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Problems with function recursion in Haskell
by Henson 14 May '16

14 May '16

Hi, I'm new in Haskell and I need help in recursion. That function below is returning "*** Exception: Prelude.head: empty list" and I need resolve that: execNTimes 0 [] = return() execNTimes n xs = if n<=0 || null xs then return() else do si <- getLine let s = words si l = read (s !! 1) :: Int r = read (s !! 2) :: Int if head s=="Q" then do let z = slice l r xs m = foldl lcm 1 z print (m `mod` (toInteger 1000000007)) else do let s1 = update l r xs execNTimes (n-1) s1 execNTimes (n-1) xs Anybody can me help? Thank you, Josenildo Silva

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Using parallels for fibonacci
by Michael Litchard 11 May '16

11 May '16

I am trying to efficiently use multicores for my fizzbuzz <https://github.com/mlitchard/swiftfizz> project. My fizzbuzz uses a Fibonacci generator as input, and this is where it can get computationally heavy. I believe I have picked the best algorithm for my project (please correct this if wrong), and now I am trying to use the parallel package <https://hackage.haskell.org/package/parallel-3.2.1.0>. I am not getting any better performance in the sense that if I try to compute the 100th Fibonacci number, it is still computing , using 4 cores, several minutes later. Here is my attempt. Please show me how to use this library right. from src/FizzBuzz.hs fizzBuzzFib :: Integer -> [Text] fizzBuzzFib ub = parMap rdeepseq fizzbuzz $! fibSeq ub from src/FizzFub.hs fibSeq :: Integer -> [Integer] fibSeq ub = withStrategy (parBuffer buffer rdeepseq) $ genericTake ub fibbwhere buffer = 100 fibb :: [Integer] fibb = 0 : 1 : zipWith (+) fibb (tail fibb)

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Template Haskell Tutorial
by Dominik Bollmann 11 May '16

11 May '16

Hi, I've been diving into Template Haskell recently and thought to write down what I've learned. The result hopes to be a practical, example-driven introduction to Template Haskell's main features: https://wiki.haskell.org/A_practical_Template_Haskell_Tutorial I hope it serves others new to Template Haskell. It's all on the Haskell wiki, so feedback and improvements are welcome! Cheers, Dominik.

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Applying a Constraint to a Proxy'd type
by Ivan Lazar Miljenovic 11 May '16

11 May '16

I'm not sure if it's possible (and I have an alternate method of doing this so it isn't strictly speaking _necessary_, but would make the code cleaner if I can), but I would like to convert a "value" of kind (k -> Constraint) into one of kind (Proxy k -> Constraint). I can achieve this using a type family (note: I've also made this polymorphic in terms of the Proxy type used, but that isn't necessary): > type family LiftConstraint_ c pa :: Constraint where > LiftConstraint_ c (proxy a) = c a > LiftConstraint_ c pa = () ~ Bool -- A nonsensical failing Constraint value which has the problem that it can't be partially applied (which I also need). This can be wrapped up in a typeclass: > class (LiftConstraint_ c pa) => LiftConstraint c pa > instance (LiftConstraint_ c pa) => LiftConstraint c pa The problem with this is that if I try and use it somewhere that - having already a type of kind (c :: * -> Constraint) from the environment - that expects a function of type (forall a. (c a) => a -> b) with a fixed "b" value, then trying to use something like this function prevents it from working (for the purposes of argument, assume the old specification of Num that had an Eq superclass; this is the way I can produce the smallest example): > fn :: forall pn. (LiftConstraint Num (Proxy n)) => Proxy n -> Bool > fn = join (==) . asProxyTypeOf 0 The resulting error message is: "Could not deduce (a ~ Proxy n0) ..." This occurs even if I make the type of fn polymorphic in terms of the proxy type or if I make LiftConstraint_ specific. Note that the error goes away if I make the definition to be "fn = const True"; it's only if I try and _use_ the proxy value. I've tried defining and using an IsProxy type family + class but that fails to the same basic problem: it can't tell that the provided value can be pulled apart to get what's in the Proxy. Any ideas? -- Ivan Lazar Miljenovic Ivan.Miljenovic(a)gmail.com http://IvanMiljenovic.wordpress.com

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Haskell Weekly News
by Kim-Ee Yeoh 11 May '16

