Unfold for Maps and Easier Conversion from Foldable

[andrewthad]

Let's say I have a list of things that I want to turn into a Map. Right now, my only option for doing that is with fromList. But fromList needs a list of tuples.

fromList :: Ord k => [(k, a)] -> Map k a

So, I would have to first map over my list, converting everything to a tuple with the key and value, and then I could call fromList:

fromList . map (\d -> (dogId d, dogName, d)) :: [Dog] -> Map Int Text

or more succinctly

fromList . map (dogId &&& dogName)

It would be nice to have a more direct function for doing this:

unfoldList :: Ord k => (a -> (k,v)) -> [a] -> Map k v

This could be generalized to work with any Foldable:

unfold :: (Ord k, Foldable f) => (a -> (k,v)) -> f a -> Map k v

Similarly, the existing fromList could use a more general counterpart:

fromPairs :: (Ord k, Foldable f) => f (k, a) -> Map k a

[treeowl]

I like some of this. I don't like the name unfold, which generally suggests
building a structure from a seed. One option for fromPairs might be to
generalize fromList to any Foldable; I haven't looked into the efficiency
implications, but I don't think we rely on having an actual list as much
as we do in Data.Sequence.

On Sep 8, 2016 8:44 AM, "Andrew Martin" [email protected] wrote:

Let's say I have a list of things that I want to turn into a Map. Right
now, my only option for doing that is with fromList. But fromList needs a
list of tuples.

fromList :: Ord k => [(k, a)] -> Map k a

So, I would have to first map over my list, converting everything to a
tuple with the key and value, and then I could call fromList:

fromList . map (\d -> (dogId d, dogName, d)) :: [Dog] -> Map Int Text

or more succinctly

fromList . map (dogId &&& dogName)

It would be nice to have a more direct function for doing this:

unfoldList :: Ord k => (a -> (k,v)) -> [a] -> Map k v

This could be generalized to work with any Foldable:

unfold :: (Ord k, Foldable f) => (a -> (k,v)) -> f a -> Map k v

Similarly, the existing fromList could use a more general counterpart:

fromPairs :: (Ord k, Foldable f) => f (k, a) -> Map k a

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[andrewthad]

Thanks. You're right that unfold is a bad name. I just took a look at fromList and it's way more optimized than I assumed it was. I thought it would just be foldr (\(k,v) m -> Map.insert k v m) Map.empty or something like that. I think that's the only way you could define it to work with all Foldables.