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• matchable
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• matchable-th
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Matchable, Bimatchable classes themselves can be defined in Haskell98. Currently it requires base >= 4.10.

Hi there, I independently discovered something like your Matchable, but I'm actually using the Bifunctor version. Would you be interested in adding this typeclass to your package?

class Bifunctor f => Bimatchable f where
  bimatch :: f a x -> f b y -> Maybe (f (a, b) (x, y))
  bimatch = bimatchWith (fmap Just . (,)) (fmap Just . (,))

  bimatchWith
    :: (a -> b -> Maybe c) -> (x -> y -> Maybe z)
    -> f a x -> f b y -> Maybe (f c z)

Note that, to match your Matchable, we'd want to add an Eq2 superclass, though I'm not convinced it's necessary.

Taking an idea from https://hackage.haskell.org/package/recursion-schemes-5.2.2.2/docs/Data-Functor-Foldable-TH.html

It'll be good to be able to do this ↓

data Foo a b = Foo a [b] deriving Functor

makeMatchableInstances [d|
instance Eq a => Eq1 (Foo a)
instance Eq a => Matchable (Foo a)
|]

For my own project I wrote deriveBimatchable (like the functions you'd find in Data.Bifunctor.TH). I could adapt it and write deriveMatchable as well, if you're interested.