Good point. I've created a feature-request ticket http://hackage.haskell.org/trac/ghc/ticket/2340. Any comments from other TH folk? Simon | -----Original Message----- | From: template-haskell-bounces@haskell.org [mailto:template-haskell-bounces@haskell.org] On Behalf Of | Marc Weber | Sent: 25 May 2008 18:27 | To: template-haskell@haskell.org | Subject: [Template-haskell] recover and get error message? - parsing th syntax trees | | I'd like to implement a simple helper function | -- helper pasring the reify output | thChoice :: a -> [ a -> Q r ] -> Q r | thChoice a list = thChoice' list | where | thChoice' [] = fail $ "no parser did match" | thChoice' (p:ps) = (p a) `recover` (thChoice' ps) | | However there I can't get the result of why a parser has failed this | way: | thChoice :: [ a -> Q r ] -> Q r | thChoice a list = thChoice' [] list | where | thChoice' errors [] = fail $ "no parser did match: error messages " ++ (show errors) | thChoice' errors (p:ps) = (p a) `recover` \e -> (thChoice' (e:errors) ps) | ^^ unfotunately recover | doesn't support passing this \e | | To show a nice list | expected either | a) ... | b) ... | c) ... | but no parser did match the passed arg | ... | | I've also tried `catchError` but there was no instance.. | Is there another way handling this or should I just use | Either Error result and | return . Left $ | return . Right $ | or put kind of ErrorMonad on top of the Q monad myself? | | My use case: | Parse different settings specified by using different types such as | UniqKey or (Key Foo ()) or relation settings | | type Tickets = Table (Autoinc, Surname, TestCase.Name) | (Key Name (), Key Surname ()) | (Rel Inbound Flights, Rel Outbound ( Maybe Flights) ) | | type <name> = Table <cols> | <keys> | <master tables> | | <keys> : the primary key field is added automatically | example: key Foo (-> Map Foo row) | double index (Foo2, Bar) (-> Map Foo2 ( Map Bar row ) ) ) | ( UniqKey Foo | , Key Foo2 ( Key Bar () ) | ) | <master tables>: | ( TableMaster | , Rel IdType TableMaster2 | -- optional relation (eg a ticket may have a retour, but doesn't have to have) | , Maybe TableMaster4 | , (Rel IdType (Maybe TableMaster5)) | ) | | Is there alreay a library providing this kind of error reporting for | parsing haskell template syntax trees? | | Marc Weber | _______________________________________________ | template-haskell mailing list | template-haskell@haskell.org | http://www.haskell.org/mailman/listinfo/template-haskell