fromListWithDefault

[maxigit]

I often use Map to group list by a key. The result is of type Map k [a]. There is at the moment no easy way to build such Map even though this use case seems frequent. I have to use fromwith (++) but this means I have to wrap the value in a list instead of being able to just cons them (using :).

Given a list of tuple key-value, the code I'm using is Map.fromLift (++) [(k, [x]) | <- kxs ] but I think it
will better to be able to do something like Map.newFunction ([]) (:) kxs or equivalent. I know such a function is really to write using the existing functions. However, all solution involve having to wrap the value unnecessarily in a list.
The different options I can envisage would be

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

Or alternatively, just regroup each value in a list and let user to fold them .

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

I hope the signature is enough to describe what each function does.
I'm happy to send PR if people things is a good idea and function names are welcome !

[treeowl]

Ah, now I understand the (a -> b) function. You're using that because you surely have at least one element if you're folding. I'm leaning toward foldl'Duplicates :: (b -> a -> b) -> (a -> b) -> [(k, a)] -> Map k b foldrDuplicates :: (a -> b -> b) -> (a -> b) -> [(k, a)] -> Map k b but these are also a bit ugly. Another option is foldMapDuplicatesLeftStrict:: Semigroup s => (a -> s) -> [(k, a)] -> Map k s foldMapDuplicatesRightStrict :: Semigroup s => (a -> s) -> [(k, a)] -> Map k s but I can't seem to find good names for those.

…

On Mon, Jan 30, 2017 at 5:20 PM, Max ***@***.***> wrote: I often use Map to group list by a key. The result is of type Map k [a]. There is at the moment no easy way to build such Map even though this use case seems frequent. I have to use fromwith (++) but this means I have to wrap the value in a list instead of being able to just cons them (using : ). Given a list of tuple key-value, the code I'm using is Map.fromLift (++) [(k, [x]) | <- kxs ] but I think it will better to be able to do something like Map.newFunction ([]) (:) kxs or equivalent. I know such a function is really to write using the existing functions. However, all solution involve having to wrap the value unnecessarily in a list. The different options I can envisage would be Ord k => b -> (a -> b -> b) -> [(k, a)] -> Map k b Ord k => b -> (b -> a -> b) > [(k, a)] -> Map k b Ord k => (a -> b) -> (b -> a -> b) -> [(k, a)] -> Map k b Or alternatively, just regroup each value in a list and let user to fold them . Ord k => [(k, a)] -> Map k [a] Ord k => (a -> k) -> [a] -> Map k [a] I hope the signature is enough to describe what each function does. I'm happy to send PR if people things is a good idea and function names are welcome ! — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#386>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABzi_e3ufBOVyNDQxRNhkbDaGIf0iLJyks5rXmIsgaJpZM4Lx_ig> .

[maxigit]

About, the name, as it constructing a Map from a list, I think it should belongs to the fromList* family.
I thought initially of fromListWithDefault because we are just adding a default value to fromListWith.
In fact, we are mapping at the same time, so maybe fromListWithMap or something ...

[sjakobi]

Related: #333, haskell-unordered-containers/unordered-containers#264