{-# LANGUAGE ConstraintKinds #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE RankNTypes #-} {-# LANGUAGE RecordWildCards #-} {-# LANGUAGE TupleSections #-} {-# LANGUAGE TypeFamilies #-} {-# OPTIONS_GHC -Wno-incomplete-uni-patterns #-} {- (c) The University of Glasgow 2006 (c) The GRASP/AQUA Project, Glasgow University, 1992-1998 (c) The University of Iowa 2023 -} -- | Expand @Do@ block statements into @(>>=)@, @(>>)@ and @let@s -- After renaming but right ebefore type checking module GHC.Tc.Gen.Do (expandDoStmts) where import GHC.Prelude import GHC.Rename.Utils ( wrapGenSpan, genHsExpApps, genHsApp, genHsLet, genHsLamDoExp, genHsCaseAltDoExp, genWildPat ) import GHC.Rename.Env ( irrefutableConLikeRn ) import GHC.Tc.Utils.Monad import GHC.Tc.Utils.TcMType import GHC.Hs import GHC.Utils.Outputable import GHC.Utils.Panic import GHC.Driver.DynFlags ( DynFlags, getDynFlags ) import GHC.Driver.Ppr (showPpr) import GHC.Types.SrcLoc import GHC.Types.Basic import qualified GHC.LanguageExtensions as LangExt import Data.List ((\\)) {- ************************************************************************ * * \subsection{XXExprGhcRn for Do Statements} * * ************************************************************************ -} -- | Expand the `do`-statments into expressions right after renaming -- so that they can be typechecked. -- See Note [Expanding HsDo with XXExprGhcRn] below for `HsDo` specific commentary -- and Note [Handling overloaded and rebindable constructs] for high level commentary expandDoStmts :: HsDoFlavour -> [ExprLStmt GhcRn] -> TcM (LHsExpr GhcRn) expandDoStmts :: HsDoFlavour -> [ExprLStmt (GhcPass 'Renamed)] -> TcM (LHsExpr (GhcPass 'Renamed)) expandDoStmts HsDoFlavour doFlav [ExprLStmt (GhcPass 'Renamed)] stmts = do expanded_expr <- HsDoFlavour -> [ExprLStmt (GhcPass 'Renamed)] -> TcM (LHsExpr (GhcPass 'Renamed)) expand_do_stmts HsDoFlavour doFlav [ExprLStmt (GhcPass 'Renamed)] stmts case expanded_expr of L SrcSpanAnnA _ (XExpr (PopErrCtxt LHsExpr (GhcPass 'Renamed) e)) -> LocatedA (HsExpr (GhcPass 'Renamed)) -> IOEnv (Env TcGblEnv TcLclEnv) (LocatedA (HsExpr (GhcPass 'Renamed))) forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a forall (m :: * -> *) a. Monad m => a -> m a return LHsExpr (GhcPass 'Renamed) LocatedA (HsExpr (GhcPass 'Renamed)) e -- The first expanded stmt doesn't need a pop as -- it would otherwise pop the "In the expression do ... " from -- the error context LocatedA (HsExpr (GhcPass 'Renamed)) _ -> LocatedA (HsExpr (GhcPass 'Renamed)) -> IOEnv (Env TcGblEnv TcLclEnv) (LocatedA (HsExpr (GhcPass 'Renamed))) forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a forall (m :: * -> *) a. Monad m => a -> m a return LocatedA (HsExpr (GhcPass 'Renamed)) expanded_expr -- | The main work horse for expanding do block statements into applications of binds and thens -- See Note [Expanding HsDo with XXExprGhcRn] expand_do_stmts :: HsDoFlavour -> [ExprLStmt GhcRn] -> TcM (LHsExpr GhcRn) expand_do_stmts :: HsDoFlavour -> [ExprLStmt (GhcPass 'Renamed)] -> TcM (LHsExpr (GhcPass 'Renamed)) expand_do_stmts HsDoFlavour ListComp [ExprLStmt (GhcPass 'Renamed)] _ = String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) (LocatedA (HsExpr (GhcPass 'Renamed))) forall a. HasCallStack => String -> SDoc -> a pprPanic String "expand_do_stmts: impossible happened. ListComp" SDoc forall doc. IsOutput doc => doc empty -- handeled by `GHC.Tc.Gen.Match.tcLcStmt` expand_do_stmts HsDoFlavour _ [] = String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) (LocatedA (HsExpr (GhcPass 'Renamed))) forall a. HasCallStack => String -> SDoc -> a pprPanic String "expand_do_stmts: impossible happened. Empty stmts" SDoc forall doc. IsOutput doc => doc empty expand_do_stmts HsDoFlavour _ (stmt :: ExprLStmt (GhcPass 'Renamed) stmt@(L SrcSpanAnnA _ (TransStmt {})):[ExprLStmt (GhcPass 'Renamed)] _) = String -> SDoc -> TcM (LHsExpr (GhcPass 'Renamed)) forall a. HasCallStack => String -> SDoc -> a pprPanic String "expand_do_stmts: TransStmt" (SDoc -> TcM (LHsExpr (GhcPass 'Renamed))) -> SDoc -> TcM (LHsExpr (GhcPass 'Renamed)) forall a b. (a -> b) -> a -> b $ GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) -> SDoc forall a. Outputable a => a -> SDoc ppr ExprLStmt (GhcPass 'Renamed) GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) stmt -- handeled by `GHC.Tc.Gen.Match.tcLcStmt` expand_do_stmts HsDoFlavour _ (stmt :: ExprLStmt (GhcPass 'Renamed) stmt@(L SrcSpanAnnA _ (ParStmt {})):[ExprLStmt (GhcPass 'Renamed)] _) = String -> SDoc -> TcM (LHsExpr (GhcPass 'Renamed)) forall a. HasCallStack => String -> SDoc -> a pprPanic String "expand_do_stmts: ParStmt" (SDoc -> TcM (LHsExpr (GhcPass 'Renamed))) -> SDoc -> TcM (LHsExpr (GhcPass 'Renamed)) forall a b. (a -> b) -> a -> b $ GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) -> SDoc forall a. Outputable a => a -> SDoc ppr ExprLStmt (GhcPass 'Renamed) GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) stmt -- handeled by `GHC.Tc.Gen.Match.tcLcStmt` expand_do_stmts HsDoFlavour _ (stmt :: ExprLStmt (GhcPass 'Renamed) stmt@(L SrcSpanAnnA _ (XStmtLR ApplicativeStmt{})): [ExprLStmt (GhcPass 'Renamed)] _) = String -> SDoc -> TcM (LHsExpr (GhcPass 'Renamed)) forall a. HasCallStack => String -> SDoc -> a pprPanic String "expand_do_stmts: Applicative Stmt" (SDoc -> TcM (LHsExpr (GhcPass 'Renamed))) -> SDoc -> TcM (LHsExpr (GhcPass 'Renamed)) forall a b. (a -> b) -> a -> b $ GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) -> SDoc forall a. Outputable a => a -> SDoc ppr ExprLStmt (GhcPass 'Renamed) GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) stmt -- Handeled by tcSyntaxOp see `GHC.Tc.Gen.Match.tcStmtsAndThen` expand_do_stmts HsDoFlavour _ [stmt :: ExprLStmt (GhcPass 'Renamed) stmt@(L SrcSpanAnnA loc (LastStmt XLastStmt (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) _ (L SrcSpanAnnA body_loc HsExpr (GhcPass 'Renamed) body) Maybe Bool _ SyntaxExpr (GhcPass 'Renamed) ret_expr))] -- See Note [Expanding HsDo with XXExprGhcRn] Equation (5) below -- last statement of a list comprehension, needs to explicitly return it -- See `checkLastStmt` and `Syntax.Expr.StmtLR.LastStmt` | SyntaxExpr (GhcPass 'Renamed) SyntaxExprRn NoSyntaxExprRn <- SyntaxExpr (GhcPass 'Renamed) ret_expr -- Last statement is just body if we are not in ListComp context. See Syntax.Expr.LastStmt = do String -> SDoc -> TcRn () traceTc String "expand_do_stmts last" (SyntaxExprRn -> SDoc forall a. Outputable a => a -> SDoc ppr SyntaxExpr (GhcPass 'Renamed) SyntaxExprRn ret_expr) LHsExpr (GhcPass 'Renamed) -> TcM (LHsExpr (GhcPass 'Renamed)) forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a forall (m :: * -> *) a. Monad m => a -> m a return (LHsExpr (GhcPass 'Renamed) -> TcM (LHsExpr (GhcPass 'Renamed))) -> LHsExpr (GhcPass 'Renamed) -> TcM (LHsExpr (GhcPass 'Renamed)) forall a b. (a -> b) -> a -> b $ SrcSpanAnnA -> ExprLStmt (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) mkExpandedStmtPopAt SrcSpanAnnA loc ExprLStmt (GhcPass 'Renamed) stmt HsExpr (GhcPass 'Renamed) body | SyntaxExprRn HsExpr (GhcPass 'Renamed) ret <- SyntaxExpr (GhcPass 'Renamed) ret_expr -- -- ------------------------------------------------ -- return e ~~> return e -- to make T18324 work = do String -> SDoc -> TcRn () traceTc String "expand_do_stmts last" (SyntaxExprRn -> SDoc forall a. Outputable a => a -> SDoc ppr SyntaxExpr (GhcPass 'Renamed) SyntaxExprRn ret_expr) let expansion :: HsExpr (GhcPass 'Renamed) expansion = HsExpr (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) genHsApp HsExpr (GhcPass 'Renamed) ret (SrcSpanAnnA -> HsExpr (GhcPass 'Renamed) -> LocatedA (HsExpr (GhcPass 'Renamed)) forall l e. l -> e -> GenLocated l e L SrcSpanAnnA body_loc HsExpr (GhcPass 'Renamed) body) LHsExpr (GhcPass 'Renamed) -> TcM (LHsExpr (GhcPass 'Renamed)) forall a. a -> IOEnv (Env TcGblEnv TcLclEnv) a forall (m :: * -> *) a. Monad m => a -> m a return (LHsExpr (GhcPass 'Renamed) -> TcM (LHsExpr (GhcPass 'Renamed))) -> LHsExpr (GhcPass 'Renamed) -> TcM (LHsExpr (GhcPass 'Renamed)) forall a b. (a -> b) -> a -> b $ SrcSpanAnnA -> ExprLStmt (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) mkExpandedStmtPopAt SrcSpanAnnA loc ExprLStmt (GhcPass 'Renamed) stmt HsExpr (GhcPass 'Renamed) expansion expand_do_stmts HsDoFlavour do_or_lc (stmt :: ExprLStmt (GhcPass 'Renamed) stmt@(L SrcSpanAnnA loc (LetStmt XLetStmt (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) _ HsLocalBindsLR (GhcPass 'Renamed) (GhcPass 'Renamed) bs)) : [ExprLStmt (GhcPass 'Renamed)] lstmts) = -- See Note [Expanding HsDo with XXExprGhcRn] Equation (3) below -- stmts ~~> stmts' -- ------------------------------------------------ -- let x = e ; stmts ~~> let x = e in stmts' do expand_stmts <- HsDoFlavour -> [ExprLStmt (GhcPass 'Renamed)] -> TcM (LHsExpr (GhcPass 'Renamed)) expand_do_stmts HsDoFlavour do_or_lc [ExprLStmt (GhcPass 'Renamed)] lstmts let expansion = HsLocalBindsLR (GhcPass 'Renamed) (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) genHsLet HsLocalBindsLR (GhcPass 'Renamed) (GhcPass 'Renamed) bs LHsExpr (GhcPass 'Renamed) LocatedA (HsExpr (GhcPass 'Renamed)) expand_stmts return $ mkExpandedStmtPopAt loc stmt expansion expand_do_stmts HsDoFlavour do_or_lc (stmt :: ExprLStmt (GhcPass 'Renamed) stmt@(L SrcSpanAnnA loc (BindStmt XBindStmt (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) xbsrn