Minor containers API changes

[Henning Thielemann]

On Mon, 28 Nov 2011, Milan Straka wrote:

> First five are an attempt to unify the API of different structures. The
> documentation states IntMap is Map replacement and IntSet is Set
> replacement, but there are several shortcomings:
>
> 1) `{Map,Set}.deleteMin empty` return `empty`
>   `{IntMap,IntSet}.deleteMin empty` trigger `error "Cannot delete in empty..."`
>
>   Solutions: (a) make `{Map,Set}.deleteMin empty` throw error
>              (b) make `{IntMap,IntSet}.deleteMin empty` return empty
>
>   I vote for (b), because (a) could cause unexpected runtime errors.
>   Additionally, I expect very little programs depend on
>   `{IntMap,IntSet}.deleteMin empty` causing runtime error.


I would like that it is consistent with Map.delete on an empty set.

Prelude> Map.delete undefined Map.empty :: Map.Map Char Int
fromList []


Looks like a vote for (b).