- Haskell-Cafe - Haskell.org

Following-up on the laziness thread on Reddit
by Saurabh Nanda 06 Dec '16

06 Dec '16

Thoughts on this particular comment - https://www.reddit.com/r/haskell/comments/5fyg9e/how_important_is_laziness_… - from core GHC hackers would be very interesting and enlightening. -- Saurabh.

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Call for Haskell.org Committee Nominations
by Gershom B 06 Dec '16

06 Dec '16

Dear Haskellers, It is time to put out a call for new nominations (typically but not necessarily self-nominations) to the haskell.org committee. We have four members of our committee due for retirement -- Adam Foltzer, Nicolas Wu, Andres Loeh, and Edward Kmett (who is stepping down early). As per our bylaws, three of the slots will be for regular three year terms, and one will be a short one year term to fill out the remainder of Edward's. To nominate yourself, please send an email to committee at haskell.org by December 2, 2016. The retiring members are eligible to re-nominate themselves. Please feel free to include any information about yourself that you think will help us to make a decision. The Haskell.org committee serves as a board of directors for Haskell.org, a 501(c)3 nonprofit which oversees technical and financial resources related to Haskell community infrastructure. Being a member of the committee does not necessarily require a significant amount of time, but committee members should aim to be responsive during discussions when the committee is called upon to make a decision. Strong leadership, communication, and judgement are very important characteristics for committee members. The role is about setting policy, providing direction/guidance for Haskell.org infrastructure, planning for the long term, and being fiscally responsible with the Haskell.org funds (and donations). As overseers for policy regarding the open source side of Haskell, committee members must also be able to set aside personal or business related bias and make decisions with the good of the open source Haskell community in mind. We seek a broad representation from different segments of the Haskell world -- including but not limited to those focused on education, those focused on industrial applications, those with background in organizing users-groups, and those focused directly on our technical infrastructure. More details about the committee's roles and responsibilities are on https://wiki.haskell.org/Haskell.org_committee If you have any questions about the process, please feel free to e-mail us at committee at haskell.org, or contact one of us individually. Best, Gershom Bazerman

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[course announcement] Haskell for Life
by Sergiu Ivanov 05 Dec '16

05 Dec '16

Dear Haskell Cafe, This year I have taught and will teach a short course introducing people to Haskell. I realised that such a course might interest the subscribers of this list, so here is the course page, with all course materials (CC-BY licence): http://lacl.fr/~sivanov/doku.php?id=en:haskell_for_life Any feedback is welcome. (I will teach the next instance of the course in the following two weeks in Chișinău, Republic of Moldova, so if you happen to be around, don't hesitate to come by and give me a wink IRL ;-) ) -- Sergiu

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Fearing Haskell
by Henk-Jan van Tuyl 03 Dec '16

03 Dec '16

L.S., IBM's Watson can analyze web pages, see https://alchemy-language-demo.mybluemix.net/ . If I feed it the URL https://wiki.haskell.org/Haskell , Watson states that this page is about a person called Haskell, with a fearful sentiment. With the URL https://en.wikipedia.org/wiki/Haskell_(programming_language) , there is both joy and sadness in Haskell. :-) Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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monad-coroutine alternatives
by Alex 03 Dec '16

03 Dec '16

Hello, Are there any data structures isomorphic to Coroutine[0] available on Hackage? -- Alex [0] https://hackage.haskell.org/package/monad-coroutine-0.9.0.3/docs/Control-Mo…

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An idea on extensible effects (anonymous record)
by winter 01 Dec '16

01 Dec '16

Hi everyone! First of all, i don't know if this idea is already being discussed so if there's a discussion on this i'd like to follow. To illustrate, first i want to create a class like: class Has a t where get :: t -> a then i define instance for simple product type such as tuple: instance Has a (a, b) where get (a, _) = a instance Has b (a, b) where get (_, b) = b ... You can image i will use th to make lots of instance for difference tuple size. Now if i want an extensible reader, i use Has class like this: someReader :: Has Int t => Reader t Int someReader = do x <- ask return $ get x + 1 Then i can run it with any tuple with an Int field like: runReader someReader (0 :: Int, "adad”) -- 1 This typeclass almost solved all problem of my network application: sometime’s i want ensure a logger, a sql backend and a http client pool in my monad’s environment, but i don’t want to fix my environment into a record. We can add a set :: a -> t -> t, or use lens to define Has, so that we can have extensible states. We can also use Tagged to achieve something like: (Has (Tagged “SqlBackEndOne” SqlBackEnd) t, Has (Tagged “SqlBackEndTwo" SqlBackEnd) t) => Reader t () It there a library doing this, maybe in lens? or there’re some drawbacks i didn’t notice? All ideas are welcomed! Cheers~ Winter

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Debug tracing in Haskell
by Clinton Mead 01 Dec '16

