typecheck/TcClassDcl.lhs

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

    {-# OPTIONS -fno-warn-tabs #-}
    -- The above warning supression flag is a temporary kludge.
    -- While working on this module you are encouraged to remove it and
    -- detab the module (please do the detabbing in a separate patch). See
    --     http://ghc.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#TabsvsSpaces
    -- for details

    module TcClassDcl ( tcClassSigs, tcClassDecl2,
    		    findMethodBind, instantiateMethod, tcInstanceMethodBody,
    		    tcClassMinimalDef,
                        HsSigFun, mkHsSigFun, lookupHsSig, emptyHsSigs,
    		    tcMkDeclCtxt, tcAddDeclCtxt, badMethodErr
    		  ) where

    #include "HsVersions.h"

    import HsSyn
    import TcEnv
    import TcPat( addInlinePrags )
    import TcEvidence( idHsWrapper )
    import TcBinds
    import TcUnify
    import TcHsType
    import TcMType
    import Type     ( getClassPredTys_maybe )
    import TcType
    import TcRnMonad
    import BuildTyCl( TcMethInfo )
    import Class
    import Id
    import Name
    import NameEnv
    import NameSet
    import Var
    import Outputable
    import SrcLoc
    import Maybes
    import BasicTypes
    import Bag
    import FastString
    import BooleanFormula
    import Util

    import Control.Monad

\end{code} Dictionary handling ~~~~~~~~~~~~~~~~~~~ Every class implicitly declares a new data type, corresponding to dictionaries of that class. So, for example: class (D a) => C a where op1 :: a -> a op2 :: forall b. Ord b => a -> b -> b would implicitly declare data CDict a = CDict (D a) (a -> a) (forall b. Ord b => a -> b -> b) (We could use a record decl, but that means changing more of the existing apparatus. One step at at time!) For classes with just one superclass+method, we use a newtype decl instead: class C a where op :: forallb. a -> b -> b generates newtype CDict a = CDict (forall b. a -> b -> b) Now DictTy in Type is just a form of type synomym: DictTy c t = TyConTy CDict `AppTy` t Death to "ExpandingDicts". %************************************************************************ %* * Type-checking the class op signatures %* * %************************************************************************ \begin{code}

    tcClassSigs :: Name	             -- Name of the class
    	    -> [LSig Name]
    	    -> LHsBinds Name
    	    -> TcM ([TcMethInfo],    -- Exactly one for each method
                        NameEnv Type)    -- Types of the generic-default methods
    tcClassSigs clas sigs def_methods
      = do { traceTc "tcClassSigs 1" (ppr clas)

           ; gen_dm_prs <- concat <$> mapM (addLocM tc_gen_sig) gen_sigs
           ; let gen_dm_env = mkNameEnv gen_dm_prs

           ; op_info <- concat <$> mapM (addLocM (tc_sig gen_dm_env)) vanilla_sigs

           ; let op_names = mkNameSet [ n | (n,_,_) <- op_info ]
           ; sequence_ [ failWithTc (badMethodErr clas n)
                       | n <- dm_bind_names, not (n `elemNameSet` op_names) ]
    		   -- Value binding for non class-method (ie no TypeSig)

           ; sequence_ [ failWithTc (badGenericMethod clas n)
                       | (n,_) <- gen_dm_prs, not (n `elem` dm_bind_names) ]
    		   -- Generic signature without value binding

           ; traceTc "tcClassSigs 2" (ppr clas)
           ; return (op_info, gen_dm_env) }
      where
        vanilla_sigs = [L loc (nm,ty) | L loc (TypeSig    nm ty) <- sigs]
        gen_sigs     = [L loc (nm,ty) | L loc (GenericSig nm ty) <- sigs]
        dm_bind_names :: [Name]	-- These ones have a value binding in the class decl
        dm_bind_names = [op | (_, L _ (FunBind {fun_id = L _ op})) <- bagToList def_methods]

        tc_sig genop_env (op_names, op_hs_ty)
          = do { traceTc "ClsSig 1" (ppr op_names)
               ; op_ty <- tcClassSigType op_hs_ty	-- Class tyvars already in scope
               ; traceTc "ClsSig 2" (ppr op_names)
               ; return [ (op_name, f op_name, op_ty) | L _ op_name <- op_names ] }
               where
                 f nm | nm `elemNameEnv` genop_env = GenericDM
                      | nm `elem` dm_bind_names    = VanillaDM
                      | otherwise                  = NoDM

        tc_gen_sig (op_names, gen_hs_ty)
          = do { gen_op_ty <- tcClassSigType gen_hs_ty
               ; return [ (op_name, gen_op_ty) | L _ op_name <- op_names ] }

