Jared Updike writes:
Is there a common way (standard libs, higher order) to express the lambda part below? It's not particulary complicated but I think it is not higher-order enough
unionBy (\x y -> fst x == fst y) listOfPairs1 listOfPairs2
Something like "distribute fst (==)" where
distribute f op x y = f x `op` f y
would leave
unionBy (distribute fst (==)) listOfPairs1 listOfPairs2
I imagine something involving Arrows and/or zip/curry/uncurry but I just can't see it. Is this a case of trying to make something more complicated than it is?
If you look at it in terms of folds over pairs,
cata (&) (x,y) = x & y -- corresponds to uncurry
ana f g x = (f x, g x) -- corresponds to (&&&)
Then you can de-forest:
hylo (&) f g x = f x & g x
-- hylo (&) f g == cata (&) . ana f g
-- == uncurry (&) . f &&& g
--
-- cata (&) == hylo (&) fst snd
-- ana f g == hylo (,) f g
This seems remeniscent of pull-backs (or push-outs) in category theory,
but I don't know enough to say for certain.
--
David Menendez