% % (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % Handles @deriving@ clauses on @data@ declarations. \begin{code}

    module TcDeriv ( tcDeriving ) where

    #include "HsVersions.h"

    import HsSyn
    import DynFlags

    import Generics
    import TcRnMonad
    import TcEnv
    import TcClassDcl( tcAddDeclCtxt )	-- Small helper
    import TcGenDeriv			-- Deriv stuff
    import InstEnv
    import Inst
    import TcHsType
    import TcMType
    import TcSimplify

    import RnBinds
    import RnEnv
    import HscTypes

    import Class
    import Type
    import Coercion
    import ErrUtils
    import MkId
    import DataCon
    import Maybes
    import RdrName
    import Name
    import NameSet
    import TyCon
    import TcType
    import Var
    import VarSet
    import PrelNames
    import SrcLoc
    import Util
    import ListSetOps
    import Outputable
    import FastString
    import Bag

    import Control.Monad

\end{code} %************************************************************************ %* * Overview %* * %************************************************************************ Overall plan ~~~~~~~~~~~~ 1. Convert the decls (i.e. data/newtype deriving clauses, plus standalone deriving) to [EarlyDerivSpec] 2. Infer the missing contexts for the Left DerivSpecs 3. Add the derived bindings, generating InstInfos \begin{code}

    -- DerivSpec is purely  local to this module
    data DerivSpec  = DS { ds_loc     :: SrcSpan
    		     , ds_orig    :: CtOrigin
    		     , ds_name    :: Name
    		     , ds_tvs     :: [TyVar]
    		     , ds_theta   :: ThetaType
    		     , ds_cls     :: Class
    		     , ds_tys     :: [Type]
    		     , ds_tc      :: TyCon
    		     , ds_tc_args :: [Type]
    		     , ds_newtype :: Bool }
    	-- This spec implies a dfun declaration of the form
    	--	 df :: forall tvs. theta => C tys
    	-- The Name is the name for the DFun we'll build
    	-- The tyvars bind all the variables in the theta
    	-- For type families, the tycon in
    	--	 in ds_tys is the *family* tycon
    	--	 in ds_tc, ds_tc_args is the *representation* tycon
    	-- For non-family tycons, both are the same

    	-- ds_newtype = True  <=> Newtype deriving
    	--		False <=> Vanilla deriving

\end{code} Example: newtype instance T [a] = MkT (Tree a) deriving( C s ) ==> axiom T [a] = :RTList a axiom :RTList a = Tree a DS { ds_tvs = [a,s], ds_cls = C, ds_tys = [s, T [a]] , ds_tc = :RTList, ds_tc_args = [a] , ds_newtype = True } \begin{code}

    type DerivContext = Maybe ThetaType
       -- Nothing 	 <=> Vanilla deriving; infer the context of the instance decl
       -- Just theta <=> Standalone deriving: context supplied by programmer

    type EarlyDerivSpec = Either DerivSpec DerivSpec
    	-- Left  ds => the context for the instance should be inferred
    	--	       In this case ds_theta is the list of all the
    	--		  constraints needed, such as (Eq [a], Eq a)
    	--		  The inference process is to reduce this to a
    	--		  simpler form (e.g. Eq a)
    	--
    	-- Right ds => the exact context for the instance is supplied
    	--	       by the programmer; it is ds_theta

    pprDerivSpec :: DerivSpec -> SDoc
    pprDerivSpec (DS { ds_loc = l, ds_name = n, ds_tvs = tvs,
    		   ds_cls = c, ds_tys = tys, ds_theta = rhs })
      = parens (hsep [ppr l, ppr n, ppr tvs, ppr c, ppr tys]
    	    <+> equals <+> ppr rhs)

\end{code} Inferring missing contexts ~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider data T a b = C1 (Foo a) (Bar b) | C2 Int (T b a) | C3 (T a a) deriving (Eq) [NOTE: See end of these comments for what to do with data (C a, D b) => T a b = ... ] We want to come up with an instance declaration of the form instance (Ping a, Pong b, ...) => Eq (T a b) where x == y = ... It is pretty easy, albeit tedious, to fill in the code "...". The trick is to figure out what the context for the instance decl is, namely @Ping@, @Pong@ and friends. Let's call the context reqd for the T instance of class C at types (a,b, ...) C (T a b). Thus: Eq (T a b) = (Ping a, Pong b, ...) Now we can get a (recursive) equation from the @data@ decl: Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1 u Eq (T b a) u Eq Int -- From C2 u Eq (T a a) -- From C3 Foo and Bar may have explicit instances for @Eq@, in which case we can just substitute for them. Alternatively, either or both may have their @Eq@ instances given by @deriving@ clauses, in which case they form part of the system of equations. Now all we need do is simplify and solve the equations, iterating to find the least fixpoint. Notice that the order of the arguments can switch around, as here in the recursive calls to T. Let's suppose Eq (Foo a) = Eq a, and Eq (Bar b) = Ping b. We start with: Eq (T a b) = {} -- The empty set Next iteration: Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1 u Eq (T b a) u Eq Int -- From C2 u Eq (T a a) -- From C3 After simplification: = Eq a u Ping b u {} u {} u {} = Eq a u Ping b Next iteration: Eq (T a b) = Eq (Foo a) u Eq (Bar b) -- From C1 u Eq (T b a) u Eq Int -- From C2 u Eq (T a a) -- From C3 After simplification: = Eq a u Ping b u (Eq b u Ping a) u (Eq a u Ping a) = Eq a u Ping b u Eq b u Ping a The next iteration gives the same result, so this is the fixpoint. We need to make a canonical form of the RHS to ensure convergence. We do this by simplifying the RHS to a form in which - the classes constrain only tyvars - the list is sorted by tyvar (major key) and then class (minor key) - no duplicates, of course So, here are the synonyms for the ``equation'' structures: Note [Data decl contexts] ~~~~~~~~~~~~~~~~~~~~~~~~~ Consider data (RealFloat a) => Complex a = !a :+ !a deriving( Read ) We will need an instance decl like: instance (Read a, RealFloat a) => Read (Complex a) where ... The RealFloat in the context is because the read method for Complex is bound to construct a Complex, and doing that requires that the argument type is in RealFloat. But this ain't true for Show, Eq, Ord, etc, since they don't construct a Complex; they only take them apart. Our approach: identify the offending classes, and add the data type context to the instance decl. The "offending classes" are Read, Enum? FURTHER NOTE ADDED March 2002. In fact, Haskell98 now requires that pattern matching against a constructor from a data type with a context gives rise to the constraints for that context -- or at least the thinned version. So now all classes are "offending". Note [Newtype deriving] ~~~~~~~~~~~~~~~~~~~~~~~ Consider this: class C a b instance C [a] Char newtype T = T Char deriving( C [a] ) Notice the free 'a' in the deriving. We have to fill this out to newtype T = T Char deriving( forall a. C [a] ) And then translate it to: instance C [a] Char => C [a] T where ... Note [Newtype deriving superclasses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (See also Trac #1220 for an interesting exchange on newtype deriving and superclasses.) The 'tys' here come from the partial application in the deriving clause. The last arg is the new instance type. We must pass the superclasses; the newtype might be an instance of them in a different way than the representation type E.g. newtype Foo a = Foo a deriving( Show, Num, Eq ) Then the Show instance is not done via isomorphism; it shows Foo 3 as "Foo 3" The Num instance is derived via isomorphism, but the Show superclass dictionary must the Show instance for Foo, *not* the Show dictionary gotten from the Num dictionary. So we must build a whole new dictionary not just use the Num one. The instance we want is something like: instance (Num a, Show (Foo a), Eq (Foo a)) => Num (Foo a) where (+) = ((+)@a) ...etc... There may be a coercion needed which we get from the tycon for the newtype when the dict is constructed in TcInstDcls.tcInstDecl2 Note [Unused constructors and deriving clauses] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ See Trac #3221. Consider data T = T1 | T2 deriving( Show ) Are T1 and T2 unused? Well, no: the deriving clause expands to mention both of them. So we gather defs/uses from deriving just like anything else. %************************************************************************ %* * \subsection[TcDeriv-driver]{Top-level function for \tr{derivings}} %* * %************************************************************************ \begin{code}

    tcDeriving  :: [LTyClDecl Name]  -- All type constructors
                -> [LInstDecl Name]  -- All instance declarations
                -> [LDerivDecl Name] -- All stand-alone deriving declarations
    	    -> TcM ([InstInfo Name],	-- The generated "instance decls"
    		    HsValBinds Name,	-- Extra generated top-level bindings
                        DefUses)

    tcDeriving tycl_decls inst_decls deriv_decls
      = recoverM (return ([], emptyValBindsOut, emptyDUs)) $
        do	{   	-- Fish the "deriving"-related information out of the TcEnv
    		-- And make the necessary "equations".
    	  is_boot <- tcIsHsBoot
    	; traceTc "tcDeriving" (ppr is_boot)
    	; early_specs <- makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls

