ie, home_ie) fam tys
   = home_matches ++ pkg_matches
   where
@@ -231,7 +245,7 @@ lookupFamInstEnv (pkg_ie, home_ie) fam tys
 
         -- Proper check
       | Just subst <- tcMatchTys tpl_tvs tpl_tys tys
-      = (subst, item) : find rest
+      = (item, substTyVars subst (tyConTyVars tycon)) : find rest
 
         -- No match => try next
       | otherwise
@@ -250,7 +264,7 @@ indexed synonyms and we don't want to slow that down by needless unification.
 
 \begin{code}
 lookupFamInstEnvUnify :: (FamInstEnv, FamInstEnv) -> TyCon -> [Type]
-	              -> [(TvSubst, FamInst)]
+	              -> [(FamInstMatch)]
 lookupFamInstEnvUnify (pkg_ie, home_ie) fam tys
   = home_matches ++ pkg_matches
   where
@@ -286,7 +300,9 @@ lookupFamInstEnvUnify (pkg_ie, home_ie) fam tys
 		-- Unification will break badly if the variables overlap
 		-- They shouldn't because we allocate separate uniques for them
         case tcUnifyTys bind_fn tpl_tys tys of
-	    Just subst -> (subst, item) : find rest
+	    Just subst -> let rep_tys = substTyVars subst (tyConTyVars tycon)
+                          in
+                          (item, rep_tys) : find rest
 	    Nothing    -> find rest
 
 -- See explanation at @InstEnv.bind_fn@.