path: root/ghc/compiler/deSugar/DsExpr.lhs

%
% (c) The GRASP/AQUA Project, Glasgow University, 1992-1998
%
\section[DsExpr]{Matching expressions (Exprs)}

\begin{code}
module DsExpr ( dsExpr, dsLExpr, dsLocalBinds, dsValBinds, dsLit ) where

#include "HsVersions.h"
#if defined(GHCI) && defined(BREAKPOINT)
import Foreign.StablePtr ( newStablePtr, castStablePtrToPtr )
import GHC.Exts         ( Ptr(..), Int(..), addr2Int# )
import IOEnv            ( ioToIOEnv )
import PrelNames        ( breakpointJumpName )
import TysWiredIn       ( unitTy )
import TypeRep          ( Type(..) )
#endif

import Match		( matchWrapper, matchSinglePat, matchEquations )
import MatchLit		( dsLit, dsOverLit )
import DsBinds		( dsLHsBinds, dsCoercion )
import DsGRHSs		( dsGuarded )
import DsListComp	( dsListComp, dsPArrComp )
import DsUtils		( mkErrorAppDs, mkStringExpr, mkConsExpr, mkNilExpr,
			  extractMatchResult, cantFailMatchResult, matchCanFail,
			  mkCoreTupTy, selectSimpleMatchVarL, lookupEvidence, selectMatchVar )
import DsArrows		( dsProcExpr )
import DsMonad

#ifdef GHCI
	-- Template Haskell stuff iff bootstrapped
import DsMeta		( dsBracket )
#endif

import HsSyn
import TcHsSyn		( hsPatType, mkVanillaTuplePat )

-- NB: The desugarer, which straddles the source and Core worlds, sometimes
--     needs to see source types (newtypes etc), and sometimes not
--     So WATCH OUT; check each use of split*Ty functions.
-- Sigh.  This is a pain.

import TcType		( tcSplitAppTy, tcSplitFunTys, tcTyConAppTyCon, 
			  tcTyConAppArgs, isUnLiftedType, Type, mkAppTy )
import Type		( funArgTy, splitFunTys, isUnboxedTupleType, mkFunTy )
import CoreSyn
import CoreUtils	( exprType, mkIfThenElse, bindNonRec )

import CostCentre	( mkUserCC )
import Id		( Id, idType, idName, idDataCon )
import PrelInfo		( rEC_CON_ERROR_ID, iRREFUT_PAT_ERROR_ID )
import DataCon		( DataCon, dataConWrapId, dataConFieldLabels, dataConInstOrigArgTys )
import DataCon		( isVanillaDataCon )
import TyCon		( FieldLabel, tyConDataCons )
import TysWiredIn	( tupleCon )
import BasicTypes	( RecFlag(..), Boxity(..), ipNameName )
import PrelNames	( toPName,
			  returnMName, bindMName, thenMName, failMName,
			  mfixName )
import SrcLoc		( Located(..), unLoc, getLoc, noLoc )
import Util		( zipEqual, zipWithEqual )
import Bag		( bagToList )
import Outputable
import FastString
\end{code}


%************************************************************************
%*									*
		dsLocalBinds, dsValBinds
%*									*
%************************************************************************

\begin{code}
dsLocalBinds :: HsLocalBinds Id -> CoreExpr -> DsM CoreExpr
dsLocalBinds EmptyLocalBinds	body = return body
dsLocalBinds (HsValBinds binds) body = dsValBinds binds body
dsLocalBinds (HsIPBinds binds)  body = dsIPBinds  binds body

-------------------------
dsValBinds :: HsValBinds Id -> CoreExpr -> DsM CoreExpr
dsValBinds (ValBindsOut binds _) body = foldrDs ds_val_bind body binds

-------------------------
dsIPBinds (IPBinds ip_binds dict_binds) body
  = do	{ prs <- dsLHsBinds dict_binds
	; let inner = foldr (\(x,r) e -> Let (NonRec x r) e) body prs 
	; foldrDs ds_ip_bind inner ip_binds }
  where
    ds_ip_bind (L _ (IPBind n e)) body
      = dsLExpr e	`thenDs` \ e' ->
	returnDs (Let (NonRec (ipNameName n) e') body)

