[Haskell-cafe] Classes in type declarations in Hugs

Simon Peyton-Jones simonpj at microsoft.com
Mon Dec 6 05:15:19 EST 2004

| This is where I find ghc's newtype deriving to be incredibly useful.
| newtype CPError a = CPError (Either Error a)
|         deriving(Monad,MonadError)
| now CPError is a monad and an instance of MonadError and you are free
| override or inherit more interesting instances at will without
| conflicting with the basic types for Either :)

Aha!  I've always wondered whether anyone uses this feature, which I
implemented at John Hughes's suggestion.  I'd be interested to know if
you are alone, or whether others do so too.


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