Proposal: Add First and Last wrappers around Maybe to Data.Monoid

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Jeffrey Yasskin

4 Mar 2007 4 Mar '07

4:30 a.m.

I've created http://hackage.haskell.org/trac/ghc/ticket/1189 to suggest adding two Monoid instances around Maybe that, instead of accumulating things like most monoids, just pick the first or last piece of data they see. Would a week (March 12) be enough time to discuss this? Thanks, Jeffrey Yasskin

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David Menendez

5 Mar 5 Mar

12:08 a.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

Jeffrey Yasskin writes:

...

I've created http://hackage.haskell.org/trac/ghc/ticket/1189 to suggest adding two Monoid instances around Maybe that, instead of accumulating things like most monoids, just pick the first or last piece of data they see.

Rather than have two wrappers, I say make the a Monoid instance for Maybe using a left-biased choice (like the MonadPlus instance). Then we would get right-biased choice for free with the Dual wrapper. -- David Menendez | "In this house, we obey the laws http://www.eyrie.org/~zednenem | of thermodynamics!"

Andriy Palamarchuk

10:08 p.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

Jeffrey, would it be difficult to add an example or two for the intended usage? Thanks, Andriy ____________________________________________________________________________________ 8:00? 8:25? 8:40? Find a flick in no time with the Yahoo! Search movie showtime shortcut. http://tools.search.yahoo.com/shortcuts/#news

Jeffrey Yasskin

6 Mar 6 Mar

12:42 p.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

On 3/5/07, Andriy Palamarchuk wrote:

...

Jeffrey, would it be difficult to add an example or two for the intended usage?

Not at all. Two reasonable examples are:

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findLast :: Foldable t => (a -> Bool) -> t a -> Maybe a findLast pred = getLast . foldMap (\x -> if pred x then Last (Just x) else Last Nothing)

And, assuming Data.Map were extended with alterA :: (Applicative f, Ord k) => (Maybe a -> f (Maybe a)) -> k -> Map k a -> f (Map k a)

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instance Monoid a => Applicative ((,) a) -- from Control.Applicative

insertLookupWithKey :: Ord k => (k -> v -> v -> v) -> k -> v -> Map k v -> (Maybe v, Map k v) insertLookupWithKey combine key value = Arrow.first getFirst . alterA doChange key where doChange Nothing = (First Nothing, Just value) doChange (Just oldValue) = (First (Just oldValue), Just (combine key value oldValue))

I haven't run these yet, so they may need some tweaking. Since the rest of Data.Monoid doesn't have example uses, where do you want me to put these? Jeffrey

Andriy Palamarchuk

10:59 p.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

--- Jeffrey Yasskin wrote:

...

Since the rest of Data.Monoid doesn't have example uses, where do you want me to put these?

I include shorter examples right in the function descriptions (see http://darcs.haskell.org/libraries/base/GHC/List.lhs), or create a separate section for longer examples (see http://darcs.haskell.org/libraries/mtl/Control/Monad/Error.hs) Andriy ____________________________________________________________________________________ 8:00? 8:25? 8:40? Find a flick in no time with the Yahoo! Search movie showtime shortcut. http://tools.search.yahoo.com/shortcuts/#news

John Meacham

7 Mar 7 Mar

11:19 p.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

Would it be possible to move these wrappers and other data types out of Data.Monoid and into something like Data.Monoid.(Wrappers|Util|Combinators) or something like that? A huge amount of useful namespace is eaten by them and Data.Monoid is an extremely common thing to need to import. I mean Dual,Endo,All,Any,Sum,Product are definitely useful, but way to common and generically useful names to export from such a often imported package. John -- John Meacham - ⑆repetae.net⑆john⑈

Johannes Waldmann

8 Mar 8 Mar

6:11 a.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

John Meacham wrote:

...

A huge amount of useful namespace is eaten by them and Data.Monoid is an extremely common thing to need to import.

(as I say every half year or so) the real problem is that Haskell "import" is (speaking Ada) "with" and "use" at the same time, and there is no local "use" in Haskell. Use whatever names are appropriate in the library, and don't care too much for complaints from people that have acquired the ugly habit of importing everything unqualified :-) -- -- Johannes Waldmann -- Tel/Fax (0341) 3076 6479/80 -- ---- http://www.imn.htwk-leipzig.de/~waldmann/ -------

Jeffrey Yasskin

9 Mar 9 Mar

2:03 a.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

I've attached a second version of the patch to the ticket (http://hackage.haskell.org/trac/ghc/ticket/1189) along with the HTML Haddock generates from it. I added the two examples and the "imposed identity" instance directly on Maybe. Would one of its fans like to provide an example of using the Maybe monoid? On 3/7/07, John Meacham wrote:

...

