Re: [Haskell-cafe] Unexpected Typeable behaviour; Prelude.undefined

The problem with hiding the context in the constructor is that there's no guarantee that the context actually exists in the first place; for example, given this type data IsInt a where Proof :: IsInt Int this is a legal program: foo :: IsInt Bool foo = undefined That said, you are still just fine to hide the context in the constructor at the call site: data MyGADT m where MonadAction :: (Typeable1 m, Monad m) -> m () -> MyGADT m instance (Typeable1 m, Monad m) => Typeable (MyGADT m) where typeof t = ... getTypeRep :: MyGADT m -> TypeRep getTypeRep x@(MonadAction _) = typeof (undefined `asTypeOf` x) Here we unpack the context from x and use it to construct the 'typeof' function for MyGADT m. -- ryan On Mon, Aug 29, 2011 at 2:06 AM, Philip Holzenspies wrote:

Dear Brandon, Ozgur, et al,

Thanks very much for you explanation. This seems to be a perfectly reasonable explanation; the wrapper-types I used probably explicitly invoke typeOf with undefined. The problem here, however, is that in my actual program, I don't use ADTs, but I use GADTs, so as to carry the context (Monad, Typeable1) with the constructor. To get to this context, I must pattern-match with the constructor. It seems hiding contexts (which I really like about GADTs) isn't "available" consistently. Oh well ;)

Regards, Philip

On 29 Aug 2011, at 01:20, Brandon Allbery wrote:

On Sun, Aug 28, 2011 at 18:44, Philip Holzenspies wrote:

...

instance (Typeable1 m, Monad m) => Typeable (MyADT m) where typeOf t@(MyADT _)

typeOf is usually invoked with an undefined parameter; it should use types, never values. Here you've defined it to deconstruct what it's passed, which means that anything that uses it in the usual way (`typeOf (undefined :: someType)') will immediately throw undefined.

You don't need a deconstructor there; you (correctly) throw away the value, and it doesn't provide any type information not already available from the instance declaration. `typeOf t' should be good enough.

-- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe