Overhaul of the rewrite rules

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

diff --git a/compiler/typecheck/TcInstDcls.lhs b/compiler/typecheck/TcInstDcls.lhs
index b0ca87a..5d1e63a 100644 (file)
--- a/compiler/typecheck/TcInstDcls.lhs
+++ b/compiler/typecheck/TcInstDcls.lhs
@@ -6,6 +6,13 @@
 TcInstDecls: Typechecking instance declarations
 
 \begin{code}
 TcInstDecls: Typechecking instance declarations
 
 \begin{code}

+{-# OPTIONS -w #-}
+-- The above warning supression flag is a temporary kludge.
+-- While working on this module you are encouraged to remove it and fix
+-- any warnings in the module. See
+--     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
+-- for details
+

 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
 
 #include "HsVersions.h"
 module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where
 
 #include "HsVersions.h"

@@ -29,6 +36,7 @@ import TcSimplify
 import Type
 import Coercion
 import TyCon
 import Type
 import Coercion
 import TyCon

+import TypeRep

 import DataCon
 import Class
 import Var
 import DataCon
 import Class
 import Var

@@ -147,9 +155,8 @@ tcInstDecls1 tycl_decls inst_decls deriv_decls
                -- (they recover, so that we get more than one error each
                -- round) 
 
                -- (they recover, so that we get more than one error each
                -- round) 
 

-               -- (1) Do class instance declarations and instances of indexed
-               --     types 
-       ; let { idxty_decls = filter (isIdxTyDecl . unLoc) tycl_decls }
+               -- (1) Do class and family instance declarations
+       ; let { idxty_decls = filter (isFamInstDecl . unLoc) tycl_decls }

        ; local_info_tycons <- mappM tcLocalInstDecl1  inst_decls
        ; idx_tycons        <- mappM tcIdxTyInstDeclTL idxty_decls
 
        ; local_info_tycons <- mappM tcLocalInstDecl1  inst_decls
        ; idx_tycons        <- mappM tcIdxTyInstDeclTL idxty_decls
 

@@ -180,7 +187,10 @@ tcInstDecls1 tycl_decls inst_decls deriv_decls
                -- (4) Compute instances from "deriving" clauses; 
                -- This stuff computes a context for the derived instance
                -- decl, so it needs to know about all the instances possible
                -- (4) Compute instances from "deriving" clauses; 
                -- This stuff computes a context for the derived instance
                -- decl, so it needs to know about all the instances possible

-       ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls deriv_decls
+                -- NB: class instance declarations can contain derivings as
+                --     part of associated data type declarations
+       ; (deriv_inst_info, deriv_binds) <- tcDeriving tycl_decls inst_decls 
+                                                      deriv_decls

        ; addInsts deriv_inst_info   $ do {
 
        ; gbl_env <- getGblEnv
        ; addInsts deriv_inst_info   $ do {
 
        ; gbl_env <- getGblEnv

@@ -193,7 +203,7 @@ tcInstDecls1 tycl_decls inst_decls deriv_decls
     -- !!!TODO: Need to perform this check for the TyThing of type functions,
     --         too.
     tcIdxTyInstDeclTL ldecl@(L loc decl) =
     -- !!!TODO: Need to perform this check for the TyThing of type functions,
     --         too.
     tcIdxTyInstDeclTL ldecl@(L loc decl) =

-      do { tything <- tcIdxTyInstDecl ldecl
+      do { tything <- tcFamInstDecl ldecl

         ; setSrcSpan loc $
             when (isAssocFamily tything) $
               addErr $ assocInClassErr (tcdName decl)
         ; setSrcSpan loc $
             when (isAssocFamily tything) $
               addErr $ assocInClassErr (tcdName decl)

@@ -240,14 +250,10 @@ tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
        ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
                  badBootDeclErr
 
        ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
                  badBootDeclErr
 

-       -- Typecheck the instance type itself.  We can't use 
-       -- tcHsSigType, because it's not a valid user type.
-       ; kinded_ty <- kcHsSigType poly_ty
-       ; poly_ty'  <- tcHsKindedType kinded_ty
-       ; let (tyvars, theta, tau) = tcSplitSigmaTy poly_ty'
+       ; (tyvars, theta, tau) <- tcHsInstHead poly_ty

        
        -- Next, process any associated types.
        
