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This patch changes PrefixCon to use one list instead of two: -data HsConDetails tyarg arg rec - = PrefixCon [tyarg] [arg] +data HsConDetails arg rec + = PrefixCon [arg] | RecCon rec | InfixCon arg arg The [tyarg] list is now gone. To understand the effect of this change, recall that there are three instantiations of HsConDetails: 1. type HsConPatDetails p = HsConDetails (HsConPatTyArg (NoGhcTc p)) -- tyarg (LPat p) -- arg (HsRecFields p (LPat p)) -- rec 2. type HsConDeclH98Details pass = HsConDetails Void -- tyarg (HsScaled pass (LBangType pass)) -- arg (XRec pass [LConDeclField pass]) -- rec 3. type HsPatSynDetails pass = HsConDetails Void -- tyarg (LIdP pass) -- arg [RecordPatSynField pass] -- rec In cases (2) and (3), tyarg was instantiated to Void, so the [tyarg] list was always empty. Its removal is basically a no-op. The interesting case is (1), which is used in ConPat to represent pattern matching of the form (MkE @tp1 @tp2 p1 p2). With this patch, its representation is changed as follows: ConPat "MkE" [tp1, tp2] [p1, p2] -- old ConPat "MkE" [InvisP tp1, InvisP tp2, p1, p2] -- new The new mixed-list representation is consintent with lambdas, where InvisP is already used to deal with \ @tp1 @tp2 p1 p2 -> body. The immediate effect of the new representation is an improvement to error messages. Consider the pattern (Con x @t y). Previously it resulted in a parse error because @t could not occur after x. Now it is reported as [GHC-14964]. Test case: TyAppPat_MisplacedApplication. In the long term, this is intended as preparation for #18389 and #25127, which would make (Con x @t y) potentially valid, e.g. if its type is Con :: forall a -> forall b. (a, b) -> T The TH AST is left unchanged for the moment to avoid breakage.7b84c588This patch changes PrefixCon to use one list instead of two: -data HsConDetails tyarg arg rec - = PrefixCon [tyarg] [arg] +data HsConDetails arg rec + = PrefixCon [arg] | RecCon rec | InfixCon arg arg The [tyarg] list is now gone. To understand the effect of this change, recall that there are three instantiations of HsConDetails: 1. type HsConPatDetails p = HsConDetails (HsConPatTyArg (NoGhcTc p)) -- tyarg (LPat p) -- arg (HsRecFields p (LPat p)) -- rec 2. type HsConDeclH98Details pass = HsConDetails Void -- tyarg (HsScaled pass (LBangType pass)) -- arg (XRec pass [LConDeclField pass]) -- rec 3. type HsPatSynDetails pass = HsConDetails Void -- tyarg (LIdP pass) -- arg [RecordPatSynField pass] -- rec In cases (2) and (3), tyarg was instantiated to Void, so the [tyarg] list was always empty. Its removal is basically a no-op. The interesting case is (1), which is used in ConPat to represent pattern matching of the form (MkE @tp1 @tp2 p1 p2). With this patch, its representation is changed as follows: ConPat "MkE" [tp1, tp2] [p1, p2] -- old ConPat "MkE" [InvisP tp1, InvisP tp2, p1, p2] -- new The new mixed-list representation is consintent with lambdas, where InvisP is already used to deal with \ @tp1 @tp2 p1 p2 -> body. The immediate effect of the new representation is an improvement to error messages. Consider the pattern (Con x @t y). Previously it resulted in a parse error because @t could not occur after x. Now it is reported as [GHC-14964]. Test case: TyAppPat_MisplacedApplication. In the long term, this is intended as preparation for #18389 and #25127, which would make (Con x @t y) potentially valid, e.g. if its type is Con :: forall a -> forall b. (a, b) -> T The TH AST is left unchanged for the moment to avoid breakage.
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