coreSyn/MkCore.hs

    {-# 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, mkNaturalExpr,
            mkFloatExpr, mkDoubleExpr,
            mkCharExpr, mkStringExpr, mkStringExprFS, mkStringExprFSWith,

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

            -- * Constructing small tuples
            mkCoreVarTup, mkCoreVarTupTy, mkCoreTup, mkCoreUbxTup,
            mkCoreTupBoxity, unitExpr,

            -- * Constructing big tuples
            mkBigCoreVarTup, mkBigCoreVarTup1,
            mkBigCoreVarTupTy, mkBigCoreTupTy,
            mkBigCoreTup,

            -- * Deconstructing small tuples
            mkSmallTupleSelector, mkSmallTupleCase,

            -- * Deconstructing big tuples
            mkTupleSelector, mkTupleSelector1, mkTupleCase,

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

            -- * Constructing Maybe expressions
            mkNothingExpr, mkJustExpr,

            -- * Error Ids
            mkRuntimeErrorApp, mkImpossibleExpr, mkAbsentErrorApp, errorIds,
            rEC_CON_ERROR_ID, rUNTIME_ERROR_ID,
            nON_EXHAUSTIVE_GUARDS_ERROR_ID, nO_METHOD_BINDING_ERROR_ID,
            pAT_ERROR_ID, rEC_SEL_ERROR_ID, aBSENT_ERROR_ID,
            tYPE_ERROR_ID, aBSENT_SUM_FIELD_ERROR_ID
        ) where

    #include "HsVersions.h"

    import GhcPrelude

    import Id
    import Var      ( EvVar, setTyVarUnique )

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

    import TysWiredIn
    import PrelNames

    import HsUtils          ( mkChunkified, chunkify )
    import Type
    import Coercion         ( isCoVar )
    import TysPrim
    import DataCon          ( DataCon, dataConWorkId )
    import IdInfo
    import Demand
    import Name      hiding ( varName )
    import Outputable
    import FastString
    import UniqSupply
    import BasicTypes
    import Util
    import DynFlags
    import Data.List

    import Data.Char        ( ord )
    import Control.Monad.Fail ( MonadFail )

    infixl 4 `mkCoreApp`, `mkCoreApps`

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Basic CoreSyn construction}
    *                                                                      *
    ************************************************************************
    -}
    sortQuantVars :: [Var] -> [Var]
    -- Sort the variables, putting type and covars first, in scoped order,
    -- and then other Ids
    -- It is a deterministic sort, meaining it doesn't look at the values of
    -- Uniques. For explanation why it's important See Note [Unique Determinism]
    -- in Unique.
    sortQuantVars vs = sorted_tcvs ++ ids
      where
        (tcvs, ids) = partition (isTyVar <||> isCoVar) vs
        sorted_tcvs = toposortTyVars tcvs

    -- | 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
      , not (isJoinId bndr)
      = 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
    -- paired with its type to an argument. The result is paired with its type. This
    -- function is not exported and used in the definition of 'mkCoreApp' and
    -- 'mkCoreApps'.
    -- Respects the let/app invariant by building a case expression where necessary
    --   See CoreSyn Note [CoreSyn let/app invariant]
    mkCoreAppTyped :: SDoc -> (CoreExpr, Type) -> CoreExpr -> (CoreExpr, Type)
    mkCoreAppTyped _ (fun, fun_ty) (Type ty)
      = (App fun (Type ty), piResultTy fun_ty ty)
    mkCoreAppTyped _ (fun, fun_ty) (Coercion co)
      = (App fun (Coercion co), res_ty)
      where
        (_, res_ty) = splitFunTy fun_ty
    mkCoreAppTyped d (fun, fun_ty) arg
      = ASSERT2( isFunTy fun_ty, ppr fun $$ ppr arg $$ d )
        (mk_val_app fun arg arg_ty res_ty, res_ty)
      where
        (arg_ty, res_ty) = splitFunTy fun_ty

    -- | Construct an expression which represents the application of one expression
    -- to the other
    -- Respects the let/app invariant by building a case expression where necessary
    --   See CoreSyn Note [CoreSyn let/app invariant]
    mkCoreApp :: SDoc -> CoreExpr -> CoreExpr -> CoreExpr
    mkCoreApp s fun arg
      = fst $ mkCoreAppTyped s (fun, exprType fun) arg

