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

    Utilities for desugaring

    This module exports some utility functions of no great interest.
    -}

    {-# LANGUAGE CPP #-}

    -- | Utility functions for constructing Core syntax, principally for desugaring
    module DsUtils (
            EquationInfo(..),
            firstPat, shiftEqns,

            MatchResult(..), CanItFail(..), CaseAlt(..),
            cantFailMatchResult, alwaysFailMatchResult,
            extractMatchResult, combineMatchResults,
            adjustMatchResult,  adjustMatchResultDs,
            mkCoLetMatchResult, mkViewMatchResult, mkGuardedMatchResult,
            matchCanFail, mkEvalMatchResult,
            mkCoPrimCaseMatchResult, mkCoAlgCaseMatchResult, mkCoSynCaseMatchResult,
            wrapBind, wrapBinds,

            mkErrorAppDs, mkCoreAppDs, mkCoreAppsDs,

            seqVar,

            -- LHs tuples
            mkLHsVarPatTup, mkLHsPatTup, mkVanillaTuplePat,
            mkBigLHsVarTup, mkBigLHsTup, mkBigLHsVarPatTup, mkBigLHsPatTup,

            mkSelectorBinds,

            selectSimpleMatchVarL, selectMatchVars, selectMatchVar,
            mkOptTickBox, mkBinaryTickBox
        ) where

    #include "HsVersions.h"

    import {-# SOURCE #-}   Match ( matchSimply )

    import HsSyn
    import TcHsSyn
    import TcType( tcSplitTyConApp )
    import CoreSyn
    import DsMonad
    import {-# SOURCE #-} DsExpr ( dsLExpr )

    import CoreUtils
    import MkCore
    import MkId
    import Id
    import Literal
    import TyCon
    import ConLike
    import DataCon
    import PatSyn
    import Type
    import TysPrim
    import TysWiredIn
    import BasicTypes
    import UniqSet
    import UniqSupply
    import Module
    import PrelNames
    import Outputable
    import SrcLoc
    import Util
    import DynFlags
    import FastString

    import TcEvidence

    import Control.Monad    ( zipWithM )

    {-
    ************************************************************************
    *                                                                      *
    \subsection{ Selecting match variables}
    *                                                                      *
    ************************************************************************

    We're about to match against some patterns.  We want to make some
    @Ids@ to use as match variables.  If a pattern has an @Id@ readily at
    hand, which should indeed be bound to the pattern as a whole, then use it;
    otherwise, make one up.
    -}

    selectSimpleMatchVarL :: LPat Id -> DsM Id
    selectSimpleMatchVarL pat = selectMatchVar (unLoc pat)

    -- (selectMatchVars ps tys) chooses variables of type tys
    -- to use for matching ps against.  If the pattern is a variable,
    -- we try to use that, to save inventing lots of fresh variables.
    --
    -- OLD, but interesting note:
    --    But even if it is a variable, its type might not match.  Consider
    --      data T a where
    --        T1 :: Int -> T Int
    --        T2 :: a   -> T a
    --
    --      f :: T a -> a -> Int
    --      f (T1 i) (x::Int) = x
    --      f (T2 i) (y::a)   = 0
    --    Then we must not choose (x::Int) as the matching variable!
    -- And nowadays we won't, because the (x::Int) will be wrapped in a CoPat

    selectMatchVars :: [Pat Id] -> DsM [Id]
    selectMatchVars ps = mapM selectMatchVar ps

    selectMatchVar :: Pat Id -> DsM Id
    selectMatchVar (BangPat pat) = selectMatchVar (unLoc pat)
    selectMatchVar (LazyPat pat) = selectMatchVar (unLoc pat)
    selectMatchVar (ParPat pat)  = selectMatchVar (unLoc pat)
    selectMatchVar (VarPat var)  = return (localiseId var)  -- Note [Localise pattern binders]
    selectMatchVar (AsPat var _) = return (unLoc var)
    selectMatchVar other_pat     = newSysLocalDs (hsPatType other_pat)
                                      -- OK, better make up one...

