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Add instance Alternative (f :+: g)
by Dannyu NDos 06 Aug '19

06 Aug '19

instance Alternative f => Alternative (f :+: g) where empty = InL empty InL x <|> InL y = InL (x <|> y) InL _ <|> y = y x <|> _ = x This is analogous to the proposed Alternative Either instance.

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foldrM definition
by Dannyu NDos 06 Aug '19

06 Aug '19

Wouldn't it be friendlier to list structures if foldl' was used instead of foldl?

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Proposal: Remove Control.Monad.Instances
by David Feuer 02 Aug '19

02 Aug '19

The Control.Monad.Instances module in base has been deprecated since base-4.6.0, which was released in 2012. I believe the time has long since passed to actually remove the module, and I propose that we do so.

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Is there a function \x -> (x, x) in standard libraries?
by Ignat Insarov 30 Jul '19

30 Jul '19

Hello. I wonder if there is a function like \x -> (x, x) in the standard libraries. I looked up in Hoogle. It gave me links: 1. https://hackage.haskell.org/package/utility-ht/docs/Data-Tuple-HT.html#v:do… 2. https://hackage.haskell.org/package/extra/docs/Data-Tuple-Extra.html#v:dupe 3. https://hackage.haskell.org/package/relude/docs/Relude-Extra-Tuple.html#v:d… None of these is to the standard library. I wonder if there is an intentional omission for some important reason? If not, I think this function should be added to Data.Tuple. I know some of the persons that make decisions here like Category theory, so I point out this function is mentioned in Categories for the Working Mathematician, as δ, the diagonal function, on page 3 (second edition), although set with angular brackets. Have a great day.

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Re: Proposal: Make Semigroup and Monoid instances for Data.Functor.Compose
by Ryan Scott 26 Jul '19

26 Jul '19

Hi, You may find the discussion at https://mail.haskell.org/pipermail/libraries/2018-February/028571.html interesting. The summarized version of that post is that Data.Functor.Compose was originally brought over from the transformers library, which adheres to a very Haskell98 mindset in its design. In particular, the maintainer of transformers would likely not have added the Semigroup or Monoid instances you propose, since they require the FlexibleContexts language extension. This explains why there exists an `instance (Eq1 f, Eq1 g, Eq a) => Eq (Compose f g a)` and not an `instance Eq (f (g a)) => Eq (Compose f g a)`, among other things. Of course, Data.Functor.Compose now lives in the base library, not transformers, so we need not prescribe to the same design philosophy. I don't feel too strongly about the issue, so if other people feel like adding Semigroup/Monoid instances that require FlexibleContexts is a good idea, I could get on board with that. What do others think? Ryan S.

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Proposal: Make Semigroup and Monoid instances for Data.Functor.Compose
by Keith 25 Jul '19

25 Jul '19

With the Monoid 'First' and 'Last' newtypes going away, it would be nice to have equivalent Functor and Applicative instances. This works if Compose uses its components' instances: instance Semigroup (m (n a)) ⇒ Semigroup (Compose m n a) where Compose mnx <> Compose mny = Compose (mnx <> mny) instance Monoid (m (n a)) ⇒ Monoid (Compose m n a) where mempty = Compose mempty –Keith -- Sent from my Android device with K-9 Mail. Please excuse my brevity.

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Re: Proposal: composition util for Maps
by Johannes Waldmann 23 Jul '19

23 Jul '19

+0.9 we can have it > composition :: Ord b => Map b c -> Map a b -> Map a c > composition bc ab = flip mapMaybe ab $ flip lookup bc > Which has O(|ab| * log |bc|) performance. The given implementation traverses all of ab even if bc is empty. We could detect this. But for |bc| = 1, there is no work-around? So, O(|ab| * max(1, log |bc|)) ? I think this could only be improved if we had a representation of the inverse of ab as well. I don't think it should be computed on-the-fly, as in https://github.com/haskell/containers/issues/647#issuecomment-504798611 This would better be handled in a proper library for relations ( https://hackage.haskell.org/package/relation has this data type, but does not have composition?) bike-shedding the name and type: * composition vs. compose: the library usually favours the noun: intersection (not intersect), difference (not subtract) * order of arguments: mathematically speaking, it is of course very wrong, but it's consistent with the wrongness of (.) ... - J.W.

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ANNOUNCE: lens-family 2.0.0 anniversary edition!
by roconnor＠theorem.ca 20 Jul '19

20 Jul '19

In celebration of the 10th anniversary of Twan van Laarhoven's seminal blog post ``CPS based functional references''[1], I am releasing version 2.0 of lens-family[2] and lens-family-core[3] packages. This new release continues to explore the design of Van Laarhoven style optics with new support for adapters, grates, grids[4], and prisms. To bring support to these new optics necessarily means moving a little further away from syntactic compatibility with Kmett's lens library. In particular, lens-family's 'under' is unrelated to Kmett's lens library's 'under' combinator. Nonetheless the 'under' combinator plays a crucial role in lens-family as a dual to the 'over' combinator and this naming is hard to resist despite the conflict. This new version comes with some minor incompatibilities with the version 1.2 library that may require user updates. See <https://hackage.haskell.org/package/lens-family-2.0.0/changelog>. [1]<https://www.twanvl.nl/blog/haskell/cps-functional-references> [2]<https://hackage.haskell.org/package/lens-family-2.0.0> [3]<https://hackage.haskell.org/package/lens-family-core-2.0.0> [4]A grid is an optic that is both a grate and a traversal. -- Russell O'Connor <http://r6.ca/>

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Taking over `rungekutta`
by Marco Zocca 16 Jul '19

16 Jul '19

Hi all, I'd like to take over maintainership of this library https://hackage.haskell.org/package/rungekutta Its latest upload was in 2009 and it doesn't have a listed version control repository. I think it's safe to assume it's abandoned. The listed author email is unreachable and I cannot find an alternative email to contact the author. Thank you, Marco

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Cayenne Source Code
by Andrew Martin 09 Jul '19

09 Jul '19

I just read through Lennart's paper on Cayenne, which was written in Haskell. It would be informative for me to look at the source code for a simple dependently typed language like this. (The paper claims the typechecker is only around 5000 LoC.) However, I cannot find the source code anywhere. Has it been devoured by the cruel jaws of the ephemeral web, or does someone out there still have a copy of it? -- -Andrew Thaddeus Martin

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