    	; overlap_flag <- getOverlapFlag
    	; let (infer_specs, given_specs) = splitEithers early_specs
    	; insts1 <- mapM (genInst True overlap_flag) given_specs

    	; final_specs <- extendLocalInstEnv (map (iSpec . fst) insts1) $
    			 inferInstanceContexts overlap_flag infer_specs

    	; insts2 <- mapM (genInst False overlap_flag) final_specs

    		 -- Generate the generic to/from functions from each type declaration
    	; gen_binds <- mkGenericBinds is_boot tycl_decls
    	; (inst_info, rn_binds, rn_dus) <- renameDeriv is_boot gen_binds (insts1 ++ insts2)

            ; when (not (null inst_info)) $
              dumpDerivingInfo (ddump_deriving inst_info rn_binds)

    	; return (inst_info, rn_binds, rn_dus) }
      where
        ddump_deriving :: [InstInfo Name] -> HsValBinds Name -> SDoc
        ddump_deriving inst_infos extra_binds
          = hang (ptext (sLit "Derived instances"))
               2 (vcat (map (\i -> pprInstInfoDetails i $$ text "") inst_infos)
                  $$ ppr extra_binds)

    renameDeriv :: Bool -> LHsBinds RdrName
    	    -> [(InstInfo RdrName, DerivAuxBinds)]
     	    -> TcM ([InstInfo Name], HsValBinds Name, DefUses)
    renameDeriv is_boot gen_binds insts
      | is_boot	-- If we are compiling a hs-boot file, don't generate any derived bindings
    		-- The inst-info bindings will all be empty, but it's easier to
    		-- just use rn_inst_info to change the type appropriately
      = do	{ (rn_inst_infos, fvs) <- mapAndUnzipM rn_inst_info inst_infos
    	; return (rn_inst_infos, emptyValBindsOut, usesOnly (plusFVs fvs)) }

      | otherwise
      = discardWarnings $ 	 -- Discard warnings about unused bindings etc
        do	{ (rn_gen, dus_gen) <- setOptM Opt_ScopedTypeVariables $  -- Type signatures in patterns
    								  -- are used in the generic binds
    			       rnTopBinds (ValBindsIn gen_binds [])
    	; keepAliveSetTc (duDefs dus_gen)	-- Mark these guys to be kept alive

    		-- Generate and rename any extra not-one-inst-decl-specific binds,
    		-- notably "con2tag" and/or "tag2con" functions.
    		-- Bring those names into scope before renaming the instances themselves
    	; loc <- getSrcSpanM	-- Generic loc for shared bindings
    	; let (aux_binds, aux_sigs) = unzip $ map (genAuxBind loc) $
    	                              rm_dups [] $ concat deriv_aux_binds
                  aux_val_binds = ValBindsIn (listToBag aux_binds) aux_sigs
    	; rn_aux_lhs <- rnTopBindsLHS emptyFsEnv aux_val_binds
    	; bindLocalNames (collectHsValBinders rn_aux_lhs) $
        do	{ (rn_aux, dus_aux) <- rnTopBindsRHS rn_aux_lhs
    	; (rn_inst_infos, fvs_insts) <- mapAndUnzipM rn_inst_info inst_infos
    	; return (rn_inst_infos, rn_aux `plusHsValBinds` rn_gen,
                      dus_gen `plusDU` dus_aux `plusDU` usesOnly (plusFVs fvs_insts)) } }

      where
        (inst_infos, deriv_aux_binds) = unzip insts

    	-- Remove duplicate requests for auxilliary bindings
        rm_dups acc [] = acc
        rm_dups acc (b:bs) | any (isDupAux b) acc = rm_dups acc bs
        		       | otherwise	      = rm_dups (b:acc) bs

        rn_inst_info :: InstInfo RdrName -> TcM (InstInfo Name, FreeVars)
        rn_inst_info info@(InstInfo { iBinds = NewTypeDerived coi tc })
    	= return ( info { iBinds = NewTypeDerived coi tc }
                     , mkFVs (map dataConName (tyConDataCons tc)))
    	  -- See Note [Newtype deriving and unused constructors]

        rn_inst_info inst_info@(InstInfo { iSpec = inst, iBinds = VanillaInst binds sigs standalone_deriv })
    	= 	-- Bring the right type variables into
    		-- scope (yuk), and rename the method binds
    	   ASSERT( null sigs )
    	   bindLocalNames (map Var.varName tyvars) $
     	   do { (rn_binds, fvs) <- rnMethodBinds clas_nm (\_ -> []) [] binds
    	      ; let binds' = VanillaInst rn_binds [] standalone_deriv
                  ; return (inst_info { iBinds = binds' }, fvs) }
    	where
    	  (tyvars,_, clas,_) = instanceHead inst
    	  clas_nm            = className clas

    -----------------------------------------
    mkGenericBinds :: Bool -> [LTyClDecl Name] -> TcM (LHsBinds RdrName)
    mkGenericBinds is_boot tycl_decls
      | is_boot
      = return emptyBag
      | otherwise
      = do	{ tcs <- mapM tcLookupTyCon [ tcdName d
        	      	      		    | L _ d <- tycl_decls, isDataDecl d ]
    	; return (unionManyBags [ mkTyConGenericBinds tc
    				| tc <- tcs, tyConHasGenerics tc ]) }
    		-- We are only interested in the data type declarations,
    		-- and then only in the ones whose 'has-generics' flag is on
    		-- The predicate tyConHasGenerics finds both of these

\end{code} Note [Newtype deriving and unused constructors] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider this (see Trac #1954): module Bug(P) where newtype P a = MkP (IO a) deriving Monad If you compile with -fwarn-unused-binds you do not expect the warning "Defined but not used: data consructor MkP". Yet the newtype deriving code does not explicitly mention MkP, but it should behave as if you had written instance Monad P where return x = MkP (return x) ...etc... So we want to signal a user of the data constructor 'MkP'. That's what we do in rn_inst_info, and it's the only reason we have the TyCon stored in NewTypeDerived. %************************************************************************ %* * From HsSyn to DerivSpec %* * %************************************************************************ @makeDerivSpecs@ fishes around to find the info about needed derived instances. \begin{code}

    makeDerivSpecs :: Bool
    	       -> [LTyClDecl Name]
                   -> [LInstDecl Name]
    	       -> [LDerivDecl Name]
    	       -> TcM [EarlyDerivSpec]

    makeDerivSpecs is_boot tycl_decls inst_decls deriv_decls
      | is_boot 	-- No 'deriving' at all in hs-boot files
      = do	{ mapM_ add_deriv_err deriv_locs
    	; return [] }
      | otherwise
      = do	{ eqns1 <- mapAndRecoverM deriveTyData all_tydata
    	; eqns2 <- mapAndRecoverM deriveStandalone deriv_decls
    	; return (eqns1 ++ eqns2) }
      where
        extractTyDataPreds decls
          = [(p, d) | d@(L _ (TyData {tcdDerivs = Just preds})) <- decls, p <- preds]

        all_tydata :: [(LHsType Name, LTyClDecl Name)]
    	-- Derived predicate paired with its data type declaration
        all_tydata = extractTyDataPreds (instDeclATs inst_decls ++ tycl_decls)

        deriv_locs = map (getLoc . snd) all_tydata
    		 ++ map getLoc deriv_decls

        add_deriv_err loc = setSrcSpan loc $
    			addErr (hang (ptext (sLit "Deriving not permitted in hs-boot file"))
    				   2 (ptext (sLit "Use an instance declaration instead")))

