% % (c) The University of Glasgow 2006 % (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 % \section[TcExpr]{Typecheck an expression} \begin{code}

    -- The above warning supression flag is a temporary kludge.
    -- While working on this module you are encouraged to remove it and fix
    -- any warnings in the module. See
    --     http://hackage.haskell.org/trac/ghc/wiki/Commentary/CodingStyle#Warnings
    -- for details

    module TcExpr ( tcPolyExpr, tcPolyExprNC, tcMonoExpr, tcMonoExprNC,
                    tcInferRho, tcInferRhoNC,
                    tcSyntaxOp, tcCheckId,
                    addExprErrCtxt ) where

    #include "HsVersions.h"

    #ifdef GHCI 	/* Only if bootstrapped */
    import {-# SOURCE #-}	TcSplice( tcSpliceExpr, tcBracket )
    import qualified DsMeta
    #endif

    import HsSyn
    import TcHsSyn
    import TcRnMonad
    import TcUnify
    import BasicTypes
    import Inst
    import TcBinds
    import TcEnv
    import TcArrows
    import TcMatches
    import TcHsType
    import TcPat
    import TcMType
    import TcType
    import Id
    import DataCon
    import Name
    import TyCon
    import Type
    import Coercion
    import Var
    import VarSet
    import TysWiredIn
    import TysPrim( intPrimTy )
    import PrimOp( tagToEnumKey )
    import PrelNames
    import DynFlags
    import SrcLoc
    import Util
    import ListSetOps
    import Maybes
    import Outputable
    import FastString
    import Control.Monad

\end{code} %************************************************************************ %* * \subsection{Main wrappers} %* * %************************************************************************ \begin{code}

    tcPolyExpr, tcPolyExprNC
    	 :: LHsExpr Name	-- Expression to type check
           	 -> TcSigmaType		-- Expected type (could be a polytpye)
           	 -> TcM (LHsExpr TcId)	-- Generalised expr with expected type

    -- tcPolyExpr is a convenient place (frequent but not too frequent)
    -- place to add context information.
    -- The NC version does not do so, usually because the caller wants
    -- to do so himself.

    tcPolyExpr expr res_ty
      = addExprErrCtxt expr $
        do { traceTc "tcPolyExpr" (ppr res_ty); tcPolyExprNC expr res_ty }

    tcPolyExprNC expr res_ty
      = do { traceTc "tcPolyExprNC" (ppr res_ty)
           ; (gen_fn, expr') <- tcGen GenSigCtxt res_ty $ \ _ rho ->
    			    tcMonoExprNC expr rho
           ; return (mkLHsWrap gen_fn expr') }

    ---------------
    tcMonoExpr, tcMonoExprNC
        :: LHsExpr Name      -- Expression to type check
        -> TcRhoType         -- Expected type (could be a type variable)
    			 -- Definitely no foralls at the top
        -> TcM (LHsExpr TcId)

    tcMonoExpr expr res_ty
      = addErrCtxt (exprCtxt expr) $
        tcMonoExprNC expr res_ty

    tcMonoExprNC (L loc expr) res_ty
      = ASSERT( not (isSigmaTy res_ty) )
        setSrcSpan loc $
        do	{ expr' <- tcExpr expr res_ty
    	; return (L loc expr') }

    ---------------
    tcInferRho, tcInferRhoNC :: LHsExpr Name -> TcM (LHsExpr TcId, TcRhoType)
    -- Infer a *rho*-type.  This is, in effect, a special case
    -- for ids and partial applications, so that if
    --     f :: Int -> (forall a. a -> a) -> Int
    -- then we can infer
    --     f 3 :: (forall a. a -> a) -> Int
    -- And that in turn is useful
    --  (a) for the function part of any application (see tcApp)
    --  (b) for the special rule for '$'
    tcInferRho expr = addErrCtxt (exprCtxt expr) (tcInferRhoNC expr)

    tcInferRhoNC (L loc expr)
      = setSrcSpan loc $
        do { (expr', rho) <- tcInfExpr expr
           ; return (L loc expr', rho) }

    tcInfExpr :: HsExpr Name -> TcM (HsExpr TcId, TcRhoType)
    tcInfExpr (HsVar f) 	= tcInferId f
    tcInfExpr (HsPar e) 	= do { (e', ty) <- tcInferRhoNC e
                                 ; return (HsPar e', ty) }
    tcInfExpr (HsApp e1 e2) = tcInferApp e1 [e2]
    tcInfExpr e             = tcInfer (tcExpr e)

\end{code} %************************************************************************ %* * tcExpr: the main expression typechecker %* * %************************************************************************ \begin{code}

    tcExpr :: HsExpr Name -> TcRhoType -> TcM (HsExpr TcId)
    tcExpr e res_ty | debugIsOn && isSigmaTy res_ty     -- Sanity check
           	        = pprPanic "tcExpr: sigma" (ppr res_ty $$ ppr e)

    tcExpr (HsVar name)  res_ty = tcCheckId name res_ty

    tcExpr (HsApp e1 e2) res_ty = tcApp e1 [e2] res_ty

    tcExpr (HsLit lit)   res_ty = do { let lit_ty = hsLitType lit
    		     	         ; tcWrapResult (HsLit lit) lit_ty res_ty }

    tcExpr (HsPar expr)  res_ty = do { expr' <- tcMonoExprNC expr res_ty
    				 ; return (HsPar expr') }

    tcExpr (HsSCC lbl expr) res_ty
      = do { expr' <- tcMonoExpr expr res_ty
           ; return (HsSCC lbl expr') }

    tcExpr (HsTickPragma info expr) res_ty
      = do { expr' <- tcMonoExpr expr res_ty
           ; return (HsTickPragma info expr') }

    tcExpr (HsCoreAnn lbl expr) res_ty
      = do	{ expr' <- tcMonoExpr expr res_ty
    	; return (HsCoreAnn lbl expr') }

    tcExpr (HsOverLit lit) res_ty
      = do 	{ lit' <- newOverloadedLit (LiteralOrigin lit) lit res_ty
    	; return (HsOverLit lit') }

    tcExpr (NegApp expr neg_expr) res_ty
      = do	{ neg_expr' <- tcSyntaxOp NegateOrigin neg_expr
    				  (mkFunTy res_ty res_ty)
    	; expr' <- tcMonoExpr expr res_ty
    	; return (NegApp expr' neg_expr') }

    tcExpr (HsIPVar ip) res_ty
      = do	{ let origin = IPOccOrigin ip
    	 	-- Implicit parameters must have a *tau-type* not a
    		-- type scheme.  We enforce this by creating a fresh
    		-- type variable as its type.  (Because res_ty may not
    		-- be a tau-type.)
    	; ip_ty <- newFlexiTyVarTy argTypeKind	-- argTypeKind: it can't be an unboxed tuple
    	; ip_var <- emitWanted origin (mkIPPred ip ip_ty)
    	; tcWrapResult (HsIPVar (IPName ip_var)) ip_ty res_ty }

    tcExpr (HsLam match) res_ty
      = do	{ (co_fn, match') <- tcMatchLambda match res_ty
    	; return (mkHsWrap co_fn (HsLam match')) }

    tcExpr (ExprWithTySig expr sig_ty) res_ty
     = do { sig_tc_ty <- tcHsSigType ExprSigCtxt sig_ty

          -- Remember to extend the lexical type-variable environment
          ; (gen_fn, expr')
                <- tcGen ExprSigCtxt sig_tc_ty $ \ skol_tvs res_ty ->
          	       tcExtendTyVarEnv2 (hsExplicitTvs sig_ty `zip` mkTyVarTys skol_tvs) $
    	             	       	-- See Note [More instantiated than scoped] in TcBinds
          	       tcMonoExprNC expr res_ty

          ; let inner_expr = ExprWithTySigOut (mkLHsWrap gen_fn expr') sig_ty

          ; (inst_wrap, rho) <- deeplyInstantiate ExprSigOrigin sig_tc_ty
          ; tcWrapResult (mkHsWrap inst_wrap inner_expr) rho res_ty }

    tcExpr (HsType ty) _
      = failWithTc (text "Can't handle type argument:" <+> ppr ty)
    	-- This is the syntax for type applications that I was planning
    	-- but there are difficulties (e.g. what order for type args)
    	-- so it's not enabled yet.
    	-- Can't eliminate it altogether from the parser, because the
    	-- same parser parses *patterns*.

