basicTypes/MkId.lhs

% % (c) The University of Glasgow 2006 % (c) The AQUA Project, Glasgow University, 1998 % This module contains definitions for the IdInfo for things that have a standard form, namely: - data constructors - record selectors - method and superclass selectors - primitive operations \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 MkId (
            mkDictFunId, mkDictFunTy, mkDictSelId,

            mkPrimOpId, mkFCallId,

            wrapNewTypeBody, unwrapNewTypeBody,
            wrapFamInstBody, unwrapFamInstScrut,
            wrapTypeFamInstBody, wrapTypeUnbranchedFamInstBody, unwrapTypeFamInstScrut,
            unwrapTypeUnbranchedFamInstScrut,

            DataConBoxer(..), mkDataConRep, mkDataConWorkId,

            -- And some particular Ids; see below for why they are wired in
            wiredInIds, ghcPrimIds,
            unsafeCoerceName, unsafeCoerceId, realWorldPrimId,
            voidPrimId, voidArgId,
            nullAddrId, seqId, lazyId, lazyIdKey,
            coercionTokenId, magicDictId, coerceId,

    	-- Re-export error Ids
    	module PrelRules
        ) where

    #include "HsVersions.h"

    import Rules
    import TysPrim
    import TysWiredIn
    import PrelRules
    import Type
    import FamInstEnv
    import Coercion
    import TcType
    import MkCore
    import CoreUtils	( exprType, mkCast )
    import CoreUnfold
    import Literal
    import TyCon
    import CoAxiom
    import Class
    import NameSet
    import VarSet
    import Name
    import PrimOp
    import ForeignCall
    import DataCon
    import Id
    import IdInfo
    import Demand
    import CoreSyn
    import Unique
    import UniqSupply
    import PrelNames
    import BasicTypes       hiding ( SuccessFlag(..) )
    import Util
    import Pair
    import DynFlags
    import Outputable
    import FastString
    import ListSetOps

    import Data.Maybe       ( maybeToList )

\end{code} %************************************************************************ %* * \subsection{Wired in Ids} %* * %************************************************************************ Note [Wired-in Ids] ~~~~~~~~~~~~~~~~~~~ There are several reasons why an Id might appear in the wiredInIds: (1) The ghcPrimIds are wired in because they can't be defined in Haskell at all, although the can be defined in Core. They have compulsory unfoldings, so they are always inlined and they have no definition site. Their home module is GHC.Prim, so they also have a description in primops.txt.pp, where they are called 'pseudoops'. (2) The 'error' function, eRROR_ID, is wired in because we don't yet have a way to express in an interface file that the result type variable is 'open'; that is can be unified with an unboxed type [The interface file format now carry such information, but there's no way yet of expressing at the definition site for these error-reporting functions that they have an 'open' result type. -- sof 1/99] (3) Other error functions (rUNTIME_ERROR_ID) are wired in (a) because the desugarer generates code that mentiones them directly, and (b) for the same reason as eRROR_ID (4) lazyId is wired in because the wired-in version overrides the strictness of the version defined in GHC.Base In cases (2-4), the function has a definition in a library module, and can be called; but the wired-in version means that the details are never read from that module's interface file; instead, the full definition is right here. \begin{code}

    wiredInIds :: [Id]
    wiredInIds
      =  [lazyId, dollarId]
      ++ errorIds		-- Defined in MkCore
      ++ ghcPrimIds

    -- These Ids are exported from GHC.Prim
    ghcPrimIds :: [Id]
    ghcPrimIds
      = [   -- These can't be defined in Haskell, but they have
            -- perfectly reasonable unfoldings in Core
        realWorldPrimId,
        voidPrimId,
        unsafeCoerceId,
        nullAddrId,
        seqId,
        magicDictId,
        coerceId,
        proxyHashId
        ]

\end{code} %************************************************************************ %* * \subsection{Data constructors} %* * %************************************************************************ The wrapper for a constructor is an ordinary top-level binding that evaluates any strict args, unboxes any args that are going to be flattened, and calls the worker. We're going to build a constructor that looks like: data (Data a, C b) => T a b = T1 !a !Int b T1 = /\ a b -> \d1::Data a, d2::C b -> \p q r -> case p of { p -> case q of { q -> Con T1 [a,b] [p,q,r]}} Notice that * d2 is thrown away --- a context in a data decl is used to make sure one *could* construct dictionaries at the site the constructor is used, but the dictionary isn't actually used. * We have to check that we can construct Data dictionaries for the types a and Int. Once we've done that we can throw d1 away too. * We use (case p of q -> ...) to evaluate p, rather than "seq" because all that matters is that the arguments are evaluated. "seq" is very careful to preserve evaluation order, which we don't need to be here. You might think that we could simply give constructors some strictness info, like PrimOps, and let CoreToStg do the let-to-case transformation. But we don't do that because in the case of primops and functions strictness is a *property* not a *requirement*. In the case of constructors we need to do something active to evaluate the argument. Making an explicit case expression allows the simplifier to eliminate it in the (common) case where the constructor arg is already evaluated. Note [Wrappers for data instance tycons] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In the case of data instances, the wrapper also applies the coercion turning the representation type into the family instance type to cast the result of the wrapper. For example, consider the declarations data family Map k :: * -> * data instance Map (a, b) v = MapPair (Map a (Pair b v)) The tycon to which the datacon MapPair belongs gets a unique internal name of the form :R123Map, and we call it the representation tycon. In contrast, Map is the family tycon (accessible via tyConFamInst_maybe). A coercion allows you to move between representation and family type. It is accessible from :R123Map via tyConFamilyCoercion_maybe and has kind Co123Map a b v :: {Map (a, b) v ~ :R123Map a b v} The wrapper and worker of MapPair get the types -- Wrapper $WMapPair :: forall a b v. Map a (Map a b v) -> Map (a, b) v $WMapPair a b v = MapPair a b v `cast` sym (Co123Map a b v) -- Worker MapPair :: forall a b v. Map a (Map a b v) -> :R123Map a b v This coercion is conditionally applied by wrapFamInstBody. It's a bit more complicated if the data instance is a GADT as well! data instance T [a] where T1 :: forall b. b -> T [Maybe b] Hence we translate to -- Wrapper $WT1 :: forall b. b -> T [Maybe b] $WT1 b v = T1 (Maybe b) b (Maybe b) v `cast` sym (Co7T (Maybe b)) -- Worker T1 :: forall c b. (c ~ Maybe b) => b -> :R7T c -- Coercion from family type to representation type Co7T a :: T [a] ~ :R7T a Note [Newtype datacons] ~~~~~~~~~~~~~~~~~~~~~~~ The "data constructor" for a newtype should always be vanilla. At one point this wasn't true, because the newtype arising from class C a => D a looked like newtype T:D a = D:D (C a) so the data constructor for T:C had a single argument, namely the predicate (C a). But now we treat that as an ordinary argument, not part of the theta-type, so all is well. %************************************************************************ %* * \subsection{Dictionary selectors} %* * %************************************************************************ Selecting a field for a dictionary. If there is just one field, then there's nothing to do. Dictionary selectors may get nested forall-types. Thus: class Foo a where op :: forall b. Ord b => a -> b -> b Then the top-level type for op is op :: forall a. Foo a => forall b. Ord b => a -> b -> b This is unlike ordinary record selectors, which have all the for-alls at the outside. When dealing with classes it's very convenient to recover the original type signature from the class op selector. \begin{code}

