typecheck/TcClassDcl.hs

    {-
    (c) The University of Glasgow 2006
    (c) The GRASP/AQUA Project, Glasgow University, 1992-1998

    Typechecking class declarations
    -}

    {-# LANGUAGE CPP #-}

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

    #include "HsVersions.h"

    import HsSyn
    import TcEnv
    import TcSigs
    import TcEvidence ( idHsWrapper )
    import TcBinds
    import TcUnify
    import TcHsType
    import TcMType
    import Type     ( getClassPredTys_maybe, piResultTys )
    import TcType
    import TcRnMonad
    import DriverPhases (HscSource(..))
    import BuildTyCl( TcMethInfo )
    import Class
    import Coercion ( pprCoAxiom )
    import DynFlags
    import FamInst
    import FamInstEnv
    import Id
    import Name
    import NameEnv
    import NameSet
    import Var
    import VarEnv
    import Outputable
    import SrcLoc
    import TyCon
    import Maybes
    import BasicTypes
    import Bag
    import FastString
    import BooleanFormula
    import Util

    import Control.Monad
    import Data.List ( mapAccumL, partition )

    {-
    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
    *                                                                      *
    ************************************************************************
    -}

    illegalHsigDefaultMethod :: Name -> SDoc
    illegalHsigDefaultMethod n =
        text "Illegal default method(s) in class definition of" <+> ppr n <+> text "in hsig file"

    tcClassSigs :: Name                -- Name of the class
                -> [LSig Name]
                -> LHsBinds Name
                -> TcM [TcMethInfo]    -- Exactly one for each method
    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 :: NameEnv (SrcSpan, Type)
                 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)

           ; tcg_env <- getGblEnv
           ; if tcg_src tcg_env == HsigFile
                then
                   -- Error if we have value bindings
                   -- (Generic signatures without value bindings indicate
                   -- that a default of this form is expected to be
                   -- provided.)
                   when (not (null def_methods)) $
                    failWithTc (illegalHsigDefaultMethod clas)
                else
                   -- Error for each generic signature without value binding
                   sequence_ [ failWithTc (badGenericMethod clas n)
                             | (n,_) <- gen_dm_prs, not (n `elem` dm_bind_names) ]

           ; traceTc "tcClassSigs 2" (ppr clas)
           ; return op_info }
      where
        vanilla_sigs = [L loc (nm,ty) | L loc (ClassOpSig False nm ty) <- sigs]
        gen_sigs     = [L loc (nm,ty) | L loc (ClassOpSig True  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 :: NameEnv (SrcSpan, Type) -> ([Located Name], LHsSigType Name)
               -> TcM [TcMethInfo]
        tc_sig gen_dm_env (op_names, op_hs_ty)
          = do { traceTc "ClsSig 1" (ppr op_names)
               ; op_ty <- tcClassSigType op_names op_hs_ty   -- Class tyvars already in scope
               ; traceTc "ClsSig 2" (ppr op_names)
               ; return [ (op_name, op_ty, f op_name) | L _ op_name <- op_names ] }
               where
                 f nm | Just lty <- lookupNameEnv gen_dm_env nm = Just (GenericDM lty)
                      | nm `elem` dm_bind_names                 = Just VanillaDM
                      | otherwise                               = Nothing

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

    {-
    ************************************************************************
    *                                                                      *
                    Class Declarations
    *                                                                      *
    ************************************************************************
    -}

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

    tcClassDecl2 (L _ (ClassDecl {tcdLName = class_name, tcdSigs = sigs,
                                    tcdMeths = default_binds}))
      = recoverM (return emptyLHsBinds)     $
        setSrcSpan (getLoc class_name)      $
        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     = mkPragEnv sigs default_binds
                  sig_fn      = mkHsSigFun sigs
                  clas_tyvars = snd (tcSuperSkolTyVars tyvars)
                  pred        = mkClassPred clas (mkTyVarTys clas_tyvars)
            ; this_dict <- newEvVar pred

            ; let tc_item = tcDefMeth clas clas_tyvars this_dict
                                      default_binds sig_fn prag_fn
            ; dm_binds <- tcExtendTyVarEnv clas_tyvars $
                          mapM tc_item op_items

