Fusing loops by specializing on functions with SpecConstr?

[Sebastian Graf]

>
> That’s it. These two rules alone are enough to eliminate the redundant
> tupling. Now the optimized version of `mapMaybeSF` is beautiful!
>

Beautiful indeed! That's wonderful to hear. Good luck messing about with
your FRP framework!

Sebastian

Am Sa., 4. Apr. 2020 um 03:45 Uhr schrieb Alexis King <lexi.lambda at gmail.com
>:

>
> I fiddled with alternative representations for a while and didn’t make
> any progress—it was too easy to end up with code explosion in the
> presence of any unknown calls—but I seem to have found a RULES-based
> approach that works very well on the examples I’ve tried. It’s quite
> simple, which makes it especially appealing!
>
> I started by defining a wrapper around the `SF` constructor to attach
> rules to:
>
>     mkSF :: (a -> s -> Step s b) -> s -> SF a b
>     mkSF = SF
>     {-# INLINE CONLIKE [1] mkSF #-}
>
> I  then changed the definitions of (.), (***), (&&&), (+++), and (&&&)
> to use `mkSF` instead of `SF`, but I left the other methods alone, so
> they just use `SF` directly. Then I defined two rewrite rules:
>
>     {-# RULES
>     "mkSF @((), _)" forall f s. mkSF f ((), s) =
>       SF (\a s1 -> case f a ((), s1) of Step ((), s2) b -> Step s2 b) s
>     "mkSF @(_, ())" forall f s. mkSF f (s, ()) =
>       SF (\a s1 -> case f a (s1, ()) of Step (s2, ()) b -> Step s2 b) s
>     #-}
>
> That’s it. These two rules alone are enough to eliminate the redundant
> tupling. Now the optimized version of `mapMaybeSF` is beautiful!
>
>     mapMaybeSF = \ @ a @ b f -> case f of { SF @ s f2 s2 ->
>       SF (\ a1 s1 -> case a1 of {
>            Nothing -> case s1 of dt { __DEFAULT -> Step dt Nothing }
>            Just x -> case f2 x s1 of {
>              Step s2' c1 -> Step s2' (Just c1) }})
>          s2 }
>
> So unless this breaks down in some larger situation I’m not aware of, I
> think this solves my problem without the need for any fancy SpecConstr
> shenanigans. Many thanks to you, Sebastian, for pointing me in the right
> direction!
>
> Alexis
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