Desugaring exporessions.
\begin{code}
-{-# OPTIONS -w #-}
+{-# OPTIONS -fno-warn-incomplete-patterns #-}
-- The above warning supression flag is a temporary kludge.
-- While working on this module you are encouraged to remove it and fix
-- any warnings in the module. See
#include "HsVersions.h"
-
import Match
import MatchLit
import DsBinds
import DsArrows
import DsMonad
import Name
+import NameEnv
#ifdef GHCI
-import PrelNames
-- Template Haskell stuff iff bootstrapped
import DsMeta
#endif
-- needs to see source types
import TcType
import Type
+import Coercion
import CoreSyn
import CoreUtils
+import CoreFVs
+import MkCore
+import DynFlags
+import StaticFlags
import CostCentre
import Id
+import Var
+import VarSet
import PrelInfo
import DataCon
import TysWiredIn
import BasicTypes
import PrelNames
+import Maybes
import SrcLoc
import Util
import Bag
import Outputable
import FastString
+
+import Control.Monad
\end{code}
dsValBinds (ValBindsOut binds _) body = foldrM ds_val_bind body binds
-------------------------
+dsIPBinds :: HsIPBinds Id -> CoreExpr -> DsM CoreExpr
dsIPBinds (IPBinds ip_binds dict_binds) body
= do { prs <- dsLHsBinds dict_binds
; let inner = Let (Rec prs) body
; result <- matchEquations PatBindRhs [var] [eqn] (exprType body)
; return (scrungleMatch var rhs result) }
- other -> pprPanic "dsLet: unlifted" (pprLHsBinds hsbinds $$ ppr body)
+ _ -> pprPanic "dsLet: unlifted" (pprLHsBinds hsbinds $$ ppr body)
-- Ordinary case for bindings; none should be unlifted
-ds_val_bind (is_rec, binds) body
+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) }
+ [] -> 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
isUnboxedTupleBind :: HsBind Id -> Bool
isUnboxedTupleBind (PatBind { pat_rhs_ty = ty }) = isUnboxedTupleType ty
-isUnboxedTupleBind other = False
+isUnboxedTupleBind _ = False
scrungleMatch :: Id -> CoreExpr -> CoreExpr -> CoreExpr
-- Returns something like (let var = scrut in body)
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 <$> dsExpr neg_expr <*> dsLExpr expr
-dsExpr expr@(HsLam a_Match)
+dsExpr (HsLam a_Match)
= uncurry mkLams <$> matchWrapper LambdaExpr a_Match
-dsExpr expr@(HsApp fun arg)
- = mkDsApp <$> dsLExpr fun <*> dsLExpr arg
+dsExpr (HsApp fun arg)
+ = mkCoreAppDs <$> dsLExpr fun <*> dsLExpr arg
\end{code}
Operator sections. At first it looks as if we can convert
\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)
- = mkDsApp <$> dsLExpr op <*> dsLExpr expr
+ = mkCoreAppDs <$> dsLExpr op <*> dsLExpr expr
-- dsLExpr (SectionR op expr) -- \ x -> op x expr
dsExpr (SectionR op expr) = do
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 (HsCoreAnn fs expr)
= Note (CoreNote $ unpackFS fs) <$> dsLExpr expr
-dsExpr (HsCase discrim matches) = do
- core_discrim <- dsLExpr discrim
- ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
- return (scrungleMatch discrim_var core_discrim matching_code)
+dsExpr (HsCase discrim matches@(MatchGroup _ rhs_ty))
+ | isEmptyMatchGroup matches -- A Core 'case' is always non-empty
+ = -- So desugar empty HsCase to error call
+ mkErrorAppDs pAT_ERROR_ID (funResultTy rhs_ty) (ptext (sLit "case"))
+
+ | otherwise
+ = do { core_discrim <- dsLExpr discrim
+ ; ([discrim_var], matching_code) <- matchWrapper CaseAlt matches
+ ; return (scrungleMatch discrim_var core_discrim matching_code) }
-- Pepe: The binds are in scope in the body but NOT in the binding group
-- This is to avoid silliness in breakpoints
\underline{\bf Various data construction things}
% ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
\begin{code}
-dsExpr (ExplicitList ty xs)
- = go xs
- where
- go [] = return (mkNilExpr ty)
- go (x:xs) = mkConsExpr ty <$> dsLExpr x <*> go 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
- 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 <$> dsExpr expr <*> dsLExpr from
dsExpr (PArrSeq expr (FromThenTo from thn to))
= mkApps <$> dsExpr expr <*> mapM dsLExpr [from, thn, to]
-dsExpr (PArrSeq expr _)
+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
= 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
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
- = -- 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
- ; 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.
- ; 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
\begin{code}
-#ifdef DEBUG
-- HsSyn constructs that just shouldn't be here:
dsExpr (ExprWithTySig _ _) = panic "dsExpr:ExprWithTySig"
-#endif
findField :: [HsRecField Id arg] -> Name -> [arg]
%--------------------------------------------------------------------
+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:
+
+1. Find the tail of the list that can be allocated statically (say
+ [x_k, ..., x_n]) by later stages and ensure we desugar that
+ normally: this makes sure that we don't cause a code size increase
+ by having the cons in that expression fused (see later) and hence
+ being unable to statically allocate any more
+
+2. For the prefix of the list which cannot be allocated statically,
+ say [x_1, ..., x_(k-1)], we turn it into an expression involving
+ build so that if we find any foldrs over it it will fuse away
+ entirely!
+
+ So in this example we will desugar to:
+ build (\c n -> x_1 `c` x_2 `c` .... `c` foldr c n [x_k, ..., x_n]
+
+ 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.
+
+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.
+
+=======> 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 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
+ 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)
+ where (satisfying, rejected) = span f $ reverse xs
+\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:
-> DsM CoreExpr
dsDo stmts body result_ty
- = go (map unLoc stmts)
+ = goL stmts
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
- ; 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]) }
- go (LetStmt binds : stmts)
- = do { rest <- go stmts
+ go _ (LetStmt binds) stmts
+ = do { rest <- goL 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
- ; 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
| otherwise
= extractMatchResult match (error "It can't fail")
+mk_fail_msg :: Located e -> String
mk_fail_msg pat = "Pattern match failure in do expression at " ++
showSDoc (ppr (getLoc pat))
\end{code}
-> DsM CoreExpr
dsMDo tbl stmts body result_ty
- = go (map unLoc stmts)
+ = goL stmts
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
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 }
- go (ExprStmt rhs _ rhs_ty : stmts)
+ go _ (ExprStmt rhs _ rhs_ty) stmts
= 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]) }
- 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)
; 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 )
- go (new_bind_stmt : let_stmt : stmts)
+ pprTrace "dsMDo" (ppr later_ids) $
+ goL (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)] []))
+ 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)
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])
- (mk_ret_tup rets)
+ (mkLHsTupleExpr rets)
mk_wild_pat :: Id -> LPat Id
mk_wild_pat v = noLoc $ WildPat $ idType v
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}