    -- | 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
    mkCoreApps fun args
      = fst $
        foldl' (mkCoreAppTyped doc_string) (fun, fun_ty) args
      where
        doc_string = ppr fun_ty $$ ppr fun $$ ppr args
        fun_ty = exprType fun

    -- | 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
            -- a free wild-id.  So the only thing that can go wrong
            -- is if you take apart this case expression, and pass a
            -- fragment 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 = mkLocalIdOrCoVar 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 = mkCoreConApps 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 = mkCoreConApps 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 = mkCoreConApps wordDataCon [mkWordLit dflags w]

    -- | Create a 'CoreExpr' which will evaluate to the given @Word@
    mkWordExprWord :: DynFlags -> Word -> CoreExpr
    mkWordExprWord dflags w = mkCoreConApps 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 @Natural@
    mkNaturalExpr  :: MonadThings m => Integer -> m CoreExpr
    mkNaturalExpr i = do t <- lookupTyCon naturalTyConName
                         return (Lit (mkLitNatural i (mkTyConTy t)))

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

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

    -- | Create a 'CoreExpr' which will evaluate to the given @Char@
    mkCharExpr     :: Char             -> CoreExpr      -- Result = C# c :: Int
    mkCharExpr c = mkCoreConApps 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 = mkStringExprFSWith lookupId

    mkStringExprFSWith :: Monad m => (Name -> m Id) -> FastString -> m CoreExpr
    mkStringExprFSWith lookupM str
      | nullFS str
      = return (mkNilExpr charTy)

      | all safeChar chars
      = do unpack_id <- lookupM unpackCStringName
           return (App (Var unpack_id) lit)

      | otherwise
      = do unpack_utf8_id <- lookupM unpackCStringUtf8Name
           return (App (Var unpack_utf8_id) lit)

      where
        chars = unpackFS str
        safeChar c = ord c >= 1 && ord c <= 0x7F
        lit = Lit (MachStr (fastStringToByteString str))

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

    {-
    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.

    Note [Flattening one-tuples]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    This family of functions creates a tuple of variables/expressions/types.
      mkCoreTup [e1,e2,e3] = (e1,e2,e3)
    What if there is just one variable/expression/type in the argument?
    We could do one of two things:

    * Flatten it out, so that
        mkCoreTup [e1] = e1

    * Build a one-tuple (see Note [One-tuples] in TysWiredIn)
        mkCoreTup1 [e1] = Unit e1
      We use a suffix "1" to indicate this.

    Usually we want the former, but occasionally the latter.
    -}

    -- | Build a small tuple holding the specified variables
    -- One-tuples are flattened; see Note [Flattening one-tuples]
    mkCoreVarTup :: [Id] -> CoreExpr
    mkCoreVarTup ids = mkCoreTup (map Var ids)

    -- | Build the type of a small tuple that holds the specified variables
    -- One-tuples are flattened; see Note [Flattening one-tuples]
    mkCoreVarTupTy :: [Id] -> Type
    mkCoreVarTupTy ids = mkBoxedTupleTy (map idType ids)

    -- | Build a small tuple holding the specified expressions
    -- One-tuples are flattened; see Note [Flattening one-tuples]
    mkCoreTup :: [CoreExpr] -> CoreExpr
    mkCoreTup []  = Var unitDataConId
    mkCoreTup [c] = c
    mkCoreTup cs  = mkCoreConApps (tupleDataCon Boxed (length cs))
                                  (map (Type . exprType) cs ++ cs)

    -- | Build a small unboxed tuple holding the specified expressions,
    -- with the given types. The types must be the types of the expressions.
    -- Do not include the RuntimeRep specifiers; this function calculates them
    -- for you.
    -- Does /not/ flatten one-tuples; see Note [Flattening one-tuples]
    mkCoreUbxTup :: [Type] -> [CoreExpr] -> CoreExpr
    mkCoreUbxTup tys exps
      = ASSERT( tys `equalLength` exps)
        mkCoreConApps (tupleDataCon Unboxed (length tys))
                 (map (Type . getRuntimeRep) tys ++ map Type tys ++ exps)