-------------------------
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 _ (AbsBinds [] [] exports binds)] <- bagToList hsbinds,
    (L loc bind : null_binds) <- bagToList binds,
    isBangHsBind bind
    || isUnboxedTupleBind bind
    || or [isUnLiftedType (idType g) | (_, g, _, _) <- exports]
  = let
      body_w_exports		      = foldr bind_export body exports
      bind_export (tvs, g, l, _) body = ASSERT( null tvs )
				        bindNonRec g (Var l) body
    in
    ASSERT (null null_binds)
	-- 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.)
    putSrcSpanDs loc	$
    case bind of
      FunBind { fun_id = L _ fun, fun_matches = matches, fun_co_fn = co_fn }
	-> matchWrapper (FunRhs (idName fun)) matches 	 	`thenDs` \ (args, rhs) ->
	   ASSERT( null args )	-- Functions aren't lifted
	   ASSERT( isIdCoercion co_fn )
	   returnDs (bindNonRec fun rhs body_w_exports)

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

      other -> pprPanic "dsLet: unlifted" (pprLHsBinds hsbinds $$ ppr body)


-- Ordinary case for bindings; none should be unlifted
ds_val_bind (is_rec, binds) body
  = do	{ prs <- dsLHsBinds binds
	; ASSERT( not (any (isUnLiftedType . idType . fst) prs) )
	  case prs of
	    []    -> return body
	    other -> 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

isUnboxedTupleBind :: HsBind Id -> Bool
isUnboxedTupleBind (PatBind { pat_rhs_ty = ty }) = isUnboxedTupleType ty
isUnboxedTupleBind other			 = False

scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
-- Returns something like (let var = scrut in body)
-- but if var is an unboxed-tuple type, it inlines it in a fragile way
-- Special case to handle unboxed tuple patterns; they can't appear nested
-- The idea is that 
--	case e of (# p1, p2 #) -> rhs
-- should desugar to
--	case e of (# x1, x2 #) -> ... match p1, p2 ...
-- NOT
--	let x = e in case x of ....
--
-- But there may be a big 
--	let fail = ... in case e of ...
-- wrapping the whole case, which complicates matters slightly
-- It all seems a bit fragile.  Test is dsrun013.

scrungleMatch var scrut body
  | isUnboxedTupleType (idType var) = scrungle body
  | otherwise			    = bindNonRec var scrut body
  where
    scrungle (Case (Var x) bndr ty alts)
		    | x == var = Case scrut bndr ty alts
    scrungle (Let binds body)  = Let binds (scrungle body)
    scrungle other = panic ("scrungleMatch: tuple pattern:\n" ++ showSDoc (ppr other))
\end{code}	

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

\begin{code}
dsLExpr :: LHsExpr Id -> DsM CoreExpr
dsLExpr (L loc e) = putSrcSpanDs loc $ dsExpr e

dsExpr :: HsExpr Id -> DsM CoreExpr

dsExpr (HsPar e) 	      = dsLExpr e
dsExpr (ExprWithTySigOut e _) = dsLExpr e
dsExpr (HsVar var)     	      = returnDs (Var var)
dsExpr (HsIPVar ip)    	      = returnDs (Var (ipNameName ip))
dsExpr (HsLit lit)     	      = dsLit lit
dsExpr (HsOverLit lit) 	      = dsOverLit lit

dsExpr (NegApp expr neg_expr) 
  = do	{ core_expr <- dsLExpr expr
	; core_neg  <- dsExpr neg_expr
	; return (core_neg `App` core_expr) }

dsExpr expr@(HsLam a_Match)
  = matchWrapper LambdaExpr a_Match	`thenDs` \ (binders, matching_code) ->
    returnDs (mkLams binders matching_code)

