Minor refactoring

[ghc-hetmet.git] / compiler / deSugar / DsExpr.lhs

diff --git a/compiler/deSugar/DsExpr.lhs b/compiler/deSugar/DsExpr.lhs
index 0633717..11fddf5 100644 (file)
--- a/compiler/deSugar/DsExpr.lhs
+++ b/compiler/deSugar/DsExpr.lhs
@@ -26,9 +26,9 @@ import DsUtils
 import DsArrows
 import DsMonad
 import Name
 import DsArrows
 import DsMonad
 import Name

+import NameEnv

 
 #ifdef GHCI
 
 #ifdef GHCI

-import PrelNames

        -- Template Haskell stuff iff bootstrapped
 import DsMeta
 #endif
        -- Template Haskell stuff iff bootstrapped
 import DsMeta
 #endif

@@ -40,22 +40,31 @@ import TcHsSyn
 --     needs to see source types
 import TcType
 import Type
 --     needs to see source types
 import TcType
 import Type

+import Coercion

 import CoreSyn
 import CoreUtils
 import CoreSyn
 import CoreUtils

+import CoreFVs
+import MkCore

 
 import DynFlags
 
 import DynFlags

+import StaticFlags

 import CostCentre
 import Id
 import CostCentre
 import Id

+import Var
+import VarSet

 import PrelInfo
 import DataCon
 import TysWiredIn
 import BasicTypes
 import PrelNames
 import PrelInfo
 import DataCon
 import TysWiredIn
 import BasicTypes
 import PrelNames

+import Maybes

 import SrcLoc
 import Util
 import Bag
 import Outputable
 import FastString
 import SrcLoc
 import Util
 import Bag
 import Outputable
 import FastString

+
+import Control.Monad

 \end{code}
 
 
 \end{code}
 
 

@@ -202,7 +211,9 @@ dsExpr (HsVar var)                = return (Var var)
 dsExpr (HsIPVar ip)                  = return (Var (ipNameName ip))
 dsExpr (HsLit lit)                   = dsLit lit
 dsExpr (HsOverLit lit)               = dsOverLit lit
 dsExpr (HsIPVar ip)                  = return (Var (ipNameName ip))
 dsExpr (HsLit lit)                   = dsLit lit
 dsExpr (HsOverLit lit)               = dsOverLit lit

-dsExpr (HsWrap co_fn e)       = dsCoercion co_fn (dsExpr e)
+dsExpr (HsWrap co_fn e)       = do { co_fn' <- dsCoercion co_fn
+                                   ; e' <- dsExpr e
+                                   ; return (co_fn' e') }

 
 dsExpr (NegApp expr neg_expr) 
   = App <$> dsExpr neg_expr <*> dsLExpr expr
 
 dsExpr (NegApp expr neg_expr) 
   = App <$> dsExpr neg_expr <*> dsLExpr expr

@@ -211,7 +222,7 @@ dsExpr (HsLam a_Match)
   = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
 
 dsExpr (HsApp fun arg)
   = uncurry mkLams <$> matchWrapper LambdaExpr a_Match
 
 dsExpr (HsApp fun arg)

-  = mkDsApp <$> dsLExpr fun <*>  dsLExpr arg
+  = mkCoreAppDs <$> dsLExpr fun <*>  dsLExpr arg

 \end{code}
 
 Operator sections.  At first it looks as if we can convert
 \end{code}
 
 Operator sections.  At first it looks as if we can convert

@@ -238,10 +249,10 @@ will sort it out.
 \begin{code}
 dsExpr (OpApp e1 op _ e2)
   = -- for the type of y, we need the type of op's 2nd argument
 \begin{code}
 dsExpr (OpApp e1 op _ e2)
   = -- for the type of y, we need the type of op's 2nd argument

-    mkDsApps <$> dsLExpr op <*> mapM dsLExpr [e1, e2]
+    mkCoreAppsDs <$> dsLExpr op <*> mapM dsLExpr [e1, e2]

     
 dsExpr (SectionL expr op)      -- Desugar (e !) to ((!) e)
     
 dsExpr (SectionL expr op)      -- Desugar (e !) to ((!) e)

-  = mkDsApp <$> dsLExpr op <*> dsLExpr expr
+  = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr

 
 -- dsLExpr (SectionR op expr)  -- \ x -> op x expr
 dsExpr (SectionR op expr) = do
 