        -- Next, process any associated types.

-       ; idx_tycons <- mappM tcIdxTyInstDecl ats
+       ; idx_tycons <- mappM tcFamInstDecl ats

 
        -- Now, check the validity of the instance.
        ; (clas, inst_tys) <- checkValidInstHead tau
 
        -- Now, check the validity of the instance.
        ; (clas, inst_tys) <- checkValidInstHead tau

@@ -257,9 +263,11 @@ tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
 
        -- Finally, construct the Core representation of the instance.
        -- (This no longer includes the associated types.)
 
        -- Finally, construct the Core representation of the instance.
        -- (This no longer includes the associated types.)

-       ; dfun_name <- newDFunName clas inst_tys (srcSpanStart loc)
+       ; dfun_name <- newDFunName clas inst_tys loc

        ; overlap_flag <- getOverlapFlag
        ; overlap_flag <- getOverlapFlag

-       ; let dfun           = mkDictFunId dfun_name tyvars theta clas inst_tys
+       ; let (eq_theta,dict_theta) = partition isEqPred theta
+              theta'         = eq_theta ++ dict_theta
+              dfun           = mkDictFunId dfun_name tyvars theta' clas inst_tys

              ispec          = mkLocalInstance dfun overlap_flag
 
        ; return ([InstInfo { iSpec  = ispec, 
              ispec          = mkLocalInstance dfun overlap_flag
 
        ; return ([InstInfo { iSpec  = ispec, 

@@ -278,11 +286,11 @@ tcLocalInstDecl1 decl@(L loc (InstDecl poly_ty binds uprags ats))
     checkValidAndMissingATs clas inst_tys ats
       = do { -- Issue a warning for each class AT that is not defined in this
             -- instance.
     checkValidAndMissingATs clas inst_tys ats
       = do { -- Issue a warning for each class AT that is not defined in this
             -- instance.

-          ; let classDefATs = listToNameSet . map tyConName . classATs $ clas
-                 definedATs  = listToNameSet . map (tcdName.unLoc.fst)  $ ats
-                omitted     = classDefATs `minusNameSet` definedATs
+          ; let class_ats   = map tyConName (classATs clas)
+                 defined_ats = listToNameSet . map (tcdName.unLoc.fst)  $ ats
+                omitted     = filterOut (`elemNameSet` defined_ats) class_ats

           ; warn <- doptM Opt_WarnMissingMethods
           ; warn <- doptM Opt_WarnMissingMethods

-          ; mapM_ (warnTc warn . omittedATWarn) (nameSetToList omitted)
+          ; mapM_ (warnTc warn . omittedATWarn) omitted

           
             -- Ensure that all AT indexes that correspond to class parameters
             -- coincide with the types in the instance head.  All remaining
           
             -- Ensure that all AT indexes that correspond to class parameters
             -- coincide with the types in the instance head.  All remaining

@@ -395,39 +403,37 @@ tcInstDecls2 tycl_decls inst_decls
 
 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
 the dictionary function for this instance declaration. For example
 
 The main purpose of @tcInstDecl2@ is to return a @HsBinds@ which defines
 the dictionary function for this instance declaration. For example

-\begin{verbatim}
+

        instance Foo a => Foo [a] where
                op1 x = ...
                op2 y = ...
        instance Foo a => Foo [a] where
                op1 x = ...
                op2 y = ...

-\end{verbatim}
+

 might generate something like
 might generate something like

-\begin{verbatim}
+

        dfun.Foo.List dFoo_a = let op1 x = ...
                                   op2 y = ...
                               in
                                   Dict [op1, op2]
        dfun.Foo.List dFoo_a = let op1 x = ...
                                   op2 y = ...
                               in
                                   Dict [op1, op2]

-\end{verbatim}

 
 HOWEVER, if the instance decl has no context, then it returns a
 bigger @HsBinds@ with declarations for each method.  For example
 
 HOWEVER, if the instance decl has no context, then it returns a
 bigger @HsBinds@ with declarations for each method.  For example

-\begin{verbatim}
+

        instance Foo [a] where
                op1 x = ...
                op2 y = ...
        instance Foo [a] where
                op1 x = ...
                op2 y = ...

