    {-# LANGUAGE CPP #-}

    -- | Handy functions for creating much Core syntax
    module MkCore (
            -- * Constructing normal syntax
            mkCoreLet, mkCoreLets,
            mkCoreApp, mkCoreApps, mkCoreConApps,
            mkCoreLams, mkWildCase, mkIfThenElse,
            mkWildValBinder, mkWildEvBinder,
            sortQuantVars, castBottomExpr,

            -- * Constructing boxed literals
            mkWordExpr, mkWordExprWord,
            mkIntExpr, mkIntExprInt,
            mkIntegerExpr,
            mkFloatExpr, mkDoubleExpr,
            mkCharExpr, mkStringExpr, mkStringExprFS,

            -- * Floats
            FloatBind(..), wrapFloat,

            -- * Constructing equality evidence boxes
            mkEqBox,

            -- * Constructing general big tuples
            -- $big_tuples
            mkChunkified,

            -- * Constructing small tuples
            mkCoreVarTup, mkCoreVarTupTy, mkCoreTup,

            -- * Constructing big tuples
            mkBigCoreVarTup, mkBigCoreVarTupTy,
            mkBigCoreTup, mkBigCoreTupTy,

            -- * Deconstructing small tuples
            mkSmallTupleSelector, mkSmallTupleCase,

            -- * Deconstructing big tuples
            mkTupleSelector, mkTupleCase,

            -- * Constructing list expressions
            mkNilExpr, mkConsExpr, mkListExpr,
            mkFoldrExpr, mkBuildExpr,

            -- * Error Ids
            mkRuntimeErrorApp, mkImpossibleExpr, errorIds,
            rEC_CON_ERROR_ID, iRREFUT_PAT_ERROR_ID, rUNTIME_ERROR_ID,
            nON_EXHAUSTIVE_GUARDS_ERROR_ID, nO_METHOD_BINDING_ERROR_ID,
            pAT_ERROR_ID, eRROR_ID, rEC_SEL_ERROR_ID, aBSENT_ERROR_ID,
            uNDEFINED_ID, undefinedName
        ) where

    #include "HsVersions.h"

    import Id
    import Var      ( EvVar, setTyVarUnique )

    import CoreSyn
    import CoreUtils        ( exprType, needsCaseBinding, bindNonRec )
    import Literal
    import HscTypes

    import TysWiredIn
    import PrelNames

    import TcType           ( mkSigmaTy )
    import Type
    import Coercion
    import TysPrim
    import DataCon          ( DataCon, dataConWorkId )
    import IdInfo           ( vanillaIdInfo, setStrictnessInfo,
                              setArityInfo )
    import Demand
    import Name      hiding ( varName )
    import Outputable
    import FastString
    import UniqSupply
    import BasicTypes
    import Util
    import Pair
    import Constants
    import DynFlags

    import Data.Char        ( ord )
    import Data.List
    import Data.Ord
    #if __GLASGOW_HASKELL__ < 709
    import Data.Word        ( Word )
    #endif

    infixl 4 `mkCoreApp`, `mkCoreApps`

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Basic CoreSyn construction}
    *                                                                      *
    ************************************************************************
    -}

    sortQuantVars :: [Var] -> [Var]
    -- Sort the variables (KindVars, TypeVars, and Ids)
    -- into order: Kind, then Type, then Id
    sortQuantVars = sortBy (comparing withCategory)
      where
        withCategory v = (category v, v)
        category :: Var -> Int
        category v
         | isKindVar v = 1
         | isTyVar   v = 2
         | otherwise   = 3

    -- | Bind a binding group over an expression, using a @let@ or @case@ as
    -- appropriate (see "CoreSyn#let_app_invariant")
    mkCoreLet :: CoreBind -> CoreExpr -> CoreExpr
    mkCoreLet (NonRec bndr rhs) body        -- See Note [CoreSyn let/app invariant]
      | needsCaseBinding (idType bndr) rhs
      = Case rhs bndr (exprType body) [(DEFAULT,[],body)]
    mkCoreLet bind body
      = Let bind body

    -- | Bind a list of binding groups over an expression. The leftmost binding
    -- group becomes the outermost group in the resulting expression
    mkCoreLets :: [CoreBind] -> CoreExpr -> CoreExpr
    mkCoreLets binds body = foldr mkCoreLet body binds