 -- dsLExpr (SectionR op expr)  -- \ x -> op x expr
 dsExpr (SectionR op expr) = do

@@ -253,7 +264,26 @@ dsExpr (SectionR op expr) = do
     x_id <- newSysLocalDs x_ty
     y_id <- newSysLocalDs y_ty
     return (bindNonRec y_id y_core $
     x_id <- newSysLocalDs x_ty
     y_id <- newSysLocalDs y_ty
     return (bindNonRec y_id y_core $

-            Lam x_id (mkDsApps core_op [Var x_id, Var y_id]))
+            Lam x_id (mkCoreAppsDs core_op [Var x_id, Var y_id]))
+
+dsExpr (ExplicitTuple tup_args boxity)
+  = do { let go (lam_vars, args) (Missing ty)
+                    -- For every missing expression, we need
+                   -- another lambda in the desugaring.
+               = do { lam_var <- newSysLocalDs ty
+                    ; return (lam_var : lam_vars, Var lam_var : args) }
+            go (lam_vars, args) (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 $ 
+                  mkConApp (tupleCon boxity (length tup_args))
+                           (map (Type . exprType) args ++ args) }

 
 dsExpr (HsSCC cc expr) = do
     mod_name <- getModuleDs
 
 dsExpr (HsSCC cc expr) = do
     mod_name <- getModuleDs

@@ -265,10 +295,15 @@ dsExpr (HsSCC cc expr) = do
 dsExpr (HsCoreAnn fs expr)
   = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
 
 dsExpr (HsCoreAnn fs expr)
   = Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
 

-dsExpr (HsCase discrim matches) = do
-    core_discrim <- dsLExpr discrim
-    ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
-    return (scrungleMatch discrim_var core_discrim matching_code)
+dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty)) 
+  | isEmptyMatchGroup matches  -- A Core 'case' is always non-empty
+  =                            -- So desugar empty HsCase to error call
+    mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
+
+  | otherwise
+  = do { core_discrim <- dsLExpr discrim
+       ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
+       ; return (scrungleMatch 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
 
 -- Pepe: The binds are in scope in the body but NOT in the binding group
 --       This is to avoid silliness in breakpoints

@@ -309,25 +344,20 @@ dsExpr (HsIf guard_expr then_expr else_expr)
 dsExpr (ExplicitList elt_ty xs) 
   = dsExplicitList elt_ty xs
 
 dsExpr (ExplicitList elt_ty xs) 
   = dsExplicitList elt_ty 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
+-- We desugar [:x1, ..., xn:] as
+--   singletonP x1 +:+ ... +:+ singletonP xn

 --
 --

+dsExpr (ExplicitPArr ty []) = do
+    emptyP <- dsLookupGlobalId emptyPName
+    return (Var emptyP `App` Type ty)

 dsExpr (ExplicitPArr ty xs) = do
 dsExpr (ExplicitPArr ty xs) = do

-    toP <- dsLookupGlobalId toPName
-    coreList <- dsExpr (ExplicitList ty xs)
-    return (mkApps (Var toP) [Type ty, coreList])
-
-dsExpr (ExplicitTuple expr_list boxity) = do
-    core_exprs <- mapM dsLExpr expr_list
-    return (mkConApp (tupleCon boxity (length expr_list))
-                  (map (Type .  exprType) core_exprs ++ core_exprs))
+    singletonP <- dsLookupGlobalId singletonPName
+    appP       <- dsLookupGlobalId appPName
+    xs'        <- mapM dsLExpr xs
+    return . foldr1 (binary appP) $ map (unary singletonP) xs'
+  where
+    unary  fn x   = mkApps (Var fn) [Type ty, x]
+    binary fn x y = mkApps (Var fn) [Type ty, x, y]

 
 dsExpr (ArithSeq expr (From from))
   = App <$> dsExpr expr <*> dsLExpr from
 
 dsExpr (ArithSeq expr (From from))
   = App <$> dsExpr expr <*> dsLExpr from

@@ -386,8 +416,8 @@ dsExpr (RecordCon (L _ data_con_id) con_expr rbinds) = do
           = case findField (rec_flds rbinds) lbl of
               (rhs:rhss) -> ASSERT( null rhss )
                             dsLExpr rhs
           = case findField (rec_flds rbinds) lbl 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 ""
+              []         -> mkErrorAppDs rEC_CON_ERROR_ID arg_ty (ppr lbl)
+        unlabelled_bottom arg_ty = mkErrorAppDs rEC_CON_ERROR_ID arg_ty empty

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

@@ -420,52 +450,101 @@ RHSs, and do not generate a Core constructor application directly, because the c
 might do some argument-evaluation first; and may have to throw away some
 dictionaries.
 