    ------------------------------------------------------------------
    deriveStandalone :: LDerivDecl Name -> TcM EarlyDerivSpec
    -- Standalone deriving declarations
    --  e.g.   deriving instance Show a => Show (T a)
    -- Rather like tcLocalInstDecl
    deriveStandalone (L loc (DerivDecl deriv_ty))
      = setSrcSpan loc                   $
        addErrCtxt (standaloneCtxt deriv_ty)  $
        do { traceTc "Standalone deriving decl for" (ppr deriv_ty)
           ; (tvs, theta, cls, inst_tys) <- tcHsInstHead deriv_ty
           ; traceTc "Standalone deriving;" $ vcat
                  [ text "tvs:" <+> ppr tvs
                  , text "theta:" <+> ppr theta
                  , text "cls:" <+> ppr cls
                  , text "tys:" <+> ppr inst_tys ]
           ; checkValidInstance deriv_ty tvs theta cls inst_tys
    		-- C.f. TcInstDcls.tcLocalInstDecl1

           ; let cls_tys = take (length inst_tys - 1) inst_tys
                 inst_ty = last inst_tys
           ; traceTc "Standalone deriving:" $ vcat
                  [ text "class:" <+> ppr cls
                  , text "class types:" <+> ppr cls_tys
                  , text "type:" <+> ppr inst_ty ]
           ; mkEqnHelp StandAloneDerivOrigin tvs cls cls_tys inst_ty
                       (Just theta) }

    ------------------------------------------------------------------
    deriveTyData :: (LHsType Name, LTyClDecl Name) -> TcM EarlyDerivSpec
    deriveTyData (L loc deriv_pred, L _ decl@(TyData { tcdLName = L _ tycon_name,
    					           tcdTyVars = tv_names,
    				    	           tcdTyPats = ty_pats }))
      = setSrcSpan loc     $	-- Use the location of the 'deriving' item
        tcAddDeclCtxt decl $
        do	{ (tvs, tc, tc_args) <- get_lhs ty_pats
    	; tcExtendTyVarEnv tvs $	-- Deriving preds may (now) mention
    					-- the type variables for the type constructor

        do	{ (deriv_tvs, cls, cls_tys) <- tcHsDeriv deriv_pred
    		-- The "deriv_pred" is a LHsType to take account of the fact that for
    		-- newtype deriving we allow deriving (forall a. C [a]).

    	-- Given data T a b c = ... deriving( C d ),
    	-- we want to drop type variables from T so that (C d (T a)) is well-kinded
    	; let cls_tyvars = classTyVars cls
    	      kind = tyVarKind (last cls_tyvars)
    	      (arg_kinds, _) = splitKindFunTys kind
    	      n_args_to_drop = length arg_kinds
    	      n_args_to_keep = tyConArity tc - n_args_to_drop
    	      args_to_drop   = drop n_args_to_keep tc_args
    	      inst_ty        = mkTyConApp tc (take n_args_to_keep tc_args)
    	      inst_ty_kind   = typeKind inst_ty
    	      dropped_tvs    = mkVarSet (mapCatMaybes getTyVar_maybe args_to_drop)
    	      univ_tvs       = (mkVarSet tvs `extendVarSetList` deriv_tvs)
    					`minusVarSet` dropped_tvs

    	-- Check that the result really is well-kinded
    	; checkTc (n_args_to_keep >= 0 && (inst_ty_kind `eqKind` kind))
    		  (derivingKindErr tc cls cls_tys kind)

    	; checkTc (sizeVarSet dropped_tvs == n_args_to_drop && 		 -- (a)
    	           tyVarsOfTypes (inst_ty:cls_tys) `subVarSet` univ_tvs) -- (b)
    		  (derivingEtaErr cls cls_tys inst_ty)
    		-- Check that
    		--  (a) The data type can be eta-reduced; eg reject:
    		--		data instance T a a = ... deriving( Monad )
    		--  (b) The type class args do not mention any of the dropped type
    		--      variables
    		--		newtype T a s = ... deriving( ST s )

    	-- Type families can't be partially applied
    	-- e.g.   newtype instance T Int a = MkT [a] deriving( Monad )
    	-- Note [Deriving, type families, and partial applications]
    	; checkTc (not (isFamilyTyCon tc) || n_args_to_drop == 0)
    		  (typeFamilyPapErr tc cls cls_tys inst_ty)

    	; mkEqnHelp DerivOrigin (varSetElems univ_tvs) cls cls_tys inst_ty Nothing } }
      where
    	-- Tiresomely we must figure out the "lhs", which is awkward for type families
    	-- E.g.   data T a b = .. deriving( Eq )
    	-- 	    Here, the lhs is (T a b)
    	--	  data instance TF Int b = ... deriving( Eq )
    	--	    Here, the lhs is (TF Int b)
    	-- But if we just look up the tycon_name, we get is the *family*
    	-- tycon, but not pattern types -- they are in the *rep* tycon.
        get_lhs Nothing     = do { tc <- tcLookupTyCon tycon_name
    			     ; let tvs = tyConTyVars tc
    			     ; return (tvs, tc, mkTyVarTys tvs) }
        get_lhs (Just pats) = do { let hs_app = nlHsTyConApp tycon_name pats
    			     ; (tvs, tc_app) <- tcHsQuantifiedType tv_names hs_app
    			     ; let (tc, tc_args) = tcSplitTyConApp tc_app
    			     ; return (tvs, tc, tc_args) }

    deriveTyData _other
      = panic "derivTyData"	-- Caller ensures that only TyData can happen

\end{code} Note [Deriving, type families, and partial applications] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When there are no type families, it's quite easy: newtype S a = MkS [a] -- :CoS :: S ~ [] -- Eta-reduced instance Eq [a] => Eq (S a) -- by coercion sym (Eq (:CoS a)) : Eq [a] ~ Eq (S a) instance Monad [] => Monad S -- by coercion sym (Monad :CoS) : Monad [] ~ Monad S When type familes are involved it's trickier: data family T a b newtype instance T Int a = MkT [a] deriving( Eq, Monad ) -- :RT is the representation type for (T Int a) -- :CoF:R1T a :: T Int a ~ :RT a -- Not eta reduced -- :Co:R1T :: :RT ~ [] -- Eta-reduced instance Eq [a] => Eq (T Int a) -- easy by coercion instance Monad [] => Monad (T Int) -- only if we can eta reduce??? The "???" bit is that we don't build the :CoF thing in eta-reduced form Henc the current typeFamilyPapErr, even though the instance makes sense. After all, we can write it out instance Monad [] => Monad (T Int) -- only if we can eta reduce??? return x = MkT [x] ... etc ... \begin{code}

    mkEqnHelp :: CtOrigin -> [TyVar] -> Class -> [Type] -> Type
              -> DerivContext	-- Just    => context supplied (standalone deriving)
    				-- Nothing => context inferred (deriving on data decl)
              -> TcRn EarlyDerivSpec
    -- Make the EarlyDerivSpec for an instance
    --	forall tvs. theta => cls (tys ++ [ty])
    -- where the 'theta' is optional (that's the Maybe part)
    -- Assumes that this declaration is well-kinded

    mkEqnHelp orig tvs cls cls_tys tc_app mtheta
      | Just (tycon, tc_args) <- tcSplitTyConApp_maybe tc_app
      , isAlgTyCon tycon	-- Check for functions, primitive types etc
      = do	{ (rep_tc, rep_tc_args) <- tcLookupDataFamInst tycon tc_args
    	          -- Be careful to test rep_tc here: in the case of families,
    	          -- we want to check the instance tycon, not the family tycon

    	-- For standalone deriving (mtheta /= Nothing),
    	-- check that all the data constructors are in scope.
    	-- No need for this when deriving Typeable, becuase we don't need
    	-- the constructors for that.
    	; rdr_env <- getGlobalRdrEnv
    	; let hidden_data_cons = isAbstractTyCon rep_tc || any not_in_scope (tyConDataCons rep_tc)
    	      not_in_scope dc  = null (lookupGRE_Name rdr_env (dataConName dc))
    	; checkTc (isNothing mtheta ||
    	  	   not hidden_data_cons ||
    		   className cls `elem` typeableClassNames)
    		  (derivingHiddenErr tycon)

