% % (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % TcInstDecls: Typechecking instance declarations \begin{code}

    module TcInstDcls ( tcInstDecls1, tcInstDecls2 ) where

    import HsSyn
    import TcBinds
    import TcTyClsDecls
    import TcClassDcl
    import TcPat( addInlinePrags )
    import TcRnMonad
    import TcMType
    import TcType
    import Inst
    import InstEnv
    import FamInst
    import FamInstEnv
    import MkCore	( nO_METHOD_BINDING_ERROR_ID )
    import TcDeriv
    import TcEnv
    import RnSource ( addTcgDUs )
    import TcHsType
    import TcUnify
    import Type
    import Coercion
    import TyCon
    import DataCon
    import Class
    import Var
    import VarSet
    import CoreUtils  ( mkPiTypes )
    import CoreUnfold ( mkDFunUnfolding )
    import CoreSyn    ( Expr(Var), DFunArg(..), CoreExpr )
    import Id
    import MkId
    import Name
    import NameSet
    import DynFlags
    import SrcLoc
    import Util
    import Outputable
    import Bag
    import BasicTypes
    import HscTypes
    import FastString
    import Maybes	( orElse )
    import Data.Maybe
    import Control.Monad
    import Data.List

    #include "HsVersions.h"

\end{code} Typechecking instance declarations is done in two passes. The first pass, made by @tcInstDecls1@, collects information to be used in the second pass. This pre-processed info includes the as-yet-unprocessed bindings inside the instance declaration. These are type-checked in the second pass, when the class-instance envs and GVE contain all the info from all the instance and value decls. Indeed that's the reason we need two passes over the instance decls. Note [How instance declarations are translated] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Here is how we translation instance declarations into Core Running example: class C a where op1, op2 :: Ix b => a -> b -> b op2 = instance C a => C [a] {-# INLINE [2] op1 #-} op1 = <rhs> ===> -- Method selectors op1,op2 :: forall a. C a => forall b. Ix b => a -> b -> b op1 = ... op2 = ... -- Default methods get the 'self' dictionary as argument -- so they can call other methods at the same type -- Default methods get the same type as their method selector $dmop2 :: forall a. C a => forall b. Ix b => a -> b -> b $dmop2 = /\a. \(d:C a). /\b. \(d2: Ix b). -- NB: type variables 'a' and 'b' are *both* in scope in -- Note [Tricky type variable scoping] -- A top-level definition for each instance method -- Here op1_i, op2_i are the "instance method Ids" -- The INLINE pragma comes from the user pragma {-# INLINE [2] op1_i #-} -- From the instance decl bindings op1_i, op2_i :: forall a. C a => forall b. Ix b => [a] -> b -> b op1_i = /\a. \(d:C a). let this :: C [a] this = df_i a d -- Note [Subtle interaction of recursion and overlap] local_op1 :: forall b. Ix b => [a] -> b -> b local_op1 = <rhs> -- Source code; run the type checker on this -- NB: Type variable 'a' (but not 'b') is in scope in <rhs> -- Note [Tricky type variable scoping] in local_op1 a d op2_i = /\a \d:C a. $dmop2 [a] (df_i a d) -- The dictionary function itself {-# NOINLINE CONLIKE df_i #-} -- Never inline dictionary functions df_i :: forall a. C a -> C [a] df_i = /\a. \d:C a. MkC (op1_i a d) (op2_i a d) -- But see Note [Default methods in instances] -- We can't apply the type checker to the default-method call -- Use a RULE to short-circuit applications of the class ops {-# RULE "op1@C[a]" forall a, d:C a. op1 [a] (df_i d) = op1_i a d #-} Note [Instances and loop breakers] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ * Note that df_i may be mutually recursive with both op1_i and op2_i. It's crucial that df_i is not chosen as the loop breaker, even though op1_i has a (user-specified) INLINE pragma. * Instead the idea is to inline df_i into op1_i, which may then select methods from the MkC record, and thereby break the recursion with df_i, leaving a *self*-recurisve op1_i. (If op1_i doesn't call op at the same type, it won't mention df_i, so there won't be recursion in the first place.) * If op1_i is marked INLINE by the user there's a danger that we won't inline df_i in it, and that in turn means that (since it'll be a loop-breaker because df_i isn't), op1_i will ironically never be inlined. But this is OK: the recursion breaking happens by way of a RULE (the magic ClassOp rule above), and RULES work inside InlineRule unfoldings. See Note [RULEs enabled in SimplGently] in SimplUtils Note [ClassOp/DFun selection] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ One thing we see a lot is stuff like op2 (df d1 d2) where 'op2' is a ClassOp and 'df' is DFun. Now, we could inline *both* 'op2' and 'df' to get case (MkD ($cop1 d1 d2) ($cop2 d1 d2) ... of MkD _ op2 _ _ _ -> op2 And that will reduce to ($cop2 d1 d2) which is what we wanted. But it's tricky to make this work in practice, because it requires us to inline both 'op2' and 'df'. But neither is keen to inline without having seen the other's result; and it's very easy to get code bloat (from the big intermediate) if you inline a bit too much. Instead we use a cunning trick. * We arrange that 'df' and 'op2' NEVER inline. * We arrange that 'df' is ALWAYS defined in the sylised form df d1 d2 = MkD ($cop1 d1 d2) ($cop2 d1 d2) ... * We give 'df' a magical unfolding (DFunUnfolding [$cop1, $cop2, ..]) that lists its methods. * We make CoreUnfold.exprIsConApp_maybe spot a DFunUnfolding and return a suitable constructor application -- inlining df "on the fly" as it were. * We give the ClassOp 'op2' a BuiltinRule that extracts the right piece iff its argument satisfies exprIsConApp_maybe. This is done in MkId mkDictSelId * We make 'df' CONLIKE, so that shared uses stil match; eg let d = df d1 d2 in ...(op2 d)...(op1 d)... Note [Single-method classes] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If the class has just one method (or, more accurately, just one element of {superclasses + methods}), then we still use the *same* strategy class C a where op :: a -> a instance C a => C [a] where op = <blah> We translate the class decl into a newtype, which just gives a top-level axiom: axiom Co:C a :: C a ~ (a->a) op :: forall a. C a -> (a -> a) op a d = d |> (Co:C a) MkC :: forall a. (a->a) -> C a MkC = /\a.\op. op |> (sym Co:C a) df :: forall a. C a => C [a] {-# NOINLINE df DFun[ $cop_list ] #-} df = /\a. \d. MkC ($cop_list a d) $cop_list :: forall a. C a => [a] -> [a] $cop_list = <blah> The "constructor" MkC expands to a cast, as does the class-op selector. The RULE works just like for multi-field dictionaries: * (df a d) returns (Just (MkC,..,[$cop_list a d])) to exprIsConApp_Maybe * The RULE for op picks the right result This is a bit of a hack, because (df a d) isn't *really* a constructor application. But it works just fine in this case, exprIsConApp_maybe is otherwise used only when we hit a case expression which will have a real data constructor in it. The biggest reason for doing it this way, apart from uniformity, is that we want to be very careful when we have instance C a => C [a] where {-# INLINE op #-} op = ... then we'll get an INLINE pragma on $cop_list but it's important that $cop_list only inlines when it's applied to *two* arguments (the dictionary and the list argument The danger is that we'll get something like op_list :: C a => [a] -> [a] op_list = /\a.\d. $cop_list a d and then we'll eta expand, and then we'll inline TOO EARLY. This happened in Trac #3772 and I spent far too long fiddling around trying to fix it. Look at the test for Trac #3772. (Note: re-reading the above, I can't see how using the uniform story solves the problem.) Note [Subtle interaction of recursion and overlap] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this class C a where { op1,op2 :: a -> a } instance C a => C [a] where op1 x = op2 x ++ op2 x op2 x = ... instance C [Int] where ... When type-checking the C [a] instance, we need a C [a] dictionary (for the call of op2). If we look up in the instance environment, we find an overlap. And in *general* the right thing is to complain (see Note [Overlapping instances] in InstEnv). But in *this* case it's wrong to complain, because we just want to delegate to the op2 of this same instance. Why is this justified? Because we generate a (C [a]) constraint in a context in which 'a' cannot be instantiated to anything that matches other overlapping instances, or else we would not be excecuting this version of op1 in the first place. It might even be a bit disguised: nullFail :: C [a] => [a] -> [a] nullFail x = op2 x ++ op2 x instance C a => C [a] where op1 x = nullFail x Precisely this is used in package 'regex-base', module Context.hs. See the overlapping instances for RegexContext, and the fact that they call 'nullFail' just like the example above. The DoCon package also does the same thing; it shows up in module Fraction.hs Conclusion: when typechecking the methods in a C [a] instance, we want to treat the 'a' as an *existential* type variable, in the sense described by Note [Binding when looking up instances]. That is why isOverlappableTyVar responds True to an InstSkol, which is the kind of skolem we use in tcInstDecl2. Note [Tricky type variable scoping] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In our example class C a where op1, op2 :: Ix b => a -> b -> b op2 = instance C a => C [a] {-# INLINE [2] op1 #-} op1 = <rhs> note that 'a' and 'b' are *both* in scope in , but only 'a' is in scope in <rhs>. In particular, we must make sure that 'b' is in scope when typechecking . This is achieved by subFunTys, which brings appropriate tyvars into scope. This happens for both and for <rhs>, but that doesn't matter: the *renamer* will have complained if 'b' is mentioned in <rhs>. %************************************************************************ %* * \subsection{Extracting instance decls} %* * %************************************************************************ Gather up the instance declarations from their various sources \begin{code}