 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.
+

 \begin{code}
 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
                       cons_to_upd in_inst_tys out_inst_tys)
   | null fields
   = dsLExpr record_expr
   | otherwise
 \begin{code}
 dsExpr expr@(RecordUpd record_expr (HsRecFields { rec_flds = fields })
                       cons_to_upd in_inst_tys out_inst_tys)
   | null fields
   = dsLExpr record_expr
   | otherwise

-  =    -- 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( notNull cons_to_upd && all isVanillaDataCon cons_to_upd, ppr expr )
+  = ASSERT2( notNull cons_to_upd, ppr expr )

 
     do { record_expr' <- dsLExpr record_expr
 
     do { record_expr' <- dsLExpr record_expr

-       ; let   -- Awkwardly, for families, the match goes 
-               -- from instance type to family type
-               tycon     = dataConTyCon (head cons_to_upd)
-               in_ty     = mkTyConApp tycon in_inst_tys
-               in_out_ty = mkFunTy in_ty
-                                   (mkFamilyTyConApp tycon out_inst_tys)
-
-               mk_val_arg field old_arg_id 
-                 = case findField fields field  of
-                     (rhs:rest) -> ASSERT(null rest) rhs
-                     []         -> nlHsVar old_arg_id
-
-               mk_alt con
-                 = ASSERT( isVanillaDataCon con )
-                   do  { arg_ids <- newSysLocalsDs (dataConInstOrigArgTys con in_inst_tys)
-                       -- 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)
-                                         (nlHsTyApp (dataConWrapId con) out_inst_tys)
-                                         val_args
-                             pat = mkPrefixConPat con (map nlVarPat arg_ids) in_ty
-
-                       ; return (mkSimpleMatch [pat] rhs) }
+       ; 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 aguments.
 
        -- 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.

-       ; alts <- mapM mk_alt cons_to_upd
-       ; ([discrim_var], matching_code) <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
+       ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd
+       ; ([discrim_var], matching_code) 
+               <- matchWrapper RecUpd (MatchGroup alts in_out_ty)
+
+       ; return (add_field_binds field_binds' $
+                 bindNonRec discrim_var record_expr' matching_code) }
+  where
+    ds_field :: HsRecField Id (LHsExpr 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 lcoal binder
+      -- else we shadow other uses of the record selector
+      -- Hence 'lcl_id'.  Cf Trac #2735
+    ds_field rec_field = do { rhs <- dsLExpr (hsRecFieldArg rec_field)
+                           ; let fld_id = unLoc (hsRecFieldId 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
+    tycon     = dataConTyCon (head cons_to_upd)
+    in_ty     = mkTyConApp tycon in_inst_tys
+    in_out_ty = mkFunTy in_ty (mkFamilyTyConApp tycon out_inst_tys)
+
+    mk_alt upd_fld_env con
+      = do { let (univ_tvs, ex_tvs, eq_spec, 
+                 eq_theta, dict_theta, arg_tys, _) = dataConFullSig con
+                subst = mkTopTvSubst (univ_tvs `zip` 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 (eq_theta ++ dict_theta))
+          ; arg_ids    <- newSysLocalsDs (substTys subst arg_tys)
+          ; let val_args = zipWithEqual "dsExpr:RecordUpd" mk_val_arg
+                                        (dataConFieldLabels con) arg_ids
+                 mk_val_arg field_name pat_arg_id 
+                     = nlHsVar (lookupNameEnv upd_fld_env field_name `orElse` pat_arg_id)
+                inst_con = noLoc $ HsWrap wrap (HsVar (dataConWrapId con))
+                       -- Reconstruct with the WrapId so that unpacking happens
+                wrap = mkWpApps theta_vars `WpCompose` 
+                       mkWpTyApps (mkTyVarTys ex_tvs) `WpCompose`
+                       mkWpTyApps [ty | (tv, ty) <- univ_tvs `zip` out_inst_tys
+                                      , isNothing (lookupTyVar wrap_subst tv) ]
+                rhs = foldl (\a b -> nlHsApp a b) inst_con val_args
+
+                       -- Tediously wrap the application in a cast
+                       -- Note [Update for GADTs]
+                wrapped_rhs | null eq_spec = rhs
+                            | otherwise    = mkLHsWrap (WpCast wrap_co) rhs
+                wrap_co = mkTyConApp tycon [ lookup tv ty 
+                                           | (tv,ty) <- univ_tvs `zip` out_inst_tys]
+                lookup univ_tv ty = case lookupTyVar wrap_subst univ_tv of
+                                       Just ty' -> ty'
+                                       Nothing  -> ty
+                wrap_subst = mkTopTvSubst [ (tv,mkSymCoercion (mkTyVarTy co_var))
+                                          | ((tv,_),co_var) <- eq_spec `zip` eqs_vars ]
+                
+                pat = noLoc $ ConPatOut { pat_con = noLoc con, pat_tvs = ex_tvs
+                                        , pat_dicts = eqs_vars ++ theta_vars
+                                        , pat_binds = emptyLHsBinds 
+                                        , pat_args = PrefixCon $ map nlVarPat arg_ids
+                                        , pat_ty = in_ty }
+          ; return (mkSimpleMatch [pat] wrapped_rhs) }

