Eq and Ord instances for Prio queues

[treeowl]

• Make the Eq and Ord instances for Prio queues compare the queues in fully sorted form—that is, as key-value maps. This seems to be the only way to make these instances make any real kind of sense.

• Document the "nondeterministic" nature of Prio queues.

[treeowl]

@konsumlamm Any comments?

[konsumlamm]

Sorry, I don't have much time lately, I'll try to keep up with your PRs.

I'm a bit skeptical about requiring Ord a for Eq (MinPQueue k a). That would be a breaking change and many value types I can imagine don't necessarily have an Ord instance (although I can't think of many use cases for an Eq instance anyway...). I'd be fine with just documenting that the Eq instance is non-deterministic and recommending (==) `on` toAscList. However, then we also can't change the Ord instance, since they should be compatible.

Btw, please try to link related issues, like #78. Is this supposed to close #78?

[konsumlamm]

This should have been 'MinQueue', no?

[treeowl]

Yes, I'll fix that.

[konsumlamm]

Suggested change

	-- | \(O(n)\). @'mapKeysMonotonic' f q == 'Data.PQueue.Prio.Min.mapKeys' f q@,
	-- but only works when @f@ is strictly monotonic.
	-- /The precondition is not checked./
	-- This function has better performance than 'Data.PQueue.Prio.Min.mapKeys'.
	-- | \(O(n)\). @'mapKeysMonotonic' f q == 'Data.PQueue.Prio.Min.mapKeys' f q@,
	-- but only works when @f@ is strictly monotonic.
	-- /The precondition is not checked./
	-- This function has better performance than 'Data.PQueue.Prio.Min.mapKeys'.

[treeowl]

What am I missing?

[konsumlamm]

The change is to remove the blank line, unfortunately the diff doesn't show this.

[treeowl]

I really want the Eq instance to actually mean something. I think the original idea, when it was (mistakenly) thought that insertBehind could be implemented efficiently, was that Eq would be based on the order elements come out of the queue, and merge would be nondeterministic. But since insertBehind was a failure, I think we really want to make Eq equality of queues-as-maps, with merges considered deterministic. It's fine to require Ord. Is the instance really useful? Maybe not very, but the current one is nonsense. Let's do this please.

[treeowl]

Yes this should close #78.