instance Applicative Data.Map

[Roman Cheplyaka]

* Henning Thielemann <lemming at henning-thielemann.de> [2012-11-14 20:46:29+0100]
> An ZipList-like Applicative instance for Data.Map would be nice. I
> have an application where I like to write
>    liftA3 f amap bmap cmap
>  meaning
>    Map.intersectionWith ($) (Map.intersectionWith f amap bmap) cmap
> 
> But I cannot complete the instance implementation because there is no
> sensible definition for 'pure' for Data.Map. :-(

You can lift Map like this:

    data ZipMap k a
      = Pure a -- a "Map" that maps every possible key to 'a'
      | Zip (Map k a)

Roman