Re: Proposal: Remove Semigroup and Monoid instances for Data.Map, Data.IntMap, Data.HashMap

We can use rewrite rules to recognize and optimize unionWith in one or more special cases: unionWith f = plainUnionWith f {-# NOINLINE [1] unionWith #-} bottom1, bottom2 :: a bottom1 = undefined {-# NOINLINE bottom1 #-} bottom2 = undefined {-# NOINLINE bottom2 #-} unionWithMagic :: a -> (a -> a -> a) -> Map k a -> Map k a -> Map k a unionWithMagic _ f = plainUnionWith f {-# INLINE [1] unionWithMagic #-} RULES: unionWith f ===> unionWithMagic (f bottom1 bottom2) f unionWithMagic bottom1 f ===> union On Wed, Feb 14, 2018 at 5:16 AM, Akio Takano wrote:

Hi Oleg,

On 14 February 2018 at 09:48, Oleg Grenrus wrote:

...

I agree with Oliver, was just writing the very same.

`Map.map First = Map.map coerce = coerce`

i.e. mapping First is _zero-cost_, thanks to the fact `v` in `Map k v` has represenational role.

Yes, I agree that mapping First is zero-cost. I'm saying that `unionWith (<>)` over the First monoid costs more than `union`, due to allocation of extra thunks.

- Akio _______________________________________________ Libraries mailing list Libraries@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries