Re: Proposal: Add Data instance for Const

The proposed instance is overconstrained. You only need Typeable for the second argument of Const (because it is phantom). On Wed, Nov 2, 2016 at 7:05 PM, Edward Kmett wrote:

Definitely an oversight.

On Wed, Nov 2, 2016 at 11:40 AM, Ryan Scott wrote:

...

GHC Trac #12438 [1] exists because there's no Data instance for Const in base. I found this quite surprising, since we have Data instances for just about every other type combinator out there (Identity, Sum, Product, Compose, etc.), but not for Const. The fix for #12438 would be quite simple: I propose we add

deriving instance (Data a, Data b) => Data (Const a b)

to Data.Data in base. Any objections?

Ryan S. ----- [1] https://ghc.haskell.org/trac/ghc/ticket/12438 _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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