#if defined(GHCI) && defined(BREAKPOINT)
dsExpr (HsApp (L _ (HsApp realFun@(L _ (HsCoerce _ fun)) (L loc arg))) _)
    | HsVar funId <- fun
    , idName funId == breakpointJumpName
    , ids <- filter (not.hasTyVar.idType) (extractIds arg)
    = do dsWarn (text "Extracted ids:" <+> ppr ids <+> ppr (map idType ids))
         stablePtr <- ioToIOEnv $ newStablePtr ids
         -- Yes, I know... I'm gonna burn in hell.
         let Ptr addr# = castStablePtrToPtr stablePtr
         funCore <- dsLExpr realFun
         argCore <- dsLExpr (L loc (HsLit (HsInt (fromIntegral (I# (addr2Int# addr#))))))
         hvalCore <- dsLExpr (L loc (extractHVals ids))
         return ((funCore `App` argCore) `App` hvalCore)
    where extractIds :: HsExpr Id -> [Id]
          extractIds (HsApp fn arg)
              | HsVar argId <- unLoc arg
              = argId:extractIds (unLoc fn)
              | TyApp arg' ts <- unLoc arg
              , HsVar argId <- unLoc arg'
              = error (showSDoc (ppr ts)) -- argId:extractIds (unLoc fn)
          extractIds x = []
          extractHVals ids = ExplicitList unitTy (map (L loc . HsVar) ids)
          hasTyVar (TyVarTy _) = True
          hasTyVar (FunTy a b) = hasTyVar a || hasTyVar b
          hasTyVar (NoteTy _ t) = hasTyVar t
          hasTyVar (AppTy a b) = hasTyVar a || hasTyVar b
          hasTyVar (TyConApp _ ts) = any hasTyVar ts
          hasTyVar _ = False
#endif

dsExpr expr@(HsApp fun arg)      
  = dsLExpr fun		`thenDs` \ core_fun ->
    dsLExpr arg		`thenDs` \ core_arg ->
    returnDs (core_fun `App` core_arg)
\end{code}

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.

\begin{code}
dsExpr (OpApp e1 op _ e2)
  = dsLExpr op						`thenDs` \ core_op ->
    -- for the type of y, we need the type of op's 2nd argument
    dsLExpr e1				`thenDs` \ x_core ->
    dsLExpr e2				`thenDs` \ y_core ->
    returnDs (mkApps core_op [x_core, y_core])
    
dsExpr (SectionL expr op)
  = dsLExpr op						`thenDs` \ core_op ->
    -- for the type of y, we need the type of op's 2nd argument
    let
	(x_ty:y_ty:_, _) = splitFunTys (exprType core_op)
	-- Must look through an implicit-parameter type; 
	-- newtype impossible; hence Type.splitFunTys
    in
    dsLExpr expr				`thenDs` \ x_core ->
    newSysLocalDs x_ty			`thenDs` \ x_id ->
    newSysLocalDs y_ty			`thenDs` \ y_id ->

    returnDs (bindNonRec x_id x_core $
	      Lam y_id (mkApps core_op [Var x_id, Var y_id]))

-- dsLExpr (SectionR op expr)	-- \ x -> op x expr
dsExpr (SectionR op expr)
  = dsLExpr op			`thenDs` \ core_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
    in
    dsLExpr expr				`thenDs` \ y_core ->
    newSysLocalDs x_ty			`thenDs` \ x_id ->
    newSysLocalDs y_ty			`thenDs` \ y_id ->

    returnDs (bindNonRec y_id y_core $
	      Lam x_id (mkApps core_op [Var x_id, Var y_id]))

dsExpr (HsSCC cc expr)
  = dsLExpr expr			`thenDs` \ core_expr ->
    getModuleDs			`thenDs` \ mod_name ->
    returnDs (Note (SCC (mkUserCC cc mod_name)) core_expr)


-- hdaume: core annotation

dsExpr (HsCoreAnn fs expr)
  = dsLExpr expr        `thenDs` \ core_expr ->
    returnDs (Note (CoreNote $ unpackFS fs) core_expr)

dsExpr (HsCase discrim matches)
  = dsLExpr discrim			`thenDs` \ core_discrim ->
    matchWrapper CaseAlt matches 	`thenDs` \ ([discrim_var], matching_code) ->
    returnDs (scrungleMatch discrim_var core_discrim matching_code)

dsExpr (HsLet binds body)
  = dsLExpr body		`thenDs` \ 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 stmts body result_ty)
  =	-- Special case for list comprehensions
    dsListComp stmts body elt_ty
  where
    [elt_ty] = tcTyConAppArgs result_ty