    -- | Construct an expression which represents the application of one expression
    -- to the other
    mkCoreApp :: CoreExpr -> CoreExpr -> CoreExpr
    -- Respects the let/app invariant by building a case expression where necessary
    --   See CoreSyn Note [CoreSyn let/app invariant]
    mkCoreApp fun (Type ty) = App fun (Type ty)
    mkCoreApp fun (Coercion co) = App fun (Coercion co)
    mkCoreApp fun arg       = ASSERT2( isFunTy fun_ty, ppr fun $$ ppr arg )
                              mk_val_app fun arg arg_ty res_ty
                          where
                            fun_ty = exprType fun
                            (arg_ty, res_ty) = splitFunTy fun_ty

    -- | Construct an expression which represents the application of a number of
    -- expressions to another. The leftmost expression in the list is applied first
    -- Respects the let/app invariant by building a case expression where necessary
    --   See CoreSyn Note [CoreSyn let/app invariant]
    mkCoreApps :: CoreExpr -> [CoreExpr] -> CoreExpr
    -- Slightly more efficient version of (foldl mkCoreApp)
    mkCoreApps orig_fun orig_args
      = go orig_fun (exprType orig_fun) orig_args
      where
        go fun _      []                   = fun
        go fun fun_ty (Type ty     : args) = go (App fun (Type ty)) (applyTy fun_ty ty) args
        go fun fun_ty (Coercion co : args) = go (App fun (Coercion co)) (applyCo fun_ty co) args
        go fun fun_ty (arg         : args) = ASSERT2( isFunTy fun_ty, ppr fun_ty $$ ppr orig_fun
                                                                      $$ ppr orig_args )
                                             go (mk_val_app fun arg arg_ty res_ty) res_ty args
                                           where
                                             (arg_ty, res_ty) = splitFunTy fun_ty

    -- | Construct an expression which represents the application of a number of
    -- expressions to that of a data constructor expression. The leftmost expression
    -- in the list is applied first
    mkCoreConApps :: DataCon -> [CoreExpr] -> CoreExpr
    mkCoreConApps con args = mkCoreApps (Var (dataConWorkId con)) args

    mk_val_app :: CoreExpr -> CoreExpr -> Type -> Type -> CoreExpr
    -- Build an application (e1 e2),
    -- or a strict binding  (case e2 of x -> e1 x)
    -- using the latter when necessary to respect the let/app invariant
    --   See Note [CoreSyn let/app invariant]
    mk_val_app fun arg arg_ty res_ty
      | not (needsCaseBinding arg_ty arg)
      = App fun arg                -- The vastly common case

      | otherwise
      = Case arg arg_id res_ty [(DEFAULT,[],App fun (Var arg_id))]
      where
        arg_id = mkWildValBinder arg_ty
            -- Lots of shadowing, but it doesn't matter,
            -- because 'fun ' should not have a free wild-id
            --
            -- This is Dangerous.  But this is the only place we play this
            -- game, mk_val_app returns an expression that does not have
            -- have a free wild-id.  So the only thing that can go wrong
            -- is if you take apart this case expression, and pass a
            -- fragmet of it as the fun part of a 'mk_val_app'.

    -----------
    mkWildEvBinder :: PredType -> EvVar
    mkWildEvBinder pred = mkWildValBinder pred

    -- | Make a /wildcard binder/. This is typically used when you need a binder
    -- that you expect to use only at a *binding* site.  Do not use it at
    -- occurrence sites because it has a single, fixed unique, and it's very
    -- easy to get into difficulties with shadowing.  That's why it is used so little.
    -- See Note [WildCard binders] in SimplEnv
    mkWildValBinder :: Type -> Id
    mkWildValBinder ty = mkLocalId wildCardName ty

    mkWildCase :: CoreExpr -> Type -> Type -> [CoreAlt] -> CoreExpr
    -- Make a case expression whose case binder is unused
    -- The alts should not have any occurrences of WildId
    mkWildCase scrut scrut_ty res_ty alts
      = Case scrut (mkWildValBinder scrut_ty) res_ty alts

    mkIfThenElse :: CoreExpr -> CoreExpr -> CoreExpr -> CoreExpr
    mkIfThenElse guard then_expr else_expr
    -- Not going to be refining, so okay to take the type of the "then" clause
      = mkWildCase guard boolTy (exprType then_expr)
             [ (DataAlt falseDataCon, [], else_expr),       -- Increasing order of tag!
               (DataAlt trueDataCon,  [], then_expr) ]

    castBottomExpr :: CoreExpr -> Type -> CoreExpr
    -- (castBottomExpr e ty), assuming that 'e' diverges,
    -- return an expression of type 'ty'
    -- See Note [Empty case alternatives] in CoreSyn
    castBottomExpr e res_ty
      | e_ty `eqType` res_ty = e
      | otherwise            = Case e (mkWildValBinder e_ty) res_ty []
      where
        e_ty = exprType e