    mkDictSelId :: DynFlags
                -> Bool	     -- True <=> don't include the unfolding
    			     -- Little point on imports without -O, because the
    			     -- dictionary itself won't be visible
     	    -> Name	     -- Name of one of the *value* selectors
    	       		     -- (dictionary superclass or method)
                -> Class -> Id
    mkDictSelId dflags no_unf name clas
      = mkGlobalId (ClassOpId clas) name sel_ty info
      where
        sel_ty = mkForAllTys tyvars (mkFunTy (idType dict_id) (idType the_arg_id))
            -- We can't just say (exprType rhs), because that would give a type
            --      C a -> C a
            -- for a single-op class (after all, the selector is the identity)
            -- But it's type must expose the representation of the dictionary
            -- to get (say)         C a -> (a -> a)

        base_info = noCafIdInfo
                    `setArityInfo`         1
                    `setStrictnessInfo`    strict_sig
                    `setUnfoldingInfo`     (if no_unf then noUnfolding
    	                                else mkImplicitUnfolding dflags rhs)
    		   -- In module where class op is defined, we must add
    		   -- the unfolding, even though it'll never be inlined
    		   -- because we use that to generate a top-level binding
    		   -- for the ClassOp

        info | new_tycon = base_info `setInlinePragInfo` alwaysInlinePragma
        	   	   -- See Note [Single-method classes] in TcInstDcls
    		   -- for why alwaysInlinePragma
             | otherwise = base_info  `setSpecInfo`       mkSpecInfo [rule]
    		       		  `setInlinePragInfo` neverInlinePragma
    		   -- Add a magic BuiltinRule, and never inline it
    		   -- so that the rule is always available to fire.
    		   -- See Note [ClassOp/DFun selection] in TcInstDcls

        n_ty_args = length tyvars

        -- This is the built-in rule that goes
        -- 	    op (dfT d1 d2) --->  opT d1 d2
        rule = BuiltinRule { ru_name = fsLit "Class op " `appendFS`
        	   	       	 	     occNameFS (getOccName name)
                           , ru_fn    = name
        	               , ru_nargs = n_ty_args + 1
                           , ru_try   = dictSelRule val_index n_ty_args }

            -- The strictness signature is of the form U(AAAVAAAA) -> T
            -- where the V depends on which item we are selecting
            -- It's worth giving one, so that absence info etc is generated
            -- even if the selector isn't inlined

        strict_sig = mkClosedStrictSig [arg_dmd] topRes
        arg_dmd | new_tycon = evalDmd
                | otherwise = mkManyUsedDmd $
                              mkProdDmd [ if the_arg_id == id then evalDmd else absDmd
                                        | id <- arg_ids ]

        tycon      	   = classTyCon clas
        new_tycon  	   = isNewTyCon tycon
        [data_con] 	   = tyConDataCons tycon
        tyvars     	   = dataConUnivTyVars data_con
        arg_tys    	   = dataConRepArgTys data_con	-- Includes the dictionary superclasses

        -- 'index' is a 0-index into the *value* arguments of the dictionary
        val_index      = assoc "MkId.mkDictSelId" sel_index_prs name
        sel_index_prs  = map idName (classAllSelIds clas) `zip` [0..]

        the_arg_id     = getNth arg_ids val_index
        pred       	   = mkClassPred clas (mkTyVarTys tyvars)
        dict_id    	   = mkTemplateLocal 1 pred
        arg_ids    	   = mkTemplateLocalsNum 2 arg_tys

        rhs = mkLams tyvars  (Lam dict_id   rhs_body)
        rhs_body | new_tycon = unwrapNewTypeBody tycon (map mkTyVarTy tyvars) (Var dict_id)
                 | otherwise = Case (Var dict_id) dict_id (idType the_arg_id)
                                    [(DataAlt data_con, arg_ids, varToCoreExpr the_arg_id)]
    				-- varToCoreExpr needed for equality superclass selectors
    				--   sel a b d = case x of { MkC _ (g:a~b) _ -> CO g }

    dictSelRule :: Int -> Arity -> RuleFun
    -- Tries to persuade the argument to look like a constructor
    -- application, using exprIsConApp_maybe, and then selects
    -- from it
    --       sel_i t1..tk (D t1..tk op1 ... opm) = opi
    --
    dictSelRule val_index n_ty_args _ id_unf _ args
      | (dict_arg : _) <- drop n_ty_args args
      , Just (_, _, con_args) <- exprIsConApp_maybe id_unf dict_arg
      = Just (getNth con_args val_index)
      | otherwise
      = Nothing

\end{code} %************************************************************************ %* * Boxing and unboxing %* * %************************************************************************ \begin{code}

    mkDataConWorkId :: Name -> DataCon -> Id
    mkDataConWorkId wkr_name data_con
      | isNewTyCon tycon
      = mkGlobalId (DataConWrapId data_con) wkr_name nt_wrap_ty nt_work_info
      | otherwise
      = mkGlobalId (DataConWorkId data_con) wkr_name alg_wkr_ty wkr_info

      where
        tycon = dataConTyCon data_con

            ----------- Workers for data types --------------
        alg_wkr_ty = dataConRepType data_con
        wkr_arity = dataConRepArity data_con
        wkr_info  = noCafIdInfo
                    `setArityInfo`       wkr_arity
                    `setStrictnessInfo`  wkr_sig
                    `setUnfoldingInfo`   evaldUnfolding  -- Record that it's evaluated,
                                                         -- even if arity = 0

        wkr_sig = mkClosedStrictSig (replicate wkr_arity topDmd) (dataConCPR data_con)
            --      Note [Data-con worker strictness]
            -- Notice that we do *not* say the worker is strict
            -- even if the data constructor is declared strict
            --      e.g.    data T = MkT !(Int,Int)
            -- Why?  Because the *wrapper* is strict (and its unfolding has case
            -- expresssions that do the evals) but the *worker* itself is not.
            -- If we pretend it is strict then when we see
            --      case x of y -> $wMkT y
            -- the simplifier thinks that y is "sure to be evaluated" (because
            --  $wMkT is strict) and drops the case.  No, $wMkT is not strict.
            --
            -- When the simplifer sees a pattern
            --      case e of MkT x -> ...
            -- it uses the dataConRepStrictness of MkT to mark x as evaluated;
            -- but that's fine... dataConRepStrictness comes from the data con
            -- not from the worker Id.