\end{code} %************************************************************************ %* * Class Declarations %* * %************************************************************************ \begin{code}

    tcClassDecl2 :: LTyClDecl Name		-- The class declaration
    	     -> TcM (LHsBinds Id)

    tcClassDecl2 (L loc (ClassDecl {tcdLName = class_name, tcdSigs = sigs,
    				tcdMeths = default_binds}))
      = recoverM (return emptyLHsBinds)	$
        setSrcSpan loc		   	$
        do  { clas <- tcLookupLocatedClass class_name

    	-- We make a separate binding for each default method.
    	-- At one time I used a single AbsBinds for all of them, thus
    	-- AbsBind [d] [dm1, dm2, dm3] { dm1 = ...; dm2 = ...; dm3 = ... }
    	-- But that desugars into
    	--	ds = \d -> (..., ..., ...)
    	--	dm1 = \d -> case ds d of (a,b,c) -> a
    	-- And since ds is big, it doesn't get inlined, so we don't get good
    	-- default methods.  Better to make separate AbsBinds for each
    	; let
    	      (tyvars, _, _, op_items) = classBigSig clas
                  prag_fn     = mkPragFun sigs default_binds
    	      sig_fn	  = mkHsSigFun sigs
                  clas_tyvars = snd (tcSuperSkolTyVars tyvars)
    	      pred  	  = mkClassPred clas (mkTyVarTys clas_tyvars)
    	; this_dict <- newEvVar pred

    	; traceTc "TIM2" (ppr sigs)
    	; let tc_dm = tcDefMeth clas clas_tyvars
    				this_dict default_binds
    	      			sig_fn prag_fn

    	; dm_binds <- tcExtendTyVarEnv clas_tyvars $
                          mapM tc_dm op_items

    	; return (unionManyBags dm_binds) }

    tcClassDecl2 d = pprPanic "tcClassDecl2" (ppr d)

    tcDefMeth :: Class -> [TyVar] -> EvVar -> LHsBinds Name
              -> HsSigFun -> PragFun -> ClassOpItem
              -> TcM (LHsBinds TcId)
    -- Generate code for polymorphic default methods only (hence DefMeth)
    -- (Generic default methods have turned into instance decls by now.)
    -- This is incompatible with Hugs, which expects a polymorphic
    -- default method for every class op, regardless of whether or not
    -- the programmer supplied an explicit default decl for the class.
    -- (If necessary we can fix that, but we don't have a convenient Id to hand.)
    tcDefMeth clas tyvars this_dict binds_in hs_sig_fn prag_fn (sel_id, dm_info)
      = case dm_info of
          NoDefMeth          -> do { mapM_ (addLocM (badDmPrag sel_id)) prags
                                   ; return emptyBag }
          DefMeth dm_name    -> tc_dm dm_name
          GenDefMeth dm_name -> tc_dm dm_name
      where
        sel_name           = idName sel_id
        prags              = prag_fn sel_name
        (dm_bind,bndr_loc) = findMethodBind sel_name binds_in
                             `orElse` pprPanic "tcDefMeth" (ppr sel_id)

        -- Eg.   class C a where
        --          op :: forall b. Eq b => a -> [b] -> a
        --		gen_op :: a -> a
        -- 		generic gen_op :: D a => a -> a
        -- The "local_dm_ty" is precisely the type in the above
        -- type signatures, ie with no "forall a. C a =>" prefix

        tc_dm dm_name
          = do { dm_id <- tcLookupId dm_name
    	   ; local_dm_name <- setSrcSpan bndr_loc (newLocalName sel_name)
     	     -- Base the local_dm_name on the selector name, because
     	     -- type errors from tcInstanceMethodBody come from here

               ; dm_id_w_inline <- addInlinePrags dm_id prags
               ; spec_prags     <- tcSpecPrags dm_id prags

               ; let local_dm_ty = instantiateMethod clas dm_id (mkTyVarTys tyvars)
                     hs_ty       = lookupHsSig hs_sig_fn sel_name
                                   `orElse` pprPanic "tc_dm" (ppr sel_name)

