[Git][ghc/ghc][wip/spj-apporv-Oct24] 2 commits: add Overview note

15 Jan 2026


      Apoorv Ingle pushed to branch wip/spj-apporv-Oct24 at Glasgow Haskell Compiler / GHC


Commits:
fcac5349 by Apoorv Ingle at 2026-01-14T16:54:41-06:00
add Overview note

- - - - -
dd638011 by Apoorv Ingle at 2026-01-15T12:40:36-06:00
fix for failing test case

- - - - -


4 changed files:

- compiler/GHC/Tc/Gen/Do.hs
- compiler/GHC/Tc/Gen/Expr.hs
- compiler/GHC/Tc/Types/LclEnv.hs
- compiler/GHC/Tc/Utils/Monad.hs


Changes:

=====================================
compiler/GHC/Tc/Gen/Do.hs
=====================================
@@ -114,7 +114,7 @@ expand_do_stmts doFlavour (stmt@(L loc (BindStmt xbsrn pat e)): lstmts)
   | otherwise
   = pprPanic "expand_do_stmts: The impossible happened, missing bind operator from renamer" (text "stmt" <+> ppr  stmt)
 
-expand_do_stmts doFlavour (stmt@(L loc (BodyStmt _ (L _e_lspan e) (SyntaxExprRn then_op) _)) : lstmts) =
+expand_do_stmts doFlavour (stmt@(L loc (BodyStmt _ (L e_lspan e) (SyntaxExprRn then_op) _)) : lstmts) =
 -- See Note [BodyStmt] in Language.Haskell.Syntax.Expr
 -- See  Note [Expanding HsDo with XXExprGhcRn] Equation (1) below
 --              stmts ~~> stmts'
@@ -122,8 +122,8 @@ expand_do_stmts doFlavour (stmt@(L loc (BodyStmt _ (L _e_lspan e) (SyntaxExprRn
 --      e ; stmts ~~> (>>) e stmts'
   do expand_stmts_expr <- expand_do_stmts doFlavour lstmts
      let expansion = genHsExpApps then_op  -- (>>)
-                     [ -- L e_lspan (mkExpandedStmt stmt doFlavour e)
-                       wrapGenSpan e
+                     [ L e_lspan (mkExpandedStmt stmt doFlavour e)
+                       -- wrapGenSpan e
                      , expand_stmts_expr ]
      return $ L loc (mkExpandedStmt stmt doFlavour expansion)
 


=====================================
compiler/GHC/Tc/Gen/Expr.hs
=====================================
@@ -304,6 +304,7 @@ tcExpr :: HsExpr GhcRn
 --   - ones taken apart by GHC.Tc.Gen.Head.splitHsApps
 --   - ones understood by GHC.Tc.Gen.Head.tcInferAppHead_maybe
 -- See Note [Application chains and heads] in GHC.Tc.Gen.App
+-- Se Note [Overview of Typechecking an XExpr]
 tcExpr e@(HsVar {})              res_ty = tcApp e res_ty
 tcExpr e@(HsApp {})              res_ty = tcApp e res_ty
 tcExpr e@(OpApp {})              res_ty = tcApp e res_ty
@@ -761,6 +762,50 @@ tcExpr (SectionR {})       ty = pprPanic "tcExpr:SectionR"    (ppr ty)
 ************************************************************************
 -}
 
