PROPOSAL: Add instances to Data.Monoid newtype wrappers

Edward Kmett ekmett at
Thu Dec 11 20:20:04 UTC 2014

I'm a very strong +1 on this.

We've already sat down to add some of these, and there was a general
resolution by the core libraries committee in response to a similar raised
issue for the types in GHC.Generics that missing instances like this that
have obvious canonical implementations should be added wherever possible.


On Fri, Dec 12, 2014 at 2:30 AM, Sean Leather <sean.leather at>

> As of base, none of the newtype wrappers in Data.Monoid have
> instances for:
>    - Typeable
>    - Data
>    - Functor
> It looks like the base library repository has AutoDeriveTypeable, so I
> presume that takes care of the Typeable instances. It also looks like First
> and Last now have Functor instances, amongst others.
> In particular, I would like to add Functor instances for all parameterized
> types, where applicable (i.e. Dual, Sum, Product).
> In general, I think we should add whatever other instances make sense.
> Data came to mind. Others?
> Discussion period: 2 weeks (or perhaps until after the beginning of the
> new year)
> Regards,
> Sean
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