dsExpr (HsDo DoExpr stmts body result_ty)
  = dsDo stmts body result_ty

dsExpr (HsDo (MDoExpr tbl) stmts body result_ty)
  = dsMDo tbl stmts body result_ty

dsExpr (HsDo PArrComp stmts body result_ty)
  =	-- Special case for array comprehensions
    dsPArrComp (map unLoc stmts) body elt_ty
  where
    [elt_ty] = tcTyConAppArgs result_ty

dsExpr (HsIf guard_expr then_expr else_expr)
  = dsLExpr guard_expr	`thenDs` \ core_guard ->
    dsLExpr then_expr	`thenDs` \ core_then ->
    dsLExpr else_expr	`thenDs` \ core_else ->
    returnDs (mkIfThenElse core_guard core_then core_else)
\end{code}


\noindent
\underline{\bf Type lambda and application}
%              ~~~~~~~~~~~~~~~~~~~~~~~~~~~
\begin{code}
dsExpr (TyLam tyvars expr)
  = dsLExpr expr `thenDs` \ core_expr ->
    returnDs (mkLams tyvars core_expr)

dsExpr (TyApp expr tys)
  = dsLExpr expr		`thenDs` \ core_expr ->
    returnDs (mkTyApps core_expr tys)
\end{code}


\noindent
\underline{\bf Various data construction things}
%              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\begin{code}
dsExpr (ExplicitList ty xs)
  = go xs
  where
    go []     = returnDs (mkNilExpr ty)
    go (x:xs) = dsLExpr x				`thenDs` \ core_x ->
		go xs					`thenDs` \ core_xs ->
		returnDs (mkConsExpr ty core_x core_xs)

-- we create a list from the array elements and convert them into a list using
-- `PrelPArr.toP'
--
--  * the main disadvantage to this scheme is that `toP' traverses the list
--   twice: once to determine the length and a second time to put to elements
--   into the array; this inefficiency could be avoided by exposing some of
--   the innards of `PrelPArr' to the compiler (ie, have a `PrelPArrBase') so
--   that we can exploit the fact that we already know the length of the array
--   here at compile time
--
dsExpr (ExplicitPArr ty xs)
  = dsLookupGlobalId toPName				`thenDs` \toP      ->
    dsExpr (ExplicitList ty xs)				`thenDs` \coreList ->
    returnDs (mkApps (Var toP) [Type ty, coreList])

dsExpr (ExplicitTuple expr_list boxity)
  = mappM dsLExpr expr_list	  `thenDs` \ core_exprs  ->
    returnDs (mkConApp (tupleCon boxity (length expr_list))
	    	       (map (Type .  exprType) core_exprs ++ core_exprs))

dsExpr (ArithSeq expr (From from))
  = dsExpr expr		  `thenDs` \ expr2 ->
    dsLExpr from	  `thenDs` \ from2 ->
    returnDs (App expr2 from2)

dsExpr (ArithSeq expr (FromTo from two))
  = dsExpr expr		  `thenDs` \ expr2 ->
    dsLExpr from	  `thenDs` \ from2 ->
    dsLExpr two		  `thenDs` \ two2 ->
    returnDs (mkApps expr2 [from2, two2])

dsExpr (ArithSeq expr (FromThen from thn))
  = dsExpr expr		  `thenDs` \ expr2 ->
    dsLExpr from	  `thenDs` \ from2 ->
    dsLExpr thn		  `thenDs` \ thn2 ->
    returnDs (mkApps expr2 [from2, thn2])

dsExpr (ArithSeq expr (FromThenTo from thn two))
  = dsExpr expr		  `thenDs` \ expr2 ->
    dsLExpr from	  `thenDs` \ from2 ->
    dsLExpr thn		  `thenDs` \ thn2 ->
    dsLExpr two		  `thenDs` \ two2 ->
    returnDs (mkApps expr2 [from2, thn2, two2])

dsExpr (PArrSeq expr (FromTo from two))
  = dsExpr expr		  `thenDs` \ expr2 ->
    dsLExpr from	  `thenDs` \ from2 ->
    dsLExpr two		  `thenDs` \ two2 ->
    returnDs (mkApps expr2 [from2, two2])

dsExpr (PArrSeq expr (FromThenTo from thn two))
  = dsExpr expr		  `thenDs` \ expr2 ->
    dsLExpr from	  `thenDs` \ from2 ->
    dsLExpr thn		  `thenDs` \ thn2 ->
    dsLExpr two		  `thenDs` \ two2 ->
    returnDs (mkApps expr2 [from2, thn2, two2])

