DsExpr.hs

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


Desugaring exporessions.
-}

{-# LANGUAGE CPP, MultiWayIf #-}

module DsExpr ( dsExpr, dsLExpr, dsLExprNoLP, dsLocalBinds
              , dsValBinds, dsLit, dsSyntaxExpr ) where

#include "HsVersions.h"

import Match
import MatchLit
import DsBinds
import DsGRHSs
import DsListComp
import DsUtils
import DsArrows
import DsMonad
import Name
import NameEnv
import FamInstEnv( topNormaliseType )
import DsMeta
import HsSyn

-- NB: The desugarer, which straddles the source and Core worlds, sometimes
--     needs to see source types
import TcType
import TcEvidence
import TcRnMonad
import TcHsSyn
import Type
import CoreSyn
import CoreUtils
import MkCore

import DynFlags
import CostCentre
import Id
import MkId
import Module
import ConLike
import DataCon
import TysWiredIn
import PrelNames
import BasicTypes
import Maybes
import VarEnv
import SrcLoc
import Util
import Bag
import Outputable
import PatSyn

import Control.Monad

{-
************************************************************************
*                                                                      *
                dsLocalBinds, dsValBinds
*                                                                      *
************************************************************************
-}

dsLocalBinds :: LHsLocalBinds Id -> CoreExpr -> DsM CoreExpr
dsLocalBinds (L _   EmptyLocalBinds)    body = return body
dsLocalBinds (L loc (HsValBinds binds)) body = putSrcSpanDs loc $
                                               dsValBinds binds body
dsLocalBinds (L _ (HsIPBinds binds))    body = dsIPBinds  binds body

-------------------------
-- caller sets location
dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
dsValBinds (ValBindsOut binds _) body = foldrM ds_val_bind body binds
dsValBinds (ValBindsIn {})       _    = panic "dsValBinds ValBindsIn"

-------------------------
dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr
dsIPBinds (IPBinds ip_binds ev_binds) body
  = do  { ds_binds <- dsTcEvBinds ev_binds
        ; let inner = mkCoreLets ds_binds body
                -- The dict bindings may not be in
                -- dependency order; hence Rec
        ; foldrM ds_ip_bind inner ip_binds }
  where
    ds_ip_bind (L _ (IPBind ~(Right n) e)) body
      = do e' <- dsLExpr e
           return (Let (NonRec n e') body)

-------------------------
-- caller sets location
ds_val_bind :: (RecFlag, LHsBinds Id) -> CoreExpr -> DsM CoreExpr
-- Special case for bindings which bind unlifted variables
-- We need to do a case right away, rather than building
-- a tuple and doing selections.
-- Silently ignore INLINE and SPECIALISE pragmas...
ds_val_bind (NonRecursive, hsbinds) body
  | [L loc bind] <- bagToList hsbinds
        -- Non-recursive, non-overloaded bindings only come in ones
        -- ToDo: in some bizarre case it's conceivable that there
        --       could be dict binds in the 'binds'.  (See the notes
        --       below.  Then pattern-match would fail.  Urk.)
  , isUnliftedHsBind bind
  = putSrcSpanDs loc $
     -- see Note [Strict binds checks] in DsBinds
    if is_polymorphic bind
    then errDsCoreExpr (poly_bind_err bind)
            -- data Ptr a = Ptr Addr#
            -- f x = let p@(Ptr y) = ... in ...
            -- Here the binding for 'p' is polymorphic, but does
            -- not mix with an unlifted binding for 'y'.  You should
            -- use a bang pattern.  Trac #6078.

    else do { when (looksLazyPatBind bind) $
              warnIfSetDs Opt_WarnUnbangedStrictPatterns (unlifted_must_be_bang bind)
        -- Complain about a binding that looks lazy
        --    e.g.    let I# y = x in ...
        -- Remember, in checkStrictBinds we are going to do strict
        -- matching, so (for software engineering reasons) we insist
        -- that the strictness is manifest on each binding
        -- However, lone (unboxed) variables are ok