            ----------- Workers for newtypes --------------
        (nt_tvs, _, nt_arg_tys, _) = dataConSig data_con
        res_ty_args  = mkTyVarTys nt_tvs
        nt_wrap_ty   = dataConUserType data_con
        nt_work_info = noCafIdInfo          -- The NoCaf-ness is set by noCafIdInfo
                      `setArityInfo` 1      -- Arity 1
                      `setInlinePragInfo`    alwaysInlinePragma
                      `setUnfoldingInfo`     newtype_unf
        id_arg1      = mkTemplateLocal 1 (head nt_arg_tys)
        newtype_unf  = ASSERT2( isVanillaDataCon data_con &&
                                isSingleton nt_arg_tys, ppr data_con  )
    			      -- Note [Newtype datacons]
                       mkCompulsoryUnfolding $
                       mkLams nt_tvs $ Lam id_arg1 $
                       wrapNewTypeBody tycon res_ty_args (Var id_arg1)

    dataConCPR :: DataCon -> DmdResult
    dataConCPR con
      | isDataTyCon tycon     -- Real data types only; that is,
                              -- not unboxed tuples or newtypes
      , isVanillaDataCon con  -- No existentials
      , wkr_arity > 0
      , wkr_arity <= mAX_CPR_SIZE
      = if is_prod then vanillaCprProdRes (dataConRepArity con)
                   else cprSumRes (dataConTag con)
      | otherwise
      = topRes
      where
        is_prod = isProductTyCon tycon
        tycon = dataConTyCon con
        wkr_arity = dataConRepArity con

        mAX_CPR_SIZE :: Arity
        mAX_CPR_SIZE = 10
        -- We do not treat very big tuples as CPR-ish:
        --      a) for a start we get into trouble because there aren't
        --         "enough" unboxed tuple types (a tiresome restriction,
        --         but hard to fix),
        --      b) more importantly, big unboxed tuples get returned mainly
        --         on the stack, and are often then allocated in the heap
        --         by the caller.  So doing CPR for them may in fact make
        --         things worse.

\end{code} ------------------------------------------------- -- Data constructor representation -- -- This is where we decide how to wrap/unwrap the -- constructor fields -- -------------------------------------------------- \begin{code}

    type Unboxer = Var -> UniqSM ([Var], CoreExpr -> CoreExpr)
      -- Unbox: bind rep vars by decomposing src var

    data Boxer = UnitBox | Boxer (TvSubst -> UniqSM ([Var], CoreExpr))
      -- Box:   build src arg using these rep vars

    newtype DataConBoxer = DCB ([Type] -> [Var] -> UniqSM ([Var], [CoreBind]))
                           -- Bind these src-level vars, returning the
                           -- rep-level vars to bind in the pattern

    mkDataConRep :: DynFlags -> FamInstEnvs -> Name -> DataCon -> UniqSM DataConRep
    mkDataConRep dflags fam_envs wrap_name data_con
      | not wrapper_reqd
      = return NoDataConRep

      | otherwise
      = do { wrap_args <- mapM newLocal wrap_arg_tys
           ; wrap_body <- mk_rep_app (wrap_args `zip` dropList eq_spec unboxers)
                                     initial_wrap_app

           ; let wrap_id = mkGlobalId (DataConWrapId data_con) wrap_name wrap_ty wrap_info
                 wrap_info = noCafIdInfo
                        	 `setArityInfo`         wrap_arity
                        	     -- It's important to specify the arity, so that partial
                        	     -- applications are treated as values
    		    	 `setInlinePragInfo`    alwaysInlinePragma
                        	 `setUnfoldingInfo`     wrap_unf
                        	 `setStrictnessInfo`    wrap_sig
                        	     -- We need to get the CAF info right here because TidyPgm
                        	     -- does not tidy the IdInfo of implicit bindings (like the wrapper)
                        	     -- so it not make sure that the CAF info is sane

        	     wrap_sig = mkClosedStrictSig wrap_arg_dmds (dataConCPR data_con)
        	     wrap_arg_dmds = map mk_dmd (dropList eq_spec wrap_bangs)
        	     mk_dmd str | isBanged str = evalDmd
        	                | otherwise    = topDmd
        	         -- The Cpr info can be important inside INLINE rhss, where the
        	         -- wrapper constructor isn't inlined.
        	         -- And the argument strictness can be important too; we
        	         -- may not inline a contructor when it is partially applied.
        	         -- For example:
        	         --      data W = C !Int !Int !Int
        	         --      ...(let w = C x in ...(w p q)...)...
        	         -- we want to see that w is strict in its two arguments

        	     wrap_unf = mkInlineUnfolding (Just wrap_arity) wrap_rhs
                 wrap_tvs = (univ_tvs `minusList` map fst eq_spec) ++ ex_tvs
        	     wrap_rhs = mkLams wrap_tvs $
        	                mkLams wrap_args $
        	                wrapFamInstBody tycon res_ty_args $
                            wrap_body

           ; return (DCR { dcr_wrap_id = wrap_id
                         , dcr_boxer   = mk_boxer boxers
                         , dcr_arg_tys = rep_tys
                         , dcr_stricts = rep_strs
                         , dcr_bangs   = dropList ev_tys wrap_bangs }) }

      where
        (univ_tvs, ex_tvs, eq_spec, theta, orig_arg_tys, _) = dataConFullSig data_con
        res_ty_args  = substTyVars (mkTopTvSubst eq_spec) univ_tvs
        tycon        = dataConTyCon data_con       -- The representation TyCon (not family)
        wrap_ty      = dataConUserType data_con
        ev_tys       = eqSpecPreds eq_spec ++ theta
        all_arg_tys  = ev_tys                         ++ orig_arg_tys
        orig_bangs   = map mk_pred_strict_mark ev_tys ++ dataConStrictMarks data_con

        wrap_arg_tys = theta ++ orig_arg_tys
        wrap_arity   = length wrap_arg_tys
        	     -- The wrap_args are the arguments *other than* the eq_spec
        	     -- Because we are going to apply the eq_spec args manually in the
        	     -- wrapper