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

\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.lhs/230/op1") 
	  e 
	  (recConErr t1 "M.lhs/230/op3")
\end{verbatim}
@recConErr@ then converts its arugment string into a proper message
before printing it as
\begin{verbatim}
	M.lhs, 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.

\begin{code}
dsExpr (RecordCon (L _ data_con_id) con_expr rbinds)
  = dsExpr con_expr	`thenDs` \ con_expr' ->
    let
	(arg_tys, _) = tcSplitFunTys (exprType con_expr')
	-- A newtype in the corner should be opaque; 
	-- hence TcType.tcSplitFunTys

	mk_arg (arg_ty, lbl)	-- Selector id has the field label as its name
	  = case [rhs | (L _ sel_id, rhs) <- rbinds, lbl == idName sel_id] of
	      (rhs:rhss) -> ASSERT( null rhss )
		 	    dsLExpr rhs
	      []         -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (showSDoc (ppr lbl))
	unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty ""

	labels = dataConFieldLabels (idDataCon data_con_id)
	-- The data_con_id is guaranteed to be the wrapper id of the constructor
    in

    (if null labels
	then mappM unlabelled_bottom arg_tys
	else mappM mk_arg (zipEqual "dsExpr:RecordCon" arg_tys labels))
	`thenDs` \ con_args ->

    returnDs (mkApps con_expr' con_args)
\end{code}

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.lhs/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.

\begin{code}
dsExpr (RecordUpd record_expr [] record_in_ty record_out_ty)
  = dsLExpr record_expr

dsExpr expr@(RecordUpd record_expr rbinds record_in_ty record_out_ty)
  = dsLExpr record_expr	 	`thenDs` \ record_expr' ->

	-- Desugar the rbinds, and generate let-bindings if
	-- necessary so that we don't lose sharing

    let
	in_inst_tys  = tcTyConAppArgs record_in_ty	-- Newtype opaque
	out_inst_tys = tcTyConAppArgs record_out_ty	-- Newtype opaque
	in_out_ty    = mkFunTy record_in_ty record_out_ty

	mk_val_arg field old_arg_id 
	  = case [rhs | (L _ sel_id, rhs) <- rbinds, field == idName sel_id] of
	      (rhs:rest) -> ASSERT(null rest) rhs
	      []	 -> nlHsVar old_arg_id

	mk_alt con
	  = newSysLocalsDs (dataConInstOrigArgTys con in_inst_tys) `thenDs` \ arg_ids ->
		-- This call to dataConInstOrigArgTys won't work for existentials
		-- but existentials don't have record types anyway
	    let 
		val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
					(dataConFieldLabels con) arg_ids
		rhs = foldl (\a b -> nlHsApp a b)
			(noLoc $ TyApp (nlHsVar (dataConWrapId con)) 
				out_inst_tys)
			  val_args
	    in
	    returnDs (mkSimpleMatch [noLoc $ ConPatOut (noLoc con) [] [] emptyLHsBinds 
						       (PrefixCon (map nlVarPat arg_ids)) record_in_ty]
				    rhs)
    in
	-- Record stuff doesn't work for existentials
	-- The type checker checks for this, but we need 
	-- worry only about the constructors that are to be updated
    ASSERT2( all isVanillaDataCon cons_to_upd, ppr expr )

	-- 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 aguments.
    mappM mk_alt cons_to_upd				`thenDs` \ alts ->
    matchWrapper RecUpd (MatchGroup alts in_out_ty)	`thenDs` \ ([discrim_var], matching_code) ->

    returnDs (bindNonRec discrim_var record_expr' matching_code)

  where
    updated_fields :: [FieldLabel]
    updated_fields = [ idName sel_id | (L _ sel_id,_) <- rbinds]

