Ord instance for Data.Map?

Johannes Waldmann waldmann at imn.htwk-leipzig.de
Wed Apr 18 17:11:54 EDT 2007

Isaac Dupree wrote:

> And then what do you do with (set1 `union` set2) where they have
> different comparators? 

Nice question! Looking at the JDK source,
in the TreeSet implementation of Collection.addAll (..),
they actually compare the comparator objects
( with .equals() , so you can define that as you like )

>  But you can always directly have the (key -> key -> Ordering) function
> be part of your data structure for the same effect,

Yes, I meant that function when I said "the Ord dictionary".
Sorry for confusion.

> and then you can see
> all the problems involved (serialization, interactions and comparisons
> between maps, etc.).

Well, of course the Haskell system cannot serialize or compare
(anonymous) functions.

In Java land, as the compare function is attached
to a Comparator object, the burden of serialization
is on the programmer.
(also of writing the equality test mentioned above).
Looks rather sensible to me, under the circumstances.

Best regards, J. W.

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