\end{code} %************************************************************************ %* * Infix operators and sections %* * %************************************************************************ Note [Left sections] ~~~~~~~~~~~~~~~~~~~~ Left sections, like (4 *), are equivalent to \ x -> (*) 4 x, or, if PostfixOperators is enabled, just (*) 4 With PostfixOperators we don't actually require the function to take two arguments at all. For example, (x `not`) means (not x); you get postfix operators! Not Haskell 98, but it's less work and kind of useful. Note [Typing rule for ($)] ~~~~~~~~~~~~~~~~~~~~~~~~~~ People write runST $ blah so much, where runST :: (forall s. ST s a) -> a that I have finally given in and written a special type-checking rule just for saturated appliations of ($). * Infer the type of the first argument * Decompose it; should be of form (arg2_ty -> res_ty), where arg2_ty might be a polytype * Use arg2_ty to typecheck arg2 Note [Typing rule for seq] ~~~~~~~~~~~~~~~~~~~~~~~~~~ We want to allow x `seq` (# p,q #) which suggests this type for seq: seq :: forall (a:*) (b:??). a -> b -> b, with (b:??) meaning that be can be instantiated with an unboxed tuple. But that's ill-kinded! Function arguments can't be unboxed tuples. And indeed, you could not expect to do this with a partially-applied 'seq'; it's only going to work when it's fully applied. so it turns into case x of _ -> (# p,q #) For a while I slid by by giving 'seq' an ill-kinded type, but then the simplifier eta-reduced an application of seq and Lint blew up with a kind error. It seems more uniform to treat 'seq' as it it was a language construct. See Note [seqId magic] in MkId, and \begin{code}

    tcExpr (OpApp arg1 op fix arg2) res_ty
      | (L loc (HsVar op_name)) <- op
      , op_name `hasKey` seqIdKey		-- Note [Typing rule for seq]
      = do { arg1_ty <- newFlexiTyVarTy liftedTypeKind
           ; let arg2_ty = res_ty
           ; arg1' <- tcArg op (arg1, arg1_ty, 1)
           ; arg2' <- tcArg op (arg2, arg2_ty, 2)
           ; op_id <- tcLookupId op_name
           ; let op' = L loc (HsWrap (mkWpTyApps [arg1_ty, arg2_ty]) (HsVar op_id))
           ; return $ OpApp arg1' op' fix arg2' }

      | (L loc (HsVar op_name)) <- op
      , op_name `hasKey` dollarIdKey	-- Note [Typing rule for ($)]
      = do { traceTc "Application rule" (ppr op)
           ; (arg1', arg1_ty) <- tcInferRho arg1
           ; let doc = ptext (sLit "The first argument of ($) takes")
           ; (co_arg1, [arg2_ty], op_res_ty) <- matchExpectedFunTys doc 1 arg1_ty
           	 -- arg2_ty maybe polymorphic; that's the point
           ; arg2' <- tcArg op (arg2, arg2_ty, 2)
           ; co_res <- unifyType op_res_ty res_ty
           ; op_id <- tcLookupId op_name
           ; let op' = L loc (HsWrap (mkWpTyApps [arg2_ty, op_res_ty]) (HsVar op_id))
           ; return $ mkHsWrapCoI co_res $
             OpApp (mkLHsWrapCoI co_arg1 arg1') op' fix arg2' }

      | otherwise
      = do { traceTc "Non Application rule" (ppr op)
           ; (op', op_ty) <- tcInferFun op
           ; (co_fn, arg_tys, op_res_ty) <- unifyOpFunTys op 2 op_ty
           ; co_res <- unifyType op_res_ty res_ty
           ; [arg1', arg2'] <- tcArgs op [arg1, arg2] arg_tys
           ; return $ mkHsWrapCoI co_res $
             OpApp arg1' (mkLHsWrapCoI co_fn op') fix arg2' }

    -- Right sections, equivalent to \ x -> x `op` expr, or
    --	\ x -> op x expr

    tcExpr (SectionR op arg2) res_ty
      = do { (op', op_ty) <- tcInferFun op
           ; (co_fn, [arg1_ty, arg2_ty], op_res_ty) <- unifyOpFunTys op 2 op_ty
           ; co_res <- unifyType (mkFunTy arg1_ty op_res_ty) res_ty
           ; arg2' <- tcArg op (arg2, arg2_ty, 2)
           ; return $ mkHsWrapCoI co_res $
             SectionR (mkLHsWrapCoI co_fn op') arg2' }

    tcExpr (SectionL arg1 op) res_ty
      = do { (op', op_ty) <- tcInferFun op
           ; dflags <- getDOpts	    -- Note [Left sections]
           ; let n_reqd_args | xopt Opt_PostfixOperators dflags = 1
                             | otherwise                        = 2

           ; (co_fn, (arg1_ty:arg_tys), op_res_ty) <- unifyOpFunTys op n_reqd_args op_ty
           ; co_res <- unifyType (mkFunTys arg_tys op_res_ty) res_ty
           ; arg1' <- tcArg op (arg1, arg1_ty, 1)
           ; return $ mkHsWrapCoI co_res $
             SectionL arg1' (mkLHsWrapCoI co_fn op') }

    tcExpr (ExplicitTuple tup_args boxity) res_ty
      | all tupArgPresent tup_args
      = do { let tup_tc = tupleTyCon boxity (length tup_args)
           ; (coi, arg_tys) <- matchExpectedTyConApp tup_tc res_ty
           ; tup_args1 <- tcTupArgs tup_args arg_tys
           ; return $ mkHsWrapCoI coi (ExplicitTuple tup_args1 boxity) }

      | otherwise
      = -- The tup_args are a mixture of Present and Missing (for tuple sections)
        do { let kind = case boxity of { Boxed   -> liftedTypeKind
                                       ; Unboxed -> argTypeKind }
                 arity = length tup_args
                 tup_tc = tupleTyCon boxity arity

           ; arg_tys <- newFlexiTyVarTys (tyConArity tup_tc) kind
           ; let actual_res_ty
                     = mkFunTys [ty | (ty, Missing _) <- arg_tys `zip` tup_args]
                                (mkTyConApp tup_tc arg_tys)

           ; coi <- unifyType actual_res_ty res_ty

           -- Handle tuple sections where
           ; tup_args1 <- tcTupArgs tup_args arg_tys