    -- | Make a core tuple of the given boxity
    mkCoreTupBoxity :: Boxity -> [CoreExpr] -> CoreExpr
    mkCoreTupBoxity Boxed   exps = mkCoreTup exps
    mkCoreTupBoxity Unboxed exps = mkCoreUbxTup (map exprType exps) exps

    -- | Build a big tuple holding the specified variables
    -- One-tuples are flattened; see Note [Flattening one-tuples]
    mkBigCoreVarTup :: [Id] -> CoreExpr
    mkBigCoreVarTup ids = mkBigCoreTup (map Var ids)

    mkBigCoreVarTup1 :: [Id] -> CoreExpr
    -- Same as mkBigCoreVarTup, but one-tuples are NOT flattened
    --                          see Note [Flattening one-tuples]
    mkBigCoreVarTup1 [id] = mkCoreConApps (tupleDataCon Boxed 1)
                                          [Type (idType id), Var id]
    mkBigCoreVarTup1 ids  = mkBigCoreTup (map Var ids)

    -- | Build the type of a big tuple that holds the specified variables
    -- One-tuples are flattened; see Note [Flattening one-tuples]
    mkBigCoreVarTupTy :: [Id] -> Type
    mkBigCoreVarTupTy ids = mkBigCoreTupTy (map idType ids)

    -- | Build a big tuple holding the specified expressions
    -- One-tuples are flattened; see Note [Flattening one-tuples]
    mkBigCoreTup :: [CoreExpr] -> CoreExpr
    mkBigCoreTup = mkChunkified mkCoreTup

    -- | Build the type of a big tuple that holds the specified type of thing
    -- One-tuples are flattened; see Note [Flattening one-tuples]
    mkBigCoreTupTy :: [Type] -> Type
    mkBigCoreTupTy = mkChunkified mkBoxedTupleTy

    -- | The unit expression
    unitExpr :: CoreExpr
    unitExpr = Var unitDataConId

    {-
    ************************************************************************
    *                                                                      *
    \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, mkTupleSelector1
        :: [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 ]
    -- ^ 'mkTupleSelector1' is like 'mkTupleSelector'
    -- but one-tuples are NOT flattened (see Note [Flattening one-tuples])
    mkTupleSelector1 vars the_var scrut_var scrut
      | [_] <- vars
      = mkSmallTupleSelector1 vars the_var scrut_var scrut
      | otherwise
      = mkTupleSelector vars the_var scrut_var scrut

    -- | 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, mkSmallTupleSelector1
              :: [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  -- Special case for 1-tuples
    mkSmallTupleSelector vars the_var scrut_var scrut
      = mkSmallTupleSelector1 vars the_var scrut_var scrut

    -- ^ 'mkSmallTupleSelector1' is like 'mkSmallTupleSelector'
    -- but one-tuples are NOT flattened (see Note [Flattening one-tuples])
    mkSmallTupleSelector1 vars the_var scrut_var scrut
      = ASSERT( notNull vars )
        Case scrut scrut_var (idType the_var)
             [(DataAlt (tupleDataCon Boxed (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 (tupleDataCon Boxed (length vars)), vars, body)]

    {-
    ************************************************************************
    *                                                                      *
                    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) = text "LET" <+> ppr b
      ppr (FloatCase e b c bs) = hang (text "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{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 = mkCoreConApps nilDataCon [Type ty]

    -- | Makes a list @(:)@ for lists of the specified type
    mkConsExpr :: Type -> CoreExpr -> CoreExpr -> CoreExpr
    mkConsExpr ty hd tl = mkCoreConApps 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 :: (MonadFail m, 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)

    {-
    ************************************************************************
    *                                                                      *
                 Manipulating Maybe data type
    *                                                                      *
    ************************************************************************
    -}

    -- | Makes a Nothing for the specified type
    mkNothingExpr :: Type -> CoreExpr
    mkNothingExpr ty = mkConApp nothingDataCon [Type ty]

    -- | Makes a Just from a value of the specified type
    mkJustExpr :: Type -> CoreExpr -> CoreExpr
    mkJustExpr ty val = mkConApp justDataCon [Type ty, val]