    {-
    Note [Localise pattern binders]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider     module M where
                   [Just a] = e
    After renaming it looks like
                 module M where
                   [Just M.a] = e

    We don't generalise, since it's a pattern binding, monomorphic, etc,
    so after desugaring we may get something like
                 M.a = case e of (v:_) ->
                       case v of Just M.a -> M.a
    Notice the "M.a" in the pattern; after all, it was in the original
    pattern.  However, after optimisation those pattern binders can become
    let-binders, and then end up floated to top level.  They have a
    different *unique* by then (the simplifier is good about maintaining
    proper scoping), but it's BAD to have two top-level bindings with the
    External Name M.a, because that turns into two linker symbols for M.a.
    It's quite rare for this to actually *happen* -- the only case I know
    of is tc003 compiled with the 'hpc' way -- but that only makes it
    all the more annoying.

    To avoid this, we craftily call 'localiseId' in the desugarer, which
    simply turns the External Name for the Id into an Internal one, but
    doesn't change the unique.  So the desugarer produces this:
                 M.a{r8} = case e of (v:_) ->
                           case v of Just a{r8} -> M.a{r8}
    The unique is still 'r8', but the binding site in the pattern
    is now an Internal Name.  Now the simplifier's usual mechanisms
    will propagate that Name to all the occurrence sites, as well as
    un-shadowing it, so we'll get
                 M.a{r8} = case e of (v:_) ->
                           case v of Just a{s77} -> a{s77}
    In fact, even CoreSubst.simplOptExpr will do this, and simpleOptExpr
    runs on the output of the desugarer, so all is well by the end of
    the desugaring pass.

    ************************************************************************
    *                                                                      *
    * type synonym EquationInfo and access functions for its pieces        *
    *                                                                      *
    ************************************************************************
    \subsection[EquationInfo-synonym]{@EquationInfo@: a useful synonym}

    The ``equation info'' used by @match@ is relatively complicated and
    worthy of a type synonym and a few handy functions.
    -}

    firstPat :: EquationInfo -> Pat Id
    firstPat eqn = ASSERT( notNull (eqn_pats eqn) ) head (eqn_pats eqn)

    shiftEqns :: [EquationInfo] -> [EquationInfo]
    -- Drop the first pattern in each equation
    shiftEqns eqns = [ eqn { eqn_pats = tail (eqn_pats eqn) } | eqn <- eqns ]

    -- Functions on MatchResults

    matchCanFail :: MatchResult -> Bool
    matchCanFail (MatchResult CanFail _)  = True
    matchCanFail (MatchResult CantFail _) = False

    alwaysFailMatchResult :: MatchResult
    alwaysFailMatchResult = MatchResult CanFail (\fail -> return fail)

    cantFailMatchResult :: CoreExpr -> MatchResult
    cantFailMatchResult expr = MatchResult CantFail (\_ -> return expr)

    extractMatchResult :: MatchResult -> CoreExpr -> DsM CoreExpr
    extractMatchResult (MatchResult CantFail match_fn) _
      = match_fn (error "It can't fail!")

    extractMatchResult (MatchResult CanFail match_fn) fail_expr = do
        (fail_bind, if_it_fails) <- mkFailurePair fail_expr
        body <- match_fn if_it_fails
        return (mkCoreLet fail_bind body)

    combineMatchResults :: MatchResult -> MatchResult -> MatchResult
    combineMatchResults (MatchResult CanFail      body_fn1)
                        (MatchResult can_it_fail2 body_fn2)
      = MatchResult can_it_fail2 body_fn
      where
        body_fn fail = do body2 <- body_fn2 fail
                          (fail_bind, duplicatable_expr) <- mkFailurePair body2
                          body1 <- body_fn1 duplicatable_expr
                          return (Let fail_bind body1)

    combineMatchResults match_result1@(MatchResult CantFail _) _
      = match_result1

    adjustMatchResult :: DsWrapper -> MatchResult -> MatchResult
    adjustMatchResult encl_fn (MatchResult can_it_fail body_fn)
      = MatchResult can_it_fail (\fail -> encl_fn <$> body_fn fail)

    adjustMatchResultDs :: (CoreExpr -> DsM CoreExpr) -> MatchResult -> MatchResult
    adjustMatchResultDs encl_fn (MatchResult can_it_fail body_fn)
      = MatchResult can_it_fail (\fail -> encl_fn =<< body_fn fail)

    wrapBinds :: [(Var,Var)] -> CoreExpr -> CoreExpr
    wrapBinds [] e = e
    wrapBinds ((new,old):prs) e = wrapBind new old (wrapBinds prs e)

    wrapBind :: Var -> Var -> CoreExpr -> CoreExpr
    wrapBind new old body   -- NB: this function must deal with term
      | new==old    = body  -- variables, type variables or coercion variables
      | otherwise   = Let (NonRec new (varToCoreExpr old)) body

    seqVar :: Var -> CoreExpr -> CoreExpr
    seqVar var body = Case (Var var) var (exprType body)
                            [(DEFAULT, [], body)]

    mkCoLetMatchResult :: CoreBind -> MatchResult -> MatchResult
    mkCoLetMatchResult bind = adjustMatchResult (mkCoreLet bind)

