[Haskell-cafe] Read instance for GATD

[Edward Kmett]

It turns out that defining Read is somewhat tricky to do for a GADT.

Gershom Bazerman has some very nice slides on how to survive the process by
manual typechecking (much in the spirit of Oleg's meta-typechecking code
referenced by Stephen's follow up below)

He presented them at hac-phi this time around.

I will check with him to see if I can get permission to host them somewhere
and post a link to them here.

-Edward Kmett

On Fri, Jun 25, 2010 at 5:04 AM, <corentin.dupont at ext.mpsa.com> wrote:

>
> Hello Haskellers,
>
> I'm having trouble writing a Read Instance for my GATD.
> Arg this GATD!! It causes me more problems that it solves ;)
> Especially with no automatic deriving, it adds a lot of burden to my code.
>
> >data Obs a where
> >    ProposedBy :: Obs Int       -- The player that proposed the tested
> rule
> >    Turn       :: Obs Turn      -- The current turn
> >    Official   :: Obs Bool      -- whereas the tested rule is official
> >    Equ        :: (Eq a, Show a, Typeable a) => Obs a -> Obs a -> Obs Bool
> >    Plus       :: (Num a) => Obs a -> Obs a -> Obs a
> >    Time       :: (Num a) => Obs a -> Obs a -> Obs a
> >    Minus      :: (Num a) => Obs a -> Obs a -> Obs a
> >    And        :: Obs Bool -> Obs Bool -> Obs Bool
> >    Or         :: Obs Bool -> Obs Bool -> Obs Bool
> >    Not        :: Obs Bool -> Obs Bool
> >    Konst      :: a -> Obs a
>
>
> > instance Read a => Read (Obs a) where
> > readPrec = (prec 10 $ do
> >        Ident "ProposedBy" <- lexP
> >        return (ProposedBy))
> >     +++
> >      (prec 10 $ do
> >        Ident "Official" <- lexP
> >        return (Official))
> >  (etc...)
>
> Observable.lhs:120:8:
>    Couldn't match expected type `Int' against inferred type `Bool'
>      Expected type: ReadPrec (Obs Int)
>      Inferred type: ReadPrec (Obs Bool)
>
>
> Indeed "ProposedBy" does not have the same type that "Official".
> Mmh how to make it all gently mix altogether?
>
>
> Best,
> Corentin
>
>
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