    tcInstDecls1    -- Deal with both source-code and imported instance decls
       :: [LTyClDecl Name]          -- For deriving stuff
       -> [LInstDecl Name]          -- Source code instance decls
       -> [LDerivDecl Name]         -- Source code stand-alone deriving decls
       -> TcM (TcGblEnv,            -- The full inst env
               [InstInfo Name],     -- Source-code instance decls to process;
                                    -- contains all dfuns for this module
               HsValBinds Name)     -- Supporting bindings for derived instances

    tcInstDecls1 tycl_decls inst_decls deriv_decls
      = checkNoErrs $
        do {        -- Stop if addInstInfos etc discovers any errors
                    -- (they recover, so that we get more than one error each
                    -- round)

                    -- (1) Do class and family instance declarations
           ; idx_tycons        <- mapAndRecoverM (tcFamInstDecl TopLevel) $
           	 		      filter (isFamInstDecl . unLoc) tycl_decls
           ; local_info_tycons <- mapAndRecoverM tcLocalInstDecl1  inst_decls

           ; let { (local_info,
                    at_tycons_s)   = unzip local_info_tycons
                 ; at_idx_tycons   = concat at_tycons_s ++ idx_tycons
                 ; clas_decls      = filter (isClassDecl . unLoc) tycl_decls
                 ; implicit_things = concatMap implicitTyThings at_idx_tycons
    	     ; aux_binds       = mkRecSelBinds at_idx_tycons
                 }

                    -- (2) Add the tycons of indexed types and their implicit
                    --     tythings to the global environment
           ; tcExtendGlobalEnv (at_idx_tycons ++ implicit_things) $ do {

                    -- (3) Instances from generic class declarations
           ; generic_inst_info <- getGenericInstances clas_decls

                    -- Next, construct the instance environment so far, consisting
                    -- of
                    --   (a) local instance decls
                    --   (b) generic instances
                    --   (c) local family instance decls
           ; addInsts local_info         $
             addInsts generic_inst_info  $
             addFamInsts at_idx_tycons   $ do {