           ; return $ mkHsWrapCoI coi (ExplicitTuple tup_args1 boxity) }

    tcExpr (ExplicitList _ exprs) res_ty
      = do 	{ (coi, elt_ty) <- matchExpectedListTy res_ty
    	; exprs' <- mapM (tc_elt elt_ty) exprs
    	; return $ mkHsWrapCoI coi (ExplicitList elt_ty exprs') }
      where
        tc_elt elt_ty expr = tcPolyExpr expr elt_ty

    tcExpr (ExplicitPArr _ exprs) res_ty	-- maybe empty
      = do	{ (coi, elt_ty) <- matchExpectedPArrTy res_ty
        	; exprs' <- mapM (tc_elt elt_ty) exprs
    	; return $ mkHsWrapCoI coi (ExplicitPArr elt_ty exprs') }
      where
        tc_elt elt_ty expr = tcPolyExpr expr elt_ty

\end{code} %************************************************************************ %* * Let, case, if, do %* * %************************************************************************ \begin{code}

    tcExpr (HsLet binds expr) res_ty
      = do	{ (binds', expr') <- tcLocalBinds binds $
    			     tcMonoExpr expr res_ty
    	; return (HsLet binds' expr') }

    tcExpr (HsCase scrut matches) exp_ty
      = do	{  -- We used to typecheck the case alternatives first.
    	   -- The case patterns tend to give good type info to use
    	   -- when typechecking the scrutinee.  For example
    	   --	case (map f) of
    	   --	  (x:xs) -> ...
    	   -- will report that map is applied to too few arguments
    	   --
    	   -- But now, in the GADT world, we need to typecheck the scrutinee
    	   -- first, to get type info that may be refined in the case alternatives
    	  (scrut', scrut_ty) <- tcInferRho scrut

    	; traceTc "HsCase" (ppr scrut_ty)
    	; matches' <- tcMatchesCase match_ctxt scrut_ty matches exp_ty
    	; return (HsCase scrut' matches') }
     where
        match_ctxt = MC { mc_what = CaseAlt,
    		      mc_body = tcBody }

    tcExpr (HsIf Nothing pred b1 b2) res_ty	   -- Ordinary 'if'
      = do { pred' <- tcMonoExpr pred boolTy
           ; b1' <- tcMonoExpr b1 res_ty
           ; b2' <- tcMonoExpr b2 res_ty
           ; return (HsIf Nothing pred' b1' b2') }

    tcExpr (HsIf (Just fun) pred b1 b2) res_ty   -- Rebindable syntax
      = do { pred_ty <- newFlexiTyVarTy openTypeKind
           ; b_ty <- newFlexiTyVarTy openTypeKind
           ; let if_ty = mkFunTys [pred_ty, b_ty, b_ty] res_ty
           ; fun' <- tcSyntaxOp IfOrigin fun if_ty
           ; pred' <- tcMonoExpr pred pred_ty
           ; b1' <- tcMonoExpr b1 b_ty
           ; b2' <- tcMonoExpr b2 b_ty
           ; return (HsIf (Just fun') pred' b1' b2') }

    tcExpr (HsDo do_or_lc stmts body _) res_ty
      = tcDoStmts do_or_lc stmts body res_ty

    tcExpr (HsProc pat cmd) res_ty
      = do	{ (pat', cmd', coi) <- tcProc pat cmd res_ty
    	; return $ mkHsWrapCoI coi (HsProc pat' cmd') }

    tcExpr e@(HsArrApp _ _ _ _ _) _
      = failWithTc (vcat [ptext (sLit "The arrow command"), nest 2 (ppr e),
                          ptext (sLit "was found where an expression was expected")])

    tcExpr e@(HsArrForm _ _ _) _
      = failWithTc (vcat [ptext (sLit "The arrow command"), nest 2 (ppr e),
                          ptext (sLit "was found where an expression was expected")])

\end{code} %************************************************************************ %* * Record construction and update %* * %************************************************************************ \begin{code}

    tcExpr (RecordCon (L loc con_name) _ rbinds) res_ty
      = do	{ data_con <- tcLookupDataCon con_name

     	-- Check for missing fields
    	; checkMissingFields data_con rbinds

    	; (con_expr, con_tau) <- tcInferId con_name
    	; let arity = dataConSourceArity data_con
    	      (arg_tys, actual_res_ty) = tcSplitFunTysN con_tau arity
    	      con_id = dataConWrapId data_con

            ; co_res <- unifyType actual_res_ty res_ty
            ; rbinds' <- tcRecordBinds data_con arg_tys rbinds
    	; return $ mkHsWrapCoI co_res $
              RecordCon (L loc con_id) con_expr rbinds' }

\end{code} Note [Type of a record update] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The main complication with RecordUpd is that we need to explicitly handle the *non-updated* fields. Consider: data T a b c = MkT1 { fa :: a, fb :: (b,c) } | MkT2 { fa :: a, fb :: (b,c), fc :: c -> c } | MkT3 { fd :: a } upd :: T a b c -> (b',c) -> T a b' c upd t x = t { fb = x} The result type should be (T a b' c) not (T a b c), because 'b' *is not* mentioned in a non-updated field not (T a b' c'), becuase 'c' *is* mentioned in a non-updated field NB that it's not good enough to look at just one constructor; we must look at them all; cf Trac #3219 After all, upd should be equivalent to: upd t x = case t of MkT1 p q -> MkT1 p x MkT2 a b -> MkT2 p b MkT3 d -> error ... So we need to give a completely fresh type to the result record, and then constrain it by the fields that are *not* updated ("p" above). We call these the "fixed" type variables, and compute them in getFixedTyVars. Note that because MkT3 doesn't contain all the fields being updated, its RHS is simply an error, so it doesn't impose any type constraints. Hence the use of 'relevant_cont'. Note [Implict type sharing] ~~~~~~~~~~~~~~~~~~~~~~~~~~~ We also take into account any "implicit" non-update fields. For example data T a b where { MkT { f::a } :: T a a; ... } So the "real" type of MkT is: forall ab. (a~b) => a -> T a b Then consider upd t x = t { f=x } We infer the type upd :: T a b -> a -> T a b upd (t::T a b) (x::a) = case t of { MkT (co:a~b) (_:a) -> MkT co x } We can't give it the more general type upd :: T a b -> c -> T c b Note [Criteria for update] ~~~~~~~~~~~~~~~~~~~~~~~~~~ We want to allow update for existentials etc, provided the updated field isn't part of the existential. For example, this should be ok. data T a where { MkT { f1::a, f2::b->b } :: T a } f :: T a -> b -> T b f t b = t { f1=b } The criterion we use is this: The types of the updated fields mention only the universally-quantified type variables of the data constructor NB: this is not (quite) the same as being a "naughty" record selector (See Note [Naughty record selectors]) in TcTyClsDecls), at least in the case of GADTs. Consider data T a where { MkT :: { f :: a } :: T [a] } Then f is not "naughty" because it has a well-typed record selector. But we don't allow updates for 'f'. (One could consider trying to allow this, but it makes my head hurt. Badly. And no one has asked for it.) In principle one could go further, and allow g :: T a -> T a g t = t { f2 = \x -> x } because the expression is polymorphic...but that seems a bridge too far. Note [Data family example] ~~~~~~~~~~~~~~~~~~~~~~~~~~ data instance T (a,b) = MkT { x::a, y::b } ---> data :TP a b = MkT { a::a, y::b } coTP a b :: T (a,b) ~ :TP a b Suppose r :: T (t1,t2), e :: t3 Then r { x=e } :: T (t3,t1) ---> case r |> co1 of MkT x y -> MkT e y |> co2 where co1 :: T (t1,t2) ~ :TP t1 t2 co2 :: :TP t3 t2 ~ T (t3,t2) The wrapping with co2 is done by the constructor wrapper for MkT Outgoing invariants ~~~~~~~~~~~~~~~~~~~ In the outgoing (HsRecordUpd scrut binds cons in_inst_tys out_inst_tys): * cons are the data constructors to be updated * in_inst_tys, out_inst_tys have same length, and instantiate the *representation* tycon of the data cons. In Note [Data family example], in_inst_tys = [t1,t2], out_inst_tys = [t3,t2] \begin{code}