    	; dflags <- getDOpts
    	; if isDataTyCon rep_tc then
    		mkDataTypeEqn orig dflags tvs cls cls_tys
    			      tycon tc_args rep_tc rep_tc_args mtheta
    	  else
    		mkNewTypeEqn orig dflags tvs cls cls_tys
    			     tycon tc_args rep_tc rep_tc_args mtheta }
      | otherwise
      = failWithTc (derivingThingErr False cls cls_tys tc_app
    	       (ptext (sLit "The last argument of the instance must be a data or newtype application")))

\end{code} %************************************************************************ %* * Deriving data types %* * %************************************************************************ \begin{code}

    mkDataTypeEqn :: CtOrigin
                  -> DynFlags
                  -> [Var]                  -- Universally quantified type variables in the instance
                  -> Class                  -- Class for which we need to derive an instance
                  -> [Type]                 -- Other parameters to the class except the last
                  -> TyCon                  -- Type constructor for which the instance is requested
    					--    (last parameter to the type class)
                  -> [Type]                 -- Parameters to the type constructor
                  -> TyCon                  -- rep of the above (for type families)
                  -> [Type]                 -- rep of the above
                  -> DerivContext        -- Context of the instance, for standalone deriving
                  -> TcRn EarlyDerivSpec    -- Return 'Nothing' if error

    mkDataTypeEqn orig dflags tvs cls cls_tys
                  tycon tc_args rep_tc rep_tc_args mtheta
      = case checkSideConditions dflags mtheta cls cls_tys rep_tc of
    	-- NB: pass the *representation* tycon to checkSideConditions
    	CanDerive               -> go_for_it
    	NonDerivableClass	-> bale_out (nonStdErr cls)
    	DerivableClassError msg -> bale_out msg
      where
        go_for_it    = mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
        bale_out msg = failWithTc (derivingThingErr False cls cls_tys (mkTyConApp tycon tc_args) msg)

    mk_data_eqn, mk_typeable_eqn
       :: CtOrigin -> [TyVar] -> Class
       -> TyCon -> [TcType] -> TyCon -> [TcType] -> DerivContext
       -> TcM EarlyDerivSpec
    mk_data_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
      | getName cls `elem` typeableClassNames
      = mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta

      | otherwise
      = do	{ dfun_name <- new_dfun_name cls tycon
      	; loc <- getSrcSpanM
    	; let inst_tys = [mkTyConApp tycon tc_args]
    	      inferred_constraints = inferConstraints tvs cls inst_tys rep_tc rep_tc_args
    	      spec = DS { ds_loc = loc, ds_orig = orig
    			, ds_name = dfun_name, ds_tvs = tvs
    			, ds_cls = cls, ds_tys = inst_tys
    			, ds_tc = rep_tc, ds_tc_args = rep_tc_args
    			, ds_theta =  mtheta `orElse` inferred_constraints
    			, ds_newtype = False }

      	; return (if isJust mtheta then Right spec	-- Specified context
    				   else Left spec) }	-- Infer context

    mk_typeable_eqn orig tvs cls tycon tc_args rep_tc rep_tc_args mtheta
    	-- The Typeable class is special in several ways
    	-- 	  data T a b = ... deriving( Typeable )
    	-- gives
    	--	  instance Typeable2 T where ...
    	-- Notice that:
    	-- 1. There are no constraints in the instance
    	-- 2. There are no type variables either
    	-- 3. The actual class we want to generate isn't necessarily
    	--	Typeable; it depends on the arity of the type
      | isNothing mtheta	-- deriving on a data type decl
      = do	{ checkTc (cls `hasKey` typeableClassKey)
    		  (ptext (sLit "Use deriving( Typeable ) on a data type declaration"))
    	; real_cls <- tcLookupClass (typeableClassNames !! tyConArity tycon)
    	; mk_typeable_eqn orig tvs real_cls tycon [] rep_tc [] (Just []) }

      | otherwise		-- standaone deriving
      = do	{ checkTc (null tc_args)
    		  (ptext (sLit "Derived typeable instance must be of form (Typeable")
    			<> int (tyConArity tycon) <+> ppr tycon <> rparen)
    	; dfun_name <- new_dfun_name cls tycon
      	; loc <- getSrcSpanM
    	; return (Right $
    		  DS { ds_loc = loc, ds_orig = orig, ds_name = dfun_name, ds_tvs = []
    		     , ds_cls = cls, ds_tys = [mkTyConApp tycon []]
    		     , ds_tc = rep_tc, ds_tc_args = rep_tc_args
    		     , ds_theta = mtheta `orElse` [], ds_newtype = False })  }

    inferConstraints :: [TyVar] -> Class -> [TcType] -> TyCon -> [TcType] -> ThetaType
    -- Generate a sufficiently large set of constraints that typechecking the
    -- generated method definitions should succeed.   This set will be simplified
    -- before being used in the instance declaration
    inferConstraints _ cls inst_tys rep_tc rep_tc_args
      = ASSERT2( equalLength rep_tc_tvs all_rep_tc_args, ppr cls <+> ppr rep_tc )
        stupid_constraints ++ extra_constraints
        ++ sc_constraints ++ con_arg_constraints
      where
           -- Constraints arising from the arguments of each constructor
        con_arg_constraints
          = [ mkClassPred cls [arg_ty]
            | data_con <- tyConDataCons rep_tc,
              arg_ty   <- ASSERT( isVanillaDataCon data_con )
        			get_constrained_tys $
        		 	dataConInstOrigArgTys data_con all_rep_tc_args,
              not (isUnLiftedType arg_ty) ]
        		-- No constraints for unlifted types
        		-- Where they are legal we generate specilised function calls

        		-- For functor-like classes, two things are different
        		-- (a) We recurse over argument types to generate constraints
        		--     See Functor examples in TcGenDeriv
        		-- (b) The rep_tc_args will be one short
        is_functor_like = getUnique cls `elem` functorLikeClassKeys

        get_constrained_tys :: [Type] -> [Type]
        get_constrained_tys tys
            | is_functor_like = concatMap (deepSubtypesContaining last_tv) tys
        	| otherwise	  = tys

        rep_tc_tvs = tyConTyVars rep_tc
        last_tv = last rep_tc_tvs
        all_rep_tc_args | is_functor_like = rep_tc_args ++ [mkTyVarTy last_tv]
        		    | otherwise       = rep_tc_args

        	-- Constraints arising from superclasses
        	-- See Note [Superclasses of derived instance]
        sc_constraints = substTheta (zipOpenTvSubst (classTyVars cls) inst_tys)
        				(classSCTheta cls)

        	-- Stupid constraints
        stupid_constraints = substTheta subst (tyConStupidTheta rep_tc)
        subst = zipTopTvSubst rep_tc_tvs all_rep_tc_args

    	-- Extra Data constraints
    	-- The Data class (only) requires that for
    	--    instance (...) => Data (T t1 t2)
    	-- IF   t1:*, t2:*
    	-- THEN (Data t1, Data t2) are among the (...) constraints
    	-- Reason: when the IF holds, we generate a method
    	-- 	       dataCast2 f = gcast2 f
    	--         and we need the Data constraints to typecheck the method
        extra_constraints
          | cls `hasKey` dataClassKey
          , all (isLiftedTypeKind . typeKind) rep_tc_args
          = [mkClassPred cls [ty] | ty <- rep_tc_args]
          | otherwise
          = []

    ------------------------------------------------------------------
    -- Check side conditions that dis-allow derivability for particular classes
    -- This is *apart* from the newtype-deriving mechanism
    --
    -- Here we get the representation tycon in case of family instances as it has
    -- the data constructors - but we need to be careful to fall back to the
    -- family tycon (with indexes) in error messages.