        (wrap_bangs, rep_tys_w_strs, wrappers)
           = unzip3 (zipWith (dataConArgRep dflags fam_envs) all_arg_tys orig_bangs)
        (unboxers, boxers) = unzip wrappers
        (rep_tys, rep_strs) = unzip (concat rep_tys_w_strs)

        wrapper_reqd = not (isNewTyCon tycon)  -- Newtypes have only a worker
                    && (any isBanged orig_bangs   -- Some forcing/unboxing
                                                  -- (includes eq_spec)
                        || isFamInstTyCon tycon)  -- Cast result

        initial_wrap_app = Var (dataConWorkId data_con)
                          `mkTyApps`  res_ty_args
        	              `mkVarApps` ex_tvs
        	              `mkCoApps`  map (mkReflCo Nominal . snd) eq_spec
        	                -- Dont box the eq_spec coercions since they are
        	                -- marked as HsUnpack by mk_dict_strict_mark

        mk_boxer :: [Boxer] -> DataConBoxer
        mk_boxer boxers = DCB (\ ty_args src_vars ->
                          do { let ex_vars = takeList ex_tvs src_vars
                                   subst1 = mkTopTvSubst (univ_tvs `zip` ty_args)
                                   subst2 = extendTvSubstList subst1 ex_tvs
                                                              (mkTyVarTys ex_vars)
                             ; (rep_ids, binds) <- go subst2 boxers (dropList ex_tvs src_vars)
                             ; return (ex_vars ++ rep_ids, binds) } )

        go _ [] src_vars = ASSERT2( null src_vars, ppr data_con ) return ([], [])
        go subst (UnitBox : boxers) (src_var : src_vars)
          = do { (rep_ids2, binds) <- go subst boxers src_vars
               ; return (src_var : rep_ids2, binds) }
        go subst (Boxer boxer : boxers) (src_var : src_vars)
          = do { (rep_ids1, arg)  <- boxer subst
               ; (rep_ids2, binds) <- go subst boxers src_vars
               ; return (rep_ids1 ++ rep_ids2, NonRec src_var arg : binds) }
        go _ (_:_) [] = pprPanic "mk_boxer" (ppr data_con)

        mk_rep_app :: [(Id,Unboxer)] -> CoreExpr -> UniqSM CoreExpr
        mk_rep_app [] con_app
          = return con_app
        mk_rep_app ((wrap_arg, unboxer) : prs) con_app
          = do { (rep_ids, unbox_fn) <- unboxer wrap_arg
               ; expr <- mk_rep_app prs (mkVarApps con_app rep_ids)
               ; return (unbox_fn expr) }

    -------------------------
    newLocal :: Type -> UniqSM Var
    newLocal ty = do { uniq <- getUniqueUs
                     ; return (mkSysLocal (fsLit "dt") uniq ty) }

    -------------------------
    dataConArgRep
       :: DynFlags
       -> FamInstEnvs
       -> Type -> HsBang
       -> ( HsBang   -- Like input but with HsUnpackFailed if necy
          , [(Type, StrictnessMark)]   -- Rep types
          , (Unboxer, Boxer) )

    dataConArgRep _ _ arg_ty HsNoBang
      = (HsNoBang, [(arg_ty, NotMarkedStrict)], (unitUnboxer, unitBoxer))

    dataConArgRep _ _ arg_ty (HsUserBang _ False)  -- No '!'
      = (HsNoBang, [(arg_ty, NotMarkedStrict)], (unitUnboxer, unitBoxer))

    dataConArgRep dflags fam_envs arg_ty
        (HsUserBang unpk_prag True)  -- {-# UNPACK #-} !
      | not (gopt Opt_OmitInterfacePragmas dflags) -- Don't unpack if -fomit-iface-pragmas
              -- Don't unpack if we aren't optimising; rather arbitrarily,
              -- we use -fomit-iface-pragmas as the indication
      , let mb_co   = topNormaliseType_maybe fam_envs arg_ty
                         -- Unwrap type families and newtypes
            arg_ty' = case mb_co of { Just (_,ty) -> ty; Nothing -> arg_ty }
      , isUnpackableType fam_envs arg_ty'
      , (rep_tys, wrappers) <- dataConArgUnpack arg_ty'
      , case unpk_prag of
          Nothing -> gopt Opt_UnboxStrictFields dflags
                  || (gopt Opt_UnboxSmallStrictFields dflags
                       && length rep_tys <= 1)  -- See Note [Unpack one-wide fields]
          Just unpack_me -> unpack_me
      = case mb_co of
          Nothing          -> (HsUnpack Nothing,   rep_tys, wrappers)
          Just (co,rep_ty) -> (HsUnpack (Just co), rep_tys, wrapCo co rep_ty wrappers)

      | otherwise  -- Record the strict-but-no-unpack decision
      = strict_but_not_unpacked arg_ty

    dataConArgRep _ _ arg_ty HsStrict
      = strict_but_not_unpacked arg_ty

    dataConArgRep _ _ arg_ty (HsUnpack Nothing)
      | (rep_tys, wrappers) <- dataConArgUnpack arg_ty
      = (HsUnpack Nothing, rep_tys, wrappers)

    dataConArgRep _ _ _ (HsUnpack (Just co))
      | let co_rep_ty = pSnd (coercionKind co)
      , (rep_tys, wrappers) <- dataConArgUnpack co_rep_ty
      = (HsUnpack (Just co), rep_tys, wrapCo co co_rep_ty wrappers)

    strict_but_not_unpacked :: Type -> (HsBang, [(Type,StrictnessMark)], (Unboxer, Boxer))
    strict_but_not_unpacked arg_ty
      = (HsStrict, [(arg_ty, MarkedStrict)], (seqUnboxer, unitBoxer))

    -------------------------
    wrapCo :: Coercion -> Type -> (Unboxer, Boxer) -> (Unboxer, Boxer)
    wrapCo co rep_ty (unbox_rep, box_rep)  -- co :: arg_ty ~ rep_ty
      = (unboxer, boxer)
      where
        unboxer arg_id = do { rep_id <- newLocal rep_ty
                            ; (rep_ids, rep_fn) <- unbox_rep rep_id
                            ; let co_bind = NonRec rep_id (Var arg_id `Cast` co)
                            ; return (rep_ids, Let co_bind . rep_fn) }
        boxer = Boxer $ \ subst ->
                do { (rep_ids, rep_expr)
                        <- case box_rep of
                             UnitBox -> do { rep_id <- newLocal (TcType.substTy subst rep_ty)
                                           ; return ([rep_id], Var rep_id) }
                             Boxer boxer -> boxer subst
                   ; let sco = substCo (tvCvSubst subst) co
                   ; return (rep_ids, rep_expr `Cast` mkSymCo sco) }

    ------------------------
    seqUnboxer :: Unboxer
    seqUnboxer v = return ([v], \e -> Case (Var v) v (exprType e) [(DEFAULT, [], e)])

    unitUnboxer :: Unboxer
    unitUnboxer v = return ([v], \e -> e)

    unitBoxer :: Boxer
    unitBoxer = UnitBox

    -------------------------
    dataConArgUnpack
       :: Type
       ->  ( [(Type, StrictnessMark)]   -- Rep types
           , (Unboxer, Boxer) )

    dataConArgUnpack arg_ty
      | Just (tc, tc_args) <- splitTyConApp_maybe arg_ty
      , Just con <- tyConSingleAlgDataCon_maybe tc
          -- NB: check for an *algebraic* data type
          -- A recursive newtype might mean that
          -- 'arg_ty' is a newtype
      , let rep_tys = dataConInstArgTys con tc_args
      = ASSERT( isVanillaDataCon con )
        ( rep_tys `zip` dataConRepStrictness con
        ,( \ arg_id ->
           do { rep_ids <- mapM newLocal rep_tys
              ; let unbox_fn body
                      = Case (Var arg_id) arg_id (exprType body)
                             [(DataAlt con, rep_ids, body)]
              ; return (rep_ids, unbox_fn) }
         , Boxer $ \ subst ->
           do { rep_ids <- mapM (newLocal . TcType.substTy subst) rep_tys
              ; return (rep_ids, Var (dataConWorkId con)
                                 `mkTyApps` (substTys subst tc_args)
                                 `mkVarApps` rep_ids ) } ) )
      | otherwise
      = pprPanic "dataConArgUnpack" (ppr arg_ty)
        -- An interface file specified Unpacked, but we couldn't unpack it

