On Fri, 2 Apr 2004, Simon Peyton-Jones wrote:
> Your word is my command. 'Tis done. > > Simon > > | -----Original Message----- > | From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] > On Behalf Of Wolfgang > | Jeltsch > | Sent: 21 March 2004 17:55 > | To: The Haskell Mailing List > | Subject: [Haskell] deriving with newtypes > | > | Hello, > | > | I'm trying to use GHC's deriving mechanism for newtypes in the > following way: > | class C a b > | instance C [a] Char > | newtype T = T Char deriving C [a] > | Unfortunately, this isn't possible. Is there a reason for this? Can I > | circumvent this restriction? > | > | Wolfgang It looks like this lets you use partially applied type classes in a deriving clause, always apllying that class to the new type last. This looks nice, but wouldn't work so well if your newtype was supposed to go first class C a b instance Char [a] newtype T = T Char deriving C ?? Maybe a deriving clause should allow full instance heads instead as well as class names, so you could write "deriving C T [a]". (Maybe with some restrictions, like ensuring the new type appears, or is one of the class arguments). It seems more regular to allow you to derive an instance of a multi-paramater typeclass with your class in any position rather than just the last. Brandon _______________________________________________ Haskell mailing list [EMAIL PROTECTED] http://www.haskell.org/mailman/listinfo/haskell