               ; local_dm_sig <- instTcTySig hs_ty local_dm_ty local_dm_name
               ; warnTc (not (null spec_prags))
                        (ptext (sLit "Ignoring SPECIALISE pragmas on default method")
                         <+> quotes (ppr sel_name))

               ; tc_bind <- tcInstanceMethodBody (ClsSkol clas) tyvars [this_dict]
                                                 dm_id_w_inline local_dm_sig
                                                 IsDefaultMethod dm_bind

               ; return (unitBag tc_bind) }

    ---------------
    tcInstanceMethodBody :: SkolemInfo -> [TcTyVar] -> [EvVar]
                         -> Id -> TcSigInfo
              	     -> TcSpecPrags -> (Origin, LHsBind Name)
              	     -> TcM (Origin, LHsBind Id)
    tcInstanceMethodBody skol_info tyvars dfun_ev_vars
                         meth_id local_meth_sig
    		     specs (origin, (L loc bind))
      = do	{ let local_meth_id = sig_id local_meth_sig
                  lm_bind = L loc (bind { fun_id = L loc (idName local_meth_id) })
                                 -- Substitute the local_meth_name for the binder
    			     -- NB: the binding is always a FunBind
    	; (ev_binds, (tc_bind, _, _))
                   <- checkConstraints skol_info tyvars dfun_ev_vars $
    	          tcPolyCheck NonRecursive no_prag_fn local_meth_sig (origin, lm_bind)

            ; let export = ABE { abe_wrap = idHsWrapper, abe_poly = meth_id
                               , abe_mono = local_meth_id, abe_prags = specs }
                  full_bind = AbsBinds { abs_tvs = tyvars, abs_ev_vars = dfun_ev_vars
                                       , abs_exports = [export]
                                       , abs_ev_binds = ev_binds
                                       , abs_binds = tc_bind }

            ; return (origin, L loc full_bind) }
      where
        no_prag_fn  _ = []		-- No pragmas for local_meth_id;
        		    		-- they are all for meth_id

    ---------------
    tcClassMinimalDef :: Name -> [LSig Name] -> [TcMethInfo] -> TcM ClassMinimalDef
    tcClassMinimalDef _clas sigs op_info
      = case findMinimalDef sigs of
          Nothing -> return defMindef
          Just mindef -> do
            -- Warn if the given mindef does not imply the default one
            -- That is, the given mindef should at least ensure that the
            -- class ops without default methods are required, since we
            -- have no way to fill them in otherwise
            whenIsJust (isUnsatisfied (mindef `impliesAtom`) defMindef) $
                       (\bf -> addWarnTc (warningMinimalDefIncomplete bf))
            return mindef
      where
        -- By default require all methods without a default
        -- implementation whose names don't start with '_'
        defMindef :: ClassMinimalDef
        defMindef = mkAnd [ mkVar name
                          | (name, NoDM, _) <- op_info
                          , not (startsWithUnderscore (getOccName name)) ]

\end{code} \begin{code}

    instantiateMethod :: Class -> Id -> [TcType] -> TcType
    -- Take a class operation, say
    --	op :: forall ab. C a => forall c. Ix c => (b,c) -> a
    -- Instantiate it at [ty1,ty2]
    -- Return the "local method type":
    --	forall c. Ix x => (ty2,c) -> ty1
    instantiateMethod clas sel_id inst_tys
      = ASSERT( ok_first_pred ) local_meth_ty
      where
        (sel_tyvars,sel_rho) = tcSplitForAllTys (idType sel_id)
        rho_ty = ASSERT( length sel_tyvars == length inst_tys )
        	     substTyWith sel_tyvars inst_tys sel_rho

        (first_pred, local_meth_ty) = tcSplitPredFunTy_maybe rho_ty
        		`orElse` pprPanic "tcInstanceMethod" (ppr sel_id)

        ok_first_pred = case getClassPredTys_maybe first_pred of
    		      Just (clas1, _tys) -> clas == clas1
                          Nothing -> False
    	      -- The first predicate should be of form (C a b)
    	      -- where C is the class in question

    ---------------------------
    type HsSigFun = NameEnv (LHsType Name)

    emptyHsSigs :: HsSigFun
    emptyHsSigs = emptyNameEnv

    mkHsSigFun :: [LSig Name] -> HsSigFun
    mkHsSigFun sigs = mkNameEnv [(n, hs_ty)
                                | L _ (TypeSig ns hs_ty) <- sigs
                                , L _ n <- ns ]

    lookupHsSig :: HsSigFun -> Name -> Maybe (LHsType Name)
    lookupHsSig = lookupNameEnv

