- Libraries - Haskell.org

Re: Functor, Applicative, Monad, Foldable, Traversable instances for (, , ) a b
by Jonathon Delgado 27 Apr '17

27 Apr '17

Code please? An actual, concrete example?

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Functor, Applicative, Monad, Foldable, Traversable instances for (, , ) a b
by Fumiaki Kinoshita 26 Apr '17

26 Apr '17

It's surprising that they are missing (forgive me, I'm not here to make people grumpy).

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Add Arg{Min,Max} pattern synonyms to Data.Semigroup
by Baldur Blöndal 21 Apr '17

21 Apr '17

Add (explicitly-bidirectional record) pattern synonyms to Data.Semigroup: pattern ArgMin :: a -> b -> ArgMin a b pattern ArgMin {minArg, minValue} = Min (Arg minArg minValue) pattern ArgMax :: a -> b -> ArgMax a b pattern ArgMax {maxArg, maxValue} = Max (Arg maxArg maxValue) As described in https://ghc.haskell.org/trac/ghc/ticket/13572

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Is mkTrApp unsafe?
by Andrew Martin 15 Apr '17

15 Apr '17

I've been playing around with the GHC 8.2 release candidate. In Type.Reflection.Unsafe, there's a really handy function named mkTrApp. Everything else in that module is pretty evidently unsafe, but I'm not so sure about mkTrApp. I'm wondering if it should actually be exported by Type.Reflection instead or if I've missed some way it could be used to create an ill-kinded TypeRep. -Andrew Martin

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Banded Matrices
by Dominic Steinitz 13 Apr '17

13 Apr '17

Hi Henning, I just noticed https://hackage.haskell.org/package/hmatrix-banded. I have just made a PR to hmatrix to add a tridiagonal solver: https://github.com/albertoruiz/hmatrix/pull/224. I don’t think there is anything to do but I thought it worth sharing. Dominic Steinitz dominic(a)steinitz.org http://idontgetoutmuch.wordpress.com

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Re: Constraints on definition of `length` should be strengthened
by Anthony Clayden 10 Apr '17

10 Apr '17

[Errk, somehow posted this with no Subject. Trying again.] > On Sun Apr 9 19:24:12 UTC 2017, Sven Panne wrote: >> 2017-04-09 2:01 GMT+02:00 Ben Franksen : >>> Am 08.04.2017 um 04:03 schrieb Anthony Clayden: >>> [...] So to take your examples from an earlier thread, >>> what do you expect Haskell to do here?: >>> >>> maximum (True,2) => ? >>> minimum ((3, 4),5) => ? -- i.e. :: ((Int, Int), Int) >>> sum (7,3.14) => ? -- i.e. :: (Int, Double) >>> product (Left $ error "Errk") => ? -- i.e. :: >>> Either e Int >> >> I (and others) think these should be type errors. >> > Exactly. OK. Do y'all think there should be a `toList` method for them? Indeed, should there be a `toList` method available for any data type that doesn't implement `fromList`? Perhaps `toList` should not be a method of Foldable? > >>> Do you expect Foldable (a, a) to behave differently >>> vs Foldable (b, a) vs Foldable ((a, a), a) vs ...? >>> >> It would be nice if we could all agree on removing these >> instances. >> > Yep. :-) >> BTW, I find it remarkable that of those who defend these >> instances, few seem to be especially interested >> in clarifying the OP's question: what >> laws exactly do we expect for Foldable? [...] >> One of the Laws appears to be that `length` etc behave as `length . toList`. AntC

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None
by Anthony Clayden 10 Apr '17

10 Apr '17

> On Sun Apr 9 19:24:12 UTC 2017, Sven Panne wrote: >> 2017-04-09 2:01 GMT+02:00 Ben Franksen : >>> Am 08.04.2017 um 04:03 schrieb Anthony Clayden: >>> [...] So to take your examples from an earlier thread, >>> what do you expect Haskell to do here?: >>> >>> maximum (True,2) => ? >>> minimum ((3, 4),5) => ? -- i.e. :: ((Int, Int), Int) >>> sum (7,3.14) => ? -- i.e. :: (Int, Double) >>> product (Left $ error "Errk") => ? -- i.e. :: >>> Either e Int >> >> I (and others) think these should be type errors. >> > Exactly. OK. Do y'all think there should be a `toList` method for them? Indeed, should there be a `toList` method available for any data type that doesn't implement `fromList`? Perhaps `toList` should not be a method of Foldable? > >>> Do you expect Foldable (a, a) to behave differently >>> vs Foldable (b, a) vs Foldable ((a, a), a) vs ...? >>> >> It would be nice if we could all agree on removing these instances. >> > Yep. :-) >> BTW, I find it remarkable that of those who defend these instances, few >> seem to be especially interested in clarifying the OP's question: what >> laws exactly do we expect for Foldable? [...] >> One of the Laws appears to be that `length` etc behave as `length . toList`. AntC