    -- (mkViewMatchResult var' viewExpr var mr) makes the expression
    -- let var' = viewExpr var in mr
    mkViewMatchResult :: Id -> CoreExpr -> Id -> MatchResult -> MatchResult
    mkViewMatchResult var' viewExpr var =
        adjustMatchResult (mkCoreLet (NonRec var' (mkCoreAppDs viewExpr (Var var))))

    mkEvalMatchResult :: Id -> Type -> MatchResult -> MatchResult
    mkEvalMatchResult var ty
      = adjustMatchResult (\e -> Case (Var var) var ty [(DEFAULT, [], e)])

    mkGuardedMatchResult :: CoreExpr -> MatchResult -> MatchResult
    mkGuardedMatchResult pred_expr (MatchResult _ body_fn)
      = MatchResult CanFail (\fail -> do body <- body_fn fail
                                         return (mkIfThenElse pred_expr body fail))

    mkCoPrimCaseMatchResult :: Id                           -- Scrutinee
                        -> Type                             -- Type of the case
                        -> [(Literal, MatchResult)]         -- Alternatives
                        -> MatchResult                      -- Literals are all unlifted
    mkCoPrimCaseMatchResult var ty match_alts
      = MatchResult CanFail mk_case
      where
        mk_case fail = do
            alts <- mapM (mk_alt fail) sorted_alts
            return (Case (Var var) var ty ((DEFAULT, [], fail) : alts))

        sorted_alts = sortWith fst match_alts       -- Right order for a Case
        mk_alt fail (lit, MatchResult _ body_fn)
           = ASSERT( not (litIsLifted lit) )
             do body <- body_fn fail
                return (LitAlt lit, [], body)

    data CaseAlt a = MkCaseAlt{ alt_pat :: a,
                                alt_bndrs :: [CoreBndr],
                                alt_wrapper :: HsWrapper,
                                alt_result :: MatchResult }

    mkCoAlgCaseMatchResult
      :: DynFlags
      -> Id                 -- Scrutinee
      -> Type               -- Type of exp
      -> [CaseAlt DataCon]  -- Alternatives (bndrs *include* tyvars, dicts)
      -> MatchResult
    mkCoAlgCaseMatchResult dflags var ty match_alts
      | isNewtype  -- Newtype case; use a let
      = ASSERT( null (tail match_alts) && null (tail arg_ids1) )
        mkCoLetMatchResult (NonRec arg_id1 newtype_rhs) match_result1

      | isPArrFakeAlts match_alts
      = MatchResult CanFail $ mkPArrCase dflags var ty (sort_alts match_alts)
      | otherwise
      = mkDataConCase var ty match_alts
      where
        isNewtype = isNewTyCon (dataConTyCon (alt_pat alt1))

            -- [Interesting: because of GADTs, we can't rely on the type of
            --  the scrutinised Id to be sufficiently refined to have a TyCon in it]

        alt1@MkCaseAlt{ alt_bndrs = arg_ids1, alt_result = match_result1 }
          = ASSERT( notNull match_alts ) head match_alts
        -- Stuff for newtype
        arg_id1       = ASSERT( notNull arg_ids1 ) head arg_ids1
        var_ty        = idType var
        (tc, ty_args) = tcSplitTyConApp var_ty      -- Don't look through newtypes
                                                    -- (not that splitTyConApp does, these days)
        newtype_rhs = unwrapNewTypeBody tc ty_args (Var var)