LPat (GhcPass 'Renamed) pat LocatedA (HsExpr (GhcPass 'Renamed)) e)): [ExprLStmt (GhcPass 'Renamed)] lstmts) | SyntaxExprRn HsExpr (GhcPass 'Renamed) bind_op <- XBindStmtRn -> SyntaxExpr (GhcPass 'Renamed) xbsrn_bindOp XBindStmt (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) XBindStmtRn xbsrn , FailOperator (GhcPass 'Renamed) fail_op <- XBindStmtRn -> FailOperator (GhcPass 'Renamed) xbsrn_failOp XBindStmt (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) XBindStmtRn xbsrn -- See Note [Expanding HsDo with XXExprGhcRn] Equation (2) below -- the pattern binding pat can fail -- stmts ~~> stmt' f = \case pat -> stmts'; -- _ -> fail "Pattern match failure .." -- ------------------------------------------------------- -- pat <- e ; stmts ~~> (>>=) e f = do expand_stmts <- HsDoFlavour -> [ExprLStmt (GhcPass 'Renamed)] -> TcM (LHsExpr (GhcPass 'Renamed)) expand_do_stmts HsDoFlavour do_or_lc [ExprLStmt (GhcPass 'Renamed)] lstmts failable_expr <- mk_failable_expr do_or_lc pat expand_stmts fail_op let expansion = HsExpr (GhcPass 'Renamed) -> [LHsExpr (GhcPass 'Renamed)] -> HsExpr (GhcPass 'Renamed) genHsExpApps HsExpr (GhcPass 'Renamed) bind_op -- (>>=) [ LHsExpr (GhcPass 'Renamed) LocatedA (HsExpr (GhcPass 'Renamed)) e , LHsExpr (GhcPass 'Renamed) LocatedA (HsExpr (GhcPass 'Renamed)) failable_expr ] return $ mkExpandedStmtPopAt loc stmt expansion | Bool otherwise = String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) (LocatedA (HsExpr (GhcPass 'Renamed))) forall a. HasCallStack => String -> SDoc -> a pprPanic String "expand_do_stmts: The impossible happened, missing bind operator from renamer" (String -> SDoc forall doc. IsLine doc => String -> doc text String "stmt" SDoc -> SDoc -> SDoc forall doc. IsLine doc => doc -> doc -> doc <+> GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) -> SDoc forall a. Outputable a => a -> SDoc ppr ExprLStmt (GhcPass 'Renamed) GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) stmt) expand_do_stmts HsDoFlavour do_or_lc (stmt :: ExprLStmt (GhcPass 'Renamed) stmt@(L SrcSpanAnnA loc (BodyStmt XBodyStmt (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) _ LocatedA (HsExpr (GhcPass 'Renamed)) e (SyntaxExprRn HsExpr (GhcPass 'Renamed) then_op) SyntaxExpr (GhcPass 'Renamed) _)) : [ExprLStmt (GhcPass 'Renamed)] lstmts) = -- See Note [BodyStmt] in Language.Haskell.Syntax.Expr -- See Note [Expanding HsDo with XXExprGhcRn] Equation (1) below -- stmts ~~> stmts' -- ---------------------------------------------- -- e ; stmts ~~> (>>) e stmts' do expand_stmts_expr <- HsDoFlavour -> [ExprLStmt (GhcPass 'Renamed)] -> TcM (LHsExpr (GhcPass 'Renamed)) expand_do_stmts HsDoFlavour do_or_lc [ExprLStmt (GhcPass 'Renamed)] lstmts let expansion = HsExpr (GhcPass 'Renamed) -> [LHsExpr (GhcPass 'Renamed)] -> HsExpr (GhcPass 'Renamed) genHsExpApps HsExpr (GhcPass 'Renamed) then_op -- (>>) [ LHsExpr (GhcPass 'Renamed) LocatedA (HsExpr (GhcPass 'Renamed)) e , LHsExpr (GhcPass 'Renamed) LocatedA (HsExpr (GhcPass 'Renamed)) expand_stmts_expr ] return $ mkExpandedStmtPopAt loc stmt expansion expand_do_stmts HsDoFlavour do_or_lc ((L SrcSpanAnnA loc (RecStmt { recS_stmts :: forall idL idR body. StmtLR idL idR body -> XRec idR [LStmtLR idL idR body] recS_stmts = L SrcSpanAnnL stmts_loc [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] rec_stmts , recS_later_ids :: forall idL idR body. StmtLR idL idR body -> [IdP idR] recS_later_ids = [IdP (GhcPass 'Renamed)] later_ids -- forward referenced local ids , recS_rec_ids :: forall idL idR body. StmtLR idL idR body -> [IdP idR] recS_rec_ids = [IdP (GhcPass 'Renamed)] local_ids -- ids referenced outside of the rec block , recS_bind_fn :: forall idL idR body. StmtLR idL idR body -> SyntaxExpr idR recS_bind_fn = SyntaxExprRn HsExpr (GhcPass 'Renamed) bind_fun -- the (>>=) expr , recS_mfix_fn :: forall idL idR body. StmtLR idL idR body -> SyntaxExpr idR recS_mfix_fn = SyntaxExprRn HsExpr (GhcPass 'Renamed) mfix_fun -- the `mfix` expr , recS_ret_fn :: forall idL idR body. StmtLR idL idR body -> SyntaxExpr idR recS_ret_fn = SyntaxExprRn HsExpr (GhcPass 'Renamed) return_fun -- the `return` expr -- use it explicitly -- at the end of expanded rec block })) : [ExprLStmt (GhcPass 'Renamed)] lstmts) = -- See Note [Typing a RecStmt] in Language.Haskell.Syntax.Expr -- See Note [Expanding HsDo with XXExprGhcRn] Equation (4) and (6) below -- stmts ~~> stmts' -- ------------------------------------------------------------------------------------------- -- rec { later_ids, local_ids, rec_block } ; stmts -- ~~> (>>=) (mfix (\[ local_only_ids ++ later_ids ] -- -> do { rec_stmts -- ; return (local_only_ids ++ later_ids) } )) -- (\ [ local_only_ids ++ later_ids ] -> stmts') do expand_stmts <- HsDoFlavour -> [ExprLStmt (GhcPass 'Renamed)] -> TcM (LHsExpr (GhcPass 'Renamed)) expand_do_stmts HsDoFlavour do_or_lc [ExprLStmt (GhcPass 'Renamed)] lstmts -- NB: No need to wrap the expansion with an ExpandedStmt -- as we want to flatten the rec block statements into its parent do block anyway return $ mkHsApps (wrapGenSpan bind_fun) -- (>>=) [ (wrapGenSpan mfix_fun) `mkHsApp` mfix_expr -- (mfix (do block)) , genHsLamDoExp do_or_lc [ mkBigLHsVarPatTup all_ids ] -- (\ x -> expand_stmts -- stmts') ] where local_only_ids :: [Name] local_only_ids = [IdP (GhcPass 'Renamed)] [Name] local_ids [Name] -> [Name] -> [Name] forall a. Eq a => [a] -> [a] -> [a] \\ [IdP (GhcPass 'Renamed)] [Name] later_ids -- get unique local rec ids; -- local rec ids and later ids can overlap all_ids :: [Name] all_ids = [Name] local_only_ids [Name] -> [Name] -> [Name] forall a. [a] -> [a] -> [a] ++ [IdP (GhcPass 'Renamed)] [Name] later_ids -- put local ids before return ids return_stmt :: ExprLStmt GhcRn return_stmt :: ExprLStmt (GhcPass 'Renamed) return_stmt = StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) -> GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) forall an a. HasAnnotation an => a -> GenLocated an a wrapGenSpan (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) -> GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))) -> StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) -> GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) forall a b. (a -> b) -> a -> b $ XLastStmt (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) -> LocatedA (HsExpr (GhcPass 'Renamed)) -> Maybe Bool -> SyntaxExpr (GhcPass 'Renamed) -> StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) forall idL idR body. XLastStmt idL idR body -> body -> Maybe Bool -> SyntaxExpr idR -> StmtLR idL idR body LastStmt XLastStmt (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) NoExtField noExtField ([LHsExpr (GhcPass 'Renamed)] -> XExplicitTuple (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) forall (id :: Pass). [LHsExpr (GhcPass id)] -> XExplicitTuple (GhcPass id) -> LHsExpr (GhcPass id) mkBigLHsTup ((Name -> LocatedA (HsExpr (GhcPass 'Renamed))) -> [Name] -> [LocatedA (HsExpr (GhcPass 'Renamed))] forall a b. (a -> b) -> [a] -> [b] map IdP (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) Name -> LocatedA (HsExpr (GhcPass 'Renamed)) forall (p :: Pass) a. IsSrcSpanAnn p a => IdP (GhcPass p) -> LHsExpr (GhcPass p) nlHsVar [Name] all_ids) XExplicitTuple (GhcPass 'Renamed) NoExtField noExtField) Maybe Bool forall a. Maybe a Nothing (HsExpr (GhcPass 'Renamed) -> SyntaxExprRn SyntaxExprRn HsExpr (GhcPass 'Renamed) return_fun) do_stmts :: XRec GhcRn [ExprLStmt GhcRn] do_stmts :: XRec (GhcPass 'Renamed) [ExprLStmt (GhcPass 'Renamed)] do_stmts = SrcSpanAnnL -> [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] -> GenLocated SrcSpanAnnL [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] forall l e. l -> e -> GenLocated l e L SrcSpanAnnL stmts_loc ([GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] -> GenLocated SrcSpanAnnL [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))]) -> [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] -> GenLocated SrcSpanAnnL [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] forall a b. (a -> b) -> a -> b $ [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] rec_stmts [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] -> [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] -> [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] forall a. [a] -> [a] -> [a] ++ [ExprLStmt (GhcPass 'Renamed) GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) return_stmt] do_block :: LHsExpr GhcRn do_block :: LHsExpr (GhcPass 'Renamed) do_block = SrcSpanAnnA -> HsExpr (GhcPass 'Renamed) -> LocatedA (HsExpr (GhcPass 'Renamed)) forall l e. l -> e -> GenLocated l e L SrcSpanAnnA loc (HsExpr (GhcPass 'Renamed) -> LocatedA (HsExpr (GhcPass 'Renamed))) -> HsExpr (GhcPass 'Renamed) -> LocatedA (HsExpr (GhcPass 'Renamed)) forall a b. (a -> b) -> a -> b $ XDo (GhcPass 'Renamed) -> HsDoFlavour -> XRec (GhcPass 'Renamed) [ExprLStmt (GhcPass 'Renamed)] -> HsExpr (GhcPass 'Renamed) forall p. XDo p -> HsDoFlavour -> XRec p [ExprLStmt p] -> HsExpr p HsDo XDo (GhcPass 'Renamed) NoExtField noExtField HsDoFlavour do_or_lc XRec (GhcPass 'Renamed) [ExprLStmt (GhcPass 'Renamed)] do_stmts mfix_expr :: LHsExpr GhcRn mfix_expr :: LHsExpr (GhcPass 'Renamed) mfix_expr = HsDoFlavour -> [LPat (GhcPass 'Renamed)] -> LHsExpr (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) forall (p :: Pass). (IsPass