diff options
Diffstat (limited to 'compiler/deSugar/DsExpr.hs')
| -rw-r--r-- | compiler/deSugar/DsExpr.hs | 20 |
1 files changed, 19 insertions, 1 deletions
diff --git a/compiler/deSugar/DsExpr.hs b/compiler/deSugar/DsExpr.hs index 0f1386d76d..45d208b3db 100644 --- a/compiler/deSugar/DsExpr.hs +++ b/compiler/deSugar/DsExpr.hs @@ -601,7 +601,7 @@ ds_expr _ expr@(RecordUpd { rupd_expr = record_expr, rupd_flds = fields -- constructor arguments. ; alts <- mapM (mk_alt upd_fld_env) cons_to_upd ; ([discrim_var], matching_code) - <- matchWrapper RecUpd Nothing + <- matchWrapper RecUpd (Just record_expr) -- See Note [Scrutinee in Record updates] (MG { mg_alts = noLoc alts , mg_ext = MatchGroupTc [in_ty] out_ty , mg_origin = FromSource }) @@ -707,6 +707,24 @@ ds_expr _ expr@(RecordUpd { rupd_expr = record_expr, rupd_flds = fields , pat_wrap = req_wrap } ; return (mkSimpleMatch RecUpd [pat] wrapped_rhs) } +{- Note [Scrutinee in Record updates] +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +Consider #17783: + + data PartialRec = No + | Yes { a :: Int, b :: Bool } + update No = No + update r@(Yes {}) = r { b = False } + +In the context of pattern-match checking, the occurrence of @r@ in +@r { b = False }@ is to be treated as if it was a scrutinee, as can be seen by +the following desugaring: + + r { b = False } ==> case r of Yes a b -> Yes a False + +Thus, we pass @r@ as the scrutinee expression to @matchWrapper@ above. +-} + -- Here is where we desugar the Template Haskell brackets and escapes -- Template Haskell stuff |