    tcExpr (RecordUpd record_expr rbinds _ _ _) res_ty
      = ASSERT( notNull upd_fld_names )
        do	{
    	-- STEP 0
    	-- Check that the field names are really field names
    	; sel_ids <- mapM tcLookupField upd_fld_names
    			-- The renamer has already checked that
    			-- selectors are all in scope
    	; let bad_guys = [ setSrcSpan loc $ addErrTc (notSelector fld_name)
    		       	 | (fld, sel_id) <- rec_flds rbinds `zip` sel_ids,
    		       	   not (isRecordSelector sel_id), 	-- Excludes class ops
    		       	   let L loc fld_name = hsRecFieldId fld ]
    	; unless (null bad_guys) (sequence bad_guys >> failM)

    	-- STEP 1
    	-- Figure out the tycon and data cons from the first field name
    	; let	-- It's OK to use the non-tc splitters here (for a selector)
    	      sel_id : _  = sel_ids
    	      (tycon, _)  = recordSelectorFieldLabel sel_id	-- We've failed already if
    	      data_cons   = tyConDataCons tycon			-- it's not a field label
    	      	-- NB: for a data type family, the tycon is the instance tycon

    	      relevant_cons   = filter is_relevant data_cons
    	      is_relevant con = all (`elem` dataConFieldLabels con) upd_fld_names
    		-- A constructor is only relevant to this process if
    		-- it contains *all* the fields that are being updated
    		-- Other ones will cause a runtime error if they occur

    		-- Take apart a representative constructor
    	      con1 = ASSERT( not (null relevant_cons) ) head relevant_cons
    	      (con1_tvs, _, _, _, _, con1_arg_tys, _) = dataConFullSig con1
    	      con1_flds = dataConFieldLabels con1
    	      con1_res_ty = mkFamilyTyConApp tycon (mkTyVarTys con1_tvs)

    	-- Step 2
    	-- Check that at least one constructor has all the named fields
    	-- i.e. has an empty set of bad fields returned by badFields
    	; checkTc (not (null relevant_cons)) (badFieldsUpd rbinds)

    	-- STEP 3    Note [Criteria for update]
    	-- Check that each updated field is polymorphic; that is, its type
    	-- mentions only the universally-quantified variables of the data con
    	; let flds1_w_tys = zipEqual "tcExpr:RecConUpd" con1_flds con1_arg_tys
    	      upd_flds1_w_tys = filter is_updated flds1_w_tys
    	      is_updated (fld,_) = fld `elem` upd_fld_names

    	      bad_upd_flds = filter bad_fld upd_flds1_w_tys
    	      con1_tv_set = mkVarSet con1_tvs
    	      bad_fld (fld, ty) = fld `elem` upd_fld_names &&
    				      not (tyVarsOfType ty `subVarSet` con1_tv_set)
    	; checkTc (null bad_upd_flds) (badFieldTypes bad_upd_flds)

    	-- STEP 4  Note [Type of a record update]
    	-- Figure out types for the scrutinee and result
    	-- Both are of form (T a b c), with fresh type variables, but with
    	-- common variables where the scrutinee and result must have the same type
    	-- These are variables that appear in *any* arg of *any* of the
    	-- relevant constructors *except* in the updated fields
    	--
    	; let fixed_tvs = getFixedTyVars con1_tvs relevant_cons
    	      is_fixed_tv tv = tv `elemVarSet` fixed_tvs
    	      mk_inst_ty tv result_inst_ty
    	        | is_fixed_tv tv = return result_inst_ty	    -- Same as result type
    	        | otherwise      = newFlexiTyVarTy (tyVarKind tv)  -- Fresh type, of correct kind

    	; (_, result_inst_tys, result_inst_env) <- tcInstTyVars con1_tvs
    	; scrut_inst_tys <- zipWithM mk_inst_ty con1_tvs result_inst_tys

    	; let rec_res_ty    = substTy result_inst_env con1_res_ty
    	      con1_arg_tys' = map (substTy result_inst_env) con1_arg_tys
    	      scrut_subst   = zipTopTvSubst con1_tvs scrut_inst_tys
    	      scrut_ty      = substTy scrut_subst con1_res_ty

            ; co_res <- unifyType rec_res_ty res_ty

    	-- STEP 5
    	-- Typecheck the thing to be updated, and the bindings
    	; record_expr' <- tcMonoExpr record_expr scrut_ty
    	; rbinds'      <- tcRecordBinds con1 con1_arg_tys' rbinds

    	-- STEP 6: Deal with the stupid theta
    	; let theta' = substTheta scrut_subst (dataConStupidTheta con1)
    	; instStupidTheta RecordUpdOrigin theta'

    	-- Step 7: make a cast for the scrutinee, in the case that it's from a type family
    	; let scrut_co | Just co_con <- tyConFamilyCoercion_maybe tycon
    		       = WpCast $ mkTyConApp co_con scrut_inst_tys
    		       | otherwise
    		       = idHsWrapper
    	-- Phew!
            ; return $ mkHsWrapCoI co_res $
              RecordUpd (mkLHsWrap scrut_co record_expr') rbinds'
    			           relevant_cons scrut_inst_tys result_inst_tys  }
      where
        upd_fld_names = hsRecFields rbinds

        getFixedTyVars :: [TyVar] -> [DataCon] -> TyVarSet
        -- These tyvars must not change across the updates
        getFixedTyVars tvs1 cons
          = mkVarSet [tv1 | con <- cons
        		      , let (tvs, theta, arg_tys, _) = dataConSig con
    		      	    flds = dataConFieldLabels con
        			    fixed_tvs = exactTyVarsOfTypes fixed_tys
    			    	    -- fixed_tys: See Note [Type of a record update]
    			    	        `unionVarSet` tyVarsOfTheta theta
    				    -- Universally-quantified tyvars that
    				    -- appear in any of the *implicit*
    				    -- arguments to the constructor are fixed
    			    	    -- See Note [Implict type sharing]

    		            fixed_tys = [ty | (fld,ty) <- zip flds arg_tys
                                                , not (fld `elem` upd_fld_names)]
                          , (tv1,tv) <- tvs1 `zip` tvs	-- Discards existentials in tvs
            	      , tv `elemVarSet` fixed_tvs ]