                    -- (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
                    -- NB: class instance declarations can contain derivings as
                    --     part of associated data type declarations
    	 failIfErrsM		-- If the addInsts stuff gave any errors, don't
    				-- try the deriving stuff, becuase that may give
    				-- more errors still
           ; (deriv_inst_info, deriv_binds, deriv_dus)
                  <- tcDeriving tycl_decls inst_decls deriv_decls
           ; gbl_env <- addInsts deriv_inst_info getGblEnv
           ; return ( addTcgDUs gbl_env deriv_dus,
                      generic_inst_info ++ deriv_inst_info ++ local_info,
                      aux_binds `plusHsValBinds` deriv_binds)
        }}}

    addInsts :: [InstInfo Name] -> TcM a -> TcM a
    addInsts infos thing_inside
      = tcExtendLocalInstEnv (map iSpec infos) thing_inside

    addFamInsts :: [TyThing] -> TcM a -> TcM a
    addFamInsts tycons thing_inside
      = tcExtendLocalFamInstEnv (map mkLocalFamInstTyThing tycons) thing_inside
      where
        mkLocalFamInstTyThing (ATyCon tycon) = mkLocalFamInst tycon
        mkLocalFamInstTyThing tything        = pprPanic "TcInstDcls.addFamInsts"
                                                        (ppr tything)

\end{code} \begin{code}

    tcLocalInstDecl1 :: LInstDecl Name
                     -> TcM (InstInfo Name, [TyThing])
            -- A source-file instance declaration
            -- Type-check all the stuff before the "where"
            --
            -- We check for respectable instance type, and context
    tcLocalInstDecl1 (L loc (InstDecl poly_ty binds uprags ats))
      = setSrcSpan loc		        $
        addErrCtxt (instDeclCtxt1 poly_ty)  $

        do  { is_boot <- tcIsHsBoot
            ; checkTc (not is_boot || (isEmptyLHsBinds binds && null uprags))
                      badBootDeclErr

            ; (tyvars, theta, clas, inst_tys) <- tcHsInstHead poly_ty
            ; checkValidInstance poly_ty tyvars theta clas inst_tys

            -- Next, process any associated types.
            ; idx_tycons <- recoverM (return []) $
    	  	     do { idx_tycons <- checkNoErrs $
                                            mapAndRecoverM (tcFamInstDecl NotTopLevel) ats
    		     	; checkValidAndMissingATs clas (tyvars, inst_tys)
                              			  (zip ats idx_tycons)
    			; return idx_tycons }

            -- Finally, construct the Core representation of the instance.
            -- (This no longer includes the associated types.)
            ; dfun_name <- newDFunName clas inst_tys (getLoc poly_ty)
    		-- Dfun location is that of instance *header*
            ; overlap_flag <- getOverlapFlag
            ; 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, iBinds = VanillaInst binds uprags False },
                      idx_tycons)
            }
      where
        -- We pass in the source form and the type checked form of the ATs.  We
        -- really need the source form only to be able to produce more informative
        -- error messages.
        checkValidAndMissingATs :: Class
                                -> ([TyVar], [TcType])     -- instance types
                                -> [(LTyClDecl Name,       -- source form of AT
                                     TyThing)]    	       -- Core form of AT
                                -> TcM ()
        checkValidAndMissingATs clas inst_tys ats
          = do { -- Issue a warning for each class AT that is not defined in this
                 -- instance.
               ; 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
               ; 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
                 -- AT arguments must be variables.  Also raise an error for any
                 -- type instances that are not associated with this class.
               ; mapM_ (checkIndexes clas inst_tys) ats
               }

        checkIndexes clas inst_tys (hsAT, ATyCon tycon)
    -- !!!TODO: check that this does the Right Thing for indexed synonyms, too!
          = checkIndexes' clas inst_tys hsAT
                          (tyConTyVars tycon,
                           snd . fromJust . tyConFamInst_maybe $ tycon)
        checkIndexes _ _ _ = panic "checkIndexes"

        checkIndexes' clas (instTvs, instTys) hsAT (atTvs, atTys)
          = let atName = tcdName . unLoc $ hsAT
            in
            setSrcSpan (getLoc hsAT)       $
            addErrCtxt (atInstCtxt atName) $
            case find ((atName ==) . tyConName) (classATs clas) of
              Nothing     -> addErrTc $ badATErr clas atName  -- not in this class
              Just atycon ->
                    -- The following is tricky!  We need to deal with three
                    -- complications: (1) The AT possibly only uses a subset of
                    -- the class parameters as indexes and those it uses may be in
                    -- a different order; (2) the AT may have extra arguments,
                    -- which must be type variables; and (3) variables in AT and
                    -- instance head will be different `Name's even if their
                    -- source lexemes are identical.
    		--
    		-- e.g.    class C a b c where
    		-- 	     data D b a :: * -> *           -- NB (1) b a, omits c
    		-- 	   instance C [x] Bool Char where
    		--	     data D Bool [x] v = MkD x [v]  -- NB (2) v
    		--	     	  -- NB (3) the x in 'instance C...' have differnt
    		--		  --        Names to x's in 'data D...'
                    --
                    -- Re (1), `poss' contains a permutation vector to extract the
                    -- class parameters in the right order.
                    --
                    -- Re (2), we wrap the (permuted) class parameters in a Maybe
                    -- type and use Nothing for any extra AT arguments.  (First
                    -- equation of `checkIndex' below.)
                    --
                    -- Re (3), we replace any type variable in the AT parameters
                    -- that has the same source lexeme as some variable in the
                    -- instance types with the instance type variable sharing its
                    -- source lexeme.
                    --
                    let poss :: [Int]
                        -- For *associated* type families, gives the position
                        -- of that 'TyVar' in the class argument list (0-indexed)
    	   	    -- e.g.  class C a b c where { type F c a :: *->* }
               	    --       Then we get Just [2,0]
    	            poss = catMaybes [ tv `elemIndex` classTyVars clas
                                         | tv <- tyConTyVars atycon]
                           -- We will get Nothings for the "extra" type
                           -- variables in an associated data type
                           -- e.g. class C a where { data D a :: *->* }
                           -- here D gets arity 2 and has two tyvars