            ; dsUnliftedBind bind body }
  where
    is_polymorphic (AbsBinds { abs_tvs = tvs, abs_ev_vars = evs })
                     = not (null tvs && null evs)
    is_polymorphic (AbsBindsSig { abs_tvs = tvs, abs_ev_vars = evs })
                     = not (null tvs && null evs)
    is_polymorphic _ = False

    unlifted_must_be_bang bind
      = hang (text "Pattern bindings containing unlifted types should use" $$
              text "an outermost bang pattern:")
           2 (ppr bind)

    poly_bind_err bind
      = hang (text "You can't mix polymorphic and unlifted bindings:")
           2 (ppr bind) $$
        text "Probable fix: add a type signature"

ds_val_bind (is_rec, binds) _body
  | anyBag (isUnliftedHsBind . unLoc) binds  -- see Note [Strict binds checks] in DsBinds
  = ASSERT( isRec is_rec )
    errDsCoreExpr $
    hang (text "Recursive bindings for unlifted types aren't allowed:")
       2 (vcat (map ppr (bagToList binds)))

-- Ordinary case for bindings; none should be unlifted
ds_val_bind (is_rec, binds) body
  = do  { MASSERT( isRec is_rec || isSingletonBag binds )
               -- we should never produce a non-recursive list of multiple binds

        ; (force_vars,prs) <- dsLHsBinds binds
        ; let body' = foldr seqVar body force_vars
        ; ASSERT2( not (any (isUnliftedType . idType . fst) prs), ppr is_rec $$ ppr binds )
          case prs of
            [] -> return body
            _  -> return (Let (Rec prs) body') }
        -- Use a Rec regardless of is_rec.
        -- Why? Because it allows the binds to be all
        -- mixed up, which is what happens in one rare case
        -- Namely, for an AbsBind with no tyvars and no dicts,
        --         but which does have dictionary bindings.
        -- See notes with TcSimplify.inferLoop [NO TYVARS]
        -- It turned out that wrapping a Rec here was the easiest solution
        --
        -- NB The previous case dealt with unlifted bindings, so we
        --    only have to deal with lifted ones now; so Rec is ok

------------------
dsUnliftedBind :: HsBind Id -> CoreExpr -> DsM CoreExpr
dsUnliftedBind (AbsBinds { abs_tvs = [], abs_ev_vars = []
               , abs_exports = exports
               , abs_ev_binds = ev_binds
               , abs_binds = lbinds }) body
  = do { let body1 = foldr bind_export body exports
             bind_export export b = bindNonRec (abe_poly export) (Var (abe_mono export)) b
       ; body2 <- foldlBagM (\body lbind -> dsUnliftedBind (unLoc lbind) body)
                            body1 lbinds
       ; ds_binds <- dsTcEvBinds_s ev_binds
       ; return (mkCoreLets ds_binds body2) }

dsUnliftedBind (AbsBindsSig { abs_tvs         = []
                            , abs_ev_vars     = []
                            , abs_sig_export  = poly
                            , abs_sig_ev_bind = ev_bind
                            , abs_sig_bind    = L _ bind }) body
  = do { ds_binds <- dsTcEvBinds ev_bind
       ; body' <- dsUnliftedBind (bind { fun_id = noLoc poly }) body
       ; return (mkCoreLets ds_binds body') }

dsUnliftedBind (FunBind { fun_id = L l fun
                        , fun_matches = matches
                        , fun_co_fn = co_fn
                        , fun_tick = tick }) body
               -- Can't be a bang pattern (that looks like a PatBind)
               -- so must be simply unboxed
  = do { (args, rhs) <- matchWrapper (FunRhs (L l $ idName fun) Prefix)
                                     Nothing matches
       ; MASSERT( null args ) -- Functions aren't lifted
       ; MASSERT( isIdHsWrapper co_fn )
       ; let rhs' = mkOptTickBox tick rhs
       ; return (bindNonRec fun rhs' body) }

dsUnliftedBind (PatBind {pat_lhs = pat, pat_rhs = grhss, pat_rhs_ty = ty }) body
  =     -- let C x# y# = rhs in body
        -- ==> case rhs of C x# y# -> body
    do { rhs <- dsGuarded grhss ty
       ; let upat = unLoc pat
             eqn = EqnInfo { eqn_pats = [upat],
                             eqn_rhs = cantFailMatchResult body }
       ; var    <- selectMatchVar upat
       ; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
       ; return (bindNonRec var rhs result) }

dsUnliftedBind bind body = pprPanic "dsLet: unlifted" (ppr bind $$ ppr body)

{-
************************************************************************
*                                                                      *
\subsection[DsExpr-vars-and-cons]{Variables, constructors, literals}
*                                                                      *
************************************************************************
-}

dsLExpr :: LHsExpr Id -> DsM CoreExpr

dsLExpr (L loc e)
  = putSrcSpanDs loc $
    do { core_expr <- dsExpr e
   -- uncomment this check to test the hsExprType function in TcHsSyn
   --    ; MASSERT2( exprType core_expr `eqType` hsExprType e
   --              , ppr e <+> dcolon <+> ppr (hsExprType e) $$
   --                ppr core_expr <+> dcolon <+> ppr (exprType core_expr) )
       ; return core_expr }