    data DerivStatus = CanDerive
    		 | DerivableClassError SDoc	-- Standard class, but can't do it
         		 | NonDerivableClass		-- Non-standard class

    checkSideConditions :: DynFlags -> DerivContext -> Class -> [TcType] -> TyCon -> DerivStatus
    checkSideConditions dflags mtheta cls cls_tys rep_tc
      | Just cond <- sideConditions mtheta cls
      = case (cond (dflags, rep_tc)) of
    	Just err -> DerivableClassError err	-- Class-specific error
    	Nothing  | null cls_tys -> CanDerive	-- All derivable classes are unary, so
    						-- cls_tys (the type args other than last)
    						-- should be null
    		 | otherwise    -> DerivableClassError ty_args_why	-- e.g. deriving( Eq s )
      | otherwise = NonDerivableClass	-- Not a standard class
      where
        ty_args_why	= quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "is not a class")

    nonStdErr :: Class -> SDoc
    nonStdErr cls = quotes (ppr cls) <+> ptext (sLit "is not a derivable class")

    sideConditions :: DerivContext -> Class -> Maybe Condition
    sideConditions mtheta cls
      | cls_key == eqClassKey      	   = Just cond_std
      | cls_key == ordClassKey     	   = Just cond_std
      | cls_key == showClassKey    	   = Just cond_std
      | cls_key == readClassKey    	   = Just (cond_std `andCond` cond_noUnliftedArgs)
      | cls_key == enumClassKey    	   = Just (cond_std `andCond` cond_isEnumeration)
      | cls_key == ixClassKey      	   = Just (cond_std `andCond` cond_enumOrProduct)
      | cls_key == boundedClassKey 	   = Just (cond_std `andCond` cond_enumOrProduct)
      | cls_key == dataClassKey    	   = Just (checkFlag Opt_DeriveDataTypeable `andCond`
                                               cond_std `andCond` cond_noUnliftedArgs)
      | cls_key == functorClassKey 	   = Just (checkFlag Opt_DeriveFunctor `andCond`
        	                                   cond_functorOK True)	 -- NB: no cond_std!
      | cls_key == foldableClassKey	   = Just (checkFlag Opt_DeriveFoldable `andCond`
        	                                   cond_functorOK False) -- Functor/Fold/Trav works ok for rank-n types
      | cls_key == traversableClassKey = Just (checkFlag Opt_DeriveTraversable `andCond`
        	                                   cond_functorOK False)
      | getName cls `elem` typeableClassNames = Just (checkFlag Opt_DeriveDataTypeable `andCond` cond_typeableOK)
      | otherwise = Nothing
      where
        cls_key = getUnique cls
        cond_std = cond_stdOK mtheta

    type Condition = (DynFlags, TyCon) -> Maybe SDoc
    	-- first Bool is whether or not we are allowed to derive Data and Typeable
    	-- second Bool is whether or not we are allowed to derive Functor
    	-- TyCon is the *representation* tycon if the
    	--	data type is an indexed one
    	-- Nothing => OK

    orCond :: Condition -> Condition -> Condition
    orCond c1 c2 tc
      = case c1 tc of
    	Nothing -> Nothing	    -- c1 succeeds
    	Just x  -> case c2 tc of    -- c1 fails
    		     Nothing -> Nothing
    		     Just y  -> Just (x $$ ptext (sLit "  and") $$ y)
    			            -- Both fail

    andCond :: Condition -> Condition -> Condition
    andCond c1 c2 tc = case c1 tc of
    		     Nothing -> c2 tc	-- c1 succeeds
    		     Just x  -> Just x	-- c1 fails

    cond_stdOK :: DerivContext -> Condition
    cond_stdOK (Just _) _
      = Nothing	-- Don't check these conservative conditions for
    		-- standalone deriving; just generate the code
    		-- and let the typechecker handle the result
    cond_stdOK Nothing (_, rep_tc)
      | null data_cons      = Just (no_cons_why rep_tc $$ suggestion)
      | not (null con_whys) = Just (vcat con_whys $$ suggestion)
      | otherwise      	= Nothing
      where
        suggestion  = ptext (sLit "Possible fix: use a standalone deriving declaration instead")
        data_cons   = tyConDataCons rep_tc
        con_whys = mapCatMaybes check_con data_cons

        check_con :: DataCon -> Maybe SDoc
        check_con con
          | isVanillaDataCon con
          , all isTauTy (dataConOrigArgTys con) = Nothing
          | otherwise = Just (badCon con (ptext (sLit "does not have a Haskell-98 type")))

    no_cons_why :: TyCon -> SDoc
    no_cons_why rep_tc = quotes (pprSourceTyCon rep_tc) <+>
    		     ptext (sLit "has no data constructors")

    cond_enumOrProduct :: Condition
    cond_enumOrProduct = cond_isEnumeration `orCond`
    		       (cond_isProduct `andCond` cond_noUnliftedArgs)

    cond_noUnliftedArgs :: Condition
    -- For some classes (eg Eq, Ord) we allow unlifted arg types
    -- by generating specilaised code.  For others (eg Data) we don't.
    cond_noUnliftedArgs (_, tc)
      | null bad_cons = Nothing
      | otherwise     = Just why
      where
        bad_cons = [ con | con <- tyConDataCons tc
    		     , any isUnLiftedType (dataConOrigArgTys con) ]
        why = badCon (head bad_cons) (ptext (sLit "has arguments of unlifted type"))

    cond_isEnumeration :: Condition
    cond_isEnumeration (_, rep_tc)
      | isEnumerationTyCon rep_tc   = Nothing
      | otherwise		        = Just why
      where
        why = sep [ quotes (pprSourceTyCon rep_tc) <+>
    	          ptext (sLit "is not an enumeration type")
                  , ptext (sLit "(an enumeration consists of one or more nullary, non-GADT constructors)") ]
    		  -- See Note [Enumeration types] in TyCon

    cond_isProduct :: Condition
    cond_isProduct (_, rep_tc)
      | isProductTyCon rep_tc = Nothing
      | otherwise	          = Just why
      where
        why = quotes (pprSourceTyCon rep_tc) <+>
    	  ptext (sLit "does not have precisely one constructor")

    cond_typeableOK :: Condition
    -- OK for Typeable class
    -- Currently: (a) args all of kind *
    --	      (b) 7 or fewer args
    cond_typeableOK (_, rep_tc)
      | tyConArity rep_tc > 7	= Just too_many
      | not (all (isSubArgTypeKind . tyVarKind) (tyConTyVars rep_tc))
                                    = Just bad_kind
      | isFamInstTyCon rep_tc	= Just fam_inst  -- no Typable for family insts
      | otherwise	  		= Nothing
      where
        too_many = quotes (pprSourceTyCon rep_tc) <+>
    	       ptext (sLit "has too many arguments")
        bad_kind = quotes (pprSourceTyCon rep_tc) <+>
    	       ptext (sLit "has arguments of kind other than `*'")
        fam_inst = quotes (pprSourceTyCon rep_tc) <+>
    	       ptext (sLit "is a type family")

    functorLikeClassKeys :: [Unique]
    functorLikeClassKeys = [functorClassKey, foldableClassKey, traversableClassKey]

    cond_functorOK :: Bool -> Condition
    -- OK for Functor/Foldable/Traversable class
    -- Currently: (a) at least one argument
    --            (b) don't use argument contravariantly
    --            (c) don't use argument in the wrong place, e.g. data T a = T (X a a)
    --            (d) optionally: don't use function types
    --            (e) no "stupid context" on data type
    cond_functorOK allowFunctions (_, rep_tc)
      | null tc_tvs
      = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
              <+> ptext (sLit "has no parameters"))

      | not (null bad_stupid_theta)
      = Just (ptext (sLit "Data type") <+> quotes (ppr rep_tc)
              <+> ptext (sLit "has a class context") <+> pprTheta bad_stupid_theta)

      | otherwise
      = msum (map check_con data_cons)	-- msum picks the first 'Just', if any
      where
        tc_tvs            = tyConTyVars rep_tc
        Just (_, last_tv) = snocView tc_tvs
        bad_stupid_theta  = filter is_bad (tyConStupidTheta rep_tc)
        is_bad pred       = last_tv `elemVarSet` tyVarsOfPred pred

        data_cons = tyConDataCons rep_tc
        check_con con = msum (check_vanilla con : foldDataConArgs (ft_check con) con)

        check_vanilla :: DataCon -> Maybe SDoc
        check_vanilla con | isVanillaDataCon con = Nothing
        		      | otherwise	     = Just (badCon con existential)