+{- Note [Overview of Typechecking an XExpr]
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+Certain constructs undergo expansion right before type checking.
+
+   tcExpr ue@(RecordUpd{}) rho = do { ee <- expand e; tcExpr ee rho }
+
+See Note [Handling overloaded and rebindable constructs] and
+Note [Doing XXExprGhcRn in the Renamer vs Typechecker]
+for details about which constructs are expanded.
+
+The expansion process typically takes a user written thing
+       L lspan ue
+and returns
+       L lspan (XExpr (ExpandedThingRn { xrn_orig = ue
+                                       , xrn_expanded = ee } ))
+
+where `ee` is the expansion of the user written thing `ue`
+
+Now, when a `tcMonoLHsExpr :: LHsExpr GhcRn -> ExpRhoType -> TcM (HsExpr GhcTc)`
+gets a located expression, It does 2 things:
+1. calls `addLExprCtxt` to perform error context management, and;
+2. calls tcExpr to typecheck the expression.
+
+The type checker context has 2 key fields:
+
+     TcLclCtxt { tcl_loc         :: RealSrcSpan
+               , tcl_err_ctxt    :: [ErrCtxt]
+               , ... }
+
+When called on an XExpr, `addLExprCtxt` updates the location of `tcl_loc` with
+the `lspan` above and adds an ErrCtxt on top of the `tcl_err_ctxt`. If the
+`lspan` is generated, then `addLExprCtxt` is a no-op.
+
+The type checker error stack element `GHC.Tc.Types.ErrCtxt.ErrCtxt` has two fields
+
+     ErrCtxt = EC CodeSrcFlag ErrCtxtMsgM
+
+`CodeSrcFlag` says whether we are typechecking an expanded thing, and what that expanded thing is
+`ErrCtxtMsgM` stores the pre-text error message itself. When called on an `XExpr`, `addLExprCtxt`,
+adds the user written thing `ue`, and the error message provided by the caller on the `ErrCtxtStack`
+See Note [ErrCtxtStack Manipulation] for more details.
+
+-}
+
 tcXExpr :: XXExprGhcRn -> ExpRhoType -> TcM (HsExpr GhcTc)
 tcXExpr (ExpandedThingRn o e) res_ty
    = mkExpandedTc o <$> -- necessary for hpc ticks
@@ -1482,7 +1527,7 @@ expandRecordUpd record_expr possible_parents rbnds res_ty
              ds_expr = HsLet noExtField let_binds (wrapGenSpan case_expr)
 
              case_expr :: HsExpr GhcRn
-             case_expr = HsCase RecUpd (wrapGenSpan (unLoc record_expr))
+             case_expr = HsCase RecUpd record_expr
                        $ mkMatchGroup (Generated OtherExpansion DoPmc) (wrapGenSpan matches)
              matches :: [LMatch GhcRn (LHsExpr GhcRn)]
              matches = map make_pat (NE.toList relevant_cons)


=====================================
compiler/GHC/Tc/Types/LclEnv.hs
=====================================
@@ -199,6 +199,7 @@ getLclEnvErrCtxt = tcl_err_ctxt . tcl_lcl_ctxt
 setLclEnvErrCtxt :: ErrCtxtStack -> TcLclEnv -> TcLclEnv
 setLclEnvErrCtxt ctxt = modifyLclCtxt (\env -> env { tcl_err_ctxt = ctxt })
 
+-- See Note [ErrCtxtStack Manipulation]
 addLclEnvErrCtxt :: ErrCtxt -> TcLclEnv -> TcLclEnv
 addLclEnvErrCtxt ec = setLclEnvSrcCodeOrigin ec
 


=====================================
compiler/GHC/Tc/Utils/Monad.hs
=====================================
@@ -1333,6 +1333,7 @@ addErrCtxt :: ErrCtxtMsg -> TcM a -> TcM a
 {-# INLINE addErrCtxt #-}   -- Note [Inlining addErrCtxt]
 addErrCtxt msg = addErrCtxtM (\env -> return (env, msg))
 
+-- See Note [ErrCtxtStack Manipulation]
 addExpansionErrCtxt :: SrcCodeOrigin -> ErrCtxtMsg -> TcM a -> TcM a
 {-# INLINE addExpansionErrCtxt #-}   -- Note [Inlining addErrCtxt]
 addExpansionErrCtxt o msg = addExpansionErrCtxtM o (\env -> return (env, msg))
@@ -1343,6 +1344,7 @@ addErrCtxtM :: (TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)) -> TcM a -> TcM a
 {-# INLINE addErrCtxtM #-}  -- Note [Inlining addErrCtxt]
 addErrCtxtM ctxt = pushCtxt (MkErrCtxt VanillaUserSrcCode ctxt)
 
+-- See Note [ErrCtxtStack Manipulation]
 addExpansionErrCtxtM :: SrcCodeOrigin -> (TidyEnv -> ZonkM (TidyEnv, ErrCtxtMsg)) -> TcM a -> TcM a
 {-# INLINE addExpansionErrCtxtM #-}  -- Note [Inlining addErrCtxt]
 addExpansionErrCtxtM o ctxt = pushCtxt (MkErrCtxt (ExpansionCodeCtxt o) ctxt)



View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/6b8198f1d3f0d7fd7d599c449ede1d7...

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/6b8198f1d3f0d7fd7d599c449ede1d7...
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Apoorv Ingle (＠ani)

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