-- | Variant of 'dsLExpr' that ensures that the result is not levity
-- polymorphic. This should be used when the resulting expression will
-- be an argument to some other function.
-- See Note [Levity polymorphism checking] in DsMonad
-- See Note [Levity polymorphism invariants] in CoreSyn
dsLExprNoLP :: LHsExpr Id -> DsM CoreExpr
dsLExprNoLP (L loc e)
  = putSrcSpanDs loc $
    do { e' <- dsExpr e
       ; dsNoLevPolyExpr e' (text "In the type of expression:" <+> ppr e)
       ; return e' }

dsExpr :: HsExpr Id -> DsM CoreExpr
dsExpr (HsPar e)              = dsLExpr e
dsExpr (ExprWithTySigOut e _) = dsLExpr e
dsExpr (HsVar (L _ var))      = return (varToCoreExpr var)
                                -- See Note [Desugaring vars]
dsExpr (HsUnboundVar {})      = panic "dsExpr: HsUnboundVar" -- Typechecker eliminates them
dsExpr (HsConLikeOut con)     = return (dsConLike con)
dsExpr (HsIPVar _)            = panic "dsExpr: HsIPVar"
dsExpr (HsOverLabel{})        = panic "dsExpr: HsOverLabel"
dsExpr (HsLit lit)            = dsLit lit
dsExpr (HsOverLit lit)        = dsOverLit lit

dsExpr (HsWrap co_fn e)
  = do { e' <- dsExpr e
       ; wrap' <- dsHsWrapper co_fn
       ; dflags <- getDynFlags
       ; let wrapped_e = wrap' e'
       ; warnAboutIdentities dflags e' (exprType wrapped_e)
       ; return wrapped_e }

dsExpr (NegApp (L loc (HsOverLit lit@(OverLit { ol_val = HsIntegral src i })))
                neg_expr)
  = do { expr' <- putSrcSpanDs loc $ do
          { dflags <- getDynFlags
          ; warnAboutOverflowedLiterals dflags
                                        (lit { ol_val = HsIntegral src (-i) })
          ; dsOverLit' dflags lit }
       ; dsSyntaxExpr neg_expr [expr'] }

dsExpr (NegApp expr neg_expr)
  = do { expr' <- dsLExpr expr
       ; dsSyntaxExpr neg_expr [expr'] }

dsExpr (HsLam a_Match)
  = uncurry mkLams <$> matchWrapper LambdaExpr Nothing a_Match

dsExpr (HsLamCase matches)
  = do { ([discrim_var], matching_code) <- matchWrapper CaseAlt Nothing matches
       ; return $ Lam discrim_var matching_code }

dsExpr e@(HsApp fun arg)
  = mkCoreAppDs (text "HsApp" <+> ppr e) <$> dsLExpr fun <*> dsLExprNoLP arg

dsExpr (HsAppTypeOut e _)
    -- ignore type arguments here; they're in the wrappers instead at this point
  = dsLExpr e