    {-
    The functions from this point don't really do anything cleverer than
    their counterparts in CoreSyn, but they are here for consistency
    -}

    -- | Create a lambda where the given expression has a number of variables
    -- bound over it. The leftmost binder is that bound by the outermost
    -- lambda in the result
    mkCoreLams :: [CoreBndr] -> CoreExpr -> CoreExpr
    mkCoreLams = mkLams

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Making literals}
    *                                                                      *
    ************************************************************************
    -}

    -- | Create a 'CoreExpr' which will evaluate to the given @Int@
    mkIntExpr :: DynFlags -> Integer -> CoreExpr        -- Result = I# i :: Int
    mkIntExpr dflags i = mkConApp intDataCon  [mkIntLit dflags i]

    -- | Create a 'CoreExpr' which will evaluate to the given @Int@
    mkIntExprInt :: DynFlags -> Int -> CoreExpr         -- Result = I# i :: Int
    mkIntExprInt dflags i = mkConApp intDataCon  [mkIntLitInt dflags i]

    -- | Create a 'CoreExpr' which will evaluate to the a @Word@ with the given value
    mkWordExpr :: DynFlags -> Integer -> CoreExpr
    mkWordExpr dflags w = mkConApp wordDataCon [mkWordLit dflags w]

    -- | Create a 'CoreExpr' which will evaluate to the given @Word@
    mkWordExprWord :: DynFlags -> Word -> CoreExpr
    mkWordExprWord dflags w = mkConApp wordDataCon [mkWordLitWord dflags w]

    -- | Create a 'CoreExpr' which will evaluate to the given @Integer@
    mkIntegerExpr  :: MonadThings m => Integer -> m CoreExpr  -- Result :: Integer
    mkIntegerExpr i = do t <- lookupTyCon integerTyConName
                         return (Lit (mkLitInteger i (mkTyConTy t)))

    -- | Create a 'CoreExpr' which will evaluate to the given @Float@
    mkFloatExpr :: Float -> CoreExpr
    mkFloatExpr f = mkConApp floatDataCon [mkFloatLitFloat f]

    -- | Create a 'CoreExpr' which will evaluate to the given @Double@
    mkDoubleExpr :: Double -> CoreExpr
    mkDoubleExpr d = mkConApp doubleDataCon [mkDoubleLitDouble d]

    -- | Create a 'CoreExpr' which will evaluate to the given @Char@
    mkCharExpr     :: Char             -> CoreExpr      -- Result = C# c :: Int
    mkCharExpr c = mkConApp charDataCon [mkCharLit c]

    -- | Create a 'CoreExpr' which will evaluate to the given @String@
    mkStringExpr   :: MonadThings m => String     -> m CoreExpr  -- Result :: String
    -- | Create a 'CoreExpr' which will evaluate to a string morally equivalent to the given @FastString@
    mkStringExprFS :: MonadThings m => FastString -> m CoreExpr  -- Result :: String

    mkStringExpr str = mkStringExprFS (mkFastString str)

    mkStringExprFS str
      | nullFS str
      = return (mkNilExpr charTy)

      | all safeChar chars
      = do unpack_id <- lookupId unpackCStringName
           return (App (Var unpack_id) (Lit (MachStr (fastStringToByteString str))))

      | otherwise
      = do unpack_id <- lookupId unpackCStringUtf8Name
           return (App (Var unpack_id) (Lit (MachStr (fastStringToByteString str))))

      where
        chars = unpackFS str
        safeChar c = ord c >= 1 && ord c <= 0x7F