\end{code} %************************************************************************ %* * Arithmetic sequences e.g. [a,b..] and their parallel-array counterparts e.g. [: a,b.. :] %* * %************************************************************************ \begin{code}

    tcExpr (ArithSeq _ seq@(From expr)) res_ty
      = do	{ (coi, elt_ty) <- matchExpectedListTy res_ty
    	; expr' <- tcPolyExpr expr elt_ty
    	; enum_from <- newMethodFromName (ArithSeqOrigin seq)
    			      enumFromName elt_ty
    	; return $ mkHsWrapCoI coi (ArithSeq enum_from (From expr')) }

    tcExpr (ArithSeq _ seq@(FromThen expr1 expr2)) res_ty
      = do	{ (coi, elt_ty) <- matchExpectedListTy res_ty
    	; expr1' <- tcPolyExpr expr1 elt_ty
    	; expr2' <- tcPolyExpr expr2 elt_ty
    	; enum_from_then <- newMethodFromName (ArithSeqOrigin seq)
    			      enumFromThenName elt_ty
    	; return $ mkHsWrapCoI coi
                        (ArithSeq enum_from_then (FromThen expr1' expr2')) }

    tcExpr (ArithSeq _ seq@(FromTo expr1 expr2)) res_ty
      = do	{ (coi, elt_ty) <- matchExpectedListTy res_ty
    	; expr1' <- tcPolyExpr expr1 elt_ty
    	; expr2' <- tcPolyExpr expr2 elt_ty
    	; enum_from_to <- newMethodFromName (ArithSeqOrigin seq)
    		  	      enumFromToName elt_ty
    	; return $ mkHsWrapCoI coi
                         (ArithSeq enum_from_to (FromTo expr1' expr2')) }

    tcExpr (ArithSeq _ seq@(FromThenTo expr1 expr2 expr3)) res_ty
      = do	{ (coi, elt_ty) <- matchExpectedListTy res_ty
    	; expr1' <- tcPolyExpr expr1 elt_ty
    	; expr2' <- tcPolyExpr expr2 elt_ty
    	; expr3' <- tcPolyExpr expr3 elt_ty
    	; eft <- newMethodFromName (ArithSeqOrigin seq)
    		      enumFromThenToName elt_ty
    	; return $ mkHsWrapCoI coi
                         (ArithSeq eft (FromThenTo expr1' expr2' expr3')) }

    tcExpr (PArrSeq _ seq@(FromTo expr1 expr2)) res_ty
      = do	{ (coi, elt_ty) <- matchExpectedPArrTy res_ty
    	; expr1' <- tcPolyExpr expr1 elt_ty
    	; expr2' <- tcPolyExpr expr2 elt_ty
    	; enum_from_to <- newMethodFromName (PArrSeqOrigin seq)
    				 enumFromToPName elt_ty
    	; return $ mkHsWrapCoI coi
                         (PArrSeq enum_from_to (FromTo expr1' expr2')) }

    tcExpr (PArrSeq _ seq@(FromThenTo expr1 expr2 expr3)) res_ty
      = do	{ (coi, elt_ty) <- matchExpectedPArrTy res_ty
    	; expr1' <- tcPolyExpr expr1 elt_ty
    	; expr2' <- tcPolyExpr expr2 elt_ty
    	; expr3' <- tcPolyExpr expr3 elt_ty
    	; eft <- newMethodFromName (PArrSeqOrigin seq)
    		      enumFromThenToPName elt_ty
    	; return $ mkHsWrapCoI coi
                         (PArrSeq eft (FromThenTo expr1' expr2' expr3')) }

    tcExpr (PArrSeq _ _) _
      = panic "TcExpr.tcMonoExpr: Infinite parallel array!"
        -- the parser shouldn't have generated it and the renamer shouldn't have
        -- let it through

\end{code} %************************************************************************ %* * Template Haskell %* * %************************************************************************ \begin{code}

    #ifdef GHCI	/* Only if bootstrapped */
    	-- Rename excludes these cases otherwise
    tcExpr (HsSpliceE splice) res_ty = tcSpliceExpr splice res_ty
    tcExpr (HsBracket brack)  res_ty = do	{ e <- tcBracket brack res_ty
    					; return (unLoc e) }
    tcExpr e@(HsQuasiQuoteE _) _ =
        pprPanic "Should never see HsQuasiQuoteE in type checker" (ppr e)
    #endif /* GHCI */

\end{code} %************************************************************************ %* * Catch-all %* * %************************************************************************ \begin{code}

    tcExpr other _ = pprPanic "tcMonoExpr" (ppr other)

\end{code} %************************************************************************ %* * Applications %* * %************************************************************************ \begin{code}

    tcApp :: LHsExpr Name -> [LHsExpr Name] -- Function and args
          -> TcRhoType -> TcM (HsExpr TcId) -- Translated fun and args

    tcApp (L _ (HsPar e)) args res_ty
      = tcApp e args res_ty

    tcApp (L _ (HsApp e1 e2)) args res_ty
      = tcApp e1 (e2:args) res_ty	-- Accumulate the arguments

    tcApp (L loc (HsVar fun)) args res_ty
      | fun `hasKey` tagToEnumKey
      , [arg] <- args
      = tcTagToEnum loc fun arg res_ty

    tcApp fun args res_ty
      = do	{   -- Type-check the function
    	; (fun1, fun_tau) <- tcInferFun fun

    	    -- Extract its argument types
    	; (co_fun, expected_arg_tys, actual_res_ty)
    	      <- matchExpectedFunTys (mk_app_msg fun) (length args) fun_tau

    	-- Typecheck the result, thereby propagating
            -- info (if any) from result into the argument types
            -- Both actual_res_ty and res_ty are deeply skolemised
            ; co_res <- addErrCtxt (funResCtxt fun) $
                        unifyType actual_res_ty res_ty

    	-- Typecheck the arguments
    	; args1 <- tcArgs fun args expected_arg_tys

            -- Assemble the result
    	; let fun2 = mkLHsWrapCoI co_fun fun1
                  app  = mkLHsWrapCoI co_res (foldl mkHsApp fun2 args1)

            ; return (unLoc app) }

    mk_app_msg :: LHsExpr Name -> SDoc
    mk_app_msg fun = sep [ ptext (sLit "The function") <+> quotes (ppr fun)
                         , ptext (sLit "is applied to")]

    ----------------
    tcInferApp :: LHsExpr Name -> [LHsExpr Name] -- Function and args
               -> TcM (HsExpr TcId, TcRhoType) -- Translated fun and args

    tcInferApp (L _ (HsPar e))     args = tcInferApp e args
    tcInferApp (L _ (HsApp e1 e2)) args = tcInferApp e1 (e2:args)
    tcInferApp fun args
      = -- Very like the tcApp version, except that there is
        -- no expected result type passed in
        do	{ (fun1, fun_tau) <- tcInferFun fun
    	; (co_fun, expected_arg_tys, actual_res_ty)
    	      <- matchExpectedFunTys (mk_app_msg fun) (length args) fun_tau
    	; args1 <- tcArgs fun args expected_arg_tys
    	; let fun2 = mkLHsWrapCoI co_fun fun1
                  app  = foldl mkHsApp fun2 args1
            ; return (unLoc app, actual_res_ty) }

    ----------------
    tcInferFun :: LHsExpr Name -> TcM (LHsExpr TcId, TcRhoType)
    -- Infer and instantiate the type of a function
    tcInferFun (L loc (HsVar name))
      = do { (fun, ty) <- setSrcSpan loc (tcInferId name)
           	       -- Don't wrap a context around a plain Id
           ; return (L loc fun, ty) }

    tcInferFun fun
      = do { (fun, fun_ty) <- tcInfer (tcMonoExpr fun)

             -- Zonk the function type carefully, to expose any polymorphism
    	 -- E.g. (( \(x::forall a. a->a). blah ) e)
    	 -- We can see the rank-2 type of the lambda in time to genrealise e
           ; fun_ty' <- zonkTcTypeCarefully fun_ty

           ; (wrap, rho) <- deeplyInstantiate AppOrigin fun_ty'
           ; return (mkLHsWrap wrap fun, rho) }

    ----------------
    tcArgs :: LHsExpr Name				-- The function (for error messages)
           -> [LHsExpr Name] -> [TcSigmaType]	-- Actual arguments and expected arg types
           -> TcM [LHsExpr TcId]			-- Resulting args

    tcArgs fun args expected_arg_tys
      = mapM (tcArg fun) (zip3 args expected_arg_tys [1..])