            --- Stuff for parallel arrays
            --
            -- Concerning `isPArrFakeAlts':
            --
            --  * it is *not* sufficient to just check the type of the type
            --   constructor, as we have to be careful not to confuse the real
            --   representation of parallel arrays with the fake constructors;
            --   moreover, a list of alternatives must not mix fake and real
            --   constructors (this is checked earlier on)
            --
            -- FIXME: We actually go through the whole list and make sure that
            --        either all or none of the constructors are fake parallel
            --        array constructors.  This is to spot equations that mix fake
            --        constructors with the real representation defined in
            --        `PrelPArr'.  It would be nicer to spot this situation
            --        earlier and raise a proper error message, but it can really
            --        only happen in `PrelPArr' anyway.
            --

        isPArrFakeAlts :: [CaseAlt DataCon] -> Bool
        isPArrFakeAlts [alt] = isPArrFakeCon (alt_pat alt)
        isPArrFakeAlts (alt:alts) =
          case (isPArrFakeCon (alt_pat alt), isPArrFakeAlts alts) of
            (True , True ) -> True
            (False, False) -> False
            _              -> panic "DsUtils: you may not mix `[:...:]' with `PArr' patterns"
        isPArrFakeAlts [] = panic "DsUtils: unexpectedly found an empty list of PArr fake alternatives"

    mkCoSynCaseMatchResult :: Id -> Type -> CaseAlt PatSyn -> MatchResult
    mkCoSynCaseMatchResult var ty alt = MatchResult CanFail $ mkPatSynCase var ty alt

    sort_alts :: [CaseAlt DataCon] -> [CaseAlt DataCon]
    sort_alts = sortWith (dataConTag . alt_pat)

    mkPatSynCase :: Id -> Type -> CaseAlt PatSyn -> CoreExpr -> DsM CoreExpr
    mkPatSynCase var ty alt fail = do
        matcher <- dsLExpr $ mkLHsWrap wrapper $ nlHsTyApp matcher [ty]
        let MatchResult _ mkCont = match_result
        cont <- mkCoreLams bndrs <$> mkCont fail
        return $ mkCoreAppsDs matcher [Var var, ensure_unstrict cont, Lam voidArgId fail]
      where
        MkCaseAlt{ alt_pat = psyn,
                   alt_bndrs = bndrs,
                   alt_wrapper = wrapper,
                   alt_result = match_result} = alt
        (matcher, needs_void_lam) = patSynMatcher psyn

        -- See Note [Matchers and builders for pattern synonyms] in PatSyns
        -- on these extra Void# arguments
        ensure_unstrict cont | needs_void_lam = Lam voidArgId cont
                             | otherwise      = cont

    mkDataConCase :: Id -> Type -> [CaseAlt DataCon] -> MatchResult
    mkDataConCase _   _  []            = panic "mkDataConCase: no alternatives"
    mkDataConCase var ty alts@(alt1:_) = MatchResult fail_flag mk_case
      where
        con1          = alt_pat alt1
        tycon         = dataConTyCon con1
        data_cons     = tyConDataCons tycon
        match_results = map alt_result alts

        sorted_alts :: [CaseAlt DataCon]
        sorted_alts  = sort_alts alts

        var_ty       = idType var
        (_, ty_args) = tcSplitTyConApp var_ty -- Don't look through newtypes
                                              -- (not that splitTyConApp does, these days)

        mk_case :: CoreExpr -> DsM CoreExpr
        mk_case fail = do
            alts <- mapM (mk_alt fail) sorted_alts
            return $ mkWildCase (Var var) (idType var) ty (mk_default fail ++ alts)

        mk_alt :: CoreExpr -> CaseAlt DataCon -> DsM CoreAlt
        mk_alt fail MkCaseAlt{ alt_pat = con,
                               alt_bndrs = args,
                               alt_result = MatchResult _ body_fn }
          = do { body <- body_fn fail
               ; case dataConBoxer con of {
                    Nothing -> return (DataAlt con, args, body) ;
                    Just (DCB boxer) ->
            do { us <- newUniqueSupply
               ; let (rep_ids, binds) = initUs_ us (boxer ty_args args)
               ; return (DataAlt con, rep_ids, mkLets binds body) } } }

        mk_default :: CoreExpr -> [CoreAlt]
        mk_default fail | exhaustive_case = []
                        | otherwise       = [(DEFAULT, [], fail)]

        fail_flag :: CanItFail
        fail_flag | exhaustive_case
                  = foldr orFail CantFail [can_it_fail | MatchResult can_it_fail _ <- match_results]
                  | otherwise
                  = CanFail

        mentioned_constructors = mkUniqSet $ map alt_pat alts
        un_mentioned_constructors
            = mkUniqSet data_cons `minusUniqSet` mentioned_constructors
        exhaustive_case = isEmptyUniqSet un_mentioned_constructors