{-
Note [Desugaring vars]
~~~~~~~~~~~~~~~~~~~~~~
In one situation we can get a *coercion* variable in a HsVar, namely
the support method for an equality superclass:
   class (a~b) => C a b where ...
   instance (blah) => C (T a) (T b) where ..
Then we get
   $dfCT :: forall ab. blah => C (T a) (T b)
   $dfCT ab blah = MkC ($c$p1C a blah) ($cop a blah)

   $c$p1C :: forall ab. blah => (T a ~ T b)
   $c$p1C ab blah = let ...; g :: T a ~ T b = ... } in g

That 'g' in the 'in' part is an evidence variable, and when
converting to core it must become a CO.

Operator sections.  At first it looks as if we can convert
\begin{verbatim}
        (expr op)
\end{verbatim}
to
\begin{verbatim}
        \x -> op expr x
\end{verbatim}

But no!  expr might be a redex, and we can lose laziness badly this
way.  Consider
\begin{verbatim}
        map (expr op) xs
\end{verbatim}
for example.  So we convert instead to
\begin{verbatim}
        let y = expr in \x -> op y x
\end{verbatim}
If \tr{expr} is actually just a variable, say, then the simplifier
will sort it out.
-}

dsExpr e@(OpApp e1 op _ e2)
  = -- for the type of y, we need the type of op's 2nd argument
    mkCoreAppsDs (text "opapp" <+> ppr e) <$> dsLExpr op <*> mapM dsLExprNoLP [e1, e2]

dsExpr (SectionL expr op)       -- Desugar (e !) to ((!) e)
  = mkCoreAppDs (text "sectionl" <+> ppr expr) <$> dsLExpr op <*> dsLExprNoLP expr

-- dsLExpr (SectionR op expr)   -- \ x -> op x expr
dsExpr e@(SectionR op expr) = do
    core_op <- dsLExpr op
    -- for the type of x, we need the type of op's 2nd argument
    let (x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
        -- See comment with SectionL
    y_core <- dsLExpr expr
    x_id <- newSysLocalDsNoLP x_ty
    y_id <- newSysLocalDsNoLP y_ty
    return (bindNonRec y_id y_core $
            Lam x_id (mkCoreAppsDs (text "sectionr" <+> ppr e) core_op [Var x_id, Var y_id]))

dsExpr (ExplicitTuple tup_args boxity)
  = do { let go (lam_vars, args) (L _ (Missing ty))
                    -- For every missing expression, we need
                    -- another lambda in the desugaring.
               = do { lam_var <- newSysLocalDsNoLP ty
                    ; return (lam_var : lam_vars, Var lam_var : args) }
             go (lam_vars, args) (L _ (Present expr))
                    -- Expressions that are present don't generate
                    -- lambdas, just arguments.
               = do { core_expr <- dsLExpr expr
                    ; return (lam_vars, core_expr : args) }

       ; (lam_vars, args) <- foldM go ([], []) (reverse tup_args)
                -- The reverse is because foldM goes left-to-right

       ; return $ mkCoreLams lam_vars $
                  mkCoreTupBoxity boxity args }

dsExpr (ExplicitSum alt arity expr types)
  = do { core_expr <- dsLExpr expr
       ; return $ mkCoreConApps (sumDataCon alt arity)
                                (map (Type . getRuntimeRep "dsExpr ExplicitSum") types ++
                                 map Type types ++
                                 [core_expr]) }

dsExpr (HsSCC _ cc expr@(L loc _)) = do
    dflags <- getDynFlags
    if gopt Opt_SccProfilingOn dflags
      then do
        mod_name <- getModule
        count <- goptM Opt_ProfCountEntries
        uniq <- newUnique
        Tick (ProfNote (mkUserCC (sl_fs cc) mod_name loc uniq) count True)
               <$> dsLExpr expr
      else dsLExpr expr

dsExpr (HsCoreAnn _ _ expr)
  = dsLExpr expr

dsExpr (HsCase discrim matches)
  = do { core_discrim <- dsLExpr discrim
       ; ([discrim_var], matching_code) <- matchWrapper CaseAlt (Just discrim) matches
       ; return (bindNonRec discrim_var core_discrim matching_code) }

-- Pepe: The binds are in scope in the body but NOT in the binding group
--       This is to avoid silliness in breakpoints
dsExpr (HsLet binds body) = do
    body' <- dsLExpr body
    dsLocalBinds binds body'

-- We need the `ListComp' form to use `deListComp' (rather than the "do" form)
-- because the interpretation of `stmts' depends on what sort of thing it is.
--
dsExpr (HsDo ListComp     (L _ stmts) res_ty) = dsListComp stmts res_ty
dsExpr (HsDo PArrComp     (L _ stmts) _)      = dsPArrComp (map unLoc stmts)
dsExpr (HsDo DoExpr       (L _ stmts) _)      = dsDo stmts
dsExpr (HsDo GhciStmtCtxt (L _ stmts) _)      = dsDo stmts
dsExpr (HsDo MDoExpr      (L _ stmts) _)      = dsDo stmts
dsExpr (HsDo MonadComp    (L _ stmts) _)      = dsMonadComp stmts

dsExpr (HsIf mb_fun guard_expr then_expr else_expr)
  = do { pred <- dsLExpr guard_expr
       ; b1 <- dsLExpr then_expr
       ; b2 <- dsLExpr else_expr
       ; case mb_fun of
           Just fun -> dsSyntaxExpr fun [pred, b1, b2]
           Nothing  -> return $ mkIfThenElse pred b1 b2 }

dsExpr (HsMultiIf res_ty alts)
  | null alts
  = mkErrorExpr

  | otherwise
  = do { match_result <- liftM (foldr1 combineMatchResults)
                               (mapM (dsGRHS IfAlt res_ty) alts)
       ; error_expr   <- mkErrorExpr
       ; extractMatchResult match_result error_expr }
  where
    mkErrorExpr = mkErrorAppDs nON_EXHAUSTIVE_GUARDS_ERROR_ID res_ty
                               (text "multi-way if")

{-
\noindent
\underline{\bf Various data construction things}
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-}

dsExpr (ExplicitList elt_ty wit xs)
  = dsExplicitList elt_ty wit xs

-- We desugar [:x1, ..., xn:] as
--   singletonP x1 +:+ ... +:+ singletonP xn
--
dsExpr (ExplicitPArr ty []) = do
    emptyP <- dsDPHBuiltin emptyPVar
    return (Var emptyP `App` Type ty)
dsExpr (ExplicitPArr ty xs) = do
    singletonP <- dsDPHBuiltin singletonPVar
    appP       <- dsDPHBuiltin appPVar
    xs'        <- mapM dsLExprNoLP xs
    let unary  fn x   = mkApps (Var fn) [Type ty, x]
        binary fn x y = mkApps (Var fn) [Type ty, x, y]

    return . foldr1 (binary appP) $ map (unary singletonP) xs'

dsExpr (ArithSeq expr witness seq)
  = case witness of
     Nothing -> dsArithSeq expr seq
     Just fl -> do { newArithSeq <- dsArithSeq expr seq
                   ; dsSyntaxExpr fl [newArithSeq] }

dsExpr (PArrSeq expr (FromTo from to))
  = mkApps <$> dsExpr expr <*> mapM dsLExprNoLP [from, to]

dsExpr (PArrSeq expr (FromThenTo from thn to))
  = mkApps <$> dsExpr expr <*> mapM dsLExprNoLP [from, thn, to]

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

{-
Static Pointers
~~~~~~~~~~~~~~~

