Re: Map with a different Monoid instance

I think we definitely don't want a Semigroup constraint on the key. Aside from the (potentially severe) performance issues, I think the semantics would be pretty awful. Inserting a key-value pair could potentially collapse a large map into a singleton! On Thu, May 25, 2017 at 4:37 PM, Michael Snoyman wrote:

That's a really good question, and could theoretically even generalize more than that: should `insert` have a `Monoid` or `Semigroup` constraint on the key as well? Should this API explicitly avoid any form of throwing away values, and insist that, if that's the behavior you want, you do something like `insertWith const`?

I think my guess is in line with yours, that the rest of the API functions should continue with the discard behavior of the current API, but it's worth at least raising the question.

On Thu, May 25, 2017 at 2:08 PM, David Feuer wrote:

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Unbiased sounds nice, but I'm a bit concerned that it might suggest bigger differences than just the Monoid instance. I assume people still want the same left-biased union function.

On Thu, May 25, 2017 at 3:43 PM, Michael Snoyman wrote:

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On Thu, May 25, 2017 at 11:38 AM, Mario Blažević wrote:

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On 2017-05-25 12:55 PM, David Feuer wrote:

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A lot of people have wrappers around Data.Map and Data.IntMap to give them more useful (Semigroup and) Monoid instances. I'd like to add such wrappers to containers. What we need to be able to do that are *names* for the new modules. I can't think of any, so I'm reaching out to the list. Please suggest names!

Data.Map.Monoidal is not strictly correct but would give a pretty good idea at first glance.

Data.Map.Symmetric would be more correct, since its Semigroup and Monoid instances would be symmetric, with no preference for the left argument as currently.

Just to throw out an option here: Unbiased. I don't feel strongly about it, but thought throwing it out may be helpful.

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Another question is whether we should take the opportunity of new modules to modernize and streamline the API a bit. I'd like, at least, to separate "safe" from "unsafe" functions, putting the unsafe ones in .Unsafe modules.

I think it would be better to keep the API exactly the same, much like Data.Map.Strict does. I don't want to think about the incidental API differences when I switch from one module to another. If you're going to modernize, modernize all the modules at once. That's what version numbers are for.

+1. I'd also argue against changing the API right now.

Michael

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