    -- This take a ~# b (or a ~# R b) and returns a ~ b (or Coercible a b)
    mkEqBox :: Coercion -> CoreExpr
    mkEqBox co = ASSERT2( typeKind ty2 `eqKind` k, ppr co $$ ppr ty1 $$ ppr ty2 $$ ppr (typeKind ty1) $$ ppr (typeKind ty2) )
                 Var (dataConWorkId datacon) `mkTyApps` [k, ty1, ty2] `App` Coercion co
      where (Pair ty1 ty2, role) = coercionKindRole co
            k = typeKind ty1
            datacon = case role of
                Nominal ->          eqBoxDataCon
                Representational -> coercibleDataCon
                Phantom ->          pprPanic "mkEqBox does not support boxing phantom coercions"
                                             (ppr co)

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Tuple constructors}
    *                                                                      *
    ************************************************************************
    -}

    -- $big_tuples
    -- #big_tuples#
    --
    -- GHCs built in tuples can only go up to 'mAX_TUPLE_SIZE' in arity, but
    -- we might concievably want to build such a massive tuple as part of the
    -- output of a desugaring stage (notably that for list comprehensions).
    --
    -- We call tuples above this size \"big tuples\", and emulate them by
    -- creating and pattern matching on >nested< tuples that are expressible
    -- by GHC.
    --
    -- Nesting policy: it's better to have a 2-tuple of 10-tuples (3 objects)
    -- than a 10-tuple of 2-tuples (11 objects), so we want the leaves of any
    -- construction to be big.
    --
    -- If you just use the 'mkBigCoreTup', 'mkBigCoreVarTupTy', 'mkTupleSelector'
    -- and 'mkTupleCase' functions to do all your work with tuples you should be
    -- fine, and not have to worry about the arity limitation at all.

    -- | Lifts a \"small\" constructor into a \"big\" constructor by recursive decompositon
    mkChunkified :: ([a] -> a)      -- ^ \"Small\" constructor function, of maximum input arity 'mAX_TUPLE_SIZE'
                 -> [a]             -- ^ Possible \"big\" list of things to construct from
                 -> a               -- ^ Constructed thing made possible by recursive decomposition
    mkChunkified small_tuple as = mk_big_tuple (chunkify as)
      where
            -- Each sub-list is short enough to fit in a tuple
        mk_big_tuple [as] = small_tuple as
        mk_big_tuple as_s = mk_big_tuple (chunkify (map small_tuple as_s))

    chunkify :: [a] -> [[a]]
    -- ^ Split a list into lists that are small enough to have a corresponding
    -- tuple arity. The sub-lists of the result all have length <= 'mAX_TUPLE_SIZE'
    -- But there may be more than 'mAX_TUPLE_SIZE' sub-lists
    chunkify xs
      | n_xs <= mAX_TUPLE_SIZE = [xs]
      | otherwise              = split xs
      where
        n_xs     = length xs
        split [] = []
        split xs = take mAX_TUPLE_SIZE xs : split (drop mAX_TUPLE_SIZE xs)

    {-
    Creating tuples and their types for Core expressions

    @mkBigCoreVarTup@ builds a tuple; the inverse to @mkTupleSelector@.

    * If it has only one element, it is the identity function.

    * If there are more elements than a big tuple can have, it nests
      the tuples.
    -}

    -- | Build a small tuple holding the specified variables
    mkCoreVarTup :: [Id] -> CoreExpr
    mkCoreVarTup ids = mkCoreTup (map Var ids)

    -- | Bulid the type of a small tuple that holds the specified variables
    mkCoreVarTupTy :: [Id] -> Type
    mkCoreVarTupTy ids = mkBoxedTupleTy (map idType ids)

    -- | Build a small tuple holding the specified expressions
    mkCoreTup :: [CoreExpr] -> CoreExpr
    mkCoreTup []  = Var unitDataConId
    mkCoreTup [c] = c
    mkCoreTup cs  = mkConApp (tupleCon BoxedTuple (length cs))
                             (map (Type . exprType) cs ++ cs)

    -- | Build a big tuple holding the specified variables
    mkBigCoreVarTup :: [Id] -> CoreExpr
    mkBigCoreVarTup ids = mkBigCoreTup (map Var ids)

    -- | Build the type of a big tuple that holds the specified variables
    mkBigCoreVarTupTy :: [Id] -> Type
    mkBigCoreVarTupTy ids = mkBigCoreTupTy (map idType ids)

    -- | Build a big tuple holding the specified expressions
    mkBigCoreTup :: [CoreExpr] -> CoreExpr
    mkBigCoreTup = mkChunkified mkCoreTup