    isUnpackableType :: FamInstEnvs -> Type -> Bool
    -- True if we can unpack the UNPACK the argument type
    -- See Note [Recursive unboxing]
    -- We look "deeply" inside rather than relying on the DataCons
    -- we encounter on the way, because otherwise we might well
    -- end up relying on ourselves!
    isUnpackableType fam_envs ty
      | Just (tc, _) <- splitTyConApp_maybe ty
      , Just con <- tyConSingleAlgDataCon_maybe tc
      , isVanillaDataCon con
      = ok_con_args (unitNameSet (getName tc)) con
      | otherwise
      = False
      where
        ok_arg tcs (ty, bang) = not (attempt_unpack bang) || ok_ty tcs norm_ty
            where
              norm_ty = topNormaliseType fam_envs ty
        ok_ty tcs ty
          | Just (tc, _) <- splitTyConApp_maybe ty
          , let tc_name = getName tc
          =  not (tc_name `elemNameSet` tcs)
          && case tyConSingleAlgDataCon_maybe tc of
                Just con | isVanillaDataCon con
                        -> ok_con_args (tcs `addOneToNameSet` getName tc) con
                _ -> True
          | otherwise
          = True

        ok_con_args tcs con
           = all (ok_arg tcs) (dataConOrigArgTys con `zip` dataConStrictMarks con)
             -- NB: dataConStrictMarks gives the *user* request;
             -- We'd get a black hole if we used dataConRepBangs

        attempt_unpack (HsUnpack {})                 = True
        attempt_unpack (HsUserBang (Just unpk) bang) = bang && unpk
        attempt_unpack (HsUserBang Nothing bang)     = bang  -- Be conservative
        attempt_unpack HsStrict                      = False
        attempt_unpack HsNoBang                      = False

\end{code} Note [Unpack one-wide fields] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The flag UnboxSmallStrictFields ensures that any field that can (safely) be unboxed to a word-sized unboxed field, should be so unboxed. For example: data A = A Int# newtype B = B A data C = C !B data D = D !C data E = E !() data F = F !D data G = G !F !F All of these should have an Int# as their representation, except G which should have two Int#s. However data T = T !(S Int) data S = S !a Here we can represent T with an Int#. Note [Recursive unboxing] ~~~~~~~~~~~~~~~~~~~~~~~~~ Consider data R = MkR {-# UNPACK #-} !S Int data S = MkS {-# UNPACK #-} !Int The representation arguments of MkR are the *representation* arguments of S (plus Int); the rep args of MkS are Int#. This is all fine. But be careful not to try to unbox this! data T = MkT {-# UNPACK #-} !T Int Because then we'd get an infinite number of arguments. Here is a more complicated case: data S = MkS {-# UNPACK #-} !T Int data T = MkT {-# UNPACK #-} !S Int Each of S and T must decide independendently whether to unpack and they had better not both say yes. So they must both say no. Also behave conservatively when there is no UNPACK pragma data T = MkS !T Int with -funbox-strict-fields or -funbox-small-strict-fields we need to behave as if there was an UNPACK pragma there. But it's the *argument* type that matters. This is fine: data S = MkS S !Int because Int is non-recursive. Note [Unpack equality predicates] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ If we have a GADT with a contructor C :: (a~[b]) => b -> T a we definitely want that equality predicate *unboxed* so that it takes no space at all. This is easily done: just give it an UNPACK pragma. The rest of the unpack/repack code does the heavy lifting. This one line makes every GADT take a word less space for each equality predicate, so it's pretty important! \begin{code}

    mk_pred_strict_mark :: PredType -> HsBang
    mk_pred_strict_mark pred
      | isEqPred pred = HsUnpack Nothing	-- Note [Unpack equality predicates]
      | otherwise     = HsNoBang

\end{code} %************************************************************************ %* * Wrapping and unwrapping newtypes and type families %* * %************************************************************************ \begin{code}

    wrapNewTypeBody :: TyCon -> [Type] -> CoreExpr -> CoreExpr
    -- The wrapper for the data constructor for a newtype looks like this:
    --      newtype T a = MkT (a,Int)
    --      MkT :: forall a. (a,Int) -> T a
    --      MkT = /\a. \(x:(a,Int)). x `cast` sym (CoT a)
    -- where CoT is the coercion TyCon assoicated with the newtype
    --
    -- The call (wrapNewTypeBody T [a] e) returns the
    -- body of the wrapper, namely
    --      e `cast` (CoT [a])
    --
    -- If a coercion constructor is provided in the newtype, then we use
    -- it, otherwise the wrap/unwrap are both no-ops
    --
    -- If the we are dealing with a newtype *instance*, we have a second coercion
    -- identifying the family instance with the constructor of the newtype
    -- instance.  This coercion is applied in any case (ie, composed with the
    -- coercion constructor of the newtype or applied by itself).

    wrapNewTypeBody tycon args result_expr
      = ASSERT( isNewTyCon tycon )
        wrapFamInstBody tycon args $
        mkCast result_expr (mkSymCo co)
      where
        co = mkUnbranchedAxInstCo Representational (newTyConCo tycon) args

    -- When unwrapping, we do *not* apply any family coercion, because this will
    -- be done via a CoPat by the type checker.  We have to do it this way as
    -- computing the right type arguments for the coercion requires more than just
    -- a spliting operation (cf, TcPat.tcConPat).

    unwrapNewTypeBody :: TyCon -> [Type] -> CoreExpr -> CoreExpr
    unwrapNewTypeBody tycon args result_expr
      = ASSERT( isNewTyCon tycon )
        mkCast result_expr (mkUnbranchedAxInstCo Representational (newTyConCo tycon) args)

    -- If the type constructor is a representation type of a data instance, wrap
    -- the expression into a cast adjusting the expression type, which is an
    -- instance of the representation type, to the corresponding instance of the
    -- family instance type.
    -- See Note [Wrappers for data instance tycons]
    wrapFamInstBody :: TyCon -> [Type] -> CoreExpr -> CoreExpr
    wrapFamInstBody tycon args body
      | Just co_con <- tyConFamilyCoercion_maybe tycon
      = mkCast body (mkSymCo (mkUnbranchedAxInstCo Representational co_con args))
      | otherwise
      = body