 
 

-       ; return (bindNonRec discrim_var record_expr' matching_code) }

 \end{code}
 
 Here is where we desugar the Template Haskell brackets and escapes
 \end{code}
 
 Here is where we desugar the Template Haskell brackets and escapes

@@ -505,10 +584,8 @@ dsExpr (HsBinTick ixT ixF e) = do
 
 \begin{code}
 
 
 \begin{code}
 

-#ifdef DEBUG

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

-#endif

 
 
 findField :: [HsRecField Id arg] -> Name -> [arg]
 
 
 findField :: [HsRecField Id arg] -> Name -> [arg]

@@ -541,31 +618,60 @@ fruitless allocations.  Essentially, whenever we see a list literal
    
    If fusion fails to occur then build will get inlined and (since we
    defined a RULE for foldr (:) []) we will get back exactly the
    
    If fusion fails to occur then build will get inlined and (since we
    defined a RULE for foldr (:) []) we will get back exactly the

-   normal desugaring for an explicit list! However, if it does occur
-   then we can potentially save quite a bit of allocation (up to 25\%
-   of the total in some nofib programs!)
+   normal desugaring for an explicit list.
+
+This optimisation can be worth a lot: up to 25% of the total
+allocation 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%

 
 Of course, if rules aren't turned on then there is pretty much no
 point doing this fancy stuff, and it may even be harmful.
 
 
 Of course, if rules aren't turned on then there is pretty much no
 point doing this fancy stuff, and it may even be harmful.
 

-\begin{code}
+=======>  Note by SLPJ Dec 08.

 
 

+I'm unconvinced that we should *ever* generate a build for an explicit
+list.  See the comments in GHC.Base about the foldr/cons rule, which 
+points out that (foldr k z [a,b,c]) may generate *much* less code than
+(a `k` b `k` c `k` z).
+
+Furthermore generating builds messes up the LHS of RULES. 
+Example: the foldr/single rule in GHC.Base
+   foldr k z [x] = ...
+We do not want to generate a build invocation on the LHS of this RULE!
+
+To test this I've added a (static) flag -fsimple-list-literals, which
+makes all list literals be generated via the simple route.  
+
+
+\begin{code}

 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
 -- See Note [Desugaring explicit lists]
 dsExplicitList :: PostTcType -> [LHsExpr Id] -> DsM CoreExpr
 -- See Note [Desugaring explicit lists]

-dsExplicitList elt_ty xs = do
-    dflags <- getDOptsDs
-    xs' <- mapM dsLExpr xs
-    if not (dopt Opt_RewriteRules dflags)
-        then return $ mkListExpr elt_ty xs'
-        else mkBuildExpr elt_ty (mkSplitExplicitList (thisPackage dflags) xs')
+dsExplicitList elt_ty xs
+  = do { dflags <- getDOptsDs
+       ; xs' <- mapM dsLExpr xs
+       ; let (dynamic_prefix, static_suffix) = spanTail is_static xs'
+       ; if opt_SimpleListLiterals                     -- -fsimple-list-literals
+         || not (dopt Opt_EnableRewriteRules dflags)   -- Rewrite rules off
+         || null dynamic_prefix   -- Avoid build (\c n. foldr c n xs)!
+         then return $ mkListExpr elt_ty xs'
+         else mkBuildExpr elt_ty (mkSplitExplicitList dynamic_prefix static_suffix) }