    ----------------
    tcArg :: LHsExpr Name				-- The function (for error messages)
           -> (LHsExpr Name, TcSigmaType, Int)	-- Actual argument and expected arg type
           -> TcM (LHsExpr TcId)			-- Resulting argument
    tcArg fun (arg, ty, arg_no) = addErrCtxt (funAppCtxt fun arg arg_no)
    				 	 (tcPolyExprNC arg ty)

    ----------------
    tcTupArgs :: [HsTupArg Name] -> [TcSigmaType] -> TcM [HsTupArg TcId]
    tcTupArgs args tys
      = ASSERT( equalLength args tys ) mapM go (args `zip` tys)
      where
        go (Missing {},   arg_ty) = return (Missing arg_ty)
        go (Present expr, arg_ty) = do { expr' <- tcPolyExpr expr arg_ty
    			           ; return (Present expr') }

    ----------------
    unifyOpFunTys :: LHsExpr Name -> Arity -> TcRhoType
                  -> TcM (CoercionI, [TcSigmaType], TcRhoType)
    -- A wrapper for matchExpectedFunTys
    unifyOpFunTys op arity ty = matchExpectedFunTys herald arity ty
      where
        herald = ptext (sLit "The operator") <+> quotes (ppr op) <+> ptext (sLit "takes")

    ---------------------------
    tcSyntaxOp :: CtOrigin -> HsExpr Name -> TcType -> TcM (HsExpr TcId)
    -- Typecheck a syntax operator, checking that it has the specified type
    -- The operator is always a variable at this stage (i.e. renamer output)
    -- This version assumes res_ty is a monotype
    tcSyntaxOp orig (HsVar op) res_ty = do { (expr, rho) <- tcInferIdWithOrig orig op
                                           ; tcWrapResult expr rho res_ty }
    tcSyntaxOp _ other 	   _      = pprPanic "tcSyntaxOp" (ppr other)

\end{code} Note [Push result type in] ~~~~~~~~~~~~~~~~~~~~~~~~~~ Unify with expected result before type-checking the args so that the info from res_ty percolates to args. This is when we might detect a too-few args situation. (One can think of cases when the opposite order would give a better error message.) experimenting with putting this first. Here's an example where it actually makes a real difference class C t a b | t a -> b instance C Char a Bool data P t a = forall b. (C t a b) => MkP b data Q t = MkQ (forall a. P t a) f1, f2 :: Q Char; f1 = MkQ (MkP True) f2 = MkQ (MkP True :: forall a. P Char a) With the change, f1 will type-check, because the 'Char' info from the signature is propagated into MkQ's argument. With the check in the other order, the extra signature in f2 is reqd. %************************************************************************ %* * tcInferId %* * %************************************************************************ \begin{code}

    tcCheckId :: Name -> TcRhoType -> TcM (HsExpr TcId)
    tcCheckId name res_ty = do { (expr, rho) <- tcInferId name
                               ; tcWrapResult expr rho res_ty }

    ------------------------
    tcInferId :: Name -> TcM (HsExpr TcId, TcRhoType)
    -- Infer type, and deeply instantiate
    tcInferId n = tcInferIdWithOrig (OccurrenceOf n) n

    ------------------------
    tcInferIdWithOrig :: CtOrigin -> Name -> TcM (HsExpr TcId, TcRhoType)
    -- Look up an occurrence of an Id, and instantiate it (deeply)

    tcInferIdWithOrig orig id_name
      = do { id <- lookup_id
           ; (id_expr, id_rho) <- instantiateOuter orig id
           ; (wrap, rho) <- deeplyInstantiate orig id_rho
           ; return (mkHsWrap wrap id_expr, rho) }
      where
        lookup_id :: TcM TcId
        lookup_id
           = do { thing <- tcLookup id_name
    	    ; case thing of
        	    	 ATcId { tct_id = id, tct_level = lvl }
    	           -> do { check_naughty id        -- Note [Local record selectors]
                             ; checkThLocalId id lvl
                             ; return id }

        	    	 AGlobal (AnId id)
                       -> do { check_naughty id; return id }
    	    	 	-- A global cannot possibly be ill-staged
    	    	 	-- nor does it need the 'lifting' treatment
                            -- hence no checkTh stuff here

        	    	 AGlobal (ADataCon con) -> return (dataConWrapId con)

        	    	 other -> failWithTc (bad_lookup other) }

        bad_lookup thing = ppr thing <+> ptext (sLit "used where a value identifer was expected")

        check_naughty id
          | isNaughtyRecordSelector id = failWithTc (naughtyRecordSel id)
          | otherwise		   = return ()

    ------------------------
    instantiateOuter :: CtOrigin -> TcId -> TcM (HsExpr TcId, TcSigmaType)
    -- Do just the first level of instantiation of an Id
    --   a) Deal with method sharing
    --   b) Deal with stupid checks
    -- Only look at the *outer level* of quantification
    -- See Note [Multiple instantiation]

    instantiateOuter orig id
      | null tvs && null theta
      = return (HsVar id, tau)

      | otherwise
      = do { (_, tys, subst) <- tcInstTyVars tvs
           ; doStupidChecks id tys
           ; let theta' = substTheta subst theta
           ; traceTc "Instantiating" (ppr id <+> text "with" <+> (ppr tys $$ ppr theta'))
           ; wrap <- instCall orig tys theta'
           ; return (mkHsWrap wrap (HsVar id), substTy subst tau) }
      where
        (tvs, theta, tau) = tcSplitSigmaTy (idType id)

\end{code} Note [Multiple instantiation] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We are careful never to make a MethodInst that has, as its meth_id, another MethodInst. For example, consider f :: forall a. Eq a => forall b. Ord b => a -> b At a call to f, at say [Int, Bool], it's tempting to translate the call to f_m1 where f_m1 :: forall b. Ord b => Int -> b f_m1 = f Int dEqInt f_m2 :: Int -> Bool f_m2 = f_m1 Bool dOrdBool But notice that f_m2 has f_m1 as its meth_id. Now the danger is that if we do a tcSimplCheck with a Given f_mx :: f Int dEqInt, we may make a binding f_m1 = f_mx But it's entirely possible that f_m2 will continue to float out, because it mentions no type variables. Result, f_m1 isn't in scope. Here's a concrete example that does this (test tc200): class C a where f :: Eq b => b -> a -> Int baz :: Eq a => Int -> a -> Int instance C Int where baz = f Current solution: only do the "method sharing" thing for the first type/dict application, not for the iterated ones. A horribly subtle point. Note [No method sharing] ~~~~~~~~~~~~~~~~~~~~~~~~ The -fno-method-sharing flag controls what happens so far as the LIE is concerned. The default case is that for an overloaded function we generate a "method" Id, and add the Method Inst to the LIE. So you get something like f :: Num a => a -> a f = /\a (d:Num a) -> let m = (+) a d in \ (x:a) -> m x x If you specify -fno-method-sharing, the dictionary application isn't shared, so we get f :: Num a => a -> a f = /\a (d:Num a) (x:a) -> (+) a d x x This gets a bit less sharing, but a) it's better for RULEs involving overloaded functions b) perhaps fewer separated lambdas \begin{code}