See Note [Grand plan for static forms] in StaticPtrTable for an overview.

    g = ... static f ...
==>
    g = ... makeStatic loc f ...
-}

dsExpr (HsStatic _ expr@(L loc _)) = do
    expr_ds <- dsLExprNoLP expr
    let ty = exprType expr_ds
    makeStaticId <- dsLookupGlobalId makeStaticName

    dflags <- getDynFlags
    let (line, col) = case loc of
           RealSrcSpan r -> ( srcLocLine $ realSrcSpanStart r
                            , srcLocCol  $ realSrcSpanStart r
                            )
           _             -> (0, 0)
        srcLoc = mkCoreConApps (tupleDataCon Boxed 2)
                     [ Type intTy              , Type intTy
                     , mkIntExprInt dflags line, mkIntExprInt dflags col
                     ]

    putSrcSpanDs loc $ return $
      mkCoreApps (Var makeStaticId) [ Type ty, srcLoc, expr_ds ]

{-
\noindent
\underline{\bf Record construction and update}
             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
For record construction we do this (assuming T has three arguments)
\begin{verbatim}
        T { op2 = e }
==>
        let err = /\a -> recConErr a
        T (recConErr t1 "M.hs/230/op1")
          e
          (recConErr t1 "M.hs/230/op3")
\end{verbatim}
@recConErr@ then converts its argument string into a proper message
before printing it as
\begin{verbatim}
        M.hs, line 230: missing field op1 was evaluated
\end{verbatim}

We also handle @C{}@ as valid construction syntax for an unlabelled
constructor @C@, setting all of @C@'s fields to bottom.
-}

dsExpr (RecordCon { rcon_con_expr = con_expr, rcon_flds = rbinds
                  , rcon_con_like = con_like })
  = do { con_expr' <- dsExpr con_expr
       ; let
             (arg_tys, _) = tcSplitFunTys (exprType con_expr')
             -- A newtype in the corner should be opaque;
             -- hence TcType.tcSplitFunTys

             mk_arg (arg_ty, fl)
               = case findField (rec_flds rbinds) (flSelector fl) of
                   (rhs:rhss) -> ASSERT( null rhss )
                                 dsLExprNoLP rhs
                   []         -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr (flLabel fl))
             unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty Outputable.empty

             labels = conLikeFieldLabels con_like

       ; con_args <- if null labels
                     then mapM unlabelled_bottom arg_tys
                     else mapM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels)