-\end{verbatim}
+

 might produce
 might produce

-\begin{verbatim}
+

        dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
        const.Foo.op1.List a x = ...
        const.Foo.op2.List a y = ...
        dfun.Foo.List a = Dict [Foo.op1.List a, Foo.op2.List a]
        const.Foo.op1.List a x = ...
        const.Foo.op2.List a y = ...

-\end{verbatim}
+

 This group may be mutually recursive, because (for example) there may
 be no method supplied for op2 in which case we'll get
 This group may be mutually recursive, because (for example) there may
 be no method supplied for op2 in which case we'll get

-\begin{verbatim}
+

        const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)
        const.Foo.op2.List a = default.Foo.op2 (dfun.Foo.List a)

-\end{verbatim}
-that is, the default method applied to the dictionary at this type.

 
 

+that is, the default method applied to the dictionary at this type.

 What we actually produce in either case is:
 
        AbsBinds [a] [dfun_theta_dicts]
 What we actually produce in either case is:
 
        AbsBinds [a] [dfun_theta_dicts]

@@ -439,7 +445,6 @@ What we actually produce in either case is:
 
 The "maybe" says that we only ask AbsBinds to make global constant methods
 if the dfun_theta is empty.
 
 The "maybe" says that we only ask AbsBinds to make global constant methods
 if the dfun_theta is empty.

-

                
 For an instance declaration, say,
 
                
 For an instance declaration, say,
 

@@ -455,8 +460,6 @@ Notice that we pass it the superclass dictionaries at the instance type; this
 is the ``Mark Jones optimisation''.  The stuff before the "=>" here
 is the @dfun_theta@ below.
 
 is the ``Mark Jones optimisation''.  The stuff before the "=>" here
 is the @dfun_theta@ below.
 

-First comes the easy case of a non-local instance decl.
-

 
 \begin{code}
 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
 
 \begin{code}
 tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)

@@ -465,23 +468,23 @@ tcInstDecl2 :: InstInfo -> TcM (LHsBinds Id)
 ------------------------
 -- Derived newtype instances; surprisingly tricky!
 --
 ------------------------
 -- Derived newtype instances; surprisingly tricky!
 --

--- In the case of a newtype, things are rather easy

 --     class Show a => Foo a b where ...
 --     class Show a => Foo a b where ...

---     newtype T a = MkT (Tree [a]) deriving( Foo Int )
+--     newtype N a = MkN (Tree [a]) deriving( Foo Int )
+--

 -- The newtype gives an FC axiom looking like
 -- The newtype gives an FC axiom looking like

---     axiom CoT a ::  T a :=: Tree [a]
+--     axiom CoN a ::  N a :=: Tree [a]

 --   (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
 --
 -- So all need is to generate a binding looking like: 
 --   (see Note [Newtype coercions] in TyCon for this unusual form of axiom)
 --
 -- So all need is to generate a binding looking like: 

---     dfunFooT :: forall a. (Foo Int (Tree [a], Show (T a)) => Foo Int (T a)
---     dfunFooT = /\a. \(ds:Show (T a)) (df:Foo (Tree [a])).
---               case df `cast` (Foo Int (sym (CoT a))) of
+--     dfunFooT :: forall a. (Foo Int (Tree [a], Show (N a)) => Foo Int (N a)
+--     dfunFooT = /\a. \(ds:Show (N a)) (df:Foo (Tree [a])).
+--               case df `cast` (Foo Int (sym (CoN a))) of

 --                  Foo _ op1 .. opn -> Foo ds op1 .. opn
 --
 -- If there are no superclasses, matters are simpler, because we don't need the case
 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
 
 --                  Foo _ op1 .. opn -> Foo ds op1 .. opn
 --
 -- If there are no superclasses, matters are simpler, because we don't need the case
 -- see Note [Newtype deriving superclasses] in TcDeriv.lhs
 

-tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
+tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived })

   = do { let dfun_id      = instanceDFunId ispec 
              rigid_info   = InstSkol
              origin       = SigOrigin rigid_info
   = do { let dfun_id      = instanceDFunId ispec 
              rigid_info   = InstSkol
              origin       = SigOrigin rigid_info

@@ -489,46 +492,43 @@ tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
        ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
                -- inst_head_ty is a PredType
 