    {-
    ************************************************************************
    *                                                                      *
                          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 (getRuntimeRep res_ty)
                            , 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
      = [ rUNTIME_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,
          tYPE_ERROR_ID   -- Used with Opt_DeferTypeErrors, see #10284
          ]

    recSelErrorName, runtimeErrorName, absentErrorName :: Name
    recConErrorName, patErrorName :: Name
    nonExhaustiveGuardsErrorName, noMethodBindingErrorName :: Name
    typeErrorName :: Name
    absentSumFieldErrorName :: Name

    recSelErrorName     = err_nm "recSelError"     recSelErrorIdKey     rEC_SEL_ERROR_ID
    absentErrorName     = err_nm "absentError"     absentErrorIdKey     aBSENT_ERROR_ID
    absentSumFieldErrorName = err_nm "absentSumFieldError"  absentSumFieldErrorIdKey
                                aBSENT_SUM_FIELD_ERROR_ID
    runtimeErrorName    = err_nm "runtimeError"    runtimeErrorIdKey    rUNTIME_ERROR_ID
    recConErrorName     = err_nm "recConError"     recConErrorIdKey     rEC_CON_ERROR_ID
    patErrorName        = err_nm "patError"        patErrorIdKey        pAT_ERROR_ID
    typeErrorName       = err_nm "typeError"       typeErrorIdKey       tYPE_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, rEC_CON_ERROR_ID :: Id
    pAT_ERROR_ID, nO_METHOD_BINDING_ERROR_ID, nON_EXHAUSTIVE_GUARDS_ERROR_ID :: Id
    tYPE_ERROR_ID, aBSENT_ERROR_ID, aBSENT_SUM_FIELD_ERROR_ID :: Id
    rEC_SEL_ERROR_ID                = mkRuntimeErrorId recSelErrorName
    rUNTIME_ERROR_ID                = mkRuntimeErrorId runtimeErrorName
    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
    tYPE_ERROR_ID                   = mkRuntimeErrorId typeErrorName

    -- Note [aBSENT_SUM_FIELD_ERROR_ID]
    -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    -- Absent argument error for unused unboxed sum fields are different than absent
    -- error used in dummy worker functions (see `mkAbsentErrorApp`):
    --
    -- - `absentSumFieldError` can't take arguments because it's used in unarise for
    --   unused pointer fields in unboxed sums, and applying an argument would
    --   require allocating a thunk.
    --
    -- - `absentSumFieldError` can't be CAFFY because that would mean making some
    --   non-CAFFY definitions that use unboxed sums CAFFY in unarise.
    --
    --   To make `absentSumFieldError` non-CAFFY we get a stable pointer to it in
    --   RtsStartup.c and mark it as non-CAFFY here.
    --
    -- Getting this wrong causes hard-to-debug runtime issues, see #15038.
    --
    -- TODO: Remove stable pointer hack after fixing #9718.
    --       However, we should still be careful about not making things CAFFY just
    --       because they use unboxed sums. Unboxed objects are supposed to be
    --       efficient, and none of the other unboxed literals make things CAFFY.

    aBSENT_SUM_FIELD_ERROR_ID
      = mkVanillaGlobalWithInfo absentSumFieldErrorName
          (mkSpecForAllTys [alphaTyVar] (mkTyVarTy alphaTyVar)) -- forall a . a
          (vanillaIdInfo `setStrictnessInfo` mkClosedStrictSig [] exnRes
                         `setArityInfo` 0
                         `setCafInfo` NoCafRefs) -- #15038

    mkRuntimeErrorId :: Name -> Id
    -- Error function
    --   with type:  forall (r:RuntimeRep) (a:TYPE r). Addr# -> a
    --   with arity: 1
    -- which diverges after being given one argument
    -- The Addr# is expected to be the address of
    --   a UTF8-encoded error string
    mkRuntimeErrorId name
     = mkVanillaGlobalWithInfo name runtimeErrorTy 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] exnRes
                  -- exnRes: these throw an exception, not just diverge

    runtimeErrorTy :: Type
    -- forall (rr :: RuntimeRep) (a :: rr). Addr# -> a
    --   See Note [Error and friends have an "open-tyvar" forall]
    runtimeErrorTy = mkSpecForAllTys [runtimeRep1TyVar, openAlphaTyVar]
                                     (mkFunTy addrPrimTy openAlphaTy)

    {- Note [Error and friends have an "open-tyvar" forall]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    'error' and 'undefined' have types
            error     :: forall (v :: RuntimeRep) (a :: TYPE v). String -> a
            undefined :: forall (v :: RuntimeRep) (a :: TYPE v). a
    Notice the runtime-representation polymorphism. This ensures that
    "error" can be instantiated at unboxed as well as boxed types.
    This is OK because it never returns, so the return type is irrelevant.

