Proposal: add generic helpers for MonadZip
David Feuer
david.feuer at gmail.com
Sun Aug 22 01:44:06 UTC 2021
We can offer generic helpers for writing MonadZip instances:
genericmzipWith :: (Generic1 m, GMonadZip (Rep1 m)) => (a -> b -> c)
-> m a -> m b -> m c
genericmzipWith f m n = to1 $ gmzipWith f (from1 m) (from1 n)
genericmunzip :: (Generic1 m, GMonadZip (Rep1 m)) => m (a, b) -> (m a, m b)
genericmunzip m = case gmunzip (from1 m) of (r1, r2) -> (to1 r1, to1 r2)
These can be used with appropriate product types (there's no way to
zip sums generically). I propose adding these to Control.Monad.Zip.
The GMonadZip class and its instances are below.
class GMonadZip f where
gmzipWith :: (a -> b -> c) -> f a -> f b -> f c
gmunzip :: f (a, b) -> (f a, f b)
instance MonadZip f => GMonadZip (Rec1 f) where
gmzipWith f (Rec1 fa) (Rec1 fb) = Rec1 (mzipWith f fa fb)
gmunzip (Rec1 fab) = (Rec1 fa, Rec1 fb)
where
-- We want to be lazy here, because this might actually
-- be recursive. We do a lot of NOINLINE to get selector
-- thunks to avoid space leaks.
{-# NOINLINE mufab #-}
{-# NOINLINE fa #-}
{-# NOINLINE fb #-}
mufab = munzip fab
(fa, fb) = mufab
instance (Functor g, MonadZip f, GMonadZip g) => GMonadZip (f :.: g) where
gmzipWith f (Comp1 fga1) (Comp1 fga2) = Comp1 $ mzipWith (gmzipWith
f) fga1 fga2
gmunzip (Comp1 fgc) = case munzip . fmap gmunzip $ fgc of
(p, q) -> (Comp1 p, Comp1 q)
-- | @since 4.9.0.0
instance GMonadZip U1 where
gmzipWith _ _ _ = U1
gmunzip _ = (U1, U1)
-- | @since 4.9.0.0
instance GMonadZip Par1 where
gmzipWith = coerce
gmunzip = coerce
-- | @since 4.9.0.0
deriving instance GMonadZip f => GMonadZip (M1 i c f)
-- | @since 4.9.0.0
instance (GMonadZip f, GMonadZip g) => GMonadZip (f :*: g) where
gmzipWith f (x1 :*: y1) (x2 :*: y2) = gmzipWith f x1 x2 :*:
gmzipWith f y1 y2
-- Why don't we need to be lazy in this munzip? If we're working with
-- Rep1, then laziness will be added by the Rec1 instance. If we're working
-- with Rep, then we can't have any K1s because K1 isn't an instance of
-- Monad, let alone MonadZip!
gmunzip (fab :*: gab)
| (fa, fb) <- gmunzip fab
, (ga, gb) <- gmunzip gab
= (fa :*: ga, fb :*: gb)
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