                        relevantInstTys = map (instTys !!) poss
                        instArgs        = map Just relevantInstTys ++
                                          repeat Nothing  -- extra arguments
                        renaming        = substSameTyVar atTvs instTvs
                    in
                    zipWithM_ checkIndex (substTys renaming atTys) instArgs

        checkIndex ty Nothing
          | isTyVarTy ty         = return ()
          | otherwise            = addErrTc $ mustBeVarArgErr ty
        checkIndex ty (Just instTy)
          | ty `tcEqType` instTy = return ()
          | otherwise            = addErrTc $ wrongATArgErr ty instTy

        listToNameSet = addListToNameSet emptyNameSet

        substSameTyVar []       _            = emptyTvSubst
        substSameTyVar (tv:tvs) replacingTvs =
          let replacement = case find (tv `sameLexeme`) replacingTvs of
                            Nothing  -> mkTyVarTy tv
                            Just rtv -> mkTyVarTy rtv
              --
              tv1 `sameLexeme` tv2 =
                nameOccName (tyVarName tv1) == nameOccName (tyVarName tv2)
          in
          extendTvSubst (substSameTyVar tvs replacingTvs) tv replacement

\end{code} %************************************************************************ %* * Type-checking instance declarations, pass 2 %* * %************************************************************************ \begin{code}

    tcInstDecls2 :: [LTyClDecl Name] -> [InstInfo Name]
                 -> TcM (LHsBinds Id)
    -- (a) From each class declaration,
    --      generate any default-method bindings
    -- (b) From each instance decl
    --      generate the dfun binding

    tcInstDecls2 tycl_decls inst_decls
      = do  { -- (a) Default methods from class decls
              let class_decls = filter (isClassDecl . unLoc) tycl_decls
            ; dm_binds_s <- mapM tcClassDecl2 class_decls
            ; let dm_binds = unionManyBags dm_binds_s

              -- (b) instance declarations
    	; let dm_ids = collectHsBindsBinders dm_binds
    	      -- Add the default method Ids (again)
    	      -- See Note [Default methods and instances]
            ; inst_binds_s <- tcExtendIdEnv dm_ids $
                              mapM tcInstDecl2 inst_decls

              -- Done
            ; return (dm_binds `unionBags` unionManyBags inst_binds_s) }

\end{code} See Note [Default methods and instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The default method Ids are already in the type environment (see Note [Default method Ids and Template Haskell] in TcTyClsDcls), BUT they don't have their InlinePragmas yet. Usually that would not matter, because the simplifier propagates information from binding site to use. But, unusually, when compiling instance decls we *copy* the INLINE pragma from the default method to the method for that particular operation (see Note [INLINE and default methods] below). So right here in tcInstDecl2 we must re-extend the type envt with the default method Ids replete with their INLINE pragmas. Urk. \begin{code}

    tcInstDecl2 :: InstInfo Name -> TcM (LHsBinds Id)
                -- Returns a binding for the dfun
    tcInstDecl2 (InstInfo { iSpec = ispec, iBinds = ibinds })
      = recoverM (return emptyLHsBinds)             $
        setSrcSpan loc                              $
        addErrCtxt (instDeclCtxt2 (idType dfun_id)) $
        do {  -- Instantiate the instance decl with skolem constants
           ; (inst_tyvars, dfun_theta, inst_head) <- tcSkolDFunType (idType dfun_id)
           ; let (clas, inst_tys) = tcSplitDFunHead inst_head
                 (class_tyvars, sc_theta, _, op_items) = classBigSig clas
                 sc_theta' = substTheta (zipOpenTvSubst class_tyvars inst_tys) sc_theta
                 n_ty_args = length inst_tyvars
                 n_silent  = dfunNSilent dfun_id
                 (silent_theta, orig_theta) = splitAt n_silent dfun_theta

           ; silent_ev_vars <- mapM newSilentGiven silent_theta
           ; orig_ev_vars   <- newEvVars orig_theta
           ; let dfun_ev_vars = silent_ev_vars ++ orig_ev_vars

           ; (sc_dicts, sc_args)
                 <- mapAndUnzipM (tcSuperClass n_ty_args dfun_ev_vars) sc_theta'

           -- Check that any superclasses gotten from a silent arguemnt
           -- can be deduced from the originally-specified dfun arguments
           ; ct_loc <- getCtLoc ScOrigin
           ; _ <- checkConstraints skol_info inst_tyvars orig_ev_vars $
                  emitFlats $ listToBag $
                  [ mkEvVarX sc ct_loc | sc <- sc_dicts, isSilentEvVar sc ]

           -- Deal with 'SPECIALISE instance' pragmas
           -- See Note [SPECIALISE instance pragmas]
           ; spec_info@(spec_inst_prags,_) <- tcSpecInstPrags dfun_id ibinds

            -- Typecheck the methods
           ; (meth_ids, meth_binds)
               <- tcExtendTyVarEnv inst_tyvars $
                    -- The inst_tyvars scope over the 'where' part
                    -- Those tyvars are inside the dfun_id's type, which is a bit
                    -- bizarre, but OK so long as you realise it!
                  tcInstanceMethods dfun_id clas inst_tyvars dfun_ev_vars
                                    inst_tys spec_info
                                    op_items ibinds

           -- Create the result bindings
           ; self_dict <- newEvVar (ClassP clas inst_tys)
           ; let dict_constr       = classDataCon clas
    	     dict_bind         = mkVarBind self_dict dict_rhs
                 dict_rhs          = foldl mk_app inst_constr $
                                     map HsVar sc_dicts ++ map (wrapId arg_wrapper) meth_ids
                 inst_constr       = L loc $ wrapId (mkWpTyApps inst_tys)
                                                    (dataConWrapId dict_constr)
                         -- 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
                         -- it means that the special cases (e.g. dictionary with only one
                         -- member) are dealt with by the common MkId.mkDataConWrapId code rather
                         -- than needing to be repeated here.

                 mk_app :: LHsExpr Id -> HsExpr Id -> LHsExpr Id
                 mk_app fun arg = L loc (HsApp fun (L loc arg))

                 arg_wrapper = mkWpEvVarApps dfun_ev_vars <.> mkWpTyApps (mkTyVarTys inst_tyvars)