    -- | Build the type of a big tuple that holds the specified type of thing
    mkBigCoreTupTy :: [Type] -> Type
    mkBigCoreTupTy = mkChunkified mkBoxedTupleTy

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

    data FloatBind
      = FloatLet  CoreBind
      | FloatCase CoreExpr Id AltCon [Var]
          -- case e of y { C ys -> ... }
          -- See Note [Floating cases] in SetLevels

    instance Outputable FloatBind where
      ppr (FloatLet b) = ptext (sLit "LET") <+> ppr b
      ppr (FloatCase e b c bs) = hang (ptext (sLit "CASE") <+> ppr e <+> ptext (sLit "of") <+> ppr b)
                                    2 (ppr c <+> ppr bs)

    wrapFloat :: FloatBind -> CoreExpr -> CoreExpr
    wrapFloat (FloatLet defns)       body = Let defns body
    wrapFloat (FloatCase e b con bs) body = Case e b (exprType body) [(con, bs, body)]

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Tuple destructors}
    *                                                                      *
    ************************************************************************
    -}

    -- | Builds a selector which scrutises the given
    -- expression and extracts the one name from the list given.
    -- If you want the no-shadowing rule to apply, the caller
    -- is responsible for making sure that none of these names
    -- are in scope.
    --
    -- If there is just one 'Id' in the tuple, then the selector is
    -- just the identity.
    --
    -- If necessary, we pattern match on a \"big\" tuple.
    mkTupleSelector :: [Id]         -- ^ The 'Id's to pattern match the tuple against
                    -> Id           -- ^ The 'Id' to select
                    -> Id           -- ^ A variable of the same type as the scrutinee
                    -> CoreExpr     -- ^ Scrutinee
                    -> CoreExpr     -- ^ Selector expression

    -- mkTupleSelector [a,b,c,d] b v e
    --          = case e of v {
    --                (p,q) -> case p of p {
    --                           (a,b) -> b }}
    -- We use 'tpl' vars for the p,q, since shadowing does not matter.
    --
    -- In fact, it's more convenient to generate it innermost first, getting
    --
    --        case (case e of v
    --                (p,q) -> p) of p
    --          (a,b) -> b
    mkTupleSelector vars the_var scrut_var scrut
      = mk_tup_sel (chunkify vars) the_var
      where
        mk_tup_sel [vars] the_var = mkSmallTupleSelector vars the_var scrut_var scrut
        mk_tup_sel vars_s the_var = mkSmallTupleSelector group the_var tpl_v $
                                    mk_tup_sel (chunkify tpl_vs) tpl_v
            where
              tpl_tys = [mkBoxedTupleTy (map idType gp) | gp <- vars_s]
              tpl_vs  = mkTemplateLocals tpl_tys
              [(tpl_v, group)] = [(tpl,gp) | (tpl,gp) <- zipEqual "mkTupleSelector" tpl_vs vars_s,
                                             the_var `elem` gp ]

    -- | Like 'mkTupleSelector' but for tuples that are guaranteed
    -- never to be \"big\".
    --
    -- > mkSmallTupleSelector [x] x v e = [| e |]
    -- > mkSmallTupleSelector [x,y,z] x v e = [| case e of v { (x,y,z) -> x } |]
    mkSmallTupleSelector :: [Id]        -- The tuple args
              -> Id         -- The selected one
              -> Id         -- A variable of the same type as the scrutinee
              -> CoreExpr        -- Scrutinee
              -> CoreExpr
    mkSmallTupleSelector [var] should_be_the_same_var _ scrut
      = ASSERT(var == should_be_the_same_var)
        scrut
    mkSmallTupleSelector vars the_var scrut_var scrut
      = ASSERT( notNull vars )
        Case scrut scrut_var (idType the_var)
             [(DataAlt (tupleCon BoxedTuple (length vars)), vars, Var the_var)]