    ************************************************************************
    *                                                                      *
                         aBSENT_ERROR_ID
    *                                                                      *
    ************************************************************************

    Note [aBSENT_ERROR_ID]
    ~~~~~~~~~~~~~~~~~~~~~~
    We use aBSENT_ERROR_ID to build dummy values in workers.  E.g.

       f x = (case x of (a,b) -> b) + 1::Int

    The demand analyser figures ot that only the second component of x is
    used, and does a w/w split thus

       f x = case x of (a,b) -> $wf b

       $wf b = let a = absentError "blah"
                   x = (a,b)
               in <the original RHS of f>

    After some simplification, the (absentError "blah") thunk goes away.

    ------ Tricky wrinkle -------
    Trac #14285 had, roughly

       data T a = MkT a !a
       {-# INLINABLE f #-}
       f x = case x of MkT a b -> g (MkT b a)

    It turned out that g didn't use the second component, and hence f doesn't use
    the first.  But the stable-unfolding for f looks like
       \x. case x of MkT a b -> g ($WMkT b a)
    where $WMkT is the wrapper for MkT that evaluates its arguments.  We
    apply the same w/w split to this unfolding (see Note [Worker-wrapper
    for INLINEABLE functions] in WorkWrap) so the template ends up like
       \b. let a = absentError "blah"
               x = MkT a b
            in case x of MkT a b -> g ($WMkT b a)

    After doing case-of-known-constructor, and expanding $WMkT we get
       \b -> g (case absentError "blah" of a -> MkT b a)

    Yikes!  That bogusly appears to evaluate the absentError!

    This is extremely tiresome.  Another way to think of this is that, in
    Core, it is an invariant that a strict data contructor, like MkT, must
    be applied only to an argument in HNF. So (absentError "blah") had
    better be non-bottom.

    So the "solution" is to add a special case for absentError to exprIsHNFlike.
    This allows Simplify.rebuildCase, in the Note [Case to let transformation]
    branch, to convert the case on absentError into a let. We also make
    absentError *not* be diverging, unlike the other error-ids, so that we
    can be sure not to remove the case branches before converting the case to
    a let.

    If, by some bug or bizarre happenstance, we ever call absentError, we should
    throw an exception.  This should never happen, of course, but we definitely
    can't return anything.  e.g. if somehow we had
        case absentError "foo" of
           Nothing -> ...
           Just x  -> ...
    then if we return, the case expression will select a field and continue.
    Seg fault city. Better to throw an exception. (Even though we've said
    it is in HNF :-)

    It might seem a bit surprising that seq on absentError is simply erased

        absentError "foo" `seq` x ==> x

    but that should be okay; since there's no pattern match we can't really
    be relying on anything from it.
    -}

    aBSENT_ERROR_ID
     = mkVanillaGlobalWithInfo absentErrorName absent_ty arity_info
     where
       absent_ty = mkSpecForAllTys [alphaTyVar] (mkFunTy addrPrimTy alphaTy)
       -- Not runtime-rep polymorphic. aBSENT_ERROR_ID is only used for
       -- lifted-type things; see Note [Absent errors] in WwLib
       arity_info = vanillaIdInfo `setArityInfo` 1
       -- NB: no bottoming strictness info, unlike other error-ids.
       -- See Note [aBSENT_ERROR_ID]

    mkAbsentErrorApp :: Type         -- The type to instantiate 'a'
                     -> String       -- The string to print
                     -> CoreExpr

    mkAbsentErrorApp res_ty err_msg
      = mkApps (Var aBSENT_ERROR_ID) [ Type res_ty, err_string ]
      where
        err_string = Lit (mkMachString err_msg)