        ft_check :: DataCon -> FFoldType (Maybe SDoc)
        ft_check con = FT { ft_triv = Nothing, ft_var = Nothing
                          , ft_co_var = Just (badCon con covariant)
    	      	      , ft_fun = \x y -> if allowFunctions then x `mplus` y
                                                               else Just (badCon con functions)
                          , ft_tup = \_ xs  -> msum xs
                          , ft_ty_app = \_ x   -> x
                          , ft_bad_app = Just (badCon con wrong_arg)
                          , ft_forall = \_ x   -> x }

        existential = ptext (sLit "has existential arguments")
        covariant 	= ptext (sLit "uses the type variable in a function argument")
        functions 	= ptext (sLit "contains function types")
        wrong_arg 	= ptext (sLit "uses the type variable in an argument other than the last")

    checkFlag :: ExtensionFlag -> Condition
    checkFlag flag (dflags, _)
      | xopt flag dflags = Nothing
      | otherwise        = Just why
      where
        why = ptext (sLit "You need -X") <> text flag_str
              <+> ptext (sLit "to derive an instance for this class")
        flag_str = case [ s | (s, f, _) <- xFlags, f==flag ] of
                     [s]   -> s
                     other -> pprPanic "checkFlag" (ppr other)

    std_class_via_iso :: Class -> Bool
    -- These standard classes can be derived for a newtype
    -- using the isomorphism trick *even if no -XGeneralizedNewtypeDeriving
    -- because giving so gives the same results as generating the boilerplate
    std_class_via_iso clas
      = classKey clas `elem` [eqClassKey, ordClassKey, ixClassKey, boundedClassKey]
    	-- Not Read/Show because they respect the type
    	-- Not Enum, because newtypes are never in Enum

    non_iso_class :: Class -> Bool
    -- *Never* derive Read,Show,Typeable,Data by isomorphism,
    -- even with -XGeneralizedNewtypeDeriving
    non_iso_class cls
      = classKey cls `elem` ([readClassKey, showClassKey, dataClassKey] ++
    			 typeableClassKeys)

    typeableClassKeys :: [Unique]
    typeableClassKeys = map getUnique typeableClassNames

    new_dfun_name :: Class -> TyCon -> TcM Name
    new_dfun_name clas tycon 	-- Just a simple wrapper
      = do { loc <- getSrcSpanM	-- The location of the instance decl, not of the tycon
    	; newDFunName clas [mkTyConApp tycon []] loc }
    	-- The type passed to newDFunName is only used to generate
    	-- a suitable string; hence the empty type arg list

    badCon :: DataCon -> SDoc -> SDoc
    badCon con msg = ptext (sLit "Constructor") <+> quotes (ppr con) <+> msg

\end{code} Note [Superclasses of derived instance] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In general, a derived instance decl needs the superclasses of the derived class too. So if we have data T a = ...deriving( Ord ) then the initial context for Ord (T a) should include Eq (T a). Often this is redundant; we'll also generate an Ord constraint for each constructor argument, and that will probably generate enough constraints to make the Eq (T a) constraint be satisfied too. But not always; consider: data S a = S instance Eq (S a) instance Ord (S a) data T a = MkT (S a) deriving( Ord ) instance Num a => Eq (T a) The derived instance for (Ord (T a)) must have a (Num a) constraint! Similarly consider: data T a = MkT deriving( Data, Typeable ) Here there *is* no argument field, but we must nevertheless generate a context for the Data instances: instance Typable a => Data (T a) where ... %************************************************************************ %* * Deriving newtypes %* * %************************************************************************ \begin{code}

    mkNewTypeEqn :: CtOrigin -> DynFlags -> [Var] -> Class
                 -> [Type] -> TyCon -> [Type] -> TyCon -> [Type]
                 -> DerivContext
                 -> TcRn EarlyDerivSpec
    mkNewTypeEqn orig dflags tvs
                 cls cls_tys tycon tc_args rep_tycon rep_tc_args mtheta
    -- Want: instance (...) => cls (cls_tys ++ [tycon tc_args]) where ...
      | can_derive_via_isomorphism && (newtype_deriving || std_class_via_iso cls)
      = do	{ traceTc "newtype deriving:" (ppr tycon <+> ppr rep_tys <+> ppr all_preds)
    	; dfun_name <- new_dfun_name cls tycon
      	; loc <- getSrcSpanM
    	; let spec = DS { ds_loc = loc, ds_orig = orig
    			, ds_name = dfun_name, ds_tvs = varSetElems dfun_tvs
    			, ds_cls = cls, ds_tys = inst_tys
    			, ds_tc = rep_tycon, ds_tc_args = rep_tc_args
    			, ds_theta =  mtheta `orElse` all_preds
    			, ds_newtype = True }
    	; return (if isJust mtheta then Right spec
    				   else Left spec) }

      | otherwise
      = case checkSideConditions dflags mtheta cls cls_tys rep_tycon of
          CanDerive -> go_for_it 	-- Use the standard H98 method
          DerivableClassError msg 	-- Error with standard class
            | can_derive_via_isomorphism -> bale_out (msg $$ suggest_nd)
            | otherwise                  -> bale_out msg
          NonDerivableClass 	-- Must use newtype deriving
          	| newtype_deriving           -> bale_out cant_derive_err  -- Too hard, even with newtype deriving
            | can_derive_via_isomorphism -> bale_out (non_std $$ suggest_nd) -- Try newtype deriving!
          	| otherwise                  -> bale_out non_std
      where
            newtype_deriving = xopt Opt_GeneralizedNewtypeDeriving dflags
            go_for_it        = mk_data_eqn orig tvs cls tycon tc_args rep_tycon rep_tc_args mtheta
    	bale_out msg     = failWithTc (derivingThingErr newtype_deriving cls cls_tys inst_ty msg)

    	non_std    = nonStdErr cls
            suggest_nd = ptext (sLit "Try -XGeneralizedNewtypeDeriving for GHC's newtype-deriving extension")

    	-- Here is the plan for newtype derivings.  We see
    	--	  newtype T a1...an = MkT (t ak+1...an) deriving (.., C s1 .. sm, ...)
    	-- where t is a type,
    	-- 	 ak+1...an is a suffix of a1..an, and are all tyars
    	--	 ak+1...an do not occur free in t, nor in the s1..sm
    	-- 	 (C s1 ... sm) is a  *partial applications* of class C
    	--			with the last parameter missing
    	--	 (T a1 .. ak) matches the kind of C's last argument
    	--		(and hence so does t)
    	-- The latter kind-check has been done by deriveTyData already,
    	-- and tc_args are already trimmed
    	--
    	-- We generate the instance
    	--	 instance forall ({a1..ak} u fvs(s1..sm)).
    	--		  C s1 .. sm t => C s1 .. sm (T a1...ak)
    	-- where T a1...ap is the partial application of
    	-- 	 the LHS of the correct kind and p >= k
    	--
    	--	NB: the variables below are:
    	--		tc_tvs = [a1, ..., an]
    	--		tyvars_to_keep = [a1, ..., ak]
    	--		rep_ty = t ak .. an
    	--		deriv_tvs = fvs(s1..sm) \ tc_tvs
    	--		tys = [s1, ..., sm]
    	--		rep_fn' = t
    	--
    	-- Running example: newtype T s a = MkT (ST s a) deriving( Monad )
    	-- We generate the instance
    	--	instance Monad (ST s) => Monad (T s) where

    	nt_eta_arity = length (fst (newTyConEtadRhs rep_tycon))
    		-- For newtype T a b = MkT (S a a b), the TyCon machinery already
    		-- eta-reduces the representation type, so we know that
    		-- 	T a ~ S a a
    		-- That's convenient here, because we may have to apply
    		-- it to fewer than its original complement of arguments

    	-- Note [Newtype representation]
    	-- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    	-- Need newTyConRhs (*not* a recursive representation finder)
    	-- to get the representation type. For example
    	--	newtype B = MkB Int
    	--	newtype A = MkA B deriving( Num )
    	-- We want the Num instance of B, *not* the Num instance of Int,
    	-- when making the Num instance of A!
    	rep_inst_ty = newTyConInstRhs rep_tycon rep_tc_args
    	rep_tys     = cls_tys ++ [rep_inst_ty]
    	rep_pred    = mkClassPred cls rep_tys
    		-- rep_pred is the representation dictionary, from where
    		-- we are gong to get all the methods for the newtype
    		-- dictionary