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Giving up boxes package
by David Feuer 10 Apr '17

10 Apr '17

I've been way too busy with GHC and containers to have time to devote to boxes. If anyone wants to take over, they are welcome. The main priority right now is improving documentation, but someone also needs to make sure it works with 8.2 as soon as its dependencies do.

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Re: Constraints on definition of `length` should be strengthened
by Anthony Clayden 10 Apr '17

10 Apr '17

> On Tue Apr 4 06:50:55 UTC 2017, Sven Panne wrote: >> 2017-04-04 8:15 GMT+02:00 Vladislav Zavialov: >>> Tuples are unbiased cartesian products, full stop. >> >> This statement is not correct. > According to probably all the math books in the world, > the statement is correct, > at least if we want to see tuples as cartesian products. .. Sven, I have a lot of sympathy with the digruntlement about Foldable `length` over tuples. But if you're going to start appealing to math, you'd better get your math correct. Whether or not tuples are unbiased, even wikipedia will tell you they're not commutative. So to take your examples from an earlier thread, what do you expect Haskell to do here?: maximum (True,2) => ? minimum ((3, 4),5) => ? -- i.e. :: ((Int, Int), Int) sum (7,3.14) => ? -- i.e. :: (Int, Double) product (Left $ error "Errk") => ? -- i.e. :: Either e Int Do you expect Foldable (a, a) to behave differently vs Foldable (b, a) vs Foldable ((a, a), a) vs ...? Let's get everybody agreed on that. Then we can talk about our currying/kind system. Note that there's been an `instance Foldable Maybe` for ever, and `toList` applied to that returns a 0-or-1 length list. >> Look at their kind: >> >> > :k (,) >> > (,) :: * -> * -> * >> >> The same currying business is going on here. [...] > That's an artifact of our kind system, > not a consequence of the usual definition of cartesian products. AntC

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Constraints on definition of `length` should be strengthened
by Paolo Giarrusso 10 Apr '17

10 Apr '17

TL;DR. One (implicit?) assumption in recent debates on `length` seems to be unfounded, because current docs for Foldable are meant to be much stronger than their actual language, unless I'm missing something. Clarifications welcome. If my understanding is right, I'd suggest somebody fixes the docs. One point is not clear to me, so at present I could not volunteer to fix them myself. In recent debates, it was assumed or implied that `length` must be equivalent to length = length . toList > we also have a kind > system, so we can ignore the name. > length :: f a -> Int > We immediately know that values of the kind (* -> *) slot in to the > value (f), with a kind checker to ensure we get it correct. Therefore, > we can easily reason about the length of values of kind ((,) a) I don't quite get how that argument is supposed to proceed. However that's meant, that seems incorrect because length is a typeclass method, so even the following strawman instance typechecks: instance Foldable ((,,) a b) where length _ = 42 I assume this should violate some law, but the relevant law seems to be forgotten. The most constraining language I can find is the following: > sum, product, maximum, and minimum should all be essentially equivalent to foldMap forms, such as > sum = getSum . foldMap Sum > but may be less defined. but (a) `length` is not even mentioned, even if it's intended (b) I think those should be laws (c) using "should" and "essentially" in "should all be essentially equivalent", seems too weak. I can infer the intention, but this seems insufficient to declare Docs for `length` don't help either: > Returns the size/length of a finite structure as an Int. The default implementation is optimized for structures that are similar to cons-lists, because there is no general way to do better. I mean, these docs use the English word "length", so that actually forbids 42, but that text is too vague to forbid 3 frankly. As I missing something? I also take issue with "may be less defined", and here I'm not sure of the intention, since that declares instance Foldable ((,,) a b) where length _ = undefined as legal. I imagine the point is about taking the length of partially undefined structures, but it's not clear to me why a custom `sum` implementation would be less defined than `sum = getSum . foldMap Sum`. Even ignoring the above instances (which I would never write down), I can't reason much about my code based on those specifications. This situation seems unfortunate. Cheers, -- Paolo G. Giarrusso - Ph.D. Student, Tübingen University http://ps.informatik.uni-tuebingen.de/team/giarrusso/

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