    --- Stuff for parallel arrays
    --
    --  * the following is to desugar cases over fake constructors for
    --   parallel arrays, which are introduced by `tidy1' in the `PArrPat'
    --   case
    --
    mkPArrCase :: DynFlags -> Id -> Type -> [CaseAlt DataCon] -> CoreExpr -> DsM CoreExpr
    mkPArrCase dflags var ty sorted_alts fail = do
        lengthP <- dsDPHBuiltin lengthPVar
        alt <- unboxAlt
        return (mkWildCase (len lengthP) intTy ty [alt])
      where
        elemTy      = case splitTyConApp (idType var) of
            (_, [elemTy]) -> elemTy
            _             -> panic panicMsg
        panicMsg    = "DsUtils.mkCoAlgCaseMatchResult: not a parallel array?"
        len lengthP = mkApps (Var lengthP) [Type elemTy, Var var]
        --
        unboxAlt = do
            l      <- newSysLocalDs intPrimTy
            indexP <- dsDPHBuiltin indexPVar
            alts   <- mapM (mkAlt indexP) sorted_alts
            return (DataAlt intDataCon, [l], mkWildCase (Var l) intPrimTy ty (dft : alts))
          where
            dft  = (DEFAULT, [], fail)

        --
        -- each alternative matches one array length (corresponding to one
        -- fake array constructor), so the match is on a literal; each
        -- alternative's body is extended by a local binding for each
        -- constructor argument, which are bound to array elements starting
        -- with the first
        --
        mkAlt indexP alt@MkCaseAlt{alt_result = MatchResult _ bodyFun} = do
            body <- bodyFun fail
            return (LitAlt lit, [], mkCoreLets binds body)
          where
            lit   = MachInt $ toInteger (dataConSourceArity (alt_pat alt))
            binds = [NonRec arg (indexExpr i) | (i, arg) <- zip [1..] (alt_bndrs alt)]
            --
            indexExpr i = mkApps (Var indexP) [Type elemTy, Var var, mkIntExpr dflags i]

    {-
    ************************************************************************
    *                                                                      *
    \subsection{Desugarer's versions of some Core functions}
    *                                                                      *
    ************************************************************************
    -}

    mkErrorAppDs :: Id              -- The error function
                 -> Type            -- Type to which it should be applied
                 -> SDoc            -- The error message string to pass
                 -> DsM CoreExpr

    mkErrorAppDs err_id ty msg = do
        src_loc <- getSrcSpanDs
        dflags <- getDynFlags
        let
            full_msg = showSDoc dflags (hcat [ppr src_loc, text "|", msg])
            core_msg = Lit (mkMachString full_msg)
            -- mkMachString returns a result of type String#
        return (mkApps (Var err_id) [Type ty, core_msg])

    {-
    'mkCoreAppDs' and 'mkCoreAppsDs' hand the special-case desugaring of 'seq'.

    Note [Desugaring seq (1)]  cf Trac #1031
    ~~~~~~~~~~~~~~~~~~~~~~~~~
       f x y = x `seq` (y `seq` (# x,y #))

    The [CoreSyn let/app invariant] means that, other things being equal, because
    the argument to the outer 'seq' has an unlifted type, we'll use call-by-value thus:

       f x y = case (y `seq` (# x,y #)) of v -> x `seq` v

    But that is bad for two reasons:
      (a) we now evaluate y before x, and
      (b) we can't bind v to an unboxed pair

    Seq is very, very special!  So we recognise it right here, and desugar to
            case x of _ -> case y of _ -> (# x,y #)

    Note [Desugaring seq (2)]  cf Trac #2273
    ~~~~~~~~~~~~~~~~~~~~~~~~~
    Consider
       let chp = case b of { True -> fst x; False -> 0 }
       in chp `seq` ...chp...
    Here the seq is designed to plug the space leak of retaining (snd x)
    for too long.

    If we rely on the ordinary inlining of seq, we'll get
       let chp = case b of { True -> fst x; False -> 0 }
       case chp of _ { I# -> ...chp... }

    But since chp is cheap, and the case is an alluring contet, we'll
    inline chp into the case scrutinee.  Now there is only one use of chp,
    so we'll inline a second copy.  Alas, we've now ruined the purpose of
    the seq, by re-introducing the space leak:
        case (case b of {True -> fst x; False -> 0}) of
          I# _ -> ...case b of {True -> fst x; False -> 0}...