        -- Next we figure out what superclass dictionaries to use
        -- See Note [Newtype deriving superclasses] above

    	cls_tyvars = classTyVars cls
    	dfun_tvs = tyVarsOfTypes inst_tys
    	inst_ty = mkTyConApp tycon tc_args
    	inst_tys = cls_tys ++ [inst_ty]
    	sc_theta = substTheta (zipOpenTvSubst cls_tyvars inst_tys)
    			      (classSCTheta cls)

    		-- If there are no tyvars, there's no need
    		-- to abstract over the dictionaries we need
    		-- Example: 	newtype T = MkT Int deriving( C )
    		-- We get the derived instance
    		--		instance C T
    		-- rather than
    		--		instance C Int => C T
    	all_preds = rep_pred : sc_theta		-- NB: rep_pred comes first

    	-------------------------------------------------------------------
    	--  Figuring out whether we can only do this newtype-deriving thing

    	can_derive_via_isomorphism
    	   =  not (non_iso_class cls)
    	   && arity_ok
    	   && eta_ok
    	   && ats_ok
    --	   && not (isRecursiveTyCon tycon)	-- Note [Recursive newtypes]

    	arity_ok = length cls_tys + 1 == classArity cls
     		-- Well kinded; eg not: newtype T ... deriving( ST )
    		--			because ST needs *2* type params

    	-- Check that eta reduction is OK
    	eta_ok = nt_eta_arity <= length rep_tc_args
    		-- The newtype can be eta-reduced to match the number
    		--     of type argument actually supplied
    		--	  newtype T a b = MkT (S [a] b) deriving( Monad )
    		--     Here the 'b' must be the same in the rep type (S [a] b)
    		--     And the [a] must not mention 'b'.  That's all handled
    		--     by nt_eta_rity.

    	ats_ok = null (classATs cls)
    	       -- No associated types for the class, because we don't
    	       -- currently generate type 'instance' decls; and cannot do
    	       -- so for 'data' instance decls

    	cant_derive_err
    	   = vcat [ ppUnless arity_ok arity_msg
    		  , ppUnless eta_ok eta_msg
    		  , ppUnless ats_ok ats_msg ]
            arity_msg = quotes (ppr (mkClassPred cls cls_tys)) <+> ptext (sLit "does not have arity 1")
    	eta_msg   = ptext (sLit "cannot eta-reduce the representation type enough")
    	ats_msg   = ptext (sLit "the class has associated types")

\end{code} Note [Recursive newtypes] ~~~~~~~~~~~~~~~~~~~~~~~~~ Newtype deriving works fine, even if the newtype is recursive. e.g. newtype S1 = S1 [T1 ()] newtype T1 a = T1 (StateT S1 IO a ) deriving( Monad ) Remember, too, that type families are curretly (conservatively) given a recursive flag, so this also allows newtype deriving to work for type famillies. We used to exclude recursive types, because we had a rather simple minded way of generating the instance decl: newtype A = MkA [A] instance Eq [A] => Eq A -- Makes typechecker loop! But now we require a simple context, so it's ok. %************************************************************************ %* * \subsection[TcDeriv-fixpoint]{Finding the fixed point of \tr{deriving} equations} %* * %************************************************************************ A ``solution'' (to one of the equations) is a list of (k,TyVarTy tv) terms, which is the final correct RHS for the corresponding original equation. \begin{itemize} \item Each (k,TyVarTy tv) in a solution constrains only a type variable, tv. \item The (k,TyVarTy tv) pairs in a solution are canonically ordered by sorting on type varible, tv, (major key) and then class, k, (minor key) \end{itemize} \begin{code}

    inferInstanceContexts :: OverlapFlag -> [DerivSpec] -> TcM [DerivSpec]

    inferInstanceContexts _ [] = return []

    inferInstanceContexts oflag infer_specs
      = do	{ traceTc "inferInstanceContexts" $ vcat (map pprDerivSpec infer_specs)
    	; iterate_deriv 1 initial_solutions }
      where
        ------------------------------------------------------------------
    	-- The initial solutions for the equations claim that each
    	-- instance has an empty context; this solution is certainly
    	-- in canonical form.
        initial_solutions :: [ThetaType]
        initial_solutions = [ [] | _ <- infer_specs ]

        ------------------------------------------------------------------
    	-- iterate_deriv calculates the next batch of solutions,
    	-- compares it with the current one; finishes if they are the
    	-- same, otherwise recurses with the new solutions.
    	-- It fails if any iteration fails
        iterate_deriv :: Int -> [ThetaType] -> TcM [DerivSpec]
        iterate_deriv n current_solns
          | n > 20 	-- Looks as if we are in an infinite loop
    		-- This can happen if we have -XUndecidableInstances
    		-- (See TcSimplify.tcSimplifyDeriv.)
          = pprPanic "solveDerivEqns: probable loop"
    		 (vcat (map pprDerivSpec infer_specs) $$ ppr current_solns)
          | otherwise
          =	do { 	  -- Extend the inst info from the explicit instance decls
    		  -- with the current set of solutions, and simplify each RHS
    	     let inst_specs = zipWithEqual "add_solns" (mkInstance oflag)
    					   current_solns infer_specs
    	   ; new_solns <- checkNoErrs $
    	     		  extendLocalInstEnv inst_specs $
    	     		  mapM gen_soln infer_specs

    	   ; if (current_solns == new_solns) then
    		return [ spec { ds_theta = soln }
                           | (spec, soln) <- zip infer_specs current_solns ]
    	     else
    		iterate_deriv (n+1) new_solns }

        ------------------------------------------------------------------
        gen_soln :: DerivSpec  -> TcM [PredType]
        gen_soln (DS { ds_loc = loc, ds_orig = orig, ds_tvs = tyvars
    		 , ds_cls = clas, ds_tys = inst_tys, ds_theta = deriv_rhs })
          = setSrcSpan loc	$
    	addErrCtxt (derivInstCtxt clas inst_tys) $
    	do {      -- Check for a bizarre corner case, when the derived instance decl should
    		  -- have form 	instance C a b => D (T a) where ...
    		  -- Note that 'b' isn't a parameter of T.  This gives rise to all sorts
    		  -- of problems; in particular, it's hard to compare solutions for
    		  -- equality when finding the fixpoint.  Moreover, simplifyDeriv
    		  -- has an assert failure because it finds a TyVar when it expects
    		  -- only TcTyVars.  So I just rule it out for now.  I'm not
    		  -- even sure how it can arise.

     	   ; let tv_set = mkVarSet tyvars
    	         weird_preds = [pred | pred <- deriv_rhs
                                         , not (tyVarsOfPred pred `subVarSet` tv_set)]
    	   ; mapM_ (addErrTc . badDerivedPred) weird_preds

               ; theta <- simplifyDeriv orig tyvars deriv_rhs
    	   	-- checkValidInstance tyvars theta clas inst_tys
    		-- Not necessary; see Note [Exotic derived instance contexts]
    		-- 		  in TcSimplify

               ; traceTc "TcDeriv" (ppr deriv_rhs $$ ppr theta)
    		-- Claim: the result instance declaration is guaranteed valid
    		-- Hence no need to call:
    		--   checkValidInstance tyvars theta clas inst_tys
    	   ; return (sortLe (<=) theta) }	-- Canonicalise before returning the solution

    ------------------------------------------------------------------
    mkInstance :: OverlapFlag -> ThetaType -> DerivSpec -> Instance
    mkInstance overlap_flag theta
    	    (DS { ds_name = dfun_name
    		, ds_tvs = tyvars, ds_cls = clas, ds_tys = tys })
      = mkLocalInstance dfun overlap_flag
      where
        dfun = mkDictFunId dfun_name tyvars theta clas tys

    extendLocalInstEnv :: [Instance] -> TcM a -> TcM a
    -- Add new locally-defined instances; don't bother to check
    -- for functional dependency errors -- that'll happen in TcInstDcls
    extendLocalInstEnv dfuns thing_inside
     = do { env <- getGblEnv
          ; let  inst_env' = extendInstEnvList (tcg_inst_env env) dfuns
    	     env'      = env { tcg_inst_env = inst_env' }
          ; setGblEnv env' thing_inside }