        ; (tvs, theta, inst_head_ty) <- tcSkolSigType rigid_info inst_ty
                -- inst_head_ty is a PredType
 

-       ; inst_loc <- getInstLoc origin
-       ; (rep_dict_id : sc_dict_ids, wrap_fn, sc_binds)
-               <- make_wrapper inst_loc tvs theta mb_preds
-               -- Here, we are relying on the order of dictionary 
-               -- arguments built by NewTypeDerived in TcDeriv; 
-               -- namely, that the rep_dict_id comes first
-          

         ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty
         ; let (cls, cls_inst_tys) = tcSplitDFunHead inst_head_ty

-             cls_tycon           = classTyCon cls
-             the_coercion        = make_coercion cls_tycon cls_inst_tys
-              coerced_rep_dict           = mkHsWrap the_coercion (HsVar rep_dict_id)
-
-       ; body <- make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
+             (class_tyvars, sc_theta, _, op_items) = classBigSig cls
+             cls_tycon = classTyCon cls
+             sc_theta' = substTheta (zipOpenTvSubst class_tyvars cls_inst_tys) sc_theta
+
+             Just (initial_cls_inst_tys, last_ty) = snocView cls_inst_tys
+             (nt_tycon, tc_args) = tcSplitTyConApp last_ty     -- Can't fail
+             rep_ty              = newTyConInstRhs nt_tycon tc_args
+
+             rep_pred     = mkClassPred cls (initial_cls_inst_tys ++ [rep_ty])
+                               -- In our example, rep_pred is (Foo Int (Tree [a]))
+             the_coercion = make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
+                               -- Coercion of kind (Foo Int (Tree [a]) ~ Foo Int (N a)

               
               

-        ; return (sc_binds `snocBag` (noLoc $ VarBind dfun_id $ noLoc $ mkHsWrap wrap_fn body)) }
+       ; inst_loc   <- getInstLoc origin
+       ; sc_loc     <- getInstLoc InstScOrigin
+       ; dfun_dicts <- newDictBndrs inst_loc theta
+       ; sc_dicts   <- newDictBndrs sc_loc sc_theta'
+       ; this_dict  <- newDictBndr inst_loc (mkClassPred cls cls_inst_tys)
+       ; rep_dict   <- newDictBndr inst_loc rep_pred
+
+       -- Figure out bindings for the superclass context from dfun_dicts
+       -- Don't include this_dict in the 'givens', else
+       -- wanted_sc_insts get bound by just selecting from this_dict!!
+       ; sc_binds <- addErrCtxt superClassCtxt $
+                     tcSimplifySuperClasses inst_loc dfun_dicts (rep_dict:sc_dicts)
+
+       ; let coerced_rep_dict = mkHsWrap the_coercion (HsVar (instToId rep_dict))
+        
+       ; body <- make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
+       ; let dict_bind = noLoc $ VarBind (instToId this_dict) (noLoc body)
+
+       ; return (unitBag $ noLoc $
+                 AbsBinds  tvs (map instToId dfun_dicts)
+                           [(tvs, dfun_id, instToId this_dict, [])] 
+                           (dict_bind `consBag` sc_binds)) }

   where
   where

-
-      -----------------------
-      --       make_wrapper
-      -- We distinguish two cases:
-      -- (a) there is no tyvar abstraction in the dfun, so all dicts are constant,
-      --     and the new dict can just be a constant
-      --       (mb_preds = Just preds)
-      -- (b) there are tyvars, so we must make a dict *fun*
-      --       (mb_preds = Nothing)
-      -- See the defn of NewTypeDerived for the meaning of mb_preds
-    make_wrapper inst_loc tvs theta (Just preds)       -- Case (a)
-      = ASSERT( null tvs && null theta )
-       do { dicts <- newDictBndrs inst_loc preds
-          ; sc_binds <- addErrCtxt superClassCtxt $
-                        tcSimplifySuperClasses inst_loc [] dicts
-               -- Use tcSimplifySuperClasses to avoid creating loops, for the
-               -- same reason as Note [SUPERCLASS-LOOP 1] in TcSimplify
-          ; return (map instToId dicts, idHsWrapper, sc_binds) }
-
-    make_wrapper inst_loc tvs theta Nothing    -- Case (b)
-      = do { dicts <- newDictBndrs inst_loc theta
-          ; let dict_ids = map instToId dicts
-          ; return (dict_ids, mkWpTyLams tvs <.> mkWpLams dict_ids, emptyBag) }
-