    doStupidChecks :: TcId
    	       -> [TcType]
    	       -> TcM ()
    -- Check two tiresome and ad-hoc cases
    -- (a) the "stupid theta" for a data con; add the constraints
    --     from the "stupid theta" of a data constructor (sigh)

    doStupidChecks fun_id tys
      | Just con <- isDataConId_maybe fun_id   -- (a)
      = addDataConStupidTheta con tys

      | fun_id `hasKey` tagToEnumKey           -- (b)
      = failWithTc (ptext (sLit "tagToEnum# must appear applied to one argument"))

      | otherwise
      = return () -- The common case

\end{code} Note [tagToEnum#] ~~~~~~~~~~~~~~~~~ Nasty check to ensure that tagToEnum# is applied to a type that is an enumeration TyCon. Unification may refine the type later, but this check won't see that, alas. It's crude, because it relies on our knowing *now* that the type is ok, which in turn relies on the eager-unification part of the type checker pushing enough information here. In theory the Right Thing to do is to have a new form of constraint but I definitely cannot face that! And it works ok as-is. Here's are two cases that should fail f :: forall a. a f = tagToEnum# 0 -- Can't do tagToEnum# at a type variable g :: Int g = tagToEnum# 0 -- Int is not an enumeration When data type families are involved it's a bit more complicated. data family F a data instance F [Int] = A | B | C Then we want to generate something like tagToEnum# R:FListInt 3# |> co :: R:FListInt ~ F [Int] Usually that coercion is hidden inside the wrappers for constructors of F [Int] but here we have to do it explicitly. It's all grotesquely complicated. \begin{code}

    tcTagToEnum :: SrcSpan -> Name -> LHsExpr Name -> TcRhoType -> TcM (HsExpr TcId)
    -- tagToEnum# :: forall a. Int# -> a
    -- See Note [tagToEnum#]   Urgh!
    tcTagToEnum loc fun_name arg res_ty
      = do	{ fun <- tcLookupId fun_name
            ; ty' <- zonkTcType res_ty

    	-- Check that the type is algebraic
            ; let mb_tc_app = tcSplitTyConApp_maybe ty'
                  Just (tc, tc_args) = mb_tc_app
    	; checkTc (isJust mb_tc_app)
                      (tagToEnumError ty' doc1)

    	-- Look through any type family
            ; (coi, rep_tc, rep_args) <- get_rep_ty ty' tc tc_args

    	; checkTc (isEnumerationTyCon rep_tc)
                      (tagToEnumError ty' doc2)

            ; arg' <- tcMonoExpr arg intPrimTy
            ; let fun' = L loc (HsWrap (WpTyApp rep_ty) (HsVar fun))
                  rep_ty = mkTyConApp rep_tc rep_args

    	; return (mkHsWrapCoI coi $ HsApp fun' arg') }
      where
        doc1 = vcat [ ptext (sLit "Specify the type by giving a type signature")
    		, ptext (sLit "e.g. (tagToEnum# x) :: Bool") ]
        doc2 = ptext (sLit "Result type must be an enumeration type")
        doc3 = ptext (sLit "No family instance for this type")

        get_rep_ty :: TcType -> TyCon -> [TcType]
                   -> TcM (CoercionI, TyCon, [TcType])
        	-- Converts a family type (eg F [a]) to its rep type (eg FList a)
    	-- and returns a coercion between the two
        get_rep_ty ty tc tc_args
          | not (isFamilyTyCon tc)
          = return (IdCo ty, tc, tc_args)
          | otherwise
          = do { mb_fam <- tcLookupFamInst tc tc_args
               ; case mb_fam of
    	       Nothing -> failWithTc (tagToEnumError ty doc3)
                   Just (rep_tc, rep_args)
                       -> return ( ACo (mkSymCoercion (mkTyConApp co_tc rep_args))
                                 , rep_tc, rep_args )
                     where
                       co_tc = expectJust "tcTagToEnum" $
                               tyConFamilyCoercion_maybe rep_tc }

    tagToEnumError :: TcType -> SDoc -> SDoc
    tagToEnumError ty what
      = hang (ptext (sLit "Bad call to tagToEnum#")
               <+> ptext (sLit "at type") <+> ppr ty)
    	 2 what

\end{code} %************************************************************************ %* * Template Haskell checks %* * %************************************************************************ \begin{code}

    checkThLocalId :: Id -> ThLevel -> TcM ()
    #ifndef GHCI  /* GHCI and TH is off */
    --------------------------------------
    -- Check for cross-stage lifting
    checkThLocalId _id _bind_lvl
      = return ()

    #else	      /* GHCI and TH is on */
    checkThLocalId id bind_lvl
      = do	{ use_stage <- getStage	-- TH case
    	; let use_lvl = thLevel use_stage
    	; checkWellStaged (quotes (ppr id)) bind_lvl use_lvl
    	; traceTc "thLocalId" (ppr id <+> ppr bind_lvl <+> ppr use_stage <+> ppr use_lvl)
    	; when (use_lvl > bind_lvl) $
              checkCrossStageLifting id bind_lvl use_stage }

    --------------------------------------
    checkCrossStageLifting :: Id -> ThLevel -> ThStage -> TcM ()
    -- We are inside brackets, and (use_lvl > bind_lvl)
    -- Now we must check whether there's a cross-stage lift to do
    -- Examples   \x -> [| x |]
    --            [| map |]

    checkCrossStageLifting _ _ Comp   = return ()
    checkCrossStageLifting _ _ Splice = return ()

    checkCrossStageLifting id _ (Brack _ ps_var lie_var)
      | thTopLevelId id
      =	-- Top-level identifiers in this module,
    	-- (which have External Names)
    	-- are just like the imported case:
    	-- no need for the 'lifting' treatment
    	-- E.g.  this is fine:
    	--   f x = x
    	--   g y = [| f 3 |]
    	-- But we do need to put f into the keep-alive
    	-- set, because after desugaring the code will
    	-- only mention f's *name*, not f itself.
        keepAliveTc id

      | otherwise	-- bind_lvl = outerLevel presumably,
    		-- but the Id is not bound at top level
      = 	-- Nested identifiers, such as 'x' in
    	-- E.g. \x -> [| h x |]
    	-- We must behave as if the reference to x was
    	--	h $(lift x)
    	-- We use 'x' itself as the splice proxy, used by
    	-- the desugarer to stitch it all back together.
    	-- If 'x' occurs many times we may get many identical
    	-- bindings of the same splice proxy, but that doesn't
    	-- matter, although it's a mite untidy.
        do 	{ let id_ty = idType id
            ; checkTc (isTauTy id_ty) (polySpliceErr id)
    	       -- If x is polymorphic, its occurrence sites might
    	       -- have different instantiations, so we can't use plain
    	       -- 'x' as the splice proxy name.  I don't know how to
    	       -- solve this, and it's probably unimportant, so I'm
    	       -- just going to flag an error for now

    	; lift <- if isStringTy id_ty then
    	       	     do { sid <- tcLookupId DsMeta.liftStringName
    		     		     -- See Note [Lifting strings]
                            ; return (HsVar sid) }
    	          else
                         setConstraintVar lie_var	$ do
    		          -- Put the 'lift' constraint into the right LIE
                         newMethodFromName (OccurrenceOf (idName id))
                                           DsMeta.liftName id_ty