p, XMG (GhcPass p) (LHsExpr (GhcPass p)) ~ Origin) => HsDoFlavour -> [LPat (GhcPass p)] -> LHsExpr (GhcPass p) -> LHsExpr (GhcPass p) genHsLamDoExp HsDoFlavour do_or_lc [ Pat (GhcPass 'Renamed) -> GenLocated SrcSpanAnnA (Pat (GhcPass 'Renamed)) forall an a. HasAnnotation an => a -> GenLocated an a wrapGenSpan (XLazyPat (GhcPass 'Renamed) -> LPat (GhcPass 'Renamed) -> Pat (GhcPass 'Renamed) forall p. XLazyPat p -> LPat p -> Pat p LazyPat XLazyPat (GhcPass 'Renamed) NoExtField noExtField (LPat (GhcPass 'Renamed) -> Pat (GhcPass 'Renamed)) -> LPat (GhcPass 'Renamed) -> Pat (GhcPass 'Renamed) forall a b. (a -> b) -> a -> b $ [IdP (GhcPass 'Renamed)] -> LPat (GhcPass 'Renamed) mkBigLHsVarPatTup [IdP (GhcPass 'Renamed)] [Name] all_ids) ] (LHsExpr (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed)) -> LHsExpr (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) forall a b. (a -> b) -> a -> b $ LHsExpr (GhcPass 'Renamed) do_block -- NB: LazyPat because we do not want to eagerly evaluate the pattern -- and potentially loop forever expand_do_stmts HsDoFlavour _ [ExprLStmt (GhcPass 'Renamed)] stmts = String -> SDoc -> TcM (LHsExpr (GhcPass 'Renamed)) forall a. HasCallStack => String -> SDoc -> a pprPanic String "expand_do_stmts: impossible happened" (SDoc -> TcM (LHsExpr (GhcPass 'Renamed))) -> SDoc -> TcM (LHsExpr (GhcPass 'Renamed)) forall a b. (a -> b) -> a -> b $ ([GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] -> SDoc forall a. Outputable a => a -> SDoc ppr [ExprLStmt (GhcPass 'Renamed)] [GenLocated SrcSpanAnnA (StmtLR (GhcPass 'Renamed) (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))))] stmts) -- checks the pattern `pat` for irrefutability which decides if we need to wrap it with a fail block mk_failable_expr :: HsDoFlavour -> LPat GhcRn -> LHsExpr GhcRn -> FailOperator GhcRn -> TcM (LHsExpr GhcRn) mk_failable_expr :: HsDoFlavour -> LPat (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) -> FailOperator (GhcPass 'Renamed) -> TcM (LHsExpr (GhcPass 'Renamed)) mk_failable_expr HsDoFlavour doFlav pat :: LPat (GhcPass 'Renamed) pat@(L SrcSpanAnnA loc Pat (GhcPass 'Renamed) _) LHsExpr (GhcPass 'Renamed) expr FailOperator (GhcPass 'Renamed) fail_op = do { is_strict <- Extension -> TcRnIf TcGblEnv TcLclEnv Bool forall gbl lcl. Extension -> TcRnIf gbl lcl Bool xoptM Extension LangExt.Strict ; hscEnv <- getTopEnv ; rdrEnv <- getGlobalRdrEnv ; comps <- getCompleteMatchesTcM ; let irrf_pat = Bool -> (ConLikeP (GhcPass 'Renamed) -> Bool) -> LPat (GhcPass 'Renamed) -> Bool forall (p :: Pass). IsPass p => Bool -> (ConLikeP (GhcPass p) -> Bool) -> LPat (GhcPass p) -> Bool isIrrefutableHsPat Bool is_strict (HasDebugCallStack => HscEnv -> GlobalRdrEnv -> CompleteMatches -> Name -> Bool HscEnv -> GlobalRdrEnv -> CompleteMatches -> Name -> Bool irrefutableConLikeRn HscEnv hscEnv GlobalRdrEnv rdrEnv CompleteMatches comps) LPat (GhcPass 'Renamed) pat ; traceTc "mk_failable_expr" (vcat [ text "pat:" <+> ppr pat , text "isIrrefutable:" <+> ppr irrf_pat ]) ; if irrf_pat -- don't wrap with fail block if -- the pattern is irrefutable then return $ genHsLamDoExp doFlav [pat] expr else L loc <$> mk_fail_block doFlav pat expr fail_op } -- makes the fail block with a given fail_op mk_fail_block :: HsDoFlavour -> LPat GhcRn -> LHsExpr GhcRn -> FailOperator GhcRn -> TcM (HsExpr GhcRn) mk_fail_block :: HsDoFlavour -> LPat (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) -> FailOperator (GhcPass 'Renamed) -> IOEnv (Env TcGblEnv TcLclEnv) (HsExpr (GhcPass 'Renamed)) mk_fail_block HsDoFlavour doFlav pat :: LPat (GhcPass 'Renamed) pat@(L SrcSpanAnnA ploc Pat (GhcPass 'Renamed) _) LHsExpr (GhcPass 'Renamed) e (Just (SyntaxExprRn HsExpr (GhcPass 'Renamed) fail_op)) = do dflags <- IOEnv (Env TcGblEnv TcLclEnv) DynFlags forall (m :: * -> *). HasDynFlags m => m DynFlags getDynFlags return $ HsLam noAnn LamCases $ mkMatchGroup (doExpansionOrigin doFlav) -- \ (wrapGenSpan [ genHsCaseAltDoExp doFlav pat e -- pat -> expr , fail_alt_case dflags pat fail_op -- _ -> fail "fail pattern" ]) where fail_alt_case :: DynFlags -> LPat GhcRn -> HsExpr GhcRn -> LMatch GhcRn (LHsExpr GhcRn) fail_alt_case :: DynFlags -> LPat (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) -> LMatch (GhcPass 'Renamed) (LHsExpr (GhcPass 'Renamed)) fail_alt_case DynFlags dflags LPat (GhcPass 'Renamed) pat HsExpr (GhcPass 'Renamed) fail_op = HsDoFlavour -> LPat (GhcPass 'Renamed) -> LocatedA (HsExpr (GhcPass 'Renamed)) -> LMatch (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) forall (p :: Pass) (body :: * -> *). (Anno (GRHS (GhcPass p) (LocatedA (body (GhcPass p)))) ~ EpAnnCO, Anno (Match (GhcPass p) (LocatedA (body (GhcPass