    	        -- Do not inline the dfun; instead give it a magic DFunFunfolding
    	        -- See Note [ClassOp/DFun selection]
    		-- See also note [Single-method classes]
                 dfun_id_w_fun = dfun_id
                                 `setIdUnfolding`  mkDFunUnfolding dfun_ty (sc_args ++ meth_args)
                                 `setInlinePragma` dfunInlinePragma
                 meth_args = map (DFunPolyArg . Var) meth_ids

                 main_bind = AbsBinds { abs_tvs = inst_tyvars
                                      , abs_ev_vars = dfun_ev_vars
                                      , abs_exports = [(inst_tyvars, dfun_id_w_fun, self_dict,
                                                        SpecPrags spec_inst_prags)]
                                      , abs_ev_binds = emptyTcEvBinds
                                      , abs_binds = unitBag dict_bind }

           ; return (unitBag (L loc main_bind) `unionBags`
                     listToBag meth_binds)
           }
     where
       skol_info = InstSkol         -- See Note [Subtle interaction of recursion and overlap]
       dfun_ty   = idType dfun_id
       dfun_id   = instanceDFunId ispec
       loc       = getSrcSpan dfun_id

    ------------------------------
    tcSuperClass :: Int -> [EvVar] -> PredType -> TcM (EvVar, DFunArg CoreExpr)
    -- All superclasses should be either
    --   (a) be one of the arguments to the dfun, of
    --   (b) be a constant, soluble at top level
    tcSuperClass n_ty_args ev_vars pred
      | Just (ev, i) <- find n_ty_args ev_vars
      = return (ev, DFunLamArg i)
      | otherwise
      = ASSERT2( isEmptyVarSet (tyVarsOfPred pred), ppr pred)       -- Constant!
        do { sc_dict  <- emitWanted ScOrigin pred
           ; return (sc_dict, DFunConstArg (Var sc_dict)) }
      where
        find _ [] = Nothing
        find i (ev:evs) | pred `tcEqPred` evVarPred ev = Just (ev, i)
                        | otherwise                    = find (i+1) evs

    ------------------------------
    tcSpecInstPrags :: DFunId -> InstBindings Name
                    -> TcM ([Located TcSpecPrag], PragFun)
    tcSpecInstPrags _ (NewTypeDerived {})
      = return ([], \_ -> [])
    tcSpecInstPrags dfun_id (VanillaInst binds uprags _)
      = do { spec_inst_prags <- mapM (wrapLocM (tcSpecInst dfun_id)) $
                                filter isSpecInstLSig uprags
    	     -- The filter removes the pragmas for methods
           ; return (spec_inst_prags, mkPragFun uprags binds) }

\end{code} Note [Silent Superclass Arguments] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider the following (extreme) situation: class C a => D a where ... instance D [a] => D [a] where ... Although this looks wrong (assume D [a] to prove D [a]), it is only a more extreme case of what happens with recursive dictionaries. To implement the dfun we must generate code for the superclass C [a], which we can get by superclass selection from the supplied argument! So we’d generate: dfun :: forall a. D [a] -> D [a] dfun = \d::D [a] -> MkD (scsel d) .. However this means that if we later encounter a situation where we have a [Wanted] dw::D [a] we could solve it thus: dw := dfun dw Although recursive, this binding would pass the TcSMonadisGoodRecEv check because it appears as guarded. But in reality, it will make a bottom superclass. The trouble is that isGoodRecEv can't "see" the superclass-selection inside dfun. Our solution to this problem is to change the way ‘dfuns’ are created for instances, so that we pass as first arguments to the dfun some ``silent superclass arguments’’, which are the immediate superclasses of the dictionary we are trying to construct. In our example: dfun :: forall a. (C [a], D [a] -> D [a] dfun = \(dc::C [a]) (dd::D [a]) -> DOrd dc ... This gives us: ----------------------------------------------------------- DFun Superclass Invariant ~~~~~~~~~~~~~~~~~~~~~~~~ In the body of a DFun, every superclass argument to the returned dictionary is either * one of the arguments of the DFun, or * constant, bound at top level ----------------------------------------------------------- This means that no superclass is hidden inside a dfun application, so the counting argument in isGoodRecEv (more dfun calls than superclass selections) works correctly. The extra arguments required to satisfy the DFun Superclass Invariant always come first, and are called the "silent" arguments. DFun types are built (only) by MkId.mkDictFunId, so that is where we decide what silent arguments are to be added. This net effect is that it is safe to treat a dfun application as wrapping a dictionary constructor around its arguments (in particular, a dfun never picks superclasses from the arguments under the dictionary constructor). In our example, if we had [Wanted] dw :: D [a] we would get via the instance: dw := dfun d1 d2 [Wanted] (d1 :: C [a]) [Wanted] (d2 :: D [a]) [Derived] (d :: D [a]) [Derived] (scd :: C [a]) scd := scsel d [Derived] (scd2 :: C [a]) scd2 := scsel d2 And now, though we *can* solve: d2 := dw we will get an isGoodRecEv failure when we try to solve: d1 := scsel d or d1 := scsel d2 Test case SCLoop tests this fix. Note [SPECIALISE instance pragmas] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider instance (Ix a, Ix b) => Ix (a,b) where {-# SPECIALISE instance Ix (Int,Int) #-} range (x,y) = ... We do *not* want to make a specialised version of the dictionary function. Rather, we want specialised versions of each method. Thus we should generate something like this: $dfIx :: (Ix a, Ix x) => Ix (a,b) {- DFUN [$crange, ...] -} $dfIx da db = Ix ($crange da db) (...other methods...) $dfIxPair :: (Ix a, Ix x) => Ix (a,b) {- DFUN [$crangePair, ...] -} $dfIxPair = Ix ($crangePair da db) (...other methods...) $crange :: (Ix a, Ix b) -> ((a,b),(a,b)) -> [(a,b)] {-# SPECIALISE $crange :: ((Int,Int),(Int,Int)) -> [(Int,Int)] #-} $crange da db = <blah> {-# RULE range ($dfIx da db) = $crange da db #-} Note that * The RULE is unaffected by the specialisation. We don't want to specialise $dfIx, because then it would need a specialised RULE which is a pain. The single RULE works fine at all specialisations. See Note [How instance declarations are translated] above * Instead, we want to specialise the *method*, $crange In practice, rather than faking up a SPECIALISE pragama for each method (which is painful, since we'd have to figure out its specialised type), we call tcSpecPrag *as if* were going to specialise $dfIx -- you can see that in the call to tcSpecInst. That generates a SpecPrag which, as it turns out, can be used unchanged for each method. The "it turns out" bit is delicate, but it works fine! \begin{code}

    tcSpecInst :: Id -> Sig Name -> TcM TcSpecPrag
    tcSpecInst dfun_id prag@(SpecInstSig hs_ty)
      = addErrCtxt (spec_ctxt prag) $
        do  { let name = idName dfun_id
            ; (tyvars, theta, clas, tys) <- tcHsInstHead hs_ty
            ; let (_, spec_dfun_ty) = mkDictFunTy tyvars theta clas tys