    We can try to avoid doing this by ensuring that the binder-swap in the
    case happens, so we get his at an early stage:
       case chp of chp2 { I# -> ...chp2... }
    But this is fragile.  The real culprit is the source program.  Perhaps we
    should have said explicitly
       let !chp2 = chp in ...chp2...

    But that's painful.  So the code here does a little hack to make seq
    more robust: a saturated application of 'seq' is turned *directly* into
    the case expression, thus:
       x  `seq` e2 ==> case x of x -> e2    -- Note shadowing!
       e1 `seq` e2 ==> case x of _ -> e2

    So we desugar our example to:
       let chp = case b of { True -> fst x; False -> 0 }
       case chp of chp { I# -> ...chp... }
    And now all is well.

    The reason it's a hack is because if you define mySeq=seq, the hack
    won't work on mySeq.

    Note [Desugaring seq (3)] cf Trac #2409
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    The isLocalId ensures that we don't turn
            True `seq` e
    into
            case True of True { ... }
    which stupidly tries to bind the datacon 'True'.
    -}

    mkCoreAppDs  :: CoreExpr -> CoreExpr -> CoreExpr
    mkCoreAppDs (Var f `App` Type ty1 `App` Type ty2 `App` arg1) arg2
      | f `hasKey` seqIdKey            -- Note [Desugaring seq (1), (2)]
      = Case arg1 case_bndr ty2 [(DEFAULT,[],arg2)]
      where
        case_bndr = case arg1 of
                       Var v1 | isLocalId v1 -> v1        -- Note [Desugaring seq (2) and (3)]
                       _                     -> mkWildValBinder ty1

    mkCoreAppDs fun arg = mkCoreApp fun arg  -- The rest is done in MkCore

    mkCoreAppsDs :: CoreExpr -> [CoreExpr] -> CoreExpr
    mkCoreAppsDs fun args = foldl mkCoreAppDs fun args

    {-
    ************************************************************************
    *                                                                      *
    \subsection[mkSelectorBind]{Make a selector bind}
    *                                                                      *
    ************************************************************************

    This is used in various places to do with lazy patterns.
    For each binder $b$ in the pattern, we create a binding:
    \begin{verbatim}
        b = case v of pat' -> b'
    \end{verbatim}
    where @pat'@ is @pat@ with each binder @b@ cloned into @b'@.

    ToDo: making these bindings should really depend on whether there's
    much work to be done per binding.  If the pattern is complex, it
    should be de-mangled once, into a tuple (and then selected from).
    Otherwise the demangling can be in-line in the bindings (as here).

    Boring!  Boring!  One error message per binder.  The above ToDo is
    even more helpful.  Something very similar happens for pattern-bound
    expressions.

    Note [mkSelectorBinds]
    ~~~~~~~~~~~~~~~~~~~~~~
    Given   p = e, where p binds x,y
    we are going to make EITHER

    EITHER (A)   v = e   (where v is fresh)
                 x = case v of p -> x
                 y = case v of p -> y

    OR (B)       t = case e of p -> (x,y)
                 x = case t of (x,_) -> x
                 y = case t of (_,y) -> y

    We do (A) when
     * Matching the pattern is cheap so we don't mind
       doing it twice.
     * Or if the pattern binds only one variable (so we'll only
       match once)
     * AND the pattern can't fail (else we tiresomely get two inexhaustive
       pattern warning messages)

    Otherwise we do (B).  Really (A) is just an optimisation for very common
    cases like
         Just x = e
         (p,q) = e
    -}

    mkSelectorBinds :: [[Tickish Id]] -- ticks to add, possibly
                    -> LPat Id      -- The pattern
                    -> CoreExpr     -- Expression to which the pattern is bound
                    -> DsM [(Id,CoreExpr)]

    mkSelectorBinds ticks (L _ (VarPat v)) val_expr
      = return [(v, case ticks of
                      [t] -> mkOptTickBox t val_expr
                      _   -> val_expr)]

    mkSelectorBinds ticks pat val_expr
      | null binders
      = return []

      | isSingleton binders || is_simple_lpat pat
        -- See Note [mkSelectorBinds]
      = do { val_var <- newSysLocalDs (hsLPatType pat)
            -- Make up 'v' in Note [mkSelectorBinds]
            -- NB: give it the type of *pattern* p, not the type of the *rhs* e.
            -- This does not matter after desugaring, but there's a subtle
            -- issue with implicit parameters. Consider
            --      (x,y) = ?i
            -- Then, ?i is given type {?i :: Int}, a PredType, which is opaque
            -- to the desugarer.  (Why opaque?  Because newtypes have to be.  Why
            -- does it get that type?  So that when we abstract over it we get the
            -- right top-level type  (?i::Int) => ...)
            --
            -- So to get the type of 'v', use the pattern not the rhs.  Often more
            -- efficient too.