       -----------------------
       --       make_coercion
       -- The inst_head looks like (C s1 .. sm (T a1 .. ak))
       -----------------------
       --       make_coercion
       -- The inst_head looks like (C s1 .. sm (T a1 .. ak))

@@ -538,25 +538,24 @@ tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
       -- So we just replace T with CoT, and insert a 'sym'
       -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
 
       -- So we just replace T with CoT, and insert a 'sym'
       -- NB: we know that k will be >= arity of CoT, because the latter fully eta-reduced
 

-    make_coercion cls_tycon cls_inst_tys
-       | Just (all_tys_but_last, last_ty) <- snocView cls_inst_tys
-       , (tycon, tc_args) <- tcSplitTyConApp last_ty   -- Should not fail
-       , Just co_con <- newTyConCo_maybe tycon
+    make_coercion cls_tycon initial_cls_inst_tys nt_tycon tc_args
+       | Just co_con <- newTyConCo_maybe nt_tycon

        , let co = mkSymCoercion (mkTyConApp co_con tc_args)
        , let co = mkSymCoercion (mkTyConApp co_con tc_args)

-        = WpCo (mkTyConApp cls_tycon (all_tys_but_last ++ [co]))
+        = WpCo (mkTyConApp cls_tycon (initial_cls_inst_tys ++ [co]))

         | otherwise    -- The newtype is transparent; no need for a cast
         = idHsWrapper
 
       -----------------------
         | otherwise    -- The newtype is transparent; no need for a cast
         = idHsWrapper
 
       -----------------------

-      --       make_body
-      -- Two cases; see Note [Newtype deriving superclasses] in TcDeriv.lhs
-      -- (a) no superclasses; then we can just use the coerced dict
-      -- (b) one or more superclasses; then new need to do the unpack/repack
+      --     (make_body C tys scs coreced_rep_dict)
+      --               returns 
+      --     (case coerced_rep_dict of { C _ ops -> C scs ops })
+      -- But if there are no superclasses, it returns just coerced_rep_dict
+      -- See Note [Newtype deriving superclasses] in TcDeriv.lhs

        
        

-    make_body cls_tycon cls_inst_tys sc_dict_ids coerced_rep_dict
-       | null sc_dict_ids              -- Case (a)
+    make_body cls_tycon cls_inst_tys sc_dicts coerced_rep_dict
+       | null sc_dicts         -- Case (a)

        = return coerced_rep_dict
        = return coerced_rep_dict

-       | otherwise                     -- Case (b)
+       | otherwise             -- Case (b)

        = do { op_ids            <- newSysLocalIds FSLIT("op") op_tys
             ; dummy_sc_dict_ids <- newSysLocalIds FSLIT("sc") (map idType sc_dict_ids)
             ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],
        = do { op_ids            <- newSysLocalIds FSLIT("op") op_tys
             ; dummy_sc_dict_ids <- newSysLocalIds FSLIT("sc") (map idType sc_dict_ids)
             ; let the_pat = ConPatOut { pat_con = noLoc cls_data_con, pat_tvs = [],

@@ -574,6 +573,7 @@ tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = NewTypeDerived mb_preds })
             ; return (HsCase (noLoc coerced_rep_dict) $
                       MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
        where
             ; return (HsCase (noLoc coerced_rep_dict) $
                       MatchGroup [the_match] (mkFunTy pat_ty pat_ty)) }
        where

+         sc_dict_ids  = map instToId sc_dicts

          pat_ty       = mkTyConApp cls_tycon cls_inst_tys
           cls_data_con = head (tyConDataCons cls_tycon)
           cls_arg_tys  = dataConInstArgTys cls_data_con cls_inst_tys 
          pat_ty       = mkTyConApp cls_tycon cls_inst_tys
           cls_data_con = head (tyConDataCons cls_tycon)
           cls_arg_tys  = dataConInstArgTys cls_data_con cls_inst_tys 

