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I recently added an alterF function to Data.Map, which is a flipped form of Control.Lens.At.at.

alterF :: (Ord k, Functor f)
       => (Maybe a -> f (Maybe a))
       -> k
       -> Map k a
       -> f (Map k a)

alterF is designed to be able to offer reasonable performance even for very "heavy" functors like [] and even when the keys are fairly expensive to compare. Unfortunately, it's somewhat slower than one might wish in the more common case of a light-weight functor and cheap-to-compare keys.

To patch this up, I've added GHC rewrite rules for the Const b and Identity functors, rewriting them to simpler implementations that are usually faster. I'm about to add yet another one, for (,) b. I added one for (,) b and then removed it. See the update below.

However, I'm a bit annoyed by the fact that I need a specific rule for (,) b, because there are many other functors that follow the same pattern. In particular, any time a functor is defined

data F b1 b2 ... a = F e1 ... a ... e_n

where none of the e_k mention a, I should be able to rewrite it the same way. Is there some way to do this with GHC RULES? Or will I have to wait for some more general rewriting system?

Update

Sadly, I realized that my rewrite rule for pairs was invalid (it was too strict), and repairing that made it much less clearly beneficial. So I've scrapped that rule for now. I think the general question remains interesting, however.

dfeuer
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  • Can you offer more of the code? I'm willing to check out the core if I can get it down to a digestible chunk – Michael Klein May 23 '16 at 06:24
  • @MichaelKlein, which code are you referring to exactly? It's all in https://github.com/haskell/containers/blob/master/Data/Map/Base.hs except the bitstring implementation in `Data.Utils.BitQueue` in that repo. I suspect the performance problems without rewrite tools are insurmountable, but you're welcome to try. – dfeuer May 23 '16 at 07:03
  • I think I might have found a way. `Coercible a b` seems to be the only way to get whether a type is representational/phantom and convert to/from it. I have a somewhat working POC using `coerce`. (I'll be checking the room all occasionally) – Michael Klein May 24 '16 at 17:33

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