    -- | A generalization of 'mkTupleSelector', allowing the body
    -- of the case to be an arbitrary expression.
    --
    -- To avoid shadowing, we use uniques to invent new variables.
    --
    -- If necessary we pattern match on a \"big\" tuple.
    mkTupleCase :: UniqSupply       -- ^ For inventing names of intermediate variables
                -> [Id]             -- ^ The tuple identifiers to pattern match on
                -> CoreExpr         -- ^ Body of the case
                -> Id               -- ^ A variable of the same type as the scrutinee
                -> CoreExpr         -- ^ Scrutinee
                -> CoreExpr
    -- ToDo: eliminate cases where none of the variables are needed.
    --
    --         mkTupleCase uniqs [a,b,c,d] body v e
    --           = case e of v { (p,q) ->
    --             case p of p { (a,b) ->
    --             case q of q { (c,d) ->
    --             body }}}
    mkTupleCase uniqs vars body scrut_var scrut
      = mk_tuple_case uniqs (chunkify vars) body
      where
        -- This is the case where don't need any nesting
        mk_tuple_case _ [vars] body
          = mkSmallTupleCase vars body scrut_var scrut

        -- This is the case where we must make nest tuples at least once
        mk_tuple_case us vars_s body
          = let (us', vars', body') = foldr one_tuple_case (us, [], body) vars_s
                in mk_tuple_case us' (chunkify vars') body'

        one_tuple_case chunk_vars (us, vs, body)
          = let (uniq, us') = takeUniqFromSupply us
                scrut_var = mkSysLocal (fsLit "ds") uniq
                  (mkBoxedTupleTy (map idType chunk_vars))
                body' = mkSmallTupleCase chunk_vars body scrut_var (Var scrut_var)
            in (us', scrut_var:vs, body')

    -- | As 'mkTupleCase', but for a tuple that is small enough to be guaranteed
    -- not to need nesting.
    mkSmallTupleCase
            :: [Id]         -- ^ The tuple args
            -> CoreExpr     -- ^ Body of the case
            -> Id           -- ^ A variable of the same type as the scrutinee
            -> CoreExpr     -- ^ Scrutinee
            -> CoreExpr

    mkSmallTupleCase [var] body _scrut_var scrut
      = bindNonRec var scrut body
    mkSmallTupleCase vars body scrut_var scrut
    -- One branch no refinement?
      = Case scrut scrut_var (exprType body) [(DataAlt (tupleCon BoxedTuple (length vars)), vars, body)]

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Common list manipulation expressions}
    *                                                                      *
    ************************************************************************

    Call the constructor Ids when building explicit lists, so that they
    interact well with rules.
    -}

    -- | Makes a list @[]@ for lists of the specified type
    mkNilExpr :: Type -> CoreExpr
    mkNilExpr ty = mkConApp nilDataCon [Type ty]

    -- | Makes a list @(:)@ for lists of the specified type
    mkConsExpr :: Type -> CoreExpr -> CoreExpr -> CoreExpr
    mkConsExpr ty hd tl = mkConApp consDataCon [Type ty, hd, tl]

    -- | Make a list containing the given expressions, where the list has the given type
    mkListExpr :: Type -> [CoreExpr] -> CoreExpr
    mkListExpr ty xs = foldr (mkConsExpr ty) (mkNilExpr ty) xs

    -- | Make a fully applied 'foldr' expression
    mkFoldrExpr :: MonadThings m
                => Type             -- ^ Element type of the list
                -> Type             -- ^ Fold result type
                -> CoreExpr         -- ^ "Cons" function expression for the fold
                -> CoreExpr         -- ^ "Nil" expression for the fold
                -> CoreExpr         -- ^ List expression being folded acress
                -> m CoreExpr
    mkFoldrExpr elt_ty result_ty c n list = do
        foldr_id <- lookupId foldrName
        return (Var foldr_id `App` Type elt_ty
               `App` Type result_ty
               `App` c
               `App` n
               `App` list)

    -- | Make a 'build' expression applied to a locally-bound worker function
    mkBuildExpr :: (MonadThings m, MonadUnique m)
                => Type                                     -- ^ Type of list elements to be built
                -> ((Id, Type) -> (Id, Type) -> m CoreExpr) -- ^ Function that, given information about the 'Id's
                                                            -- of the binders for the build worker function, returns
                                                            -- the body of that worker
                -> m CoreExpr
    mkBuildExpr elt_ty mk_build_inside = do
        [n_tyvar] <- newTyVars [alphaTyVar]
        let n_ty = mkTyVarTy n_tyvar
            c_ty = mkFunTys [elt_ty, n_ty] n_ty
        [c, n] <- sequence [mkSysLocalM (fsLit "c") c_ty, mkSysLocalM (fsLit "n") n_ty]

        build_inside <- mk_build_inside (c, c_ty) (n, n_ty)

        build_id <- lookupId buildName
        return $ Var build_id `App` Type elt_ty `App` mkLams [n_tyvar, c, n] build_inside
      where
        newTyVars tyvar_tmpls = do
          uniqs <- getUniquesM
          return (zipWith setTyVarUnique tyvar_tmpls uniqs)