p)))) ~ SrcSpanAnnA) => HsDoFlavour -> LPat (GhcPass p) -> LocatedA (body (GhcPass p)) -> LMatch (GhcPass p) (LocatedA (body (GhcPass p))) genHsCaseAltDoExp HsDoFlavour doFlav LPat (GhcPass 'Renamed) genWildPat (LocatedA (HsExpr (GhcPass 'Renamed)) -> LMatch (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed)))) -> LocatedA (HsExpr (GhcPass 'Renamed)) -> LMatch (GhcPass 'Renamed) (LocatedA (HsExpr (GhcPass 'Renamed))) forall a b. (a -> b) -> a -> b $ SrcSpanAnnA -> HsExpr (GhcPass 'Renamed) -> LocatedA (HsExpr (GhcPass 'Renamed)) forall l e. l -> e -> GenLocated l e L SrcSpanAnnA ploc (DynFlags -> LPat (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) fail_op_expr DynFlags dflags LPat (GhcPass 'Renamed) pat HsExpr (GhcPass 'Renamed) fail_op) fail_op_expr :: DynFlags -> LPat GhcRn -> HsExpr GhcRn -> HsExpr GhcRn fail_op_expr :: DynFlags -> LPat (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) fail_op_expr DynFlags dflags LPat (GhcPass 'Renamed) pat HsExpr (GhcPass 'Renamed) fail_op = LPat (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) mkExpandedPatRn LPat (GhcPass 'Renamed) pat (HsExpr (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed)) -> HsExpr (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) forall a b. (a -> b) -> a -> b $ HsExpr (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) -> HsExpr (GhcPass 'Renamed) genHsApp HsExpr (GhcPass 'Renamed) fail_op (DynFlags -> LPat (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) mk_fail_msg_expr DynFlags dflags LPat (GhcPass 'Renamed) pat) mk_fail_msg_expr :: DynFlags -> LPat GhcRn -> LHsExpr GhcRn mk_fail_msg_expr :: DynFlags -> LPat (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) mk_fail_msg_expr DynFlags dflags LPat (GhcPass 'Renamed) pat = HsLit (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) forall (p :: Pass). HsLit (GhcPass p) -> LHsExpr (GhcPass p) nlHsLit (HsLit (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed)) -> HsLit (GhcPass 'Renamed) -> LHsExpr (GhcPass 'Renamed) forall a b. (a -> b) -> a -> b $ String -> HsLit (GhcPass 'Renamed) forall (p :: Pass). String -> HsLit (GhcPass p) mkHsString (String -> HsLit (GhcPass 'Renamed)) -> String -> HsLit (GhcPass 'Renamed) forall a b. (a -> b) -> a -> b $ DynFlags -> SDoc -> String forall a. Outputable a => DynFlags -> a -> String showPpr DynFlags dflags (SDoc -> String) -> SDoc -> String forall a b. (a -> b) -> a -> b $ String -> SDoc forall doc. IsLine doc => String -> doc text String "Pattern match failure in" SDoc -> SDoc -> SDoc forall doc. IsLine doc => doc -> doc -> doc <+> HsDoFlavour -> SDoc pprHsDoFlavour (Maybe ModuleName -> HsDoFlavour DoExpr Maybe ModuleName forall a. Maybe a Nothing) SDoc -> SDoc -> SDoc forall doc. IsLine doc => doc -> doc -> doc <+> String -> SDoc forall doc. IsLine doc => String -> doc text String "at" SDoc -> SDoc -> SDoc forall doc. IsLine doc => doc -> doc -> doc <+> SrcSpan -> SDoc forall a. Outputable a => a -> SDoc ppr (GenLocated SrcSpanAnnA (Pat (GhcPass 'Renamed)) -> SrcSpan forall a e. HasLoc a => GenLocated a e -> SrcSpan getLocA LPat (GhcPass 'Renamed) GenLocated SrcSpanAnnA (Pat (GhcPass 'Renamed)) pat) mk_fail_block HsDoFlavour _ LPat (GhcPass 'Renamed) _ LHsExpr (GhcPass 'Renamed) _ FailOperator (GhcPass 'Renamed) _ = String -> SDoc -> IOEnv (Env TcGblEnv TcLclEnv) (HsExpr (GhcPass 'Renamed)) forall a. HasCallStack => String -> SDoc -> a pprPanic String "mk_fail_block: impossible happened" SDoc forall doc. IsOutput doc => doc empty {- Note [Expanding HsDo with XXExprGhcRn] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ We expand `do`-blocks before typechecking it, by re-using the existing `XXExprGhcRns` and `RebindableSyntax` machinery. This is very similar to: 1. Expansions done in `GHC.Rename.Expr.rnHsIf` for expanding `HsIf`; and 2. `desugarRecordUpd` in `GHC.Tc.Gen.Expr.tcExpr` for expanding `RecordUpd` See Note [Handling overloaded and rebindable constructs] in GHC.Rename.Expr To disabmiguate desugaring (`HsExpr GhcTc -> Core.Expr`) we use the phrase expansion (`HsExpr GhcRn -> HsExpr GhcRn`) This expansion is done right before typechecking and after renaming See Part 2. of Note [Doing XXExprGhcRn in the Renamer vs Typechecker] in `GHC.Rename.Expr` Historical note START --------------------- In previous versions of GHC, the `do`-notation wasn't expanded before typechecking, instead the typechecker would operate directly on the original. Why? because it ensured that type error messages were explained in terms of what the programmer has written. In practice, however, this didn't work very well: * Attempts to typecheck the original source code turned out to be buggy, and