   where
   where

-    mkSplitExplicitList this_package xs' (c, _) (n, n_ty) = do
-        let (dynamic_prefix, static_suffix) = spanTail (rhsIsStatic this_package) xs'
-            static_suffix' = mkListExpr elt_ty static_suffix
-        
-        folded_static_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) static_suffix'
-        let build_body = foldr (App . App (Var c)) folded_static_suffix dynamic_prefix
-        return build_body
+    is_static :: CoreExpr -> Bool
+    is_static e = all is_static_var (varSetElems (exprFreeVars e))
+
+    is_static_var :: Var -> Bool
+    is_static_var v 
+      | isId v = isExternalName (idName v)  -- Top-level things are given external names
+      | otherwise = False                   -- Type variables
+
+    mkSplitExplicitList prefix suffix (c, _) (n, n_ty)
+      = do { let suffix' = mkListExpr elt_ty suffix
+           ; folded_suffix <- mkFoldrExpr elt_ty n_ty (Var c) (Var n) suffix'
+           ; return (foldr (App . App (Var c)) folded_suffix prefix) }

 
 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
 spanTail f xs = (reverse rejected, reverse satisfying)
 
 spanTail :: (a -> Bool) -> [a] -> ([a], [a])
 spanTail f xs = (reverse rejected, reverse satisfying)

@@ -582,34 +688,69 @@ dsDo      :: [LStmt Id]
        -> Type                 -- Type of the whole expression
        -> DsM CoreExpr
 
        -> Type                 -- Type of the whole expression
        -> DsM CoreExpr
 

-dsDo stmts body _result_ty
-  = go (map unLoc stmts)
+dsDo stmts body result_ty
+  = goL stmts

   where
   where

-    go [] = dsLExpr body
-    
-    go (ExprStmt rhs then_expr _ : stmts)
+    -- result_ty must be of the form (m b)
+    (m_ty, _b_ty) = tcSplitAppTy result_ty
+
+    goL [] = dsLExpr body
+    goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
+  
+    go _ (ExprStmt rhs then_expr _) stmts

       = do { rhs2 <- dsLExpr rhs
       = do { rhs2 <- dsLExpr rhs

-          ; then_expr2 <- dsExpr then_expr
-          ; rest <- go stmts
+           ; case tcSplitAppTy_maybe (exprType rhs2) of
+                Just (container_ty, returning_ty) -> warnDiscardedDoBindings rhs container_ty returning_ty
+                _                                 -> return ()
+           ; then_expr2 <- dsExpr then_expr
+          ; rest <- goL stmts

           ; return (mkApps then_expr2 [rhs2, rest]) }
     
           ; return (mkApps then_expr2 [rhs2, rest]) }
     

-    go (LetStmt binds : stmts)
-      = do { rest <- go stmts
+    go _ (LetStmt binds) stmts
+      = do { rest <- goL stmts

           ; dsLocalBinds binds rest }
 
           ; dsLocalBinds binds rest }
 

-    go (BindStmt pat rhs bind_op fail_op : stmts)
-      = 
-       do  { body     <- go stmts
-           ; rhs'     <- dsLExpr rhs
-          ; bind_op' <- dsExpr bind_op
-          ; var   <- selectSimpleMatchVarL pat
-          ; let bind_ty = exprType bind_op'    -- rhs -> (pat -> res1) -> res2
-                res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
-          ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
-                                    res1_ty (cantFailMatchResult body)
-          ; match_code <- handle_failure pat match fail_op
-          ; return (mkApps bind_op' [rhs', Lam var match_code]) }
+    go _ (BindStmt pat rhs bind_op fail_op) stmts
+      = do  { body     <- goL stmts
+            ; rhs'     <- dsLExpr rhs
+           ; bind_op' <- dsExpr bind_op
+           ; var   <- selectSimpleMatchVarL pat
+           ; let bind_ty = exprType bind_op'   -- rhs -> (pat -> res1) -> res2
+                 res1_ty = funResultTy (funArgTy (funResultTy bind_ty))
+           ; match <- matchSinglePat (Var var) (StmtCtxt DoExpr) pat
+                                     res1_ty (cantFailMatchResult body)
+           ; match_code <- handle_failure pat match fail_op
+           ; return (mkApps bind_op' [rhs', Lam var match_code]) }

     
     

