[Haskell-cafe] Automatically Deriving Numeric Type Class Instances

[adam vogt]

Hi Jake

https://hackage.haskell.org/package/applicative-numbers can generate those
instances.

Regards
Adam
On Apr 22, 2016 10:23 AM, "Jake" <jake.waksbaum at gmail.com> wrote:

> Is it possible to automatically derive instances of Numeric type classes
> like Num, Fractional, Real, Floating, etc?
>
> I currently have two datatypes, Pair and Triple, that are defined like
> this:
>
> data Pair a = Pair a a
> data Triple a = Triple a a a
>
> I wrote these pretty trivial instances for Num and Floating:
>
> instance Num a => Num (Pair a) where
>   (+) = liftA2 (+)
>   (*) = liftA2 (*)
>   abs = liftA abs
>   negate = liftA negate
>   signum = liftA signum
>   fromInteger = pure . fromInteger
>
> instance Fractional a => Fractional (Pair a) where
>   (/) = liftA2 (/)
>   recip = liftA recip
>   fromRational = pure . fromRational
>
> and practically identical instances for Triple as well.
>
> Is there anyway to have GHC derive these instances and the other numeric
> type classes?
>
> Thanks,
> Jake
>
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