01 Dec '16

I've been debugging some Haskell code, including a lot of work on Mutable Vectors, so naturally I've found `trace` useful, and in particular `traceM`, because it fits in nicely with monadic code. The problem is that unlike `assert`, both `trace` and `traceM` execute unconditionally, I can't toggle them with a compiler flag. I could however do something like this in every file I want to do tracing. {-# LANGUAGE CPP #-} #ifdef TRACE trace = Debug.Trace.trace traceM = Debug.Trace.traceM #else trace _ = id traceM _ = pure () #endif But already, that's a lot of boilerplate. I'd also like to trace based on debug levels. For example, at TRACE level 2, only print trace statements at level 1 or 2, but at TRACE level 4, print trace statements at level 1, 2, 3 and 4, providing more detail (but more noise). This makes the above code even more complex. This wouldn't be a problem if I could put the code in a separate package, but I can't, as then whether tracing is on or not depends on the compiler settings of the tracing package, not on the calling package, which defeats the purpose somewhat. I considered using implicit parameters to quietly pass whether I want tracing and at what level into the tracing module, but it seems to be that implicit parameters can't be defined at the top level. It gets even more complex. When debugging, I might want to print something of type 'a'. Obviously 'a' will need some sort of show method for this to work, but outside of debug mode I don't want to restrict my function's types to things which are showable. So I considered doing this: #ifdef TRACE type DebugShow a = Show a debugShow = show #else type DebugShow a = () debugShow _ = error "DEBUG SHOWING OUTSIDE OF DEBUG MODE" #endif And of course, if you're only using `debugShow` as part of an argument to `trace`, lazy evaluation will avoid `debugShow` ever being called when tracing is not enabled. But put all this together and you've got around a dozen lines of boilerplate just to do tracing, without even having tracing levels yet, that have to be put in every module you want to use tracing, but don't want a whole lot of debug data being spat out in a release compile. Also, adding any other tracing functions just makes this longer. The only approach I can think of so far is to whack this all in a template haskell module and add code that splices it all into the current module: e.g. #ifdef TRACE $(traceFunctions True) #else $(traceFunctions False) #endif Where `traceFunctions` is a template haskell function that dumps all the appropriate functions and type definitions mentioned above (and maybe more) at the top level of the calling module. So my questions are: 1. Is there a better way? And 2. Has this problem already been solved?

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complexity of functions in PriorityQ.hs
by Michael Litchard 01 Dec '16

01 Dec '16

I'm trying to figure out the complexity of the functions in Melissa O'Neill's Priority Queue library, Priority.Q.hs Below find definitions, and what I think the complexity is. I'd appreciate feedback, especially on the ones I have marked as guesses. data PriorityQ k v = Lf | Br {-# UNPACK #-} !k v !(PriorityQ k v) !(PriorityQ k v) deriving (Eq, Ord, Read, Show) O(log n) . This is a guess but alternating which side of the sub-tree is being traversed is my hint. insert :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v insert wk wv (Br vk vv t1 t2) | wk <= vk = Br wk wv (insert vk vv t2) t1 | otherwise = Br vk vv (insert wk wv t2) t1 insert wk wv Lf = Br wk wv Lf Lf I have no idea. Reasoning help here please. siftdown :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v -> PriorityQ k v siftdown wk wv Lf _ = Br wk wv Lf Lf siftdown wk wv (t @ (Br vk vv _ _)) Lf | wk <= vk = Br wk wv t Lf | otherwise = Br vk vv (Br wk wv Lf Lf) Lf siftdown wk wv (t1 @ (Br vk1 vv1 p1 q1)) (t2 @ (Br vk2 vv2 p2 q2)) | wk <= vk1 && wk <= vk2 = Br wk wv t1 t2 | vk1 <= vk2 = Br vk1 vv1 (siftdown wk wv p1 q1) t2 | otherwise = Br vk2 vv2 t1 (siftdown wk wv p2 q2) Whatever the cost of siftdown is. deleteMinAndInsert :: Ord k => k -> v -> PriorityQ k v -> PriorityQ k v deleteMinAndInsert wk wv Lf = error "Empty PriorityQ" deleteMinAndInsert wk wv (Br _ _ t1 t2) = siftdown wk wv t1 t2 This looks like O(log N) to me. leftrem :: PriorityQ k v -> (k, v, PriorityQ k v) leftrem (Br vk vv Lf Lf) = (vk, vv, Lf) leftrem (Br vk vv t1 t2) = (wk, wv, Br vk vv t2 t) where (wk, wv, t) = leftrem t1 leftrem _ = error "Empty heap!" The cost of siftdown, which is probably more expensive than leftrem. deleteMin :: Ord k => PriorityQ k v -> PriorityQ k v deleteMin (Br vk vv Lf _) = Lf deleteMin (Br vk vv t1 t2) = siftdown wk wv t2 t where (wk,wv,t) = leftrem t1 deleteMin _ = error "Empty heap!" Thanks for the feedback. I'm new to computational analysis and this is the most complicated code I have tried to tackle thus far. I have Okasaki's book, but am having trouble making best use of it.

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HWWG
by Ben Spencer 30 Nov '16

30 Nov '16

Hello all, Is there a way to get involved with the HWWG? Is there a slack channel or a mailing list? Best, Ben Spencer Sent from my iPhone

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[ANN] Sprinkles - zero-programming web dev platform
by Tobias Dammers 30 Nov '16

30 Nov '16

A little something I've been working on: https://sprinkles.tobiasdammers.nl/ You feed it a bunch of templates, content in files, databases, APIs, etc., define routes and map them onto data sources and templates using a YAML configuration file, and it'll serve that as a full blown website. It sits somewhere in between a classic CMS and a static site generator. The code is available on github: https://github.com/tdammers/sprinkles Both the sprinkles website and that for ginger (https://ginger.tobiasdammers.nl/) are built on Sprinkles (following some advice about dog food I read somewhere). Before I unleash this to a more general audience, I'd like to get some honest (harsh) criticism here, on both the code itself and the available materials (website, documentation). Feel free to roast me relentlessly. -- Tobias Dammers - tdammers(a)gmail.com

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