+    go loc (RecStmt { recS_stmts = rec_stmts, recS_later_ids = later_ids
+                    , recS_rec_ids = rec_ids, recS_ret_fn = return_op
+                    , recS_mfix_fn = mfix_op, recS_bind_fn = bind_op
+                    , recS_rec_rets = rec_rets, recS_dicts = binds }) stmts 
+      = ASSERT( length rec_ids > 0 )
+        goL (new_bind_stmt : let_stmt : stmts)
+      where
+        -- returnE <- dsExpr return_id
+        -- mfixE <- dsExpr mfix_id
+        new_bind_stmt = L loc $ BindStmt (mkLHsPatTup later_pats) mfix_app
+                                         bind_op 
+                                            noSyntaxExpr  -- Tuple cannot fail
+
+        let_stmt = L loc $ LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))
+
+        tup_ids      = rec_ids ++ filterOut (`elem` rec_ids) later_ids
+        rec_tup_pats = map nlVarPat tup_ids
+        later_pats   = rec_tup_pats
+        rets         = map noLoc rec_rets
+
+        mfix_app   = nlHsApp (noLoc mfix_op) mfix_arg
+        mfix_arg   = noLoc $ HsLam (MatchGroup [mkSimpleMatch [mfix_pat] body]
+                                             (mkFunTy tup_ty body_ty))
+        mfix_pat   = noLoc $ LazyPat $ mkLHsPatTup rec_tup_pats
+        body       = noLoc $ HsDo DoExpr rec_stmts return_app body_ty
+        return_app = nlHsApp (noLoc return_op) (mkLHsTupleExpr rets)
+       body_ty    = mkAppTy m_ty tup_ty
+        tup_ty     = mkBoxedTupleTy (map idType tup_ids) -- Deals with singleton case
+

     -- In a do expression, pattern-match failure just calls
     -- the monadic 'fail' rather than throwing an exception
     handle_failure pat match fail_op
     -- In a do expression, pattern-match failure just calls
     -- the monadic 'fail' rather than throwing an exception
     handle_failure pat match fail_op

@@ -640,8 +781,11 @@ dsMDo      :: PostTcTable
        -> DsM CoreExpr
 
 dsMDo tbl stmts body result_ty
        -> DsM CoreExpr
 
 dsMDo tbl stmts body result_ty

-  = go (map unLoc stmts)
+  = goL stmts

   where
   where

+    goL [] = dsLExpr body
+    goL ((L loc stmt):lstmts) = putSrcSpanDs loc (go loc stmt lstmts)
+  

     (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
     (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

@@ -650,19 +794,18 @@ dsMDo tbl stmts body result_ty
     fail_id   = lookupEvidence tbl failMName
     ctxt      = MDoExpr tbl
 
     fail_id   = lookupEvidence tbl failMName
     ctxt      = MDoExpr tbl
 

-    go [] = dsLExpr body
-    
-    go (LetStmt binds : stmts)
-      = do { rest <- go stmts
+    go _ (LetStmt binds) stmts
+      = do { rest <- goL stmts

           ; dsLocalBinds binds rest }
 
           ; dsLocalBinds binds rest }
 

-    go (ExprStmt rhs _ rhs_ty : stmts)
+    go _ (ExprStmt rhs _ rhs_ty) stmts

       = do { rhs2 <- dsLExpr rhs
       = do { rhs2 <- dsLExpr rhs

-          ; rest <- go stmts
+          ; warnDiscardedDoBindings rhs m_ty rhs_ty
+           ; rest <- goL stmts

           ; return (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
     
           ; return (mkApps (Var then_id) [Type rhs_ty, Type b_ty, rhs2, rest]) }
     

-    go (BindStmt pat rhs _ _ : stmts)
-      = do { body  <- go stmts
+    go _ (BindStmt pat rhs _ _) stmts
+      = do { body  <- goL stmts

           ; var   <- selectSimpleMatchVarL pat
           ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
                                  result_ty (cantFailMatchResult body)
           ; var   <- selectSimpleMatchVarL pat
           ; match <- matchSinglePat (Var var) (StmtCtxt ctxt) pat
                                  result_ty (cantFailMatchResult body)

@@ -674,13 +817,14 @@ dsMDo tbl stmts body result_ty
           ; return (mkApps (Var bind_id) [Type (hsLPatType pat), Type b_ty, 
                                             rhs', Lam var match_code]) }
     