	-- Get the type constructor from the record_in_ty
	-- so that we are sure it'll have all its DataCons
	-- (In GHCI, it's possible that some TyCons may not have all
	--  their constructors, in a module-loop situation.)
    tycon       = tcTyConAppTyCon record_in_ty
    data_cons   = tyConDataCons tycon
    cons_to_upd = filter has_all_fields data_cons

    has_all_fields :: DataCon -> Bool
    has_all_fields con_id 
      = all (`elem` con_fields) updated_fields
      where
	con_fields = dataConFieldLabels con_id
\end{code}


\noindent
\underline{\bf Dictionary lambda and application}
%              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
@DictLam@ and @DictApp@ turn into the regular old things.
(OLD:) @DictFunApp@ also becomes a curried application, albeit slightly more
complicated; reminiscent of fully-applied constructors.
\begin{code}
dsExpr (DictLam dictvars expr)
  = dsLExpr expr `thenDs` \ core_expr ->
    returnDs (mkLams dictvars core_expr)

------------------

dsExpr (DictApp expr dicts)	-- becomes a curried application
  = dsLExpr expr			`thenDs` \ core_expr ->
    returnDs (foldl (\f d -> f `App` (Var d)) core_expr dicts)

dsExpr (HsCoerce co_fn e) = dsCoercion co_fn (dsExpr e)
\end{code}

Here is where we desugar the Template Haskell brackets and escapes

\begin{code}
-- Template Haskell stuff

#ifdef GHCI	/* Only if bootstrapping */
dsExpr (HsBracketOut x ps) = dsBracket x ps
dsExpr (HsSpliceE s)       = pprPanic "dsExpr:splice" (ppr s)
#endif

-- Arrow notation extension
dsExpr (HsProc pat cmd) = dsProcExpr pat cmd
\end{code}


\begin{code}

#ifdef DEBUG
-- HsSyn constructs that just shouldn't be here:
dsExpr (ExprWithTySig _ _)  = panic "dsExpr:ExprWithTySig"
#endif

\end{code}

%--------------------------------------------------------------------

Desugar 'do' and 'mdo' expressions (NOT list comprehensions, they're
handled in DsListComp).  Basically does the translation given in the
Haskell 98 report:

\begin{code}
dsDo	:: [LStmt Id]
	-> LHsExpr Id
	-> Type			-- Type of the whole expression
	-> DsM CoreExpr

dsDo stmts body result_ty
  = go (map unLoc stmts)
  where
    go [] = dsLExpr body
    
    go (ExprStmt rhs then_expr _ : stmts)
      = do { rhs2 <- dsLExpr rhs
	   ; then_expr2 <- dsExpr then_expr
	   ; rest <- go stmts
	   ; returnDs (mkApps then_expr2 [rhs2, rest]) }
    
    go (LetStmt binds : stmts)
      = do { rest <- go stmts
	   ; dsLocalBinds binds rest }
        
    go (BindStmt pat rhs bind_op fail_op : stmts)
      = do { body  <- go stmts
	   ; var   <- selectSimpleMatchVarL pat
	   ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
    				  result_ty (cantFailMatchResult body)
	   ; match_code <- handle_failure pat match fail_op
	   ; rhs'       <- dsLExpr rhs
	   ; bind_op'   <- dsExpr bind_op
	   ; returnDs (mkApps bind_op' [rhs', Lam var match_code]) }
    
    -- In a do expression, pattern-match failure just calls
    -- the monadic 'fail' rather than throwing an exception
    handle_failure pat match fail_op
      | matchCanFail match
      = do { fail_op' <- dsExpr fail_op
	   ; fail_msg <- mkStringExpr (mk_fail_msg pat)
    	   ; extractMatchResult match (App fail_op' fail_msg) }
      | otherwise
      = extractMatchResult match (error "It can't fail") 

mk_fail_msg pat = "Pattern match failure in do expression at " ++ 
		  showSDoc (ppr (getLoc pat))
\end{code}

Translation for RecStmt's: 
-----------------------------
We turn (RecStmt [v1,..vn] stmts) into:
  