            ; return (unionManyBags dm_binds) }

    tcClassDecl2 d = pprPanic "tcClassDecl2" (ppr d)

    tcDefMeth :: Class -> [TyVar] -> EvVar -> LHsBinds Name
              -> HsSigFun -> TcPragEnv -> ClassOpItem
              -> TcM (LHsBinds TcId)
    -- Generate code for default methods
    -- 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 _ _ _ _ _ prag_fn (sel_id, Nothing)
      = do { -- No default method
             mapM_ (addLocM (badDmPrag sel_id))
                   (lookupPragEnv prag_fn (idName sel_id))
           ; return emptyBag }

    tcDefMeth clas tyvars this_dict binds_in hs_sig_fn prag_fn
              (sel_id, Just (dm_name, dm_spec))
      | Just (L bind_loc dm_bind, bndr_loc, prags) <- findMethodBind sel_name binds_in prag_fn
      = do { -- First look up the default method; it should be there!
             -- It can be the orinary default method
             -- or the generic-default method.  E.g of the latter
             --      class C a where
             --        op :: a -> a -> Bool
             --        default op :: Eq a => a -> a -> Bool
             --        op x y = x==y
             -- The default method we generate is
             --    $gm :: (C a, Eq a) => a -> a -> Bool
             --    $gm x y = x==y

             global_dm_id  <- tcLookupId dm_name
           ; global_dm_id  <- addInlinePrags global_dm_id prags
           ; local_dm_name <- newNameAt (getOccName sel_name) bndr_loc
                -- Base the local_dm_name on the selector name, because
                -- type errors from tcInstanceMethodBody come from here

           ; spec_prags <- discardConstraints $
                           tcSpecPrags global_dm_id prags
           ; warnTc NoReason
                    (not (null spec_prags))
                    (text "Ignoring SPECIALISE pragmas on default method"
                     <+> quotes (ppr sel_name))

           ; let hs_ty = hs_sig_fn sel_name
                         `orElse` pprPanic "tc_dm" (ppr sel_name)
                 -- We need the HsType so that we can bring the right
                 -- type variables into scope
                 --
                 -- 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

                 local_dm_ty = instantiateMethod clas global_dm_id (mkTyVarTys tyvars)

                 lm_bind     = dm_bind { fun_id = L bind_loc local_dm_name }
                                 -- Substitute the local_meth_name for the binder
                                 -- NB: the binding is always a FunBind

                 warn_redundant = case dm_spec of
                                    GenericDM {} -> True
                                    VanillaDM    -> False
                    -- For GenericDM, warn if the user specifies a signature
                    -- with redundant constraints; but not for VanillaDM, where
                    -- the default method may well be 'error' or something

                 ctxt = FunSigCtxt sel_name warn_redundant

           ; let local_dm_id = mkLocalId local_dm_name local_dm_ty
                 local_dm_sig = CompleteSig { sig_bndr = local_dm_id
                                            , sig_ctxt  = ctxt
                                            , sig_loc   = getLoc (hsSigType hs_ty) }

           ; (ev_binds, (tc_bind, _))
                   <- checkConstraints (ClsSkol clas) tyvars [this_dict] $
                      tcPolyCheck no_prag_fn local_dm_sig
                                  (L bind_loc lm_bind)

           ; let export = ABE { abe_poly   = global_dm_id
                               , abe_mono  = local_dm_id
                               , abe_wrap  = idHsWrapper
                               , abe_prags = IsDefaultMethod }
                 full_bind = AbsBinds { abs_tvs      = tyvars
                                      , abs_ev_vars  = [this_dict]
                                      , abs_exports  = [export]
                                      , abs_ev_binds = [ev_binds]
                                      , abs_binds    = tc_bind }

           ; return (unitBag (L bind_loc full_bind)) }

      | otherwise = pprPanic "tcDefMeth" (ppr sel_id)
      where
        sel_name = idName sel_id
        no_prag_fn = emptyPragEnv   -- 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
            tcg_env <- getGblEnv
            -- However, only do this test when it's not an hsig file,
            -- since you can't write a default implementation.
            when (tcg_src tcg_env /= HsigFile) $
                whenIsJust (isUnsatisfied (mindef `impliesAtom`) defMindef) $
                           (\bf -> addWarnTc NoReason (warningMinimalDefIncomplete bf))
            return mindef
      where
        -- By default require all methods without a default implementation
        defMindef :: ClassMinimalDef
        defMindef = mkAnd [ noLoc (mkVar name)
                          | (name, _, Nothing) <- op_info ]

    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
        rho_ty = piResultTys (idType sel_id) inst_tys
        (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 = Name -> Maybe (LHsSigType Name)

    mkHsSigFun :: [LSig Name] -> HsSigFun
    mkHsSigFun sigs = lookupNameEnv env
      where
        env = mkHsSigEnv get_classop_sig sigs

        get_classop_sig :: LSig Name -> Maybe ([Located Name], LHsSigType Name)
        get_classop_sig  (L _ (ClassOpSig _ ns hs_ty)) = Just (ns, hs_ty)
        get_classop_sig  _                             = Nothing