virtually impossible to fix (#14963, #15598, #21206 and others) * The typechecker expected the `>>=` operator to have a type that matches `m _ -> (_ -> m _) -> m _` for some `m`. With `RebindableSyntax` or `QualifiedDo` the `>>=` operator might not have the standard type. It might have a type like (>>=) :: Wombat m => m a1 a2 b -> (b -> m a2 a3 c) -> m a1 a3 c Typechecking the term `(>>=) e1 (\x -> e2)` deals with all of this automatically. * With `ImpredicativeTypes` the programmer will expect Quick Look to instantiate the quantifiers impredicatively (#18324). Again, that happens automatically if you typecheck the expanded expression. Historical note END ------------------- Do Expansions Equationally -------------------------- We have the following schema for expanding `do`-statements. They capture the essence of statement expansions as implemented in `expand_do_stmts` DO【 _ 】 maps a sequence of do statements and recursively converts them into expressions (1) DO【 s; ss 】 = ‹ExpansionStmt s›((>>) s (‹PopErrCtxt›DO【 ss 】)) (2) DO【 p <- e; ss 】 = if p is irrefutable then ‹ExpansionStmt (p <- e)› (>>=) s (‹PopExprCtxt›(\ p -> DO【 ss 】)) else ‹ExpansionStmt (p <- e)› (>>=) s (‹PopExprCtxt›(\case p -> DO【 ss 】 _ -> fail "pattern p failure")) (3) DO【 let x = e; ss 】 = ‹ExpansionStmt (let x = e)› (let x = e in (‹PopErrCtxt›DO【 ss 】)) (4) DO【 rec ss; sss 】 = (>>=) e (\vars -> ‹PopErrCtxt›DO【 sss 】)) where (vars, e) = RECDO【 ss 】 (5) DO【 s 】 = s RECDO【 _ 】 maps a sequence of recursively dependent monadic statements and converts it into an expression paired with the variables that the rec finds a fix point of. (6) RECDO【 ss 】 = (vars, mfix (\~vars -> (>>=) (DO【 ss 】) (return vars))) where vars are all the variables free in ss For a concrete example, consider a `do`-block written by the user f = {l0} do {l1} {pl}p <- {l1'} e1 {l2} g p {l3} return {l3'} p The expanded version (performed by `expand_do_stmts`) looks like: f = {g1} (>>=) ({l1'} e1) (\ {pl}p -> {g2} (>>) ({l2} g p) ({l3} return p)) The {l1} etc are location/source span information stored in the AST by the parser, {g1} are compiler generated source spans. The 3 non-obvious points to consider are: 1. Wrap the expression with a `fail` block if the pattern match is not irrefutable. See Part 1. below 2. Generate appropriate warnings for discarded results in a body statement eg. say `do { .. ; (g p :: m Int) ; ... }` See Part 2. below 3. Generating appropriate type error messages which blame the correct source spans See Part 3. below Part 1. Expanding Patterns Bindings ----------------------------------- If `p` is a failable pattern---checked by `GHC.Tc.Gen.Pat.isIrrefutableHsPatRnTcM`--- we need to wrap it with a `fail`-block. See Equation (2) above. The expansion of the `do`-block do { Just p <- e1; e2 } (ignoring the location information) will be (>>=) (e1) (\case -- anonymous continuation lambda Just p -> e2 _ -> fail "failable pattern p at location") The `fail`-block wrapping is done by `GHC.Tc.Gen.Do.mk_failable_expr`. * Note the explicit call to `fail`, in the monad of the `do`-block. Part of the specification of do-notation is that if the pattern match fails, we fail in the monad, *not* just crash at runtime. * According to the language specification, when the pattern is irrefutable, we should not add the `fail` alternative. This is important because the occurrence of `fail` means that the typechecker will generate a `MonadFail` constraint, and irrefutable patterns shouldn't need a fail alternative. * _Wrinkel 1_: Note that pattern synonyms count as refutable during type checking, (see `isIrrefutableHsPat`). They will hence generate a `MonadFail` constraint and they will always be wrapped in a `fail`able-block. Consider a patten synonym declaration (testcase T24552): pattern MyJust :: a -> Maybe a pattern MyJust x <- Just x where MyJust = Just and a `do`-block with the following bind and return statement do { MyJust x <- [MyNothing, MyJust ()] ; return x } The `do`-expansion will generate the expansion (>>=) ([MyNothing, MyJust ()]) (\case MyJust x -> return x -- (1) _ -> fail "failable pattern .. " -- (2) ) This code (specifically the `match` spanning lines (1) and (2)) is a compiler generated code; the associated `Origin` in tagged `Generated` The alternative statements will thus be ignored by the pattern match check (c.f. `isMatchContextPmChecked`). This ensures we do not generate spurious redundant-pattern-match warnings due to the line (2) above. See Note [Generated code and pattern-match checking] See Note [Long-distance information in matchWrapper] * _Wrinkle 2_: The call to `fail` will give rise to a `MonadFail` constraint. What `CtOrigin` do