       ; return (mkCoreApps con_expr' con_args) }

{-
Record update is a little harder. Suppose we have the decl:
\begin{verbatim}
        data T = T1 {op1, op2, op3 :: Int}
               | T2 {op4, op2 :: Int}
               | T3
\end{verbatim}
Then we translate as follows:
\begin{verbatim}
        r { op2 = e }
===>
        let op2 = e in
        case r of
          T1 op1 _ op3 -> T1 op1 op2 op3
          T2 op4 _     -> T2 op4 op2
          other        -> recUpdError "M.hs/230"
\end{verbatim}
It's important that we use the constructor Ids for @T1@, @T2@ etc on the
RHSs, and do not generate a Core constructor application directly, because the constructor
might do some argument-evaluation first; and may have to throw away some
dictionaries.

Note [Update for GADTs]
~~~~~~~~~~~~~~~~~~~~~~~
Consider
   data T a b where
     T1 :: { f1 :: a } -> T a Int

Then the wrapper function for T1 has type
   $WT1 :: a -> T a Int
But if x::T a b, then
   x { f1 = v } :: T a b   (not T a Int!)
So we need to cast (T a Int) to (T a b).  Sigh.

-}

dsExpr expr@(RecordUpd { rupd_expr = record_expr, rupd_flds = fields
                       , rupd_cons = cons_to_upd
                       , rupd_in_tys = in_inst_tys, rupd_out_tys = out_inst_tys
                       , rupd_wrap = dict_req_wrap } )
  | null fields
  = dsLExpr record_expr
  | otherwise
  = ASSERT2( notNull cons_to_upd, ppr expr )

    do  { record_expr' <- dsLExpr record_expr
        ; field_binds' <- mapM ds_field fields
        ; let upd_fld_env :: NameEnv Id -- Maps field name to the LocalId of the field binding
              upd_fld_env = mkNameEnv [(f,l) | (f,l,_) <- field_binds']

        -- It's important to generate the match with matchWrapper,
        -- and the right hand sides with applications of the wrapper Id
        -- so that everything works when we are doing fancy unboxing on the
        -- constructor arguments.
        ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
        ; ([discrim_var], matching_code)
                <- matchWrapper RecUpd Nothing (MG { mg_alts = noLoc alts
                                                   , mg_arg_tys = [in_ty]
                                                   , mg_res_ty = out_ty, mg_origin = FromSource })
                                                   -- FromSource is not strictly right, but we
                                                   -- want incomplete pattern-match warnings

        ; return (add_field_binds field_binds' $
                  bindNonRec discrim_var record_expr' matching_code) }
  where
    ds_field :: LHsRecUpdField Id -> DsM (Name, Id, CoreExpr)
      -- Clone the Id in the HsRecField, because its Name is that
      -- of the record selector, and we must not make that a local binder
      -- else we shadow other uses of the record selector
      -- Hence 'lcl_id'.  Cf Trac #2735
    ds_field (L _ rec_field) = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
                                  ; let fld_id = unLoc (hsRecUpdFieldId rec_field)
                                  ; lcl_id <- newSysLocalDs (idType fld_id)
                                  ; return (idName fld_id, lcl_id, rhs) }

    add_field_binds [] expr = expr
    add_field_binds ((_,b,r):bs) expr = bindNonRec b r (add_field_binds bs expr)

        -- Awkwardly, for families, the match goes
        -- from instance type to family type
    (in_ty, out_ty) =
      case (head cons_to_upd) of
        RealDataCon data_con ->
          let tycon = dataConTyCon data_con in
          (mkTyConApp tycon in_inst_tys, mkFamilyTyConApp tycon out_inst_tys)
        PatSynCon pat_syn ->
          ( patSynInstResTy pat_syn in_inst_tys
          , patSynInstResTy pat_syn out_inst_tys)
    mk_alt upd_fld_env con
      = do { let (univ_tvs, ex_tvs, eq_spec,
                  prov_theta, _req_theta, arg_tys, _) = conLikeFullSig con
                 subst = zipTvSubst univ_tvs in_inst_tys

                -- I'm not bothering to clone the ex_tvs
           ; eqs_vars   <- mapM newPredVarDs (substTheta subst (eqSpecPreds eq_spec))
           ; theta_vars <- mapM newPredVarDs (substTheta subst prov_theta)
           ; arg_ids    <- newSysLocalsDs (substTysUnchecked subst arg_tys)
           ; let field_labels = conLikeFieldLabels con
                 val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
