G'day all.
Quoting Tomasz Zielonka
It doesn't work when the type Foo doesn't belong to Ord class.
Right.
There is a simple technique which helped me when I wanted to make a priority queue of elements with Double key, and IO () value. I wanted to use something from Edison, but most of relevant data structures didn't distinguish key from value and insisted that element type belong to Ord class.
Yes. Edison priority queues are collections, not associations. This is a sufficiently common case that this might be worth thinking about for those working on a new collection system.
I just introduced a wrapper type that provides a trivial "Egalite" ordering.
Just added this to PreludeExts, though I called it OrdWrapper. (NoOrd suggests that it doesn't support Ord, whereas it's the underlying type which doesn't support Ord, or at least, you want to suppress its default Ord behaviour.) Cheers, Andrew Bromage