deriving Typeable

[Simon Peyton-Jones]

You can just write the instance of Typeable by hand
	instance Typeable1 e => Typeable (Y e) where ...

(Use the instances in the library as a guide.)

Yes, GHC could do this for you, but it would then stumble when e had kind ((*->*) -> *).  We need kind polymorphism!

Simon

| -----Original Message-----
| From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-
| bounces at haskell.org] On Behalf Of John Meacham
| Sent: 02 June 2004 21:53
| To: glasgow-haskell-users at haskell.org
| Subject: deriving Typeable
| 
| newtype Y e = Y { unY :: (e (Y e)) }
|     deriving(Data,Typeable,Show,Read,Eq)
| 
| gives
| E.hs:64:
|     Can't make a derived instance of `Typeable (Y e)'
|     (`Y' is parameterised over arguments of kind other than `*')
|     When deriving instances for type `Y'
| 
| Is there any way around this limitation other than manually expanding Y
| everywhere I want to use it (which I really don't want to do)? Is the
| limitation inherent to the way Typeable works, or is it just that no one
| has implemented it yet?
| 
| --
| John Meacham - ⑆repetae.net⑆john⑈
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