            -- For the error message we make one error-app, to avoid duplication.
            -- But we need it at different types, so we make it polymorphic:
            --     err_var = /\a. iRREFUT_PAT_ERR a "blah blah blah"
           ; err_app <- mkErrorAppDs iRREFUT_PAT_ERROR_ID alphaTy (ppr pat)
           ; err_var <- newSysLocalDs (mkForAllTy alphaTyVar alphaTy)
           ; binds   <- zipWithM (mk_bind val_var err_var) ticks' binders
           ; return ( (val_var, val_expr) :
                      (err_var, Lam alphaTyVar err_app) :
                      binds ) }

      | otherwise
      = do { error_expr <- mkErrorAppDs iRREFUT_PAT_ERROR_ID   tuple_ty (ppr pat)
           ; tuple_expr <- matchSimply val_expr PatBindRhs pat local_tuple error_expr
           ; tuple_var <- newSysLocalDs tuple_ty
           ; let mk_tup_bind tick binder
                  = (binder, mkOptTickBox tick $
                                mkTupleSelector local_binders binder
                                                tuple_var (Var tuple_var))
           ; return ( (tuple_var, tuple_expr) : zipWith mk_tup_bind ticks' binders ) }
      where
        binders       = collectPatBinders pat
        ticks'        = ticks ++ repeat []

        local_binders = map localiseId binders      -- See Note [Localise pattern binders]
        local_tuple   = mkBigCoreVarTup binders
        tuple_ty      = exprType local_tuple

        mk_bind scrut_var err_var tick bndr_var = do
        -- (mk_bind sv err_var) generates
        --          bv = case sv of { pat -> bv; other -> err_var @ type-of-bv }
        -- Remember, pat binds bv
            rhs_expr <- matchSimply (Var scrut_var) PatBindRhs pat
                                    (Var bndr_var) error_expr
            return (bndr_var, mkOptTickBox tick rhs_expr)
          where
            error_expr = Var err_var `App` Type (idType bndr_var)

        is_simple_lpat p = is_simple_pat (unLoc p)

        is_simple_pat (TuplePat ps Boxed _) = all is_triv_lpat ps
        is_simple_pat pat@(ConPatOut{})     = case unLoc (pat_con pat) of
            RealDataCon con -> isProductTyCon (dataConTyCon con)
                               && all is_triv_lpat (hsConPatArgs (pat_args pat))
            PatSynCon _     -> False
        is_simple_pat (VarPat _)                   = True
        is_simple_pat (ParPat p)                   = is_simple_lpat p
        is_simple_pat _                                    = False

        is_triv_lpat p = is_triv_pat (unLoc p)

        is_triv_pat (VarPat _)  = True
        is_triv_pat (WildPat _) = True
        is_triv_pat (ParPat p)  = is_triv_lpat p
        is_triv_pat _           = False

    {-
    Creating big tuples and their types for full Haskell expressions.
    They work over *Ids*, and create tuples replete with their types,
    which is whey they are not in HsUtils.
    -}

    mkLHsPatTup :: [LPat Id] -> LPat Id
    mkLHsPatTup []     = noLoc $ mkVanillaTuplePat [] Boxed
    mkLHsPatTup [lpat] = lpat
    mkLHsPatTup lpats  = L (getLoc (head lpats)) $
                         mkVanillaTuplePat lpats Boxed

    mkLHsVarPatTup :: [Id] -> LPat Id
    mkLHsVarPatTup bs  = mkLHsPatTup (map nlVarPat bs)

    mkVanillaTuplePat :: [OutPat Id] -> Boxity -> Pat Id
    -- A vanilla tuple pattern simply gets its type from its sub-patterns
    mkVanillaTuplePat pats box = TuplePat pats box (map hsLPatType pats)