                                         field_labels arg_ids
                 mk_val_arg fl pat_arg_id
                     = nlHsVar (lookupNameEnv upd_fld_env (flSelector fl) `orElse` pat_arg_id)

                 inst_con = noLoc $ HsWrap wrap (HsConLikeOut con)
                        -- Reconstruct with the WrapId so that unpacking happens
                 -- The order here is because of the order in `TcPatSyn`.
                 wrap = mkWpEvVarApps theta_vars                                <.>
                        dict_req_wrap                                           <.>
                        mkWpTyApps    (mkTyVarTys ex_tvs)                       <.>
                        mkWpTyApps    [ ty
                                      | (tv, ty) <- univ_tvs `zip` out_inst_tys
                                      , not (tv `elemVarEnv` wrap_subst) ]
                 rhs = foldl (\a b -> nlHsApp a b) inst_con val_args

                        -- Tediously wrap the application in a cast
                        -- Note [Update for GADTs]
                 wrapped_rhs =
                  case con of
                    RealDataCon data_con ->
                      let
                        wrap_co =
                          mkTcTyConAppCo Nominal
                            (dataConTyCon data_con)
                            [ lookup tv ty
                              | (tv,ty) <- univ_tvs `zip` out_inst_tys ]
                        lookup univ_tv ty =
                          case lookupVarEnv wrap_subst univ_tv of
                            Just co' -> co'
                            Nothing  -> mkTcReflCo Nominal ty
                        in if null eq_spec
                             then rhs
                             else mkLHsWrap (mkWpCastN wrap_co) rhs
                    -- eq_spec is always null for a PatSynCon
                    PatSynCon _ -> rhs

                 wrap_subst =
                  mkVarEnv [ (tv, mkTcSymCo (mkTcCoVarCo eq_var))
                           | (spec, eq_var) <- eq_spec `zip` eqs_vars
                           , let tv = eqSpecTyVar spec ]

                 req_wrap = dict_req_wrap <.> mkWpTyApps in_inst_tys

                 pat = noLoc $ ConPatOut { pat_con = noLoc con
                                         , pat_tvs = ex_tvs
                                         , pat_dicts = eqs_vars ++ theta_vars
                                         , pat_binds = emptyTcEvBinds
                                         , pat_args = PrefixCon $ map nlVarPat arg_ids
                                         , pat_arg_tys = in_inst_tys
                                         , pat_wrap = req_wrap }
           ; return (mkSimpleMatch RecUpd [pat] wrapped_rhs) }

-- Here is where we desugar the Template Haskell brackets and escapes

-- Template Haskell stuff

dsExpr (HsRnBracketOut _ _) = panic "dsExpr HsRnBracketOut"
dsExpr (HsTcBracketOut x ps) = dsBracket x ps
dsExpr (HsSpliceE s)  = pprPanic "dsExpr:splice" (ppr s)

-- Arrow notation extension
dsExpr (HsProc pat cmd) = dsProcExpr pat cmd

-- Hpc Support

dsExpr (HsTick tickish e) = do
  e' <- dsLExpr e
  return (Tick tickish e')

-- There is a problem here. The then and else branches
-- have no free variables, so they are open to lifting.
-- We need someway of stopping this.
-- This will make no difference to binary coverage
-- (did you go here: YES or NO), but will effect accurate
-- tick counting.

dsExpr (HsBinTick ixT ixF e) = do
  e2 <- dsLExpr e
  do { ASSERT(exprType e2 `eqType` boolTy)
       mkBinaryTickBox ixT ixF e2
     }

dsExpr (HsTickPragma _ _ _ expr) = do
  dflags <- getDynFlags
  if gopt Opt_Hpc dflags
    then panic "dsExpr:HsTickPragma"
    else dsLExpr expr

-- HsSyn constructs that just shouldn't be here:
dsExpr (ExprWithTySig {})  = panic "dsExpr:ExprWithTySig"
dsExpr (HsBracket     {})  = panic "dsExpr:HsBracket"
dsExpr (HsArrApp      {})  = panic "dsExpr:HsArrApp"
dsExpr (HsArrForm     {})  = panic "dsExpr:HsArrForm"
dsExpr (EWildPat      {})  = panic "dsExpr:EWildPat"
dsExpr (EAsPat        {})  = panic "dsExpr:EAsPat"
dsExpr (EViewPat      {})  = panic "dsExpr:EViewPat"
dsExpr (ELazyPat      {})  = panic "dsExpr:ELazyPat"
dsExpr (HsAppType     {})  = panic "dsExpr:HsAppType" -- removed by typechecker
dsExpr (HsDo          {})  = panic "dsExpr:HsDo"
dsExpr (HsRecFld      {})  = panic "dsExpr:HsRecFld"