    ---------------------------
    findMethodBind	:: Name  	        -- Selector name
              	-> LHsBinds Name 	-- A group of bindings
    		-> Maybe ((Origin, LHsBind Name), SrcSpan)
              	-- Returns the binding, and the binding
                    -- site of the method binder
    findMethodBind sel_name binds
      = foldlBag mplus Nothing (mapBag f binds)
      where
        f bind@(_, L _ (FunBind { fun_id = L bndr_loc op_name }))
                 | op_name == sel_name
        	     = Just (bind, bndr_loc)
        f _other = Nothing

    ---------------------------
    findMinimalDef :: [LSig Name] -> Maybe ClassMinimalDef
    findMinimalDef = firstJusts . map toMinimalDef
      where
        toMinimalDef (L _ (MinimalSig bf)) = Just (fmap unLoc bf)
        toMinimalDef _ = Nothing

\end{code} Note [Polymorphic methods] ~~~~~~~~~~~~~~~~~~~~~~~~~~ Consider class Foo a where op :: forall b. Ord b => a -> b -> b -> b instance Foo c => Foo [c] where op = e When typechecking the binding 'op = e', we'll have a meth_id for op whose type is op :: forall c. Foo c => forall b. Ord b => [c] -> b -> b -> b So tcPolyBinds must be capable of dealing with nested polytypes; and so it is. See TcBinds.tcMonoBinds (with type-sig case). Note [Silly default-method bind] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ When we pass the default method binding to the type checker, it must look like op2 = e not $dmop2 = e otherwise the "$dm" stuff comes out error messages. But we want the "$dm" to come out in the interface file. So we typecheck the former, and wrap it in a let, thus $dmop2 = let op2 = e in op2 This makes the error messages right. %************************************************************************ %* * Error messages %* * %************************************************************************ \begin{code}

    tcMkDeclCtxt :: TyClDecl Name -> SDoc
    tcMkDeclCtxt decl = hsep [ptext (sLit "In the"), pprTyClDeclFlavour decl,
                          ptext (sLit "declaration for"), quotes (ppr (tcdName decl))]

    tcAddDeclCtxt :: TyClDecl Name -> TcM a -> TcM a
    tcAddDeclCtxt decl thing_inside
      = addErrCtxt (tcMkDeclCtxt decl) thing_inside

    badMethodErr :: Outputable a => a -> Name -> SDoc
    badMethodErr clas op
      = hsep [ptext (sLit "Class"), quotes (ppr clas),
    	  ptext (sLit "does not have a method"), quotes (ppr op)]

    badGenericMethod :: Outputable a => a -> Name -> SDoc
    badGenericMethod clas op
      = hsep [ptext (sLit "Class"), quotes (ppr clas),
    	  ptext (sLit "has a generic-default signature without a binding"), quotes (ppr op)]

    {-
    badGenericInstanceType :: LHsBinds Name -> SDoc
    badGenericInstanceType binds
      = vcat [ptext (sLit "Illegal type pattern in the generic bindings"),
    	  nest 2 (ppr binds)]

    missingGenericInstances :: [Name] -> SDoc
    missingGenericInstances missing
      = ptext (sLit "Missing type patterns for") <+> pprQuotedList missing

    dupGenericInsts :: [(TyCon, InstInfo a)] -> SDoc
    dupGenericInsts tc_inst_infos
      = vcat [ptext (sLit "More than one type pattern for a single generic type constructor:"),
    	  nest 2 (vcat (map ppr_inst_ty tc_inst_infos)),
    	  ptext (sLit "All the type patterns for a generic type constructor must be identical")
        ]
      where
        ppr_inst_ty (_,inst) = ppr (simpleInstInfoTy inst)
    -}
    badDmPrag :: Id -> Sig Name -> TcM ()
    badDmPrag sel_id prag
      = addErrTc (ptext (sLit "The") <+> hsSigDoc prag <+> ptext (sLit "for default method")
                  <+> quotes (ppr sel_id)
                  <+> ptext (sLit "lacks an accompanying binding"))

    warningMinimalDefIncomplete :: ClassMinimalDef -> SDoc
    warningMinimalDefIncomplete mindef
      = vcat [ ptext (sLit "The MINIMAL pragma does not require:")
             , nest 2 (pprBooleanFormulaNice mindef)
             , ptext (sLit "but there is no default implementation.") ]

\end{code}