@@ -587,10 +587,11 @@ tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
        dfun_id    = instanceDFunId ispec
        rigid_info = InstSkol
        inst_ty    = idType dfun_id
        dfun_id    = instanceDFunId ispec
        rigid_info = InstSkol
        inst_ty    = idType dfun_id

+       loc        = srcLocSpan (getSrcLoc dfun_id)

     in
         -- Prime error recovery
     recoverM (returnM emptyLHsBinds)           $
     in
         -- Prime error recovery
     recoverM (returnM emptyLHsBinds)           $

-    setSrcSpan (srcLocSpan (getSrcLoc dfun_id))        $
+    setSrcSpan loc                             $

     addErrCtxt (instDeclCtxt2 (idType dfun_id))        $
 
        -- Instantiate the instance decl with skolem constants 
     addErrCtxt (instDeclCtxt2 (idType dfun_id))        $
 
        -- Instantiate the instance decl with skolem constants 

@@ -604,20 +605,30 @@ tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
 
         -- Instantiate the super-class context with inst_tys
        sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta
 
         -- Instantiate the super-class context with inst_tys
        sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys') sc_theta

+       (eq_sc_theta',dict_sc_theta')     = partition isEqPred sc_theta'

        origin    = SigOrigin rigid_info
        origin    = SigOrigin rigid_info

+       (eq_dfun_theta',dict_dfun_theta') = partition isEqPred dfun_theta'

     in
         -- Create dictionary Ids from the specified instance contexts.
     getInstLoc InstScOrigin                            `thenM` \ sc_loc -> 
     in
         -- Create dictionary Ids from the specified instance contexts.
     getInstLoc InstScOrigin                            `thenM` \ sc_loc -> 

-    newDictBndrs sc_loc sc_theta'                      `thenM` \ sc_dicts ->
+    newDictBndrs sc_loc dict_sc_theta'                 `thenM` \ sc_dicts ->

     getInstLoc origin                                  `thenM` \ inst_loc -> 
     getInstLoc origin                                  `thenM` \ inst_loc -> 

-    newDictBndrs inst_loc dfun_theta'                  `thenM` \ dfun_arg_dicts ->
-    newDictBndr inst_loc (mkClassPred clas inst_tys')  `thenM` \ this_dict ->
+    mkMetaCoVars eq_sc_theta'                          `thenM` \ sc_covars ->
+    mkEqInsts eq_sc_theta' (map mkWantedCo sc_covars)  `thenM` \ wanted_sc_eqs ->
+    mkCoVars eq_dfun_theta'                            `thenM` \ dfun_covars ->
+    mkEqInsts eq_dfun_theta' (map mkGivenCo $ mkTyVarTys dfun_covars)  `thenM` \ dfun_eqs    ->
+    newDictBndrs inst_loc dict_dfun_theta'             `thenM` \ dfun_dicts ->
+    newDictBndr inst_loc (mkClassPred clas inst_tys')   `thenM` \ this_dict ->

                -- Default-method Ids may be mentioned in synthesised RHSs,
                -- but they'll already be in the environment.
 
        -- Typecheck the methods
     let                -- These insts are in scope; quite a few, eh?
                -- Default-method Ids may be mentioned in synthesised RHSs,
                -- but they'll already be in the environment.
 
        -- Typecheck the methods
     let                -- These insts are in scope; quite a few, eh?

-       avail_insts = [this_dict] ++ dfun_arg_dicts ++ sc_dicts
+       dfun_insts      = dfun_eqs ++ dfun_dicts
+       wanted_sc_insts = wanted_sc_eqs   ++ sc_dicts
+       given_sc_eqs    = map (updateEqInstCoercion (mkGivenCo . TyVarTy . fromWantedCo "tcInstDecl2") ) wanted_sc_eqs
+       given_sc_insts  = given_sc_eqs   ++ sc_dicts
+       avail_insts     = [this_dict] ++ dfun_insts ++ given_sc_insts

     in
     tcMethods origin clas inst_tyvars' 
              dfun_theta' inst_tys' avail_insts 
     in
     tcMethods origin clas inst_tyvars' 
              dfun_theta' inst_tys' avail_insts 

@@ -625,10 +636,10 @@ tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
 
        -- Figure out bindings for the superclass context
        -- Don't include this_dict in the 'givens', else
 