            ; co_fn <- tcSubType (SpecPragOrigin name) SpecInstCtxt
                                 (idType dfun_id) spec_dfun_ty
            ; return (SpecPrag dfun_id co_fn defaultInlinePragma) }
      where
        spec_ctxt prag = hang (ptext (sLit "In the SPECIALISE pragma")) 2 (ppr prag)

    tcSpecInst _  _ = panic "tcSpecInst"

\end{code} %************************************************************************ %* * Type-checking an instance method %* * %************************************************************************ tcInstanceMethod - Make the method bindings, as a [(NonRec, HsBinds)], one per method - Remembering to use fresh Name (the instance method Name) as the binder - Bring the instance method Ids into scope, for the benefit of tcInstSig - Use sig_fn mapping instance method Name -> instance tyvars - Ditto prag_fn - Use tcValBinds to do the checking \begin{code}

    tcInstanceMethods :: DFunId -> Class -> [TcTyVar]
                      -> [EvVar]
    	 	  -> [TcType]
                      -> ([Located TcSpecPrag], PragFun)
    	  	  -> [(Id, DefMeth)]
                      -> InstBindings Name
              	  -> TcM ([Id], [LHsBind Id])
    	-- The returned inst_meth_ids all have types starting
    	--	forall tvs. theta => ...
    tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys
                      (spec_inst_prags, prag_fn)
                      op_items (VanillaInst binds _ standalone_deriv)
      = mapAndUnzipM tc_item op_items
      where
        ----------------------
        tc_item :: (Id, DefMeth) -> TcM (Id, LHsBind Id)
        tc_item (sel_id, dm_info)
          = case findMethodBind (idName sel_id) binds of
      	    Just user_bind -> tc_body sel_id standalone_deriv user_bind
      	    Nothing	   -> tc_default sel_id dm_info

        ----------------------
        tc_body :: Id -> Bool -> LHsBind Name -> TcM (TcId, LHsBind Id)
        tc_body sel_id generated_code rn_bind
          = add_meth_ctxt sel_id generated_code rn_bind $
            do { (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars
                                                       inst_tys sel_id
               ; let prags = prag_fn (idName sel_id)
               ; meth_id1 <- addInlinePrags meth_id prags
               ; spec_prags <- tcSpecPrags meth_id1 prags
               ; bind <- tcInstanceMethodBody InstSkol
                              tyvars dfun_ev_vars
                              meth_id1 local_meth_id meth_sig_fn
                              (mk_meth_spec_prags meth_id1 spec_prags)
                              rn_bind
               ; return (meth_id1, bind) }

        ----------------------
        tc_default :: Id -> DefMeth -> TcM (TcId, LHsBind Id)
        tc_default sel_id GenDefMeth    -- Derivable type classes stuff
          = do { meth_bind <- mkGenericDefMethBind clas inst_tys sel_id
               ; tc_body sel_id False {- Not generated code? -} meth_bind }

        tc_default sel_id NoDefMeth	    -- No default method at all
          = do { warnMissingMethod sel_id
        	   ; (meth_id, _) <- mkMethIds clas tyvars dfun_ev_vars
                                             inst_tys sel_id
               ; return (meth_id, mkVarBind meth_id $
                                  mkLHsWrap lam_wrapper error_rhs) }
          where
        	error_rhs    = L loc $ HsApp error_fun error_msg
        	error_fun    = L loc $ wrapId (WpTyApp meth_tau) nO_METHOD_BINDING_ERROR_ID
        	error_msg    = L loc (HsLit (HsStringPrim (mkFastString error_string)))
        	meth_tau     = funResultTy (applyTys (idType sel_id) inst_tys)
        	error_string = showSDoc (hcat [ppr loc, text "|", ppr sel_id ])
            lam_wrapper  = mkWpTyLams tyvars <.> mkWpLams dfun_ev_vars

        tc_default sel_id (DefMeth dm_name)	-- A polymorphic default method
          = do {   -- Build the typechecked version directly,
        		 -- without calling typecheck_method;
        		 -- see Note [Default methods in instances]
                     -- Generate   /\as.\ds. let self = df as ds
                     --                      in $dm inst_tys self
        		 -- The 'let' is necessary only because HsSyn doesn't allow
        		 -- you to apply a function to a dictionary *expression*.

               ; self_dict <- newEvVar (ClassP clas inst_tys)
               ; let self_ev_bind = EvBind self_dict $
                                    EvDFunApp dfun_id (mkTyVarTys tyvars) dfun_ev_vars

               ; (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars
                                                       inst_tys sel_id
               ; dm_id <- tcLookupId dm_name
               ; let dm_inline_prag = idInlinePragma dm_id
                     rhs = HsWrap (mkWpEvVarApps [self_dict] <.> mkWpTyApps inst_tys) $
        		         HsVar dm_id

        	         meth_bind = L loc $ VarBind { var_id = local_meth_id
                                                 , var_rhs = L loc rhs
                                                 , var_inline = False }
                     meth_id1 = meth_id `setInlinePragma` dm_inline_prag
        		   	    -- Copy the inline pragma (if any) from the default
        			    -- method to this version. Note [INLINE and default methods]

                     bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars =  dfun_ev_vars
                                     , abs_exports = [( tyvars, meth_id1, local_meth_id
                                                      , mk_meth_spec_prags meth_id1 [])]
                                     , abs_ev_binds = EvBinds (unitBag self_ev_bind)
                                     , abs_binds    = unitBag meth_bind }
        	     -- Default methods in an instance declaration can't have their own
        	     -- INLINE or SPECIALISE pragmas. It'd be possible to allow them, but
        	     -- currently they are rejected with
        	     --		  "INLINE pragma lacks an accompanying binding"

               ; return (meth_id1, L loc bind) }

        ----------------------
        mk_meth_spec_prags :: Id -> [LTcSpecPrag] -> TcSpecPrags
    	-- Adapt the SPECIALISE pragmas to work for this method Id
            -- There are two sources:
            --   * spec_inst_prags: {-# SPECIALISE instance :: <blah> #-}
            --     These ones have the dfun inside, but [perhaps surprisingly]
            --     the correct wrapper
            --   * spec_prags_for_me: {-# SPECIALISE op :: <blah> #-}
        mk_meth_spec_prags meth_id spec_prags_for_me
          = SpecPrags (spec_prags_for_me ++
                       [ L loc (SpecPrag meth_id wrap inl)
            	   | L loc (SpecPrag _ wrap inl) <- spec_inst_prags])

        loc = getSrcSpan dfun_id
        meth_sig_fn _ = Just ([],loc)	-- The 'Just' says "yes, there's a type sig"
    	-- But there are no scoped type variables from local_method_id
    	-- Only the ones from the instance decl itself, which are already
    	-- in scope.  Example:
    	--	class C a where { op :: forall b. Eq b => ... }
    	-- 	instance C [c] where { op = <rhs> }
    	-- In <rhs>, 'c' is scope but 'b' is not!

            -- For instance decls that come from standalone deriving clauses
    	-- we want to print out the full source code if there's an error
    	-- because otherwise the user won't see the code at all
        add_meth_ctxt sel_id generated_code rn_bind thing
          | generated_code = addLandmarkErrCtxt (derivBindCtxt sel_id clas inst_tys rn_bind) thing
          | otherwise      = thing

    tcInstanceMethods dfun_id clas tyvars dfun_ev_vars inst_tys
                      _ op_items (NewTypeDerived coi _)

    -- Running example:
    --   class Show b => Foo a b where
    --     op :: a -> b -> b
    --   newtype N a = MkN (Tree [a])
    --   deriving instance (Show p, Foo Int p) => Foo Int (N p)
    --		 -- NB: standalone deriving clause means
    --		 --     that the contex is user-specified
    -- Hence op :: forall a b. Foo a b => a -> b -> b
    --
    -- We're going to make an instance like
    --   instance (Show p, Foo Int p) => Foo Int (N p)
    --      op = $copT
    --
    --   $copT :: forall p. (Show p, Foo Int p) => Int -> N p -> N p
    --   $copT p (d1:Show p) (d2:Foo Int p)
    --     = op Int (Tree [p]) rep_d |> op_co
    --     where
    --       rep_d :: Foo Int (Tree [p]) = ...d1...d2...
    --       op_co :: (Int -> Tree [p] -> Tree [p]) ~ (Int -> T p -> T p)
    -- We get op_co by substituting [Int/a] and [co/b] in type for op
    -- where co : [p] ~ T p
    --
    -- Notice that the dictionary bindings "..d1..d2.." must be generated
    -- by the constraint solver, since the <context> may be
    -- user-specified.

      = do { rep_d_stuff <- checkConstraints InstSkol tyvars dfun_ev_vars $
                            emitWanted ScOrigin rep_pred

           ; mapAndUnzipM (tc_item rep_d_stuff) op_items }
      where
         loc = getSrcSpan dfun_id

         inst_tvs = fst (tcSplitForAllTys (idType dfun_id))
         Just (init_inst_tys, _) = snocView inst_tys
         rep_ty   = fst (coercionKind co)  -- [p]
         rep_pred = mkClassPred clas (init_inst_tys ++ [rep_ty])

         -- co : [p] ~ T p
         co = substTyWith inst_tvs (mkTyVarTys tyvars) $
              case coi of { IdCo ty -> ty ;
                            ACo co  -> mkSymCoercion co }

         ----------------
         tc_item :: (TcEvBinds, EvVar) -> (Id, DefMeth) -> TcM (TcId, LHsBind TcId)
         tc_item (rep_ev_binds, rep_d) (sel_id, _)
           = do { (meth_id, local_meth_id) <- mkMethIds clas tyvars dfun_ev_vars
                                                        inst_tys sel_id

                ; let meth_rhs  = wrapId (mk_op_wrapper sel_id rep_d) sel_id
                      meth_bind = VarBind { var_id = local_meth_id
                                          , var_rhs = L loc meth_rhs
        				      , var_inline = False }

    	          bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars
                                       , abs_exports = [(tyvars, meth_id,
                                                         local_meth_id, noSpecPrags)]
    				   , abs_ev_binds = rep_ev_binds
                                       , abs_binds = unitBag $ L loc meth_bind }

                ; return (meth_id, L loc bind) }

         ----------------
         mk_op_wrapper :: Id -> EvVar -> HsWrapper
         mk_op_wrapper sel_id rep_d
           = WpCast (substTyWith sel_tvs (init_inst_tys ++ [co]) local_meth_ty)
             <.> WpEvApp (EvId rep_d)
             <.> mkWpTyApps (init_inst_tys ++ [rep_ty])
           where
             (sel_tvs, sel_rho) = tcSplitForAllTys (idType sel_id)
             (_, local_meth_ty) = tcSplitPredFunTy_maybe sel_rho
                                  `orElse` pprPanic "tcInstanceMethods" (ppr sel_id)

    ----------------------
    mkMethIds :: Class -> [TcTyVar] -> [EvVar] -> [TcType] -> Id -> TcM (TcId, TcId)
    mkMethIds clas tyvars dfun_ev_vars inst_tys sel_id
      = do  { uniq <- newUnique
      	; let meth_name = mkDerivedInternalName mkClassOpAuxOcc uniq sel_name
      	; local_meth_name <- newLocalName sel_name
      		  -- Base the local_meth_name on the selector name, becuase
      		  -- type errors from tcInstanceMethodBody come from here