    -- Same as `wrapFamInstBody`, but for type family instances, which are
    -- represented by a `CoAxiom`, and not a `TyCon`
    wrapTypeFamInstBody :: CoAxiom br -> Int -> [Type] -> CoreExpr -> CoreExpr
    wrapTypeFamInstBody axiom ind args body
      = mkCast body (mkSymCo (mkAxInstCo Representational axiom ind args))

    wrapTypeUnbranchedFamInstBody :: CoAxiom Unbranched -> [Type] -> CoreExpr -> CoreExpr
    wrapTypeUnbranchedFamInstBody axiom
      = wrapTypeFamInstBody axiom 0

    unwrapFamInstScrut :: TyCon -> [Type] -> CoreExpr -> CoreExpr
    unwrapFamInstScrut tycon args scrut
      | Just co_con <- tyConFamilyCoercion_maybe tycon
      = mkCast scrut (mkUnbranchedAxInstCo Representational co_con args) -- data instances only
      | otherwise
      = scrut

    unwrapTypeFamInstScrut :: CoAxiom br -> Int -> [Type] -> CoreExpr -> CoreExpr
    unwrapTypeFamInstScrut axiom ind args scrut
      = mkCast scrut (mkAxInstCo Representational axiom ind args)

    unwrapTypeUnbranchedFamInstScrut :: CoAxiom Unbranched -> [Type] -> CoreExpr -> CoreExpr
    unwrapTypeUnbranchedFamInstScrut axiom
      = unwrapTypeFamInstScrut axiom 0

\end{code} %************************************************************************ %* * \subsection{Primitive operations} %* * %************************************************************************ \begin{code}

    mkPrimOpId :: PrimOp -> Id
    mkPrimOpId prim_op
      = id
      where
        (tyvars,arg_tys,res_ty, arity, strict_sig) = primOpSig prim_op
        ty   = mkForAllTys tyvars (mkFunTys arg_tys res_ty)
        name = mkWiredInName gHC_PRIM (primOpOcc prim_op)
                             (mkPrimOpIdUnique (primOpTag prim_op))
                             (AnId id) UserSyntax
        id   = mkGlobalId (PrimOpId prim_op) name ty info

        info = noCafIdInfo
               `setSpecInfo`          mkSpecInfo (maybeToList $ primOpRules name prim_op)
               `setArityInfo`         arity
               `setStrictnessInfo`    strict_sig
               `setInlinePragInfo`    neverInlinePragma
                   -- We give PrimOps a NOINLINE pragma so that we don't
                   -- get silly warnings from Desugar.dsRule (the inline_shadows_rule
                   -- test) about a RULE conflicting with a possible inlining
                   -- cf Trac #7287

    -- For each ccall we manufacture a separate CCallOpId, giving it
    -- a fresh unique, a type that is correct for this particular ccall,
    -- and a CCall structure that gives the correct details about calling
    -- convention etc.
    --
    -- The *name* of this Id is a local name whose OccName gives the full
    -- details of the ccall, type and all.  This means that the interface
    -- file reader can reconstruct a suitable Id

    mkFCallId :: DynFlags -> Unique -> ForeignCall -> Type -> Id
    mkFCallId dflags uniq fcall ty
      = ASSERT( isEmptyVarSet (tyVarsOfType ty) )
        -- A CCallOpId should have no free type variables;
        -- when doing substitutions won't substitute over it
        mkGlobalId (FCallId fcall) name ty info
      where
        occ_str = showSDoc dflags (braces (ppr fcall <+> ppr ty))
        -- The "occurrence name" of a ccall is the full info about the
        -- ccall; it is encoded, but may have embedded spaces etc!

        name = mkFCallName uniq occ_str

        info = noCafIdInfo
               `setArityInfo`         arity
               `setStrictnessInfo`    strict_sig

        (_, tau)        = tcSplitForAllTys ty
        (arg_tys, _)    = tcSplitFunTys tau
        arity           = length arg_tys
        strict_sig      = mkClosedStrictSig (replicate arity evalDmd) topRes

\end{code} %************************************************************************ %* * \subsection{DictFuns and default methods} %* * %************************************************************************ Note [Dict funs and default methods] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Dict funs and default methods are *not* ImplicitIds. Their definition involves user-written code, so we can't figure out their strictness etc based on fixed info, as we can for constructors and record selectors (say). NB: See also Note [Exported LocalIds] in Id \begin{code}

    mkDictFunId :: Name      -- Name to use for the dict fun;
                -> [TyVar]
                -> ThetaType
                -> Class
                -> [Type]
                -> Id
    -- Implements the DFun Superclass Invariant (see TcInstDcls)
    -- See Note [Dict funs and default methods]

    mkDictFunId dfun_name tvs theta clas tys
      = mkExportedLocalId (DFunId n_silent is_nt)
                          dfun_name
                          dfun_ty
      where
        is_nt = isNewTyCon (classTyCon clas)
        (n_silent, dfun_ty) = mkDictFunTy tvs theta clas tys

    mkDictFunTy :: [TyVar] -> ThetaType -> Class -> [Type] -> (Int, Type)
    mkDictFunTy tvs theta clas tys
      = (length silent_theta, dfun_ty)
      where
        dfun_ty = mkSigmaTy tvs (silent_theta ++ theta) (mkClassPred clas tys)
        silent_theta
          | null tvs, null theta
          = []
          | otherwise
          = filterOut discard $
            substTheta (zipTopTvSubst (classTyVars clas) tys)
                       (classSCTheta clas)
                       -- See Note [Silent Superclass Arguments]
        discard pred = any (`eqPred` pred) theta
                     -- See the DFun Superclass Invariant in TcInstDcls

\end{code} %************************************************************************ %* * \subsection{Un-definable} %* * %************************************************************************ These Ids can't be defined in Haskell. They could be defined in unfoldings in the wired-in GHC.Prim interface file, but we'd have to ensure that they were definitely, definitely inlined, because there is no curried identifier for them. That's what mkCompulsoryUnfolding does. If we had a way to get a compulsory unfolding from an interface file, we could do that, but we don't right now. unsafeCoerce# isn't so much a PrimOp as a phantom identifier, that just gets expanded into a type coercion wherever it occurs. Hence we add it as a built-in Id with an unfolding here. The type variables we use here are "open" type variables: this means they can unify with both unlifted and lifted types. Hence we provide another gun with which to shoot yourself in the foot. \begin{code}

    lazyIdName, unsafeCoerceName, nullAddrName, seqName,
       realWorldName, voidPrimIdName, coercionTokenName,
       magicDictName, coerceName, proxyName, dollarName :: Name
    unsafeCoerceName  = mkWiredInIdName gHC_PRIM  (fsLit "unsafeCoerce#")  unsafeCoerceIdKey  unsafeCoerceId
    nullAddrName      = mkWiredInIdName gHC_PRIM  (fsLit "nullAddr#")      nullAddrIdKey      nullAddrId
    seqName           = mkWiredInIdName gHC_PRIM  (fsLit "seq")            seqIdKey           seqId
    realWorldName     = mkWiredInIdName gHC_PRIM  (fsLit "realWorld#")     realWorldPrimIdKey realWorldPrimId
    voidPrimIdName    = mkWiredInIdName gHC_PRIM  (fsLit "void#")          voidPrimIdKey      voidPrimId
    lazyIdName        = mkWiredInIdName gHC_MAGIC (fsLit "lazy")           lazyIdKey          lazyId
    coercionTokenName = mkWiredInIdName gHC_PRIM  (fsLit "coercionToken#") coercionTokenIdKey coercionTokenId
    magicDictName     = mkWiredInIdName gHC_PRIM  (fsLit "magicDict")      magicDictKey       magicDictId
    coerceName        = mkWiredInIdName gHC_PRIM  (fsLit "coerce")         coerceKey          coerceId
    proxyName         = mkWiredInIdName gHC_PRIM  (fsLit "proxy#")         proxyHashKey       proxyHashId
    dollarName        = mkWiredInIdName gHC_BASE  (fsLit "$")              dollarIdKey        dollarId