        -- Figure out bindings for the superclass context
        -- Don't include this_dict in the 'givens', else

-       -- sc_dicts get bound by just selecting  from this_dict!!
+       -- wanted_sc_insts get bound by just selecting  from this_dict!!

     addErrCtxt superClassCtxt
        (tcSimplifySuperClasses inst_loc
     addErrCtxt superClassCtxt
        (tcSimplifySuperClasses inst_loc

-                        dfun_arg_dicts sc_dicts)       `thenM` \ sc_binds ->
+                        dfun_insts wanted_sc_insts)    `thenM` \ sc_binds ->

 
        -- It's possible that the superclass stuff might unified one
        -- of the inst_tyavars' with something in the envt
 
        -- It's possible that the superclass stuff might unified one
        -- of the inst_tyavars' with something in the envt

@@ -642,8 +653,8 @@ tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
         dict_constr   = classDataCon clas
        scs_and_meths = map instToId sc_dicts ++ meth_ids
        this_dict_id  = instToId this_dict
         dict_constr   = classDataCon clas
        scs_and_meths = map instToId sc_dicts ++ meth_ids
        this_dict_id  = instToId this_dict

-       inline_prag | null dfun_arg_dicts = []
-                   | otherwise = [InlinePrag (Inline AlwaysActive True)]
+       inline_prag | null dfun_insts  = []
+                   | otherwise        = [L loc (InlinePrag (Inline AlwaysActive True))]

                -- Always inline the dfun; this is an experimental decision
                -- because it makes a big performance difference sometimes.
                -- Often it means we can do the method selection, and then
                -- Always inline the dfun; this is an experimental decision
                -- because it makes a big performance difference sometimes.
                -- Often it means we can do the method selection, and then

@@ -656,7 +667,7 @@ tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
                --      See Note [Inline dfuns] below
 
        dict_rhs
                --      See Note [Inline dfuns] below
 
        dict_rhs

-         = mkHsConApp dict_constr inst_tys' (map HsVar scs_and_meths)
+         = mkHsConApp dict_constr (inst_tys' ++ mkTyVarTys sc_covars)  (map HsVar scs_and_meths)

                -- We don't produce a binding for the dict_constr; instead we
                -- rely on the simplifier to unfold this saturated application
                -- We do this rather than generate an HsCon directly, because
                -- We don't produce a binding for the dict_constr; instead we
                -- rely on the simplifier to unfold this saturated application
                -- We do this rather than generate an HsCon directly, because

@@ -668,15 +679,25 @@ tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = VanillaInst monobinds uprags })
        all_binds  = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
 
        main_bind = noLoc $ AbsBinds
        all_binds  = dict_bind `consBag` (sc_binds `unionBags` meth_binds)
 
        main_bind = noLoc $ AbsBinds

-                           inst_tyvars'
-                           (map instToId dfun_arg_dicts)
-                           [(inst_tyvars', dfun_id, this_dict_id, 
-                                           inline_prag ++ prags)] 
+                           (inst_tyvars' ++ dfun_covars)
+                           (map instToId dfun_dicts)
+                           [(inst_tyvars' ++ dfun_covars, dfun_id, this_dict_id, inline_prag ++ prags)] 

                            all_binds
     in
     showLIE (text "instance")          `thenM_`
     returnM (unitBag main_bind)
 
                            all_binds
     in
     showLIE (text "instance")          `thenM_`
     returnM (unitBag main_bind)
 

+mkCoVars :: [PredType] -> TcM [TyVar]
+mkCoVars = newCoVars . map unEqPred
+  where
+    unEqPred (EqPred ty1 ty2) = (ty1, ty2)
+    unEqPred _                = panic "TcInstDcls.mkCoVars"
+
+mkMetaCoVars :: [PredType] -> TcM [TyVar]
+mkMetaCoVars = mappM eqPredToCoVar
+  where
+    eqPredToCoVar (EqPred ty1 ty2) = newMetaCoVar ty1 ty2
+    eqPredToCoVar _                = panic "TcInstDcls.mkMetaCoVars"

 
 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys' 
          avail_insts op_items monobinds uprags
 
 tcMethods origin clas inst_tyvars' dfun_theta' inst_tys' 
          avail_insts op_items monobinds uprags