Would it be possible to move these wrappers and other data types out of Data.Monoid and into something like Data.Monoid.(Wrappers|Util|Combinators) or something like that?

A huge amount of useful namespace is eaten by them and Data.Monoid is an extremely common thing to need to import.

I mean Dual,Endo,All,Any,Sum,Product are definitely useful, but way to common and generically useful names to export from such a often imported package.

Arguably, the monoid wrappers around a particular type should go with that type instead of in Data.Monoid. That would mean my change would be to Data.Maybe instead of here, and All, Any, Sum, and Product would go in the Data.Bool and the Prelude (or Data.Num?). I don't care a whole lot, but unless there's a consensus to move things around, I think my change should just follow the status quo. Thanks, Jeffrey

Jeffrey Yasskin

12 Mar 12 Mar

12:10 p.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

Today is the scheduled end for discussion on this proposal. Are there any final objections, suggestions, or comments before I update the ticket and ask someone to commit it? On 3/8/07, Jeffrey Yasskin wrote:

...

I've attached a second version of the patch to the ticket (http://hackage.haskell.org/trac/ghc/ticket/1189) along with the HTML Haddock generates from it. I added the two examples and the "imposed identity" instance directly on Maybe. Would one of its fans like to provide an example of using the Maybe monoid?

Spencer Janssen

3:41 p.m.

I'd like to note that the behavior for this Maybe instance is quite different from what you get with MonadPlus. This seems inconsistent with respect to the instances of MonadPlus and Monoid for lists. I realize I'm objecting in the 11th hour, but I think this requires some consideration. Cheers, Spencer Janssen On Mon, 12 Mar 2007 08:10:24 -0800 "Jeffrey Yasskin" wrote:

...

Today is the scheduled end for discussion on this proposal. Are there any final objections, suggestions, or comments before I update the ticket and ask someone to commit it?

On 3/8/07, Jeffrey Yasskin wrote:

...
I've attached a second version of the patch to the ticket (http://hackage.haskell.org/trac/ghc/ticket/1189) along with the HTML Haddock generates from it. I added the two examples and the "imposed identity" instance directly on Maybe. Would one of its fans like to provide an example of using the Maybe monoid?

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Jeffrey Yasskin

13 Mar 13 Mar

1:22 a.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

A MonadPlus instance and a Monoid instance will be used in different circumstances, so I don't see a compelling argument that they should always be the same if both exist for a particular type. Maybe you have one? Note that [] and the nearly identical Seq are the only types so far to have both instances, so there's no particular reason to think we can generalize from them. MonadPlus Maybe and Monoid First are the same though. If people want, and can provide a name for the type, I'm willing to wrap the "inject identity" instance in the same way First and Last are wrapped. But if nobody besides you speaks up in the next day or two, I'll assume it's not a widespread complaint and go with what I have. Thanks for the comment though, Jeffrey On 3/12/07, Spencer Janssen wrote:

...

I'd like to note that the behavior for this Maybe instance is quite different from what you get with MonadPlus. This seems inconsistent with respect to the instances of MonadPlus and Monoid for lists.

I realize I'm objecting in the 11th hour, but I think this requires some consideration.

Cheers, Spencer Janssen

On Mon, 12 Mar 2007 08:10:24 -0800 "Jeffrey Yasskin" wrote:

...
Today is the scheduled end for discussion on this proposal. Are there any final objections, suggestions, or comments before I update the ticket and ask someone to commit it?

On 3/8/07, Jeffrey Yasskin wrote:

...
I've attached a second version of the patch to the ticket (http://hackage.haskell.org/trac/ghc/ticket/1189) along with the HTML Haddock generates from it. I added the two examples and the "imposed identity" instance directly on Maybe. Would one of its fans like to provide an example of using the Maybe monoid?

Jeffrey Yasskin

15 Mar 15 Mar

1:01 p.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

I've updated the ticket (http://hackage.haskell.org/trac/ghc/ticket/1189) according to the instructions for library submissions. I think the general opinion was neutral to favorable, so I'd like to find someone to commit the patch. Ross, do you have commit privileges? Thanks, Jeffrey

Ian Lynagh

29 Mar 29 Mar

2:32 p.m.

New subject: Proposal: Add First and Last wrappers around Maybe to Data.Monoid

On Thu, Mar 15, 2007 at 10:01:06AM -0700, Jeffrey Yasskin wrote:

...

I've updated the ticket (http://hackage.haskell.org/trac/ghc/ticket/1189) according to the instructions for library submissions. I think the general opinion was neutral to favorable, so I'd like to find someone to commit the patch.

I've committed it. Thanks! Ian

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participants (7)

Andriy Palamarchuk
David Menendez
Ian Lynagh
Jeffrey Yasskin
Johannes Waldmann
John Meacham
Spencer Janssen