Re: instance Applicative Data.Map

On Wed, 14 Nov 2012, Roman Cheplyaka wrote:

* Henning Thielemann [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)

Yes, that would work. It is only annoying that for liftA3 I do not need 'pure', at all. Thus we may note in the record that Data.Map is an example where <*> and liftAn make sense (for n>0) but 'pure' (i.e. Pointed) cannot be defined.