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Re: [Haskell-cafe] Seeking Help with IHaskell Maintenance/Development
by Jonathan Wheaton 07 May '17

07 May '17

As a Haskell newbie and someone who uses IHaskell as a way of exploring Haskell from anywhere (work, various laptops and Chromebooks at home, my mobile phone even), I would be really sad to see IHaskell go away. I'm very much hoping the project can find a maintainer, as it's been invaluable to me. Jonathan Wheaton On May 7, 2017 11:52 AM, "Andrew Gibiansky" <andrew.gibiansky(a)gmail.com> wrote: Hey folks, For the past several months, I have not been able to put any effort into maintaining IHaskell, a front-end for Jupyter for Haskell. I do not know how useful IHaskell has been in the past to the community, but as it becomes more and more outdated it becomes less useful. If anyone is interested in helping out (or taking over) IHaskell maintenance and development, please let me know. Best, Andrew -- – Andrew _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

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[ANN] ble: Bluetooth Low Energy in Haskell
by Julian K. Arni 07 May '17

07 May '17

Dear list, I've released the package 'ble' a little while ago, but hadn't announced it so far because I wanted to make sure the API stabilized somewhat, and at least some bugs were addressed, in the course of dealing with our own needs. But now feels like a good time to announce it. 'ble' allows writing Bluetooth Low Energy peripherals in Haskell (and soon, centrals too). Currently only Linux (with Bluez >= 5.41) is supported. Mostly it exposes the GATT API. The README (in the github repo and in the Hackage main page) provide a simple example application. Github: https://github.com/plow-technologies/ble Hackage: http://hackage.haskell.org/package/ble Enjoy, ~~~ Julian Arni Haskell Consultant https://turingjump.com/

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Seeking Help with IHaskell Maintenance/Development
by Andrew Gibiansky 07 May '17

07 May '17

Hey folks, For the past several months, I have not been able to put any effort into maintaining IHaskell, a front-end for Jupyter for Haskell. I do not know how useful IHaskell has been in the past to the community, but as it becomes more and more outdated it becomes less useful. If anyone is interested in helping out (or taking over) IHaskell maintenance and development, please let me know. Best, Andrew -- – Andrew

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Partial type family application
by mniip 06 May '17

06 May '17

So I've been thinking of a type system extension: partial type family application. If we look at the type system used by modern GHC, in it, type family application is very distinct from constructor/constraint application: it is somewhat opaque in the sense that a type family must always appear fully applied. As such, it doesn't make sense to talk about a type family's kind, if we have: type family Id (a :: *) :: * where Id x = x then saying that 'Id :: * -> *' would be wrong, because Id cannot be used interchangeably with things that are truly of kind * -> *, such as Maybe or []. The reason for the "fully applied" restriction is that if we allowed partially applied type functions, then their equality would be undecidable. Still, I think, in a lot of cases we could benefit from partially applied type functions, even without extensional equality. Writing type level list folds, filters, zips, you name it. As stated above, we can't add the proposition 'Id :: * -> *' to our type system, so, what if, instead, we added a different kind of arrow kind for type families? I like the look of (~>), so 'Id :: * ~> *'. (~>) :: * -> * -> * This rule of inference, similar to the usual, would supersede all the type family kinding rules: f :: k ~> l, x :: k ------------------- f x :: l As such, the juxtaposition operator would very clearly have two different "kinds": (k -> l) --> k --> l and (k ~> l) --> k --> l, and there is no N-ary "type family application" operator. In order for the type checker to not fall apart, we need to have at least intesional (structural) equality on (~>), which we can have because it is decidable. We could then write stuff like type family Foldr (f :: k ~> l ~> l) (z :: l) (xs :: [k]) :: l where Foldr f z '[] = z Foldr f z (x ': xs) = f x (Foldr f z xs)

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Generalized concatMap for lenses?
by Tyson Whitehead 06 May '17

06 May '17

I am writing some code to do some web-page parsing using the lens library. The folded (or traverse) functions let me go sequence -> lens fold It would be nice to be able to go back again lens fold -> sequence The general idea being move it into a lens-fold sequence representation, work with it there, and then recover the original sequence representation. As a more concrete example, the following works for lists (folded .) :: Getter s [a] -> Fold s a (to . toListOf) :: Fold s a -> Getter s [a] I think I'm maybe thinking of something like a generalized concatMap for lenses magic :: Fold s a -> Getter (f s) (f a) Possibly there is already a function for this or some other way it should be done? Haven't found it yet though, so thought I would ask the list for some enlightenment. Thanks! -Tyson

