I would be happy enough with just re-exporting Down and calling it a day. Last time this topic came up it was bikeshedded to death without resolution. -Edward On Mon, Jul 9, 2012 at 5:51 PM, ARJANEN Loïc Jean David < arjanen.loic@gmail.com> wrote:

On 2012-06-28 16:40, Twan van Laarhoven wrote :

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On 2012-06-28 12:01, ARJANEN Loïc Jean David wrote:

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Hello all,

I recently noticed that despite conversations about that in this mailing-list, there isn't an order-reversing newtype in Data.Ord. That newtype would be quite useful, e.g. for reverse-sorting a list or a set, so I propose we add a newtype defined thusly:

newtype Reverse a = Reverse { fromReverse :: a } deriving (Eq)

instance Ord a => Ord (Reverse a) where compare (Reverse x) (Reverse y) = compare y x

I took the implementation from the Down newtype in GHC.Exts, so we could perhaps just re-export it. This will cause breakage to code already defining a type named Reverse, so I don't know if the damage will be important. Of course, the name and definition are open to discussion.

+1 with any sensible name except Dual.

The name Dual is also used for dual monoids. I would prefer to be able to tell from a piece of code like `f (Dual x)` whether it reverses the order of the arguments to `mappend` or of the arguments to `compare`.

As a good compromise, we could call the newtype "DualOrd" or something.

The extraction function should maybe be called getReverse (or getWhatever), which is the convention used by the wrappers in Data.Monoid.

Twan

So, what would people prefer between re-exporting GHC.Exts's order-reversing newtype (and thus calling it Down) and moving it to Data.Ord, provisionally calling it Reverse and with Twan's suggested modification ?

If we move it to Data.Ord, we would have to provide the existing Down in GHC.Exts (perhaps with a newtype, but that could become quite unwieldy) or at least deprecate it.

So, what are your thoughts ?

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On Tue, Jul 10, 2012 at 03:07:38AM -0400, wren ng thornton wrote:

On 7/9/12 7:29 PM, Edward Kmett wrote:

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I would be happy enough with just re-exporting Down and calling it a day. Last time this topic came up it was bikeshedded to death without resolution.

Indeed. I'd rather use "Reverse" than "Down", but I'd rather have Down in the standard library than have this bikeshedded to death again.

+1 for re-exporting Down and no bikeshedding. -Brent

On 10/07/12 14:31, Strake wrote:

On 09/07/2012, Edward Kmett wrote:

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I would be happy enough with just re-exporting Down and calling it a day. Last time this topic came up it was bikeshedded to death without resolution.

Why is it called "Down"? Is this a mathematical term?

I believe the Down newtype was introduced for use with extended list comprehension, things like [ name u | u <- users, then sort (Down (salary x)) ]. FWIW, I'll settle for reexporting Down if that avoids bikeshedding. Twan

On 11.07.12 4:31 AM, Conrad Parker wrote:

On 10 July 2012 23:28, Twan van Laarhoven wrote:

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On 10/07/12 14:31, Strake wrote:

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On 09/07/2012, Edward Kmett wrote:

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I would be happy enough with just re-exporting Down and calling it a day. Last time this topic came up it was bikeshedded to death without resolution.

Why is it called "Down"? Is this a mathematical term?

I believe the Down newtype was introduced for use with extended list comprehension, things like [ name u | u<- users, then sort (Down (salary x)) ].

That is illustrative; it reads much better than Reverse or DualOrd would, and I expect that sort is the most common use of an Ord instance.

+1 for re-exporting Down.

+1 for Down. (DualOrd would be also ok, but sounds artifical. Reverse has too much connotation of list-reverse for my taste.) -- Andreas Abel <>< Du bist der geliebte Mensch. Theoretical Computer Science, University of Munich Oettingenstr. 67, D-80538 Munich, GERMANY andreas.abel@ifi.lmu.de http://www2.tcs.ifi.lmu.de/~abel/