\end{code} \begin{code}

    dollarId :: Id  -- Note [dollarId magic]
    dollarId = pcMiscPrelId dollarName ty
                 (noCafIdInfo `setUnfoldingInfo` unf)
      where
        fun_ty = mkFunTy alphaTy openBetaTy
        ty     = mkForAllTys [alphaTyVar, openBetaTyVar] $
                 mkFunTy fun_ty fun_ty
        unf    = mkInlineUnfolding (Just 2) rhs
        [f,x]  = mkTemplateLocals [fun_ty, alphaTy]
        rhs    = mkLams [alphaTyVar, openBetaTyVar, f, x] $
                 App (Var f) (Var x)

    ------------------------------------------------
    -- proxy# :: forall a. Proxy# a
    proxyHashId :: Id
    proxyHashId
      = pcMiscPrelId proxyName ty
           (noCafIdInfo `setUnfoldingInfo` evaldUnfolding) -- Note [evaldUnfoldings]
      where
        ty      = mkForAllTys [kv, tv] (mkProxyPrimTy k t)
        kv      = kKiVar
        k       = mkTyVarTy kv
        tv:_    = tyVarList k
        t       = mkTyVarTy tv

    ------------------------------------------------
    -- unsafeCoerce# :: forall a b. a -> b
    unsafeCoerceId :: Id
    unsafeCoerceId
      = pcMiscPrelId unsafeCoerceName ty info
      where
        info = noCafIdInfo `setInlinePragInfo` alwaysInlinePragma
                           `setUnfoldingInfo`  mkCompulsoryUnfolding rhs

        ty  = mkForAllTys [openAlphaTyVar,openBetaTyVar]
                          (mkFunTy openAlphaTy openBetaTy)
        [x] = mkTemplateLocals [openAlphaTy]
        rhs = mkLams [openAlphaTyVar,openBetaTyVar,x] $
              Cast (Var x) (mkUnsafeCo openAlphaTy openBetaTy)

    ------------------------------------------------
    nullAddrId :: Id
    -- nullAddr# :: Addr#
    -- The reason is is here is because we don't provide
    -- a way to write this literal in Haskell.
    nullAddrId = pcMiscPrelId nullAddrName addrPrimTy info
      where
        info = noCafIdInfo `setInlinePragInfo` alwaysInlinePragma
                           `setUnfoldingInfo`  mkCompulsoryUnfolding (Lit nullAddrLit)

    ------------------------------------------------
    seqId :: Id	-- See Note [seqId magic]
    seqId = pcMiscPrelId seqName ty info
      where
        info = noCafIdInfo `setInlinePragInfo` alwaysInlinePragma
                           `setUnfoldingInfo`  mkCompulsoryUnfolding rhs
                           `setSpecInfo`       mkSpecInfo [seq_cast_rule]

        ty  = mkForAllTys [alphaTyVar,betaTyVar]
                          (mkFunTy alphaTy (mkFunTy betaTy betaTy))
                  -- NB argBetaTyVar; see Note [seqId magic]

        [x,y] = mkTemplateLocals [alphaTy, betaTy]
        rhs = mkLams [alphaTyVar,betaTyVar,x,y] (Case (Var x) x betaTy [(DEFAULT, [], Var y)])

        -- See Note [Built-in RULES for seq]
        seq_cast_rule = BuiltinRule { ru_name  = fsLit "seq of cast"
                                    , ru_fn    = seqName
                                    , ru_nargs = 4
                                    , ru_try   = match_seq_of_cast
                                    }

    match_seq_of_cast :: RuleFun
        -- See Note [Built-in RULES for seq]
    match_seq_of_cast _ _ _ [Type _, Type res_ty, Cast scrut co, expr]
      = Just (Var seqId `mkApps` [Type (pFst (coercionKind co)), Type res_ty,
                                  scrut, expr])
    match_seq_of_cast _ _ _ _ = Nothing

    ------------------------------------------------
    lazyId :: Id	-- See Note [lazyId magic]
    lazyId = pcMiscPrelId lazyIdName ty info
      where
        info = noCafIdInfo
        ty  = mkForAllTys [alphaTyVar] (mkFunTy alphaTy alphaTy)

    --------------------------------------------------------------------------------
    magicDictId :: Id  -- See Note [magicDictId magic]
    magicDictId = pcMiscPrelId magicDictName ty info
      where
      info = noCafIdInfo `setInlinePragInfo` neverInlinePragma
      ty   = mkForAllTys [alphaTyVar] alphaTy

    --------------------------------------------------------------------------------

    coerceId :: Id
    coerceId = pcMiscPrelId coerceName ty info
      where
        info = noCafIdInfo `setInlinePragInfo` alwaysInlinePragma
                           `setUnfoldingInfo`  mkCompulsoryUnfolding rhs
        kv = kKiVar
        k = mkTyVarTy kv
        a:b:_ = tyVarList k
        [aTy,bTy] = map mkTyVarTy [a,b]
        eqRTy     = mkTyConApp coercibleTyCon  [k, aTy, bTy]
        eqRPrimTy = mkTyConApp eqReprPrimTyCon [k, aTy, bTy]
        ty   = mkForAllTys [kv, a, b] (mkFunTys [eqRTy, aTy] bTy)

        [eqR,x,eq] = mkTemplateLocals [eqRTy, aTy,eqRPrimTy]
        rhs = mkLams [kv,a,b,eqR,x] $
              mkWildCase (Var eqR) eqRTy bTy $
    	  [(DataAlt coercibleDataCon, [eq], Cast (Var x) (CoVarCo eq))]