    ---------------------------
    findMethodBind  :: Name                 -- Selector
                    -> LHsBinds Name        -- A group of bindings
                    -> TcPragEnv
                    -> Maybe (LHsBind Name, SrcSpan, [LSig Name])
                    -- Returns the binding, the binding
                    -- site of the method binder, and any inline or
                    -- specialisation pragmas
    findMethodBind sel_name binds prag_fn
      = foldlBag mplus Nothing (mapBag f binds)
      where
        prags    = lookupPragEnv prag_fn sel_name

        f bind@(L _ (FunBind { fun_id = L bndr_loc op_name }))
          | op_name == sel_name
                 = Just (bind, bndr_loc, prags)
        f _other = Nothing

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

    {-
    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
    *                                                                      *
    ************************************************************************
    -}

    tcMkDeclCtxt :: TyClDecl Name -> SDoc
    tcMkDeclCtxt decl = hsep [text "In the", pprTyClDeclFlavour decl,
                          text "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 [text "Class", quotes (ppr clas),
              text "does not have a method", quotes (ppr op)]

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

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

    missingGenericInstances :: [Name] -> SDoc
    missingGenericInstances missing
      = text "Missing type patterns for" <+> pprQuotedList missing

    dupGenericInsts :: [(TyCon, InstInfo a)] -> SDoc
    dupGenericInsts tc_inst_infos
      = vcat [text "More than one type pattern for a single generic type constructor:",
              nest 2 (vcat (map ppr_inst_ty tc_inst_infos)),
              text "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 (text "The" <+> hsSigDoc prag <+> ptext (sLit "for default method")
                  <+> quotes (ppr sel_id)
                  <+> text "lacks an accompanying binding")

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

    tcATDefault :: Bool -- If a warning should be emitted when a default instance
                        -- definition is not provided by the user
                -> SrcSpan
                -> TCvSubst
                -> NameSet
                -> ClassATItem
                -> TcM [FamInst]
    -- ^ Construct default instances for any associated types that
    -- aren't given a user definition
    -- Returns [] or singleton
    tcATDefault emit_warn loc inst_subst defined_ats (ATI fam_tc defs)
      -- User supplied instances ==> everything is OK
      | tyConName fam_tc `elemNameSet` defined_ats
      = return []

      -- No user instance, have defaults ==> instantiate them
       -- Example:   class C a where { type F a b :: *; type F a b = () }
       --            instance C [x]
       -- Then we want to generate the decl:   type F [x] b = ()
      | Just (rhs_ty, _loc) <- defs
      = do { let (subst', pat_tys') = mapAccumL subst_tv inst_subst
                                                (tyConTyVars fam_tc)
                 rhs'     = substTyUnchecked subst' rhs_ty
                 tcv' = tyCoVarsOfTypesList pat_tys'
                 (tv', cv') = partition isTyVar tcv'
                 tvs'     = toposortTyVars tv'
                 cvs'     = toposortTyVars cv'
           ; rep_tc_name <- newFamInstTyConName (L loc (tyConName fam_tc)) pat_tys'
           ; let axiom = mkSingleCoAxiom Nominal rep_tc_name tvs' cvs'
                                         fam_tc pat_tys' rhs'
               -- NB: no validity check. We check validity of default instances
               -- in the class definition. Because type instance arguments cannot
               -- be type family applications and cannot be polytypes, the
               -- validity check is redundant.

           ; traceTc "mk_deflt_at_instance" (vcat [ ppr fam_tc, ppr rhs_ty
                                                  , pprCoAxiom axiom ])
           ; fam_inst <- newFamInst SynFamilyInst axiom
           ; return [fam_inst] }

       -- No defaults ==> generate a warning
      | otherwise  -- defs = Nothing
      = do { when emit_warn $ warnMissingAT (tyConName fam_tc)
           ; return [] }
      where
        subst_tv subst tc_tv
          | Just ty <- lookupVarEnv (getTvSubstEnv subst) tc_tv
          = (subst, ty)
          | otherwise
          = (extendTvSubst subst tc_tv ty', ty')
          where
            ty' = mkTyVarTy (updateTyVarKind (substTyUnchecked subst) tc_tv)

    warnMissingAT :: Name -> TcM ()
    warnMissingAT name
      = do { warn <- woptM Opt_WarnMissingMethods
           ; traceTc "warn" (ppr name <+> ppr warn)
           ; hsc_src <- fmap tcg_src getGblEnv
           -- Warn only if -Wmissing-methods AND not a signature
           ; warnTc (Reason Opt_WarnMissingMethods) (warn && hsc_src /= HsigFile)
                    (text "No explicit" <+> text "associated type"
                        <+> text "or default declaration for     "
                        <+> quotes (ppr name)) }