    {-
    ************************************************************************
    *                                                                      *
                          Error expressions
    *                                                                      *
    ************************************************************************
    -}

    mkRuntimeErrorApp
            :: Id           -- Should be of type (forall a. Addr# -> a)
                            --      where Addr# points to a UTF8 encoded string
            -> Type         -- The type to instantiate 'a'
            -> String       -- The string to print
            -> CoreExpr

    mkRuntimeErrorApp err_id res_ty err_msg
      = mkApps (Var err_id) [Type res_ty, err_string]
      where
        err_string = Lit (mkMachString err_msg)

    mkImpossibleExpr :: Type -> CoreExpr
    mkImpossibleExpr res_ty
      = mkRuntimeErrorApp rUNTIME_ERROR_ID res_ty "Impossible case alternative"

    {-
    ************************************************************************
    *                                                                      *
                         Error Ids
    *                                                                      *
    ************************************************************************

    GHC randomly injects these into the code.

    @patError@ is just a version of @error@ for pattern-matching
    failures.  It knows various ``codes'' which expand to longer
    strings---this saves space!

    @absentErr@ is a thing we put in for ``absent'' arguments.  They jolly
    well shouldn't be yanked on, but if one is, then you will get a
    friendly message from @absentErr@ (rather than a totally random
    crash).

    @parError@ is a special version of @error@ which the compiler does
    not know to be a bottoming Id.  It is used in the @_par_@ and @_seq_@
    templates, but we don't ever expect to generate code for it.
    -}

    errorIds :: [Id]
    errorIds
      = [ eRROR_ID,   -- This one isn't used anywhere else in the compiler
                      -- But we still need it in wiredInIds so that when GHC
                      -- compiles a program that mentions 'error' we don't
                      -- import its type from the interface file; we just get
                      -- the Id defined here.  Which has an 'open-tyvar' type.

          uNDEFINED_ID,   -- Ditto for 'undefined'. The big deal is to give it
                          -- an 'open-tyvar' type.

          rUNTIME_ERROR_ID,
          iRREFUT_PAT_ERROR_ID,
          nON_EXHAUSTIVE_GUARDS_ERROR_ID,
          nO_METHOD_BINDING_ERROR_ID,
          pAT_ERROR_ID,
          rEC_CON_ERROR_ID,
          rEC_SEL_ERROR_ID,
          aBSENT_ERROR_ID ]

    recSelErrorName, runtimeErrorName, absentErrorName :: Name
    irrefutPatErrorName, recConErrorName, patErrorName :: Name
    nonExhaustiveGuardsErrorName, noMethodBindingErrorName :: Name

    recSelErrorName     = err_nm "recSelError"     recSelErrorIdKey     rEC_SEL_ERROR_ID
    absentErrorName     = err_nm "absentError"     absentErrorIdKey     aBSENT_ERROR_ID
    runtimeErrorName    = err_nm "runtimeError"    runtimeErrorIdKey    rUNTIME_ERROR_ID
    irrefutPatErrorName = err_nm "irrefutPatError" irrefutPatErrorIdKey iRREFUT_PAT_ERROR_ID
    recConErrorName     = err_nm "recConError"     recConErrorIdKey     rEC_CON_ERROR_ID
    patErrorName        = err_nm "patError"        patErrorIdKey        pAT_ERROR_ID

    noMethodBindingErrorName     = err_nm "noMethodBindingError"
                                      noMethodBindingErrorIdKey nO_METHOD_BINDING_ERROR_ID
    nonExhaustiveGuardsErrorName = err_nm "nonExhaustiveGuardsError"
                                      nonExhaustiveGuardsErrorIdKey nON_EXHAUSTIVE_GUARDS_ERROR_ID

    err_nm :: String -> Unique -> Id -> Name
    err_nm str uniq id = mkWiredInIdName cONTROL_EXCEPTION_BASE (fsLit str) uniq id