\end{code} Note [dollarId magic] ~~~~~~~~~~~~~~~~~~~~~ The only reason that ($) is wired in is so that its type can be forall (a:*, b:Open). (a->b) -> a -> b That is, the return type can be unboxed. E.g. this is OK foo $ True where foo :: Bool -> Int# because ($) doesn't inspect or move the result of the call to foo. See Trac #8739. There is a special typing rule for ($) in TcExpr, so the type of ($) isn't looked at there, BUT Lint subsequently (and rightly) complains if sees ($) applied to Int# (say), unless we give it a wired-in type as we do here. Note [Unsafe coerce magic] ~~~~~~~~~~~~~~~~~~~~~~~~~~ We define a *primitive* GHC.Prim.unsafeCoerce# and then in the base library we define the ordinary function Unsafe.Coerce.unsafeCoerce :: forall (a:*) (b:*). a -> b unsafeCoerce x = unsafeCoerce# x Notice that unsafeCoerce has a civilized (albeit still dangerous) polymorphic type, whose type args have kind *. So you can't use it on unboxed values (unsafeCoerce 3#). In contrast unsafeCoerce# is even more dangerous because you *can* use it on unboxed things, (unsafeCoerce# 3#) :: Int. Its type is forall (a:OpenKind) (b:OpenKind). a -> b Note [seqId magic] ~~~~~~~~~~~~~~~~~~ 'GHC.Prim.seq' is special in several ways. a) Its second arg can have an unboxed type x `seq` (v +# w) Hence its second type variable has ArgKind b) Its fixity is set in LoadIface.ghcPrimIface c) It has quite a bit of desugaring magic. See DsUtils.lhs Note [Desugaring seq (1)] and (2) and (3) d) There is some special rule handing: Note [User-defined RULES for seq] e) See Note [Typing rule for seq] in TcExpr. Note [User-defined RULES for seq] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Roman found situations where he had case (f n) of _ -> e where he knew that f (which was strict in n) would terminate if n did. Notice that the result of (f n) is discarded. So it makes sense to transform to case n of _ -> e Rather than attempt some general analysis to support this, I've added enough support that you can do this using a rewrite rule: RULE "f/seq" forall n. seq (f n) e = seq n e You write that rule. When GHC sees a case expression that discards its result, it mentally transforms it to a call to 'seq' and looks for a RULE. (This is done in Simplify.rebuildCase.) As usual, the correctness of the rule is up to you. To make this work, we need to be careful that the magical desugaring done in Note [seqId magic] item (c) is *not* done on the LHS of a rule. Or rather, we arrange to un-do it, in DsBinds.decomposeRuleLhs. Note [Built-in RULES for seq] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We also have the following built-in rule for seq seq (x `cast` co) y = seq x y This eliminates unnecessary casts and also allows other seq rules to match more often. Notably, seq (f x `cast` co) y --> seq (f x) y and now a user-defined rule for seq (see Note [User-defined RULES for seq]) may fire. Note [lazyId magic] ~~~~~~~~~~~~~~~~~~~ lazy :: forall a?. a? -> a? (i.e. works for unboxed types too) Used to lazify pseq: pseq a b = a `seq` lazy b Also, no strictness: by being a built-in Id, all the info about lazyId comes from here, not from GHC.Base.hi. This is important, because the strictness analyser will spot it as strict! Also no unfolding in lazyId: it gets "inlined" by a HACK in CorePrep. It's very important to do this inlining *after* unfoldings are exposed in the interface file. Otherwise, the unfolding for (say) pseq in the interface file will not mention 'lazy', so if we inline 'pseq' we'll totally miss the very thing that 'lazy' was there for in the first place. See Trac #3259 for a real world example. lazyId is defined in GHC.Base, so we don't *have* to inline it. If it appears un-applied, we'll end up just calling it. Note [magicDictId magic] ~~~~~~~~~~~~~~~~~~~~~~~~~ The identifier `magicDict` is just a place-holder, which is used to implement a primitve that we cannot define in Haskell but we can write in Core. It is declared with a place-holder type: magicDict :: forall a. a The intention is that the identifier will be used in a very specific way, to create dictionaries for classes with a single method. Consider a class like this: class C a where f :: T a We are going to use `magicDict`, in conjunction with a built-in Prelude rule, to cast values of type `T a` into dictionaries for `C a`. To do this, we define a function like this in the library: data WrapC a b = WrapC (C a => Proxy a -> b) withT :: (C a => Proxy a -> b) -> T a -> Proxy a -> b withT f x y = magicDict (WrapC f) x y The purpose of `WrapC` is to avoid having `f` instantiated. Also, it avoids impredicativity, because `magicDict`'s type cannot be instantiated with a forall. The field of `WrapC` contains a `Proxy` parameter which is used to link the type of the constraint, `C a`, with the type of the `Wrap` value being made. Next, we add a built-in Prelude rule (see prelude/PrelRules.hs), which will replace the RHS of this definition with the appropriate definition in Core. The rewrite rule works as follows: magicDict@t (wrap@a@b f) x y ----> f (x `cast` co a) y The `co` coercion is the newtype-coercion extracted from the type-class. The type class is obtain by looking at the type of wrap. ------------------------------------------------------------- @realWorld#@ used to be a magic literal, \tr{void#}. If things get nasty as-is, change it back to a literal (@Literal@). voidArgId is a Local Id used simply as an argument in functions where we just want an arg to avoid having a thunk of unlifted type. E.g. x = \ void :: Void# -> (# p, q #) This comes up in strictness analysis Note [evaldUnfoldings] ~~~~~~~~~~~~~~~~~~~~~~ The evaldUnfolding makes it look that some primitive value is evaluated, which in turn makes Simplify.interestingArg return True, which in turn makes INLINE things applied to said value likely to be inlined. \begin{code}

    realWorldPrimId :: Id   -- :: State# RealWorld
    realWorldPrimId = pcMiscPrelId realWorldName realWorldStatePrimTy
                         (noCafIdInfo `setUnfoldingInfo` evaldUnfolding    -- Note [evaldUnfoldings]
                                      `setOneShotInfo` stateHackOneShot)

    voidPrimId :: Id     -- Global constant :: Void#
    voidPrimId  = pcMiscPrelId voidPrimIdName voidPrimTy
                    (noCafIdInfo `setUnfoldingInfo` evaldUnfolding)    -- Note [evaldUnfoldings]

    voidArgId :: Id       -- Local lambda-bound :: Void#
    voidArgId = mkSysLocal (fsLit "void") voidArgIdKey voidPrimTy

    coercionTokenId :: Id 	      -- :: () ~ ()
    coercionTokenId -- Used to replace Coercion terms when we go to STG
      = pcMiscPrelId coercionTokenName
                     (mkTyConApp eqPrimTyCon [liftedTypeKind, unitTy, unitTy])
                     noCafIdInfo

\end{code} \begin{code}

    pcMiscPrelId :: Name -> Type -> IdInfo -> Id
    pcMiscPrelId name ty info
      = mkVanillaGlobalWithInfo name ty info
        -- We lie and say the thing is imported; otherwise, we get into
        -- a mess with dependency analysis; e.g., core2stg may heave in
        -- random calls to GHCbase.unpackPS__.  If GHCbase is the module
        -- being compiled, then it's just a matter of luck if the definition
        -- will be in "the right place" to be in scope.

\end{code}