Re: Many functions can be generalised

I'd also be -1. Function generalization sometimes has a cost in comprehensability, and there aren't enough benefits to make it worth it to me. Tom

El 27 nov 2016, a las 01:53, David Feuer escribió:

I disagree with many of these. For example, I think of takeWhile as having a type shaped like

takeWhile :: (a -> Bool) -> f a -> f a

Implementations are available for, e.g., sequences, sets, and maps. I don't really want some silly list producer. If I want takeWhile.toList, I know where to get it. Similarly, if I want distribute (which I haven't yet), I know where to get it. Some of these proposals also have substantial performance penalties, such as the sort generalization (which also can't be written in an "obviously total" manner, unfortunately).

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On Nov 27, 2016 2:10 AM, "Baldur Blöndal" wrote: A year ago Edwardk Kmett pointed out some possible generalizations of functions [1], I made a ticket about them that led me here [2]. In addition to the functions mentioned

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maybeToList :: Foldable f => f a -> [a] maybeToList = toList

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catMaybes :: (Foldable f) => f (Maybe a) -> [a] catMaybes :: (Foldable f, Foldable g) => f (g a) -> [a] catMaybes = foldMap toList

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mapMaybes :: (a -> Maybe b) -> (forall f. Foldable f => f a -> [b]) mapMaybes :: Foldable m => (a -> m b) -> (forall f. Foldable f => f a -> [b]) mapMaybes f = foldMap (toList . f)

we also have *many* other functions (I do not propose generalising all these function ((especially when the name stops making sense)), but I will include them) that I will define in the vocabulary of ‘lens’. Some generalise to ‘Foldable’

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take :: Int -> (forall f a. Foldable f => f a -> [a]) take n = toListOf (taking n folded)

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drop :: Int -> (forall f a. Foldable f => f a -> [a]) drop n = toListOf (dropping n folded)

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takeWhile :: (a -> Bool) -> (forall f. Foldable f => f a -> [a]) takeWhile p = toListOf (takingWhile p folded)

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dropWhile :: (a -> Bool) -> (forall f. Foldable f => f a -> [a]) dropWhile p = toListOf (droppingWhile p folded)

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-- Same as ‘Control.Lens.Indexed.None’ filter :: (a -> Bool) -> (forall f. Foldable f => f a -> [a]) filter p = toListOf (folded.filtered p)

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cycle :: Foldable f => f a -> [a] cycle = toListOf (cycled folded)

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lookup :: Eq k => k -> (forall f. Foldable f => f (k, v) -> Maybe v) lookup = lookupOf folded

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listToMaybe :: Foldable f => f a -> Maybe a listToMaybe = firstOf folded

while others — to ‘Traversable’

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transpose :: Traversable f => f [b] -> [f b] transpose = transposeOf traverse

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scanl1 :: (a -> a -> a) -> (forall f. Traversable f => f a -> f a) scanl1 = scanl1Of traverse

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scanr1_ :: (a -> a -> a) -> (forall f. Traversable f => f a -> f a) scanr1_ = scanr1Of traverse

More radical suggestions (pay no heed to the hacky ‘partsOf’, assume better implementation [3]) would allow us to sort a ‘data V2 a = V2 a a deriving (…, Traversable)’ if it contains ordered values:

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sort :: (Traversable t, Ord a) => t a -> t a sort = over (partsOf traverse) Data.List.sort

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sortBy :: (a -> a -> Ordering) -> ([a] -> [a]) sortBy = over (partsOf traverse) . Data.List.sortBy

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sortOn :: Ord b => (a -> b) -> ([a] -> [a]) sortOn = over (partsOf traverse) . Data.List.sortOn

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reverse :: Traversable t => t a -> t a reverse = over (partsOf traverse) Data.List.reverse

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-- Based on ‘Control.Lens.??’ flip :: Functor f => f (a -> b) -> a -> f b flip f x = fmap ($ x) f

or

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flip :: (Functor f, Distributive g) => f (g a) -> g (f a) flip = Data.Distributive.distribute

AMP happened some years ago, does this go too far or not far enough? ;) share your thoughts

P.s. I understand those skeptical of the ‘partsOf’ solutions but they do feel magical and uses crop up in odd places, especially in compound structures (I don't have better examples):

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ghci> peopleList = Pair ["Bob", "Eve"] (Just "Alice") ghci> data Product f g a = Pair (f a) (g a) deriving (Show, Functor, Foldable, Traversable) ghci> sort peopleList Pair ["Alice","Bob"] (Just "Eve") ghci> reverse peopleList Pair ["Alice","Eve"] (Just "Bob")

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ghci> peopleMap = fromList [(1,"Bob"),(2,"Eve"),(3,"Alice")] ghci> sort peopleMap fromList [(1,"Alice"),(2,"Bob"),(3,"Eve")] ghci> reverse peopleMap fromList [(1,"Alice"),(2,"Eve"),(3,"Bob")]

[1] https://www.reddit.com/r/haskell/comments/2y2pe5/shouldnt_ftp_propagate_chan... [2] https://ghc.haskell.org/trac/ghc/ticket/12828 [3] http://stackoverflow.com/a/33320155/165806

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