           ; return (mkApps (Var bind_id) [Type (hsLPatType pat), Type b_ty, 
                                             rhs', Lam var match_code]) }
     

-    go (RecStmt rec_stmts later_ids rec_ids rec_rets binds : stmts)
+    go loc (RecStmt rec_stmts later_ids rec_ids _ _ _ rec_rets binds) stmts

       = ASSERT( length rec_ids > 0 )
         ASSERT( length rec_ids == length rec_rets )
       = ASSERT( length rec_ids > 0 )
         ASSERT( length rec_ids == length rec_rets )

-       go (new_bind_stmt : let_stmt : stmts)
+        pprTrace "dsMDo" (ppr later_ids) $
+        goL (new_bind_stmt : let_stmt : stmts)

       where
       where

-        new_bind_stmt = mkBindStmt (mk_tup_pat later_pats) mfix_app
-       let_stmt = LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))
+        new_bind_stmt = L loc $ mkBindStmt (mk_tup_pat later_pats) mfix_app
+       let_stmt = L loc $ LetStmt (HsValBinds (ValBindsOut [(Recursive, binds)] []))

 
        
                -- Remove the later_ids that appear (without fancy coercions) 
 
        
                -- Remove the later_ids that appear (without fancy coercions) 

@@ -703,11 +847,10 @@ dsMDo tbl stmts body result_ty
        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
        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
+       tup_ty  = mkBoxedTupleTy (map idType (later_ids' ++ rec_ids))  -- Deals with singleton case

 
        return_app  = nlHsApp (nlHsTyApp return_id [tup_ty]) 
 
        return_app  = nlHsApp (nlHsTyApp return_id [tup_ty]) 

-                             (mk_ret_tup rets)
+                             (mkLHsTupleExpr rets)

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

@@ -719,8 +862,41 @@ dsMDo tbl stmts body result_ty
        mk_tup_pat :: [LPat Id] -> LPat Id
        mk_tup_pat [p] = p
        mk_tup_pat ps  = noLoc $ mkVanillaTuplePat ps Boxed
        mk_tup_pat :: [LPat Id] -> LPat Id
        mk_tup_pat [p] = p
        mk_tup_pat ps  = noLoc $ mkVanillaTuplePat ps Boxed

+\end{code}
+

 
 

-       mk_ret_tup :: [LHsExpr Id] -> LHsExpr Id
-       mk_ret_tup [r] = r
-       mk_ret_tup rs  = noLoc $ ExplicitTuple rs Boxed
+%************************************************************************
+%*                                                                     *
+\subsection{Errors and contexts}
+%*                                                                     *
+%************************************************************************
+
+\begin{code}
+-- Warn about certain types of values discarded in monadic bindings (#3263)
+warnDiscardedDoBindings :: LHsExpr Id -> Type -> Type -> DsM ()
+warnDiscardedDoBindings rhs container_ty returning_ty = do {
+          -- Warn about discarding non-() things in 'monadic' binding
+        ; warn_unused <- doptDs Opt_WarnUnusedDoBind
+        ; if warn_unused && not (returning_ty `tcEqType` unitTy)
+           then warnDs (unusedMonadBind rhs returning_ty)
+           else do {
+          -- Warn about discarding m a things in 'monadic' binding of the same type,
+          -- but only if we didn't already warn due to Opt_WarnUnusedDoBind
+        ; warn_wrong <- doptDs Opt_WarnWrongDoBind
+        ; case tcSplitAppTy_maybe returning_ty of
+                  Just (returning_container_ty, _) -> when (warn_wrong && container_ty `tcEqType` returning_container_ty) $
+                                                            warnDs (wrongMonadBind rhs returning_ty)
+                  _ -> return () } }
+
+unusedMonadBind :: LHsExpr Id -> Type -> SDoc
+unusedMonadBind rhs returning_ty
+  = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
+    ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
+    ptext (sLit "or by using the flag -fno-warn-unused-do-bind")
+
+wrongMonadBind :: LHsExpr Id -> Type -> SDoc
+wrongMonadBind rhs returning_ty
+  = ptext (sLit "A do-notation statement discarded a result of type") <+> ppr returning_ty <> dot $$
+    ptext (sLit "Suppress this warning by saying \"_ <- ") <> ppr rhs <> ptext (sLit "\",") $$
+    ptext (sLit "or by using the flag -fno-warn-wrong-do-bind")

 \end{code}
 \end{code}