[Template-haskell] recover and get error message? - parsing th syntax trees

[Simon Peyton-Jones]

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 at haskell.org [mailto:template-haskell-bounces at haskell.org] On Behalf Of
| Marc Weber
| Sent: 25 May 2008 18:27
| To: template-haskell at 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
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