    rEC_SEL_ERROR_ID, rUNTIME_ERROR_ID, iRREFUT_PAT_ERROR_ID, rEC_CON_ERROR_ID :: Id
    pAT_ERROR_ID, nO_METHOD_BINDING_ERROR_ID, nON_EXHAUSTIVE_GUARDS_ERROR_ID :: Id
    aBSENT_ERROR_ID :: Id
    rEC_SEL_ERROR_ID                = mkRuntimeErrorId recSelErrorName
    rUNTIME_ERROR_ID                = mkRuntimeErrorId runtimeErrorName
    iRREFUT_PAT_ERROR_ID            = mkRuntimeErrorId irrefutPatErrorName
    rEC_CON_ERROR_ID                = mkRuntimeErrorId recConErrorName
    pAT_ERROR_ID                    = mkRuntimeErrorId patErrorName
    nO_METHOD_BINDING_ERROR_ID      = mkRuntimeErrorId noMethodBindingErrorName
    nON_EXHAUSTIVE_GUARDS_ERROR_ID  = mkRuntimeErrorId nonExhaustiveGuardsErrorName
    aBSENT_ERROR_ID                 = mkRuntimeErrorId absentErrorName

    mkRuntimeErrorId :: Name -> Id
    mkRuntimeErrorId name = pc_bottoming_Id1 name runtimeErrorTy

    runtimeErrorTy :: Type
    -- The runtime error Ids take a UTF8-encoded string as argument
    runtimeErrorTy = mkSigmaTy [openAlphaTyVar] [] (mkFunTy addrPrimTy openAlphaTy)

    errorName :: Name
    errorName = mkWiredInIdName gHC_ERR (fsLit "error") errorIdKey eRROR_ID

    eRROR_ID :: Id
    eRROR_ID = pc_bottoming_Id1 errorName errorTy

    errorTy  :: Type   -- See Note [Error and friends have an "open-tyvar" forall]
    errorTy  = mkSigmaTy [openAlphaTyVar] [] (mkFunTys [mkListTy charTy] openAlphaTy)

    undefinedName :: Name
    undefinedName = mkWiredInIdName gHC_ERR (fsLit "undefined") undefinedKey uNDEFINED_ID

    uNDEFINED_ID :: Id
    uNDEFINED_ID = pc_bottoming_Id0 undefinedName undefinedTy

    undefinedTy  :: Type   -- See Note [Error and friends have an "open-tyvar" forall]
    undefinedTy  = mkSigmaTy [openAlphaTyVar] [] openAlphaTy

    {-
    Note [Error and friends have an "open-tyvar" forall]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    'error' and 'undefined' have types
            error     :: forall (a::OpenKind). String -> a
            undefined :: forall (a::OpenKind). a
    Notice the 'OpenKind' (manifested as openAlphaTyVar in the code). This ensures that
    "error" can be instantiated at
      * unboxed as well as boxed types
      * polymorphic types
    This is OK because it never returns, so the return type is irrelevant.
    See Note [OpenTypeKind accepts foralls] in TcUnify.

    ************************************************************************
    *                                                                      *
    \subsection{Utilities}
    *                                                                      *
    ************************************************************************
    -}

    pc_bottoming_Id1 :: Name -> Type -> Id
    -- Function of arity 1, which diverges after being given one argument
    pc_bottoming_Id1 name ty
     = mkVanillaGlobalWithInfo name ty bottoming_info
     where
        bottoming_info = vanillaIdInfo `setStrictnessInfo`    strict_sig
                                       `setArityInfo`         1
                            -- Make arity and strictness agree

            -- Do *not* mark them as NoCafRefs, because they can indeed have
            -- CAF refs.  For example, pAT_ERROR_ID calls GHC.Err.untangle,
            -- which has some CAFs
            -- In due course we may arrange that these error-y things are
            -- regarded by the GC as permanently live, in which case we
            -- can give them NoCaf info.  As it is, any function that calls
            -- any pc_bottoming_Id will itself have CafRefs, which bloats
            -- SRTs.

        strict_sig = mkClosedStrictSig [evalDmd] botRes
        -- These "bottom" out, no matter what their arguments

    pc_bottoming_Id0 :: Name -> Type -> Id
    -- Same but arity zero
    pc_bottoming_Id0 name ty
     = mkVanillaGlobalWithInfo name ty bottoming_info
     where
        bottoming_info = vanillaIdInfo `setStrictnessInfo` strict_sig
        strict_sig = mkClosedStrictSig [] botRes