      	; let meth_id       = mkLocalId meth_name meth_ty
      	      local_meth_id = mkLocalId local_meth_name local_meth_ty
            ; return (meth_id, local_meth_id) }
      where
        local_meth_ty = instantiateMethod clas sel_id inst_tys
        meth_ty = mkForAllTys tyvars $ mkPiTypes dfun_ev_vars local_meth_ty
        sel_name = idName sel_id

    ----------------------
    wrapId :: HsWrapper -> id -> HsExpr id
    wrapId wrapper id = mkHsWrap wrapper (HsVar id)

    derivBindCtxt :: Id -> Class -> [Type ] -> LHsBind Name -> SDoc
    derivBindCtxt sel_id clas tys _bind
       = vcat [ ptext (sLit "When typechecking the code for ") <+> quotes (ppr sel_id)
              , nest 2 (ptext (sLit "in a standalone derived instance for")
    	  	    <+> quotes (pprClassPred clas tys) <> colon)
              , nest 2 $ ptext (sLit "To see the code I am typechecking, use -ddump-deriv") ]

    -- Too voluminous
    --	  , nest 2 $ pprSetDepth AllTheWay $ ppr bind ]

    warnMissingMethod :: Id -> TcM ()
    warnMissingMethod sel_id
      = do { warn <- doptM Opt_WarnMissingMethods
           ; warnTc (warn  -- Warn only if -fwarn-missing-methods
                     && not (startsWithUnderscore (getOccName sel_id)))
    					-- Don't warn about _foo methods
    		(ptext (sLit "No explicit method nor default method for")
                     <+> quotes (ppr sel_id)) }

\end{code} Note [Export helper functions] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We arrange to export the "helper functions" of an instance declaration, so that they are not subject to preInlineUnconditionally, even if their RHS is trivial. Reason: they are mentioned in the DFunUnfolding of the dict fun as Ids, not as CoreExprs, so we can't substitute a non-variable for them. We could change this by making DFunUnfoldings have CoreExprs, but it seems a bit simpler this way. Note [Default methods in instances] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this class Baz v x where foo :: x -> x foo y = <blah> instance Baz Int Int From the class decl we get $dmfoo :: forall v x. Baz v x => x -> x $dmfoo y = <blah> Notice that the type is ambiguous. That's fine, though. The instance decl generates $dBazIntInt = MkBaz fooIntInt fooIntInt = $dmfoo Int Int $dBazIntInt BUT this does mean we must generate the dictionary translation of fooIntInt directly, rather than generating source-code and type-checking it. That was the bug in Trac #1061. In any case it's less work to generate the translated version! Note [INLINE and default methods] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Default methods need special case. They are supposed to behave rather like macros. For exmample class Foo a where op1, op2 :: Bool -> a -> a {-# INLINE op1 #-} op1 b x = op2 (not b) x instance Foo Int where -- op1 via default method op2 b x = <blah> The instance declaration should behave just as if 'op1' had been defined with the code, and INLINE pragma, from its original definition. That is, just as if you'd written instance Foo Int where op2 b x = <blah> {-# INLINE op1 #-} op1 b x = op2 (not b) x So for the above example we generate: {-# INLINE $dmop1 #-} -- $dmop1 has an InlineCompulsory unfolding $dmop1 d b x = op2 d (not b) x $fFooInt = MkD $cop1 $cop2 {-# INLINE $cop1 #-} $cop1 = $dmop1 $fFooInt $cop2 = <blah> Note carefullly: * We *copy* any INLINE pragma from the default method $dmop1 to the instance $cop1. Otherwise we'll just inline the former in the latter and stop, which isn't what the user expected * Regardless of its pragma, we give the default method an unfolding with an InlineCompulsory source. That means that it'll be inlined at every use site, notably in each instance declaration, such as $cop1. This inlining must happen even though a) $dmop1 is not saturated in $cop1 b) $cop1 itself has an INLINE pragma It's vital that $dmop1 *is* inlined in this way, to allow the mutual recursion between $fooInt and $cop1 to be broken * To communicate the need for an InlineCompulsory to the desugarer (which makes the Unfoldings), we use the IsDefaultMethod constructor in TcSpecPrags. %************************************************************************ %* * \subsection{Error messages} %* * %************************************************************************ \begin{code}

    instDeclCtxt1 :: LHsType Name -> SDoc
    instDeclCtxt1 hs_inst_ty
      = inst_decl_ctxt (case unLoc hs_inst_ty of
                            HsForAllTy _ _ _ (L _ (HsPredTy pred)) -> ppr pred
                            HsPredTy pred                    -> ppr pred
                            _                                -> ppr hs_inst_ty)     -- Don't expect this
    instDeclCtxt2 :: Type -> SDoc
    instDeclCtxt2 dfun_ty
      = inst_decl_ctxt (ppr (mkClassPred cls tys))
      where
        (_,cls,tys) = tcSplitDFunTy dfun_ty

    inst_decl_ctxt :: SDoc -> SDoc
    inst_decl_ctxt doc = ptext (sLit "In the instance declaration for") <+> quotes doc

    atInstCtxt :: Name -> SDoc
    atInstCtxt name = ptext (sLit "In the associated type instance for") <+>
                      quotes (ppr name)

    mustBeVarArgErr :: Type -> SDoc
    mustBeVarArgErr ty =
      sep [ ptext (sLit "Arguments that do not correspond to a class parameter") <+>
            ptext (sLit "must be variables")
          , ptext (sLit "Instead of a variable, found") <+> ppr ty
          ]

    wrongATArgErr :: Type -> Type -> SDoc
    wrongATArgErr ty instTy =
      sep [ ptext (sLit "Type indexes must match class instance head")
          , ptext (sLit "Found") <+> quotes (ppr ty)
            <+> ptext (sLit "but expected") <+> quotes (ppr instTy)
          ]

\end{code}