Fix Trac #3813: unused variables in GHCi bindings

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

diff --git a/compiler/deSugar/DsExpr.lhs b/compiler/deSugar/DsExpr.lhs
index b91380d..ef69b47 100644 (file)
--- a/compiler/deSugar/DsExpr.lhs
+++ b/compiler/deSugar/DsExpr.lhs
@@ -29,7 +29,6 @@ import Name
 import NameEnv
 
 #ifdef GHCI
 import NameEnv
 
 #ifdef GHCI

-import PrelNames

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

@@ -44,11 +43,15 @@ import Type
 import Coercion
 import CoreSyn
 import CoreUtils
 import Coercion
 import CoreSyn
 import CoreUtils

+import CoreFVs

 import MkCore
 
 import DynFlags
 import MkCore
 
 import DynFlags

+import StaticFlags

 import CostCentre
 import Id
 import CostCentre
 import Id

+import Var
+import VarSet

 import PrelInfo
 import DataCon
 import TysWiredIn
 import PrelInfo
 import DataCon
 import TysWiredIn

@@ -60,6 +63,8 @@ import Util
 import Bag
 import Outputable
 import FastString
 import Bag
 import Outputable
 import FastString

+
+import Control.Monad

 \end{code}
 
 
 \end{code}
 
 

@@ -206,7 +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

@@ -215,7 +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)

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

@@ -242,10 +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

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

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

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

@@ -257,7 +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 (mkCoreApps 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

@@ -272,7 +298,7 @@ dsExpr (HsCoreAnn fs expr)
 dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty)) 
   | isEmptyMatchGroup matches  -- A Core 'case' is always non-empty
   =                            -- So desugar empty HsCase to error call
 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) "case"
+    mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))

 
   | otherwise
   = do { core_discrim <- dsLExpr discrim
 
   | otherwise
   = do { core_discrim <- dsLExpr discrim

@@ -297,6 +323,9 @@ dsExpr (HsDo ListComp stmts body result_ty)
 dsExpr (HsDo DoExpr stmts body result_ty)
   = dsDo stmts body result_ty
 
 dsExpr (HsDo DoExpr stmts body result_ty)
   = dsDo stmts body result_ty
 

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

@@ -333,11 +362,6 @@ dsExpr (ExplicitPArr ty xs) = do
     unary  fn x   = mkApps (Var fn) [Type ty, x]
     binary fn x y = mkApps (Var fn) [Type ty, x, y]
 
     unary  fn x   = mkApps (Var fn) [Type ty, x]
     binary fn x y = mkApps (Var fn) [Type ty, x, y]
 

-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))
-

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

@@ -395,8 +419,8 @@ 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

@@ -609,24 +633,53 @@ allocation in some nofib programs. Specifically
 
 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!
+
+We fix this by disabling rules in rule LHSs, and testing that
+flag here; see Note [Desugaring RULE left hand sides] in Desugar
+
+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_EnableRewriteRules 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
+               -- Don't generate a build if there are no rules to eliminate it!
+               -- See Note [Desugaring RULE left hand sides] in Desugar
+         || 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)

@@ -643,34 +696,69 @@ 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

@@ -701,8 +789,11 @@ 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

@@ -711,19 +802,18 @@ 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)

@@ -735,13 +825,14 @@ 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) 

@@ -764,11 +855,10 @@ 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

@@ -780,8 +870,41 @@ 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}
+
+
+%************************************************************************
+%*                                                                     *
+\subsection{Errors and contexts}
+%*                                                                     *
+%************************************************************************

 
 

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