------------------------------
dsSyntaxExpr :: SyntaxExpr Id -> [CoreExpr] -> DsM CoreExpr
dsSyntaxExpr (SyntaxExpr { syn_expr      = expr
                         , syn_arg_wraps = arg_wraps
                         , syn_res_wrap  = res_wrap })
             arg_exprs
  = do { fun            <- dsExpr expr
       ; core_arg_wraps <- mapM dsHsWrapper arg_wraps
       ; core_res_wrap  <- dsHsWrapper res_wrap
       ; let wrapped_args = zipWith ($) core_arg_wraps arg_exprs
       ; zipWithM_ dsNoLevPolyExpr wrapped_args [ mk_doc n | n <- [1..] ]
       ; return (core_res_wrap (mkApps fun wrapped_args)) }
  where
    mk_doc n = text "In the" <+> speakNth n <+> text "argument of" <+> quotes (ppr expr)

findField :: [LHsRecField Id arg] -> Name -> [arg]
findField rbinds sel
  = [hsRecFieldArg fld | L _ fld <- rbinds
                       , sel == idName (unLoc $ hsRecFieldId fld) ]

{-
%--------------------------------------------------------------------

Note [Desugaring explicit lists]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Explicit lists are desugared in a cleverer way to prevent some
fruitless allocations.  Essentially, whenever we see a list literal
[x_1, ..., x_n] we generate the corresponding expression in terms of
build:

Explicit lists (literals) are desugared to allow build/foldr fusion when
beneficial. This is a bit of a trade-off,

 * build/foldr fusion can generate far larger code than the corresponding
   cons-chain (e.g. see #11707)

 * even when it doesn't produce more code, build can still fail to fuse,
   requiring that the simplifier do more work to bring the expression
   back into cons-chain form; this costs compile time

 * when it works, fusion can be a significant win. Allocations are reduced
   by up to 25% in some nofib programs. Specifically,

        Program           Size    Allocs   Runtime  CompTime
        rewrite          +0.0%    -26.3%      0.02     -1.8%
           ansi          -0.3%    -13.8%      0.00     +0.0%
           lift          +0.0%     -8.7%      0.00     -2.3%

At the moment we use a simple heuristic to determine whether build will be
fruitful: for small lists we assume the benefits of fusion will be worthwhile;
for long lists we assume that the benefits will be outweighted by the cost of
code duplication. This magic length threshold is @maxBuildLength@. Also, fusion
won't work at all if rewrite rules are disabled, so we don't use the build-based
desugaring in this case.

We used to have a more complex heuristic which would try to break the list into
"static" and "dynamic" parts and only build-desugar the dynamic part.
Unfortunately, determining "static-ness" reliably is a bit tricky and the
heuristic at times produced surprising behavior (see #11710) so it was dropped.
-}

{- | The longest list length which we will desugar using @build@.

This is essentially a magic number and its setting is unfortunate rather
arbitrary. The idea here, as mentioned in Note [Desugaring explicit lists],
is to avoid deforesting large static data into large(r) code. Ideally we'd
want a smaller threshold with larger consumers and vice-versa, but we have no
way of knowing what will be consuming our list in the desugaring impossible to
set generally correctly.

The effect of reducing this number will be that 'build' fusion is applied
less often. From a runtime performance perspective, applying 'build' more
liberally on "moderately" sized lists should rarely hurt and will often it can
only expose further optimization opportunities; if no fusion is possible it will
eventually get rule-rewritten back to a list). We do, however, pay in compile
time.
-}
maxBuildLength :: Int
maxBuildLength = 32

dsExplicitList :: Type -> Maybe (SyntaxExpr Id) -> [LHsExpr Id]
               -> DsM CoreExpr
-- See Note [Desugaring explicit lists]
dsExplicitList elt_ty Nothing xs
  = do { dflags <- getDynFlags
       ; xs' <- mapM dsLExprNoLP xs
       ; if length xs' > maxBuildLength
                -- Don't generate builds if the list is very long.
         || length xs' == 0
                -- Don't generate builds when the [] constructor will do
         || not (gopt Opt_EnableRewriteRules dflags)  -- Rewrite rules off
                -- Don't generate a build if there are no rules to eliminate it!
                -- See Note [Desugaring RULE left hand sides] in Desugar