\end{code} %************************************************************************ %* * \subsection[TcDeriv-normal-binds]{Bindings for the various classes} %* * %************************************************************************ After all the trouble to figure out the required context for the derived instance declarations, all that's left is to chug along to produce them. They will then be shoved into @tcInstDecls2@, which will do all its usual business. There are lots of possibilities for code to generate. Here are various general remarks. PRINCIPLES: \begin{itemize} \item We want derived instances of @Eq@ and @Ord@ (both v common) to be ``you-couldn't-do-better-by-hand'' efficient. \item Deriving @Show@---also pretty common--- should also be reasonable good code. \item Deriving for the other classes isn't that common or that big a deal. \end{itemize} PRAGMATICS: \begin{itemize} \item Deriving @Ord@ is done mostly with the 1.3 @compare@ method. \item Deriving @Eq@ also uses @compare@, if we're deriving @Ord@, too. \item We {\em normally} generate code only for the non-defaulted methods; there are some exceptions for @Eq@ and (especially) @Ord@... \item Sometimes we use a @_con2tag_<tycon>@ function, which returns a data constructor's numeric (@Int#@) tag. These are generated by @gen_tag_n_con_binds@, and the heuristic for deciding if one of these is around is given by @hasCon2TagFun@. The examples under the different sections below will make this clearer. \item Much less often (really just for deriving @Ix@), we use a @_tag2con_<tycon>@ function. See the examples. \item We use the renamer!!! Reason: we're supposed to be producing @LHsBinds Name@ for the methods, but that means producing correctly-uniquified code on the fly. This is entirely possible (the @TcM@ monad has a @UniqueSupply@), but it is painful. So, instead, we produce @MonoBinds RdrName@ then heave 'em through the renamer. What a great hack! \end{itemize} \begin{code}

    -- Generate the InstInfo for the required instance paired with the
    --   *representation* tycon for that instance,
    -- plus any auxiliary bindings required
    --
    -- Representation tycons differ from the tycon in the instance signature in
    -- case of instances for indexed families.
    --
    genInst :: Bool 	-- True <=> standalone deriving
    	-> OverlapFlag
            -> DerivSpec -> TcM (InstInfo RdrName, DerivAuxBinds)
    genInst standalone_deriv oflag
            spec@(DS { ds_tc = rep_tycon, ds_tc_args = rep_tc_args
                     , ds_theta = theta, ds_newtype = is_newtype
                     , ds_name = name, ds_cls = clas })
      | is_newtype
      = return (InstInfo { iSpec   = inst_spec
                         , iBinds  = NewTypeDerived co rep_tycon }, [])

      | otherwise
      = do  { fix_env <- getFixityEnv
            ; let loc   = getSrcSpan name
                  (meth_binds, aux_binds) = genDerivBinds loc fix_env clas rep_tycon
                       -- In case of a family instance, we need to use the representation
                       -- tycon (after all, it has the data constructors)

            ; return (InstInfo { iSpec   = inst_spec
                               , iBinds  = VanillaInst meth_binds [] standalone_deriv }
                     , aux_binds) }
      where
        inst_spec = mkInstance oflag theta spec
        co1 = case tyConFamilyCoercion_maybe rep_tycon of
    	      Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
        	      Nothing     -> id_co
    	      -- Not a family => rep_tycon = main tycon
        co2 = case newTyConCo_maybe rep_tycon of
    	      Just co_con -> ACo (mkTyConApp co_con rep_tc_args)
                  Nothing     -> id_co  -- The newtype is transparent; no need for a cast
        co = co1 `mkTransCoI` co2
        id_co = IdCo (mkTyConApp rep_tycon rep_tc_args)

    -- Example: newtype instance N [a] = N1 (Tree a)
    --          deriving instance Eq b => Eq (N [(b,b)])
    -- From the instance, we get an implicit newtype R1:N a = N1 (Tree a)
    -- When dealing with the deriving clause
    --    co1 : N [(b,b)] ~ R1:N (b,b)
    --    co2 : R1:N (b,b) ~ Tree (b,b)
    --    co  : N [(b,b)] ~ Tree (b,b)

    genDerivBinds :: SrcSpan -> FixityEnv -> Class -> TyCon -> (LHsBinds RdrName, DerivAuxBinds)
    genDerivBinds loc fix_env clas tycon
      | className clas `elem` typeableClassNames
      = (gen_Typeable_binds loc tycon, [])

      | otherwise
      = case assocMaybe gen_list (getUnique clas) of
    	Just gen_fn -> gen_fn loc tycon
    	Nothing	    -> pprPanic "genDerivBinds: bad derived class" (ppr clas)
      where
        gen_list :: [(Unique, SrcSpan -> TyCon -> (LHsBinds RdrName, DerivAuxBinds))]
        gen_list = [(eqClassKey,       gen_Eq_binds)
     	       ,(ordClassKey,      gen_Ord_binds)
     	       ,(enumClassKey,     gen_Enum_binds)
     	       ,(boundedClassKey,  gen_Bounded_binds)
     	       ,(ixClassKey,       gen_Ix_binds)
     	       ,(showClassKey,     gen_Show_binds fix_env)
     	       ,(readClassKey,     gen_Read_binds fix_env)
    	       ,(dataClassKey,     gen_Data_binds)
    	       ,(functorClassKey,  gen_Functor_binds)
    	       ,(foldableClassKey, gen_Foldable_binds)
    	       ,(traversableClassKey, gen_Traversable_binds)
     	       ]

\end{code} %************************************************************************ %* * \subsection[TcDeriv-taggery-Names]{What con2tag/tag2con functions are available?} %* * %************************************************************************ \begin{code}

    derivingKindErr :: TyCon -> Class -> [Type] -> Kind -> Message
    derivingKindErr tc cls cls_tys cls_kind
      = hang (ptext (sLit "Cannot derive well-kinded instance of form")
    		<+> quotes (pprClassPred cls cls_tys <+> parens (ppr tc <+> ptext (sLit "..."))))
           2 (ptext (sLit "Class") <+> quotes (ppr cls)
    	    <+> ptext (sLit "expects an argument of kind") <+> quotes (pprKind cls_kind))

    derivingEtaErr :: Class -> [Type] -> Type -> Message
    derivingEtaErr cls cls_tys inst_ty
      = sep [ptext (sLit "Cannot eta-reduce to an instance of form"),
    	 nest 2 (ptext (sLit "instance (...) =>")
    		<+> pprClassPred cls (cls_tys ++ [inst_ty]))]

    typeFamilyPapErr :: TyCon -> Class -> [Type] -> Type -> Message
    typeFamilyPapErr tc cls cls_tys inst_ty
      = hang (ptext (sLit "Derived instance") <+> quotes (pprClassPred cls (cls_tys ++ [inst_ty])))
           2 (ptext (sLit "requires illegal partial application of data type family") <+> ppr tc)

    derivingThingErr :: Bool -> Class -> [Type] -> Type -> Message -> Message
    derivingThingErr newtype_deriving clas tys ty why
      = sep [(hang (ptext (sLit "Can't make a derived instance of"))
    	     2 (quotes (ppr pred))
              $$ nest 2 extra) <> colon,
    	 nest 2 why]
      where
        extra | newtype_deriving = ptext (sLit "(even with cunning newtype deriving)")
              | otherwise        = empty
        pred = mkClassPred clas (tys ++ [ty])

    derivingHiddenErr :: TyCon -> SDoc
    derivingHiddenErr tc
      = hang (ptext (sLit "The data constructors of") <+> quotes (ppr tc) <+> ptext (sLit "are not all in scope"))
           2 (ptext (sLit "so you cannot derive an instance for it"))

    standaloneCtxt :: LHsType Name -> SDoc
    standaloneCtxt ty = hang (ptext (sLit "In the stand-alone deriving instance for"))
    		       2 (quotes (ppr ty))

    derivInstCtxt :: Class -> [Type] -> Message
    derivInstCtxt clas inst_tys
      = ptext (sLit "When deriving the instance for") <+> parens (pprClassPred clas inst_tys)

    badDerivedPred :: PredType -> Message
    badDerivedPred pred
      = vcat [ptext (sLit "Can't derive instances where the instance context mentions"),
    	  ptext (sLit "type variables that are not data type parameters"),
    	  nest 2 (ptext (sLit "Offending constraint:") <+> ppr pred)]

\end{code}