    		   -- Update the pending splices
    	; ps <- readMutVar ps_var
    	; writeMutVar ps_var ((idName id, nlHsApp (noLoc lift) (nlHsVar id)) : ps)

    	; return () }
    #endif /* GHCI */

\end{code} Note [Lifting strings] ~~~~~~~~~~~~~~~~~~~~~~ If we see $(... [| s |] ...) where s::String, we don't want to generate a mass of Cons (CharL 'x') (Cons (CharL 'y') ...)) etc. So this conditional short-circuits the lifting mechanism to generate (liftString "xy") in that case. I didn't want to use overlapping instances for the Lift class in TH.Syntax, because that can lead to overlapping-instance errors in a polymorphic situation. If this check fails (which isn't impossible) we get another chance; see Note [Converting strings] in Convert.lhs Local record selectors ~~~~~~~~~~~~~~~~~~~~~~ Record selectors for TyCons in this module are ordinary local bindings, which show up as ATcIds rather than AGlobals. So we need to check for naughtiness in both branches. c.f. TcTyClsBindings.mkAuxBinds. %************************************************************************ %* * \subsection{Record bindings} %* * %************************************************************************ Game plan for record bindings ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ 1. Find the TyCon for the bindings, from the first field label. 2. Instantiate its tyvars and unify (T a1 .. an) with expected_ty. For each binding field = value 3. Instantiate the field type (from the field label) using the type envt from step 2. 4 Type check the value using tcArg, passing the field type as the expected argument type. This extends OK when the field types are universally quantified. \begin{code}

    tcRecordBinds
    	:: DataCon
    	-> [TcType]	-- Expected type for each field
    	-> HsRecordBinds Name
    	-> TcM (HsRecordBinds TcId)

    tcRecordBinds data_con arg_tys (HsRecFields rbinds dd)
      = do	{ mb_binds <- mapM do_bind rbinds
    	; return (HsRecFields (catMaybes mb_binds) dd) }
      where
        flds_w_tys = zipEqual "tcRecordBinds" (dataConFieldLabels data_con) arg_tys
        do_bind fld@(HsRecField { hsRecFieldId = L loc field_lbl, hsRecFieldArg = rhs })
          | Just field_ty <- assocMaybe flds_w_tys field_lbl
          = addErrCtxt (fieldCtxt field_lbl)	$
    	do { rhs' <- tcPolyExprNC rhs field_ty
    	   ; let field_id = mkUserLocal (nameOccName field_lbl)
    				        (nameUnique field_lbl)
    					field_ty loc
    		-- Yuk: the field_id has the *unique* of the selector Id
    		-- 	    (so we can find it easily)
    		--      but is a LocalId with the appropriate type of the RHS
    		--	    (so the desugarer knows the type of local binder to make)
    	   ; return (Just (fld { hsRecFieldId = L loc field_id, hsRecFieldArg = rhs' })) }
          | otherwise
          = do { addErrTc (badFieldCon data_con field_lbl)
    	   ; return Nothing }

    checkMissingFields :: DataCon -> HsRecordBinds Name -> TcM ()
    checkMissingFields data_con rbinds
      | null field_labels 	-- Not declared as a record;
    			-- But C{} is still valid if no strict fields
      = if any isBanged field_strs then
    	-- Illegal if any arg is strict
    	addErrTc (missingStrictFields data_con [])
        else
    	return ()

      | otherwise = do		-- A record
        unless (null missing_s_fields)
    	   (addErrTc (missingStrictFields data_con missing_s_fields))

        warn <- doptM Opt_WarnMissingFields
        unless (not (warn && notNull missing_ns_fields))
    	   (warnTc True (missingFields data_con missing_ns_fields))

      where
        missing_s_fields
    	= [ fl | (fl, str) <- field_info,
    	  	 isBanged str,
    	  	 not (fl `elem` field_names_used)
    	  ]
        missing_ns_fields
    	= [ fl | (fl, str) <- field_info,
    	  	 not (isBanged str),
    	  	 not (fl `elem` field_names_used)
    	  ]

        field_names_used = hsRecFields rbinds
        field_labels     = dataConFieldLabels data_con

        field_info = zipEqual "missingFields"
    			  field_labels
    	  		  field_strs

        field_strs = dataConStrictMarks data_con

\end{code} %************************************************************************ %* * \subsection{Errors and contexts} %* * %************************************************************************ Boring and alphabetical: \begin{code}

    addExprErrCtxt :: LHsExpr Name -> TcM a -> TcM a
    addExprErrCtxt expr = addErrCtxt (exprCtxt expr)

    exprCtxt :: LHsExpr Name -> SDoc
    exprCtxt expr
      = hang (ptext (sLit "In the expression:")) 2 (ppr expr)

    fieldCtxt :: Name -> SDoc
    fieldCtxt field_name
      = ptext (sLit "In the") <+> quotes (ppr field_name) <+> ptext (sLit "field of a record")

    funAppCtxt :: LHsExpr Name -> LHsExpr Name -> Int -> SDoc
    funAppCtxt fun arg arg_no
      = hang (hsep [ ptext (sLit "In the"), speakNth arg_no, ptext (sLit "argument of"),
    		    quotes (ppr fun) <> text ", namely"])
           2 (quotes (ppr arg))

    funResCtxt :: LHsExpr Name -> SDoc
    funResCtxt fun
      = ptext (sLit "In the return type of a call of") <+> quotes (ppr fun)

    badFieldTypes :: [(Name,TcType)] -> SDoc
    badFieldTypes prs
      = hang (ptext (sLit "Record update for insufficiently polymorphic field")
    			 <> plural prs <> colon)
           2 (vcat [ ppr f <+> dcolon <+> ppr ty | (f,ty) <- prs ])

    badFieldsUpd :: HsRecFields Name a -> SDoc
    badFieldsUpd rbinds
      = hang (ptext (sLit "No constructor has all these fields:"))
           2 (pprQuotedList (hsRecFields rbinds))

    naughtyRecordSel :: TcId -> SDoc
    naughtyRecordSel sel_id
      = ptext (sLit "Cannot use record selector") <+> quotes (ppr sel_id) <+>
        ptext (sLit "as a function due to escaped type variables") $$
        ptext (sLit "Probable fix: use pattern-matching syntax instead")

    notSelector :: Name -> SDoc
    notSelector field
      = hsep [quotes (ppr field), ptext (sLit "is not a record selector")]

    missingStrictFields :: DataCon -> [FieldLabel] -> SDoc
    missingStrictFields con fields
      = header <> rest
      where
        rest | null fields = empty	-- Happens for non-record constructors
    				-- with strict fields
    	 | otherwise   = colon <+> pprWithCommas ppr fields

        header = ptext (sLit "Constructor") <+> quotes (ppr con) <+>
    	     ptext (sLit "does not have the required strict field(s)")

    missingFields :: DataCon -> [FieldLabel] -> SDoc
    missingFields con fields
      = ptext (sLit "Fields of") <+> quotes (ppr con) <+> ptext (sLit "not initialised:")
    	<+> pprWithCommas ppr fields

    -- callCtxt fun args = ptext (sLit "In the call") <+> parens (ppr (foldl mkHsApp fun args))

    #ifdef GHCI
    polySpliceErr :: Id -> SDoc
    polySpliceErr id
      = ptext (sLit "Can't splice the polymorphic local variable") <+> quotes (ppr id)
    #endif

\end{code}