[Haskell-cafe] Data.Map.fromListWith

[Holger Siegel]

On Saturday 06 December 2008 22:07:51 Paul Johnson wrote:

> So we could have
>
>    fromListWithZero :: Ord k => (a -> b -> b) -> b -> [(k, a)] -> Map k b
>    fromListWithZero combiner zero pairs = ...
>
> The first time a key is seen the combining function is called with
> "zero" as its second argument.  E.g.
>
>    fromListWithZero (:) [] xs
>
> Or is that too much trouble?

It could be made a bit more general and efficient: Every entry x that did not 
have to be combined with another will end up as an application (combiner zero 
x). Thus, you could also write a function

fromListWithUnit :: Ord k => (a -> b -> b) -> (a -> b) -> [(k, a)] -> Map k b
fromListWithUnit combiner unit pairs = ...

and define fromListWithZero via

fromListWithZero c z ps = fromListWithUnit c (c z) ps

But there is

fromListWithUnit c u = fromListWith c . map (\p ->(fst p, u (snd p)))

So, you already have what you want.

Regards,
Holger