we attach to that constraint? When the `MonadFail` constraint can't be solved, it'll show up in error messages and it needs to be a good location. Ideally, it should identify the pattern `p`. Hence, we wrap the `fail` alternative expression with a `ExpandedPat` that tags the fail expression with the failable pattern. (See testcase MonadFailErrors.hs) Part 2. Generate warnings for discarded body statement results -------------------------------------------------------------- If the `do`-blocks' body statement is an expression that returns a value that is not of type `()`, we need to warn the user about discarded the value when `-Wunused-binds` flag is turned on. (See testcase T3263-2.hs) For example the `do`-block do { e1; e2 } -- where, e1 :: m Int expands to (>>) e1 e2 * If `e1` returns a non-() value we want to emit a warning, telling the user that they are discarding the value returned by e1. This is done by `HsToCore.dsExpr` in the `HsApp` with a call to `HsToCore.warnUnusedBindValue`. * The decision to trigger the warning is: if the function is a compiler generated `(>>)`, and its first argument `e1` has a non-() type Part 3. Blaming Offending Source Code and Generating Appropriate Error Messages ------------------------------------------------------------------------------- To ensure we correctly track source of the offending user written source code, in this case the `do`-statement, we need to keep track of which source statement's expansion the typechecker is currently typechecking. For this purpose we use the `XXExprGhcRn.ExpansionRn`. It stores the original statement (with location) and the expanded expression A. Expanding Body Statements ----------------------------- For example, the `do`-block do { e1; e2; e3 } expands (ignoring the location info) to ‹ExpandedThingRn do { e1; e2; e3 }› -- Original Do Expression -- Expanded Do Expression (‹ExpandedThingRn e1› -- Original Statement ({(>>) e1} -- Expanded Expression ‹PopErrCtxt› (‹ExpandedThingRn e2› ({(>>) e2} ‹PopErrCtxt› (‹ExpandedThingRn e3› {e3}))))) * Whenever the typechecker steps through an `ExpandedThingRn`, we push the original statement in the error context, set the error location to the location of the statement, and then typecheck the expanded expression. This is similar to vanilla `XXExprGhcRn` and rebindable syntax See Note [Rebindable syntax and XXExprGhcRn] in `GHC.Hs.Expr`. * Recall, that when a source function argument fails to typecheck, we print an error message like "In the second argument of the function f..". However, `(>>)` is generated thus, we don't want to display that to the user; it would be confusing. But also, we do not want to completely ignore it as we do want to keep the error blame carets as precise as possible, and not just blame the complete `do`-block. Thus, when we typecheck the application `(>>) e1`, we push the "In the stmt of do block e1" with the source location of `e1` in the error context stack as we walk inside an `ExpandedThingRn`. See also Note [splitHsApps]. * After the expanded expression of a `do`-statement is typechecked and before moving to the next statement of the `do`-block, we need to first pop the top of the error context stack which contains the error message for the previous statement: eg. "In the stmt of a do block: e1". This is explicitly encoded in the expansion expression using the `XXExprGhcRn.PopErrCtxt`. Whenever `GHC.Tc.Gen.Expr.tcExpr` (via `GHC.Tc.Gen.tcXExpr`) sees a `PopErrCtxt` it calls `GHC.Tc.Utils.Monad.popErrCtxt` to pop of the top of error context stack. See ‹PopErrCtxt› in the example above. Without this popping business for error context stack, if there is a type error in `e2`, we would get a spurious and confusing error message which mentions "In the stmt of a do block e1" along with the message "In the stmt of a do block e2". B. Expanding Bind Statements ----------------------------- A `do`-block with a bind statement: do { p <- e1; e2 } expands (ignoring the location information) to ‹ExpandedThingRn do{ p <- e1; e2 }› -- Original Do Expression -- (‹ExpandedThingRn (p <- e1)› -- Original Statement (((>>=) e1) -- Expanded Expression ‹PopErrCtxt› ((\ p -> ‹ExpandedThingRn (e2)› e2))) ) However, the expansion lambda `(\p -> e2)` is special as it is generated from a `do`-stmt expansion and if a type checker error occurs in the pattern `p` (which is source generated), we need to say "in a pattern binding in a do block" and not "in the pattern of a lambda" (cf. Typeable1.hs). We hence use a tag `GenReason` in `Ghc.Tc.Origin`. When typechecking a `HsLam` in `Tc.Gen.Expr.tcExpr` the `match_ctxt` is set to a `StmtCtxt` if `GenOrigin` is a `DoExpansionOrigin`. -}