Add unionsWith to combine a list of hash maps

[ppetr]

No description provided.

[sjakobi]

Uh, sorry for the late review… 😄

Probably a dumb question, by why doesn't this need both strict and lazy variants?

[sjakobi]

Probably a dumb question, but why doesn't this need both strict and lazy variants?

To answer my question: It quite certainly does! We also have lazy and strict variants of unionWith.

Before any further work on the code is done here, we need to make a decision on the correct type for this function though: See #240 (comment).

[treeowl]

I continue to wonder if this function meets the Fairbairn threshold. It's an entirely immediate application of foldl' and unionWith.

[sjakobi]

I continue to wonder if this function meets the Fairbairn threshold. It's an entirely immediate application of foldl' and unionWith.

One argument for providing the function is that we could include proper documentation on the order in which the values are combined. unionWith unionsWith has the same problem as fromListWith, I believe.

[treeowl]

That's a problem for unionWith. If we don't provide unionsWith, then we certainly don't have to document it! At all! Huge win.

[sjakobi]

Sorry, I meant to say that unionsWith has a similar gotcha as fromListWith. That's an argument for holding off on unionsWith until we've made some progress on #264 though.

[Prillan]

I continue to wonder if this function meets the Fairbairn threshold. It's an entirely immediate application of foldl' and unionWith.

Just a comment from a random passerby, I'd much prefer to have unions and unionsWith exposed by the library since I, a normal user without any intricate knowledge of the library, do not know which implementations would be best.

By not including the two functions you would leave it up to everyone to figure out whether foldl' union empty, fromList . concatMap toList or any other version is the most performant. The best implementation might not even use any exported functions!