Changes to Typeable

Edward Kmett ekmett at
Thu Oct 4 21:47:56 CEST 2012

Well, I think there is a general consensus that the existing manual
instances for Typeable are more of a liability than a feature, and were a
necessary evil given the limitations of the previous system, so either
ignoring them or making them an error would make a lot of sense.


On Thu, Oct 4, 2012 at 3:42 PM, Dominique Devriese <
dominique.devriese at> wrote:

> Small note while I happen to be reading the list:
> 2012/10/3 Simon Peyton-Jones <simonpj at>:
> >    Plan A (easy): make GHC derive Typeable for everything, deprecate
> >             all uses of 'deriving Typeable'.  Small downside: some
> programs
> >             that are currently rejected will be accepted.
> Does this imply forbidding user-written instances of Typeable? If yes,
> then I guess some currently accepted programs would also be rejected
> (those with manual instances)?  If not, then wouldn't there be
> duplicate instances? Or would manual instances somehow take priority?
> Or be ignored?
> Dominique
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