[Haskell-cafe] Re: [Haskell] recursive deriving
dons at galois.com
Tue Nov 20 23:56:10 EST 2007
> On Tue, 2007-11-20 at 19:18 -0500, Alex Jacobson wrote:
> > When you want automated deriving of show/read etc., you need all the
> > components of your type also to be instances of show/read but you won't
> > want to *require* them to be automatically generated verions.
> > Standalone deriving does the wrong thing here. Standalone deriving
> > should not cause an overlapping instance error if someone derives an
> > instance manually. Instead, the manually derived instance should be
> > treated as more specific and win out.
> > The other part of this problem is that you can't do automatic recursive
> > deriving and this results in a ridiculous amount of boilerplate. I know
> > some people have a theory that they want to avoid accidentally creating
> > instances for things that shouldn't have them, but the solution to that
> > is probably to add some declaration for types that prohibits automatic
> > deriving for those types. The 99% case is that automatic deriving is ok.
> > Proposed syntax:
> > derive instance Show T recursively
> > data T = T no-deriving (Ord,Eq)
> I would expect that if the data constructor for T is not exported then
> standalone deriving should not work. However this appears not to be the
> case which breaks module abstraction.
> module Foo ( T, t ) where
> data T = T
> t = T
> import Foo
> deriving instance Eq T
> $ ghci Bar.hs -XStandaloneDeriving
> [1 of 2] Compiling Bar ( Bar.hs, interpreted )
> [2 of 2] Compiling Main ( Baz.hs, interpreted )
> Ok, modules loaded: Bar, Main.
> *Main> t == t
> You could write that Eq instance by hand since they do not have access
> to the T constructor, then StandaloneDeriving should not be able to so
> either. I think it's a design flaw in standalone deriving.
> Does anyone else agree? Should we file a bug report?
Yes, this seems wrong.
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