Library proposal: add a Location interface for element-wise operations on Data.Map (#4887)

[Ross Paterson]

On Sat, Jan 08, 2011 at 04:21:54PM +0100, Henning Thielemann wrote:
> On Sat, 8 Jan 2011, Ross Paterson wrote:
> >That would work well for search, but then index, minLocation and maxLocation
> >would return Locations that value was always mapped to Just something.
> >Extra invariants like that feel wrong to me.
> 
> 'index' can be out of range and then return Nothing. minLocation and
> maxLocation return Nothing if the Map is empty.

But what is the key in those cases?