    -- The Big equivalents for the source tuple expressions
    mkBigLHsVarTup :: [Id] -> LHsExpr Id
    mkBigLHsVarTup ids = mkBigLHsTup (map nlHsVar ids)

    mkBigLHsTup :: [LHsExpr Id] -> LHsExpr Id
    mkBigLHsTup = mkChunkified mkLHsTupleExpr

    -- The Big equivalents for the source tuple patterns
    mkBigLHsVarPatTup :: [Id] -> LPat Id
    mkBigLHsVarPatTup bs = mkBigLHsPatTup (map nlVarPat bs)

    mkBigLHsPatTup :: [LPat Id] -> LPat Id
    mkBigLHsPatTup = mkChunkified mkLHsPatTup

    {-
    ************************************************************************
    *                                                                      *
    \subsection[mkFailurePair]{Code for pattern-matching and other failures}
    *                                                                      *
    ************************************************************************

    Generally, we handle pattern matching failure like this: let-bind a
    fail-variable, and use that variable if the thing fails:
    \begin{verbatim}
            let fail.33 = error "Help"
            in
            case x of
                    p1 -> ...
                    p2 -> fail.33
                    p3 -> fail.33
                    p4 -> ...
    \end{verbatim}
    Then
    \begin{itemize}
    \item
    If the case can't fail, then there'll be no mention of @fail.33@, and the
    simplifier will later discard it.

    \item
    If it can fail in only one way, then the simplifier will inline it.

    \item
    Only if it is used more than once will the let-binding remain.
    \end{itemize}

    There's a problem when the result of the case expression is of
    unboxed type.  Then the type of @fail.33@ is unboxed too, and
    there is every chance that someone will change the let into a case:
    \begin{verbatim}
            case error "Help" of
              fail.33 -> case ....
    \end{verbatim}

    which is of course utterly wrong.  Rather than drop the condition that
    only boxed types can be let-bound, we just turn the fail into a function
    for the primitive case:
    \begin{verbatim}
            let fail.33 :: Void -> Int#
                fail.33 = \_ -> error "Help"
            in
            case x of
                    p1 -> ...
                    p2 -> fail.33 void
                    p3 -> fail.33 void
                    p4 -> ...
    \end{verbatim}

    Now @fail.33@ is a function, so it can be let-bound.
    -}

    mkFailurePair :: CoreExpr       -- Result type of the whole case expression
                  -> DsM (CoreBind, -- Binds the newly-created fail variable
                                    -- to \ _ -> expression
                          CoreExpr) -- Fail variable applied to realWorld#
    -- See Note [Failure thunks and CPR]
    mkFailurePair expr
      = do { fail_fun_var <- newFailLocalDs (voidPrimTy `mkFunTy` ty)
           ; fail_fun_arg <- newSysLocalDs voidPrimTy
           ; let real_arg = setOneShotLambda fail_fun_arg
           ; return (NonRec fail_fun_var (Lam real_arg expr),
                     App (Var fail_fun_var) (Var voidPrimId)) }
      where
        ty = exprType expr

    {-
    Note [Failure thunks and CPR]
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    When we make a failure point we ensure that it
    does not look like a thunk. Example:

       let fail = \rw -> error "urk"
       in case x of
            [] -> fail realWorld#
            (y:ys) -> case ys of
                        [] -> fail realWorld#
                        (z:zs) -> (y,z)

    Reason: we know that a failure point is always a "join point" and is
    entered at most once.  Adding a dummy 'realWorld' token argument makes
    it clear that sharing is not an issue.  And that in turn makes it more
    CPR-friendly.  This matters a lot: if you don't get it right, you lose
    the tail call property.  For example, see Trac #3403.
    -}

    mkOptTickBox :: [Tickish Id] -> CoreExpr -> CoreExpr
    mkOptTickBox = flip (foldr Tick)

    mkBinaryTickBox :: Int -> Int -> CoreExpr -> DsM CoreExpr
    mkBinaryTickBox ixT ixF e = do
           uq <- newUnique
           this_mod <- getModule
           let bndr1 = mkSysLocal (fsLit "t1") uq boolTy
           let
               falseBox = Tick (HpcTick this_mod ixF) (Var falseDataConId)
               trueBox  = Tick (HpcTick this_mod ixT) (Var trueDataConId)
           --
           return $ Case e bndr1 boolTy
                           [ (DataAlt falseDataCon, [], falseBox)
                           , (DataAlt trueDataCon,  [], trueBox)
                           ]