  (v1,..,vn) <- mfix (\~(v1,..vn). do stmts
				      return (v1,..vn))

\begin{code}
dsMDo	:: PostTcTable
	-> [LStmt Id]
	-> LHsExpr Id
	-> Type			-- Type of the whole expression
	-> DsM CoreExpr

dsMDo tbl stmts body result_ty
  = go (map unLoc stmts)
  where
    (m_ty, b_ty) = tcSplitAppTy result_ty	-- result_ty must be of the form (m b)
    mfix_id   = lookupEvidence tbl mfixName
    return_id = lookupEvidence tbl returnMName
    bind_id   = lookupEvidence tbl bindMName
    then_id   = lookupEvidence tbl thenMName
    fail_id   = lookupEvidence tbl failMName
    ctxt      = MDoExpr tbl

    go [] = dsLExpr body
    
    go (LetStmt binds : stmts)
      = do { rest <- go stmts
	   ; dsLocalBinds binds rest }

    go (ExprStmt rhs _ rhs_ty : stmts)
      = do { rhs2 <- dsLExpr rhs
	   ; rest <- go stmts
	   ; returnDs (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
    
    go (BindStmt pat rhs _ _ : stmts)
      = do { body  <- go stmts
	   ; var   <- selectSimpleMatchVarL pat
	   ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
    				  result_ty (cantFailMatchResult body)
	   ; fail_msg   <- mkStringExpr (mk_fail_msg pat)
	   ; let fail_expr = mkApps (Var fail_id) [Type b_ty, fail_msg]
	   ; match_code <- extractMatchResult match fail_expr

	   ; rhs'       <- dsLExpr rhs
	   ; returnDs (mkApps (Var bind_id) [Type (hsPatType pat), Type b_ty, 
					     rhs', Lam var match_code]) }
    
    go (RecStmt rec_stmts later_ids rec_ids rec_rets binds : stmts)
      = ASSERT( length rec_ids > 0 )
        ASSERT( length rec_ids == length rec_rets )
	go (new_bind_stmt : let_stmt : stmts)
      where
        new_bind_stmt = mkBindStmt (mk_tup_pat later_pats) mfix_app
	let_stmt = LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))

	
		-- Remove the later_ids that appear (without fancy coercions) 
		-- in rec_rets, because there's no need to knot-tie them separately
		-- See Note [RecStmt] in HsExpr
	later_ids'   = filter (`notElem` mono_rec_ids) later_ids
	mono_rec_ids = [ id | HsVar id <- rec_rets ]
    
	mfix_app = nlHsApp (noLoc $ TyApp (nlHsVar mfix_id) [tup_ty]) mfix_arg
	mfix_arg = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
					     (mkFunTy tup_ty body_ty))

	-- The rec_tup_pat must bind the rec_ids only; remember that the 
	-- 	trimmed_laters may share the same Names
	-- Meanwhile, the later_pats must bind the later_vars
	rec_tup_pats = map mk_wild_pat later_ids' ++ map nlVarPat rec_ids
	later_pats   = map nlVarPat    later_ids' ++ map mk_later_pat rec_ids
	rets         = map nlHsVar     later_ids' ++ map noLoc rec_rets

	mfix_pat = noLoc $ LazyPat $ mk_tup_pat rec_tup_pats
	body     = noLoc $ HsDo ctxt rec_stmts return_app body_ty
	body_ty = mkAppTy m_ty tup_ty
	tup_ty  = mkCoreTupTy (map idType (later_ids' ++ rec_ids))
		  -- mkCoreTupTy deals with singleton case

	return_app  = nlHsApp (noLoc $ TyApp (nlHsVar return_id) [tup_ty]) 
			      (mk_ret_tup rets)

	mk_wild_pat :: Id -> LPat Id 
   	mk_wild_pat v = noLoc $ WildPat $ idType v

	mk_later_pat :: Id -> LPat Id
	mk_later_pat v | v `elem` later_ids' = mk_wild_pat v
		       | otherwise	     = nlVarPat v

 	mk_tup_pat :: [LPat Id] -> LPat Id
  	mk_tup_pat [p] = p
	mk_tup_pat ps  = noLoc $ mkVanillaTuplePat ps Boxed

	mk_ret_tup :: [LHsExpr Id] -> LHsExpr Id
	mk_ret_tup [r] = r
	mk_ret_tup rs  = noLoc $ ExplicitTuple rs Boxed
\end{code}