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Functor instance for FunPB
by Alex Wede 06 May '17

06 May '17

Hey Everyone, ich have a algebraic data type: data FunPB a b = FunPB { runFunPB :: a -> (a,[b]) } I got a Monoid Instance instance Monoid (FunPB a b) where mempty = FunPB $ \k -> (k,mempty) mappend pb1 pb2 = FunPB $ \n -> (,) n $ msum . (<$>) (uncurry (flip const).((flip runFunPB) n)) $ [pb1,pb2] Now I need to write a functor instance instance Functor (FunPB a) where fmap f b = ? How can I map over a->(a,[b])? Lg Alex

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Hunting for ghci manual input vs. file input differences again
by MarLinn 05 May '17

05 May '17

Hi, I've been working on a kind of constrained version of Data.Dynamic: data Dynamic (c :: * -> Constraint) where Dynamic :: (Typeable a, c a) => a -> Dynamic c The idea is that Data.Dynamic gives up all compile time type information. In this variant I can at least keep a bit of the information around that I have. (In fact I can even "lift" a type into a constraint, thereby keeping /all/ information.) Most of my functions are nailed down, including mappings and traversals. But one group of functions eludes me: functions to change only the constraint, while keeping the value. Here is one of my goal types: castDynamic :: ( Typeable a, c a, d a ) => Dynamic c -> Dynamic d I'm pretty sure I'll have to sprinkle in a Proxy a, but so be it. I've thrown all typing tricks at this function that I know of, from unsafeCoerce over case-of instead of pattern matching, to ScopedTypeVariables. But I couldn't convince ghci to accept my code. So I tried something simpler. castDynamic' :: (Typeable a, c a) => Dynamic Show -> Maybe (Dynamic c) Here's where it gets funny: I can implement this function in ghci interactively just fine. >>> :t \(d :: Dynamic Show) -> case d of Dynamic a -> Dynamic <$> cast a \(d :: Dynamic Show) -> case d of Dynamic a -> Dynamic <$> cast a :: (Typeable a, c a) => Dynamic Show -> Maybe (Dynamic c) But when I enter the exact same code in a file and load it, ghci balks at me because a is too ambiguous. Again, I've tried if it's the monomorphism restriction, or if I need to sprinkle in more (scoped) signatures or explicit forall's etc… nothing helped. What is the difference here? Am I missing an extension? Am I doing something I shouldn't and the interactive mode is just doing me a favor? There was a discussion on this list less than a month ago where it was mentioned that ghci handles polymorphic types differently depending on the source of the code. Is there some documentation on these differences? Any help would be appreciated. Cheers, MarLinn

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http-client(-tls) or http-conduit?
by Baojun Wang 05 May '17

05 May '17

Hi cafe, It seems both(all) packages are actively maintained, however, they seem to provide a lots of similar functionalities, is there any preferences to use any one of them, and the reason why (Personally I'd prefer the former because of familiarity)? I've read [ https://haskell-lang.org/library/http-client] but it doesn't have recommendation. Thanks baojun

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Re: [Haskell-cafe] Foldable for (,)
by Olaf Klinke 04 May '17

04 May '17

Look at the Stream functor in Data.Stream [1]. Below I will argue by examples that while both Foldable and Traversable instances can be defined, the Foldable instance is less likely to produce terminating functions. Streams can be given a Traversable instance easily: instance Traversable Stream where sequenceA (Cons fa fas) = liftA2 Cons fa (sequenceA fas) For some applicatives this will not terminate. For streams of Maybes, sequenceA will either yield Just an infinite stream or return Nothing. Streams of IO actions also seem fine. A possible Foldable instance would suffer the same termination issues due to recursion. But with a monoid, folds over a stream will terminate less often, e.g. instance Foldable Stream where foldMap f (Cons x xs) = mappend (f x) (foldMap f xs) Then, all (xs :: Stream Bool) will diverge for the constant True stream and terminate for all others. -- Olaf [1] http://hackage.haskell.org/package/Stream-0.4.7.2/docs/Data-Stream.html

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Haskell type declaration summary
by Andrew Butterfield 04 May '17

04 May '17

Dear Haskellers, any quick and easy way to extract all the type declarations from a Haskell source file? (for easy reference) Regards, Andrew Butterfield School of Computer Science & Statistics Trinity College Dublin 2, Ireland

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