11 May '16

*Top Picks:* - Oskar Wickström rewrites the Oden-to-Go transpiler from Racket to Haskell <http://oden-lang.org/blog/compiler/2016/01/18/the-haskell-rewrite.html>. Oden is an FP language comprising an ML type system and LISP syntax. Oskar explains that he made the migration because of several advantages that Haskell offered over Racket: exhaustive pattern-match checking, type-guided refactoring, monad transformers, and faster execution times. Apropos, the convo over at lobste.rs <https://lobste.rs/s/8hidko/the_haskell_rewrite_the_motivation_for_moving_aw…> links to this claim by Gabriel Gonzalez: "Haskell is an amazing language for writing your own compiler. If you are writing a compiler in another language you should genuinely consider switching." - Reviewing 2015 work month-by-month, Gracjan Polak tells the story of how he decided to lead the development of Haskell Mode <https://github.com/haskell/haskell-mode/wiki/Haskell-Mode-2015-retrospective>, "a bunch of Emacs major and minor modes put together in a single package." Discussion over at /r/haskell <https://www.reddit.com/r/haskell/comments/425gn4/haskell_mode_2015_retrospe…> . - Jared Tobin presents monadic versions of five recursion-schemas <http://jtobin.ca/monadic-recursion-schemes/>, namely: cata-, ana-, para-, apo-, and hylomorphisms. *Quotes of the Week:* - Tim Kellogg: <https://lobste.rs/s/erxkun/redundancy_vs_dependencies_which_is_worse/commen…> I’ve known a few old programmers nearing retirement that have a long list of very impressive accomplishments. The older and more accomplished they get, the more they prefer redundancy over dependency. The oldest and most accomplished will write their own load balancers, TCP stacks, loggers, everything if need be. Are they on to something? - From HN: <https://news.ycombinator.com/item?id=10972051#up_10979314> If you have the time, I'd advise you to learn Haskell, in order to stretch your mind and become an excellent OCaml developer, the way learning Latin makes you a better French or Italian writer. - HN markov chain parody site headline: <http://news.ycombniator.com/comments/comments_419.html> $690 for an hour minimum wage for state management in haskell *Recorded Talk of the Week:* - On Dec 17 last year <http://www.meetup.com/haskellhackersathackerdojo/events/226870399/>, Andrew Gibiansky demoed IHaskell, a Mathematica-like "capable graphical alternative" to the ghci REPL at the NorCal Hacker Dojo. Thanks go to Joe "begriffs" Nelson who recorded the talk and summarized it into bullet points <http://begriffs.com/posts/2016-01-20-ihaskell-notebook.html>. Joe's page was well-received at both Hacker News <https://news.ycombinator.com/item?id=10942969> and Haskell Reddit <https://www.reddit.com/r/haskell/comments/41xnhw/demo_of_ihaskell_notebook_…> . p.s. There will be no News next week. HWN will resume the week after. -- Kim-Ee

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OAuth2 and OpenID Connect in Haskell
by Luke Taylor 11 May '16

11 May '16

Hello all, I've written a blog article as an introduction to the Haskell project I've been working on, which may be of interest to some people here. It's an OpenID Connect identity provider. http://broch.io/posts/oauth2-openid-connect-haskell/ I'd really welcome any feedback on other/better ways of doing some of things it talks about. Cheers, Luke.

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Re: [Haskell-cafe] Automatic differentiation (AD) with respect to list of matrices in Haskell
by Miguel A. Santos 11 May '16

11 May '16

Awesome! Lots of new info! And more importantly, I got it to work. > > I still need to work on it a bit more, but things seems to be getting > clearer. > > On Sun, May 8, 2016 at 2:25 PM, Moritz Kiefer < > moritz.kiefer(a)purelyfunctional.org> wrote: > >> >> Miguel A. Santos <miguel.a.santos.l(a)gmail.com> writes: >> >> > Thanks a lot for your quick answer. >> > >> > First, I can't get your modifications through: What are Scalar & Mode? >> ghc >> > says type/class not in scope. >> >> Oh sorry you need to import Numeric.AD for that. Mode is a typeclass >> with an associated type called Scalar. You can just think of it as some >> magic that allows you to use constants in your function. >> >> > Yes. I figured out as well. I was only importing its Mode.Reverse as R. > > > >> > Some more questions to help me understand your suggestions below. >> > >> > >> > On Sun, May 8, 2016 at 1:14 PM, Moritz Kiefer < >> > moritz.kiefer(a)purelyfunctional.org> wrote: >> > >> >> >> >> There are two orthogonal errors going on here: >> >> >> >> 1. You need to use auto to embed constants. See the comments at the >> code >> >> below on how to use it. >> >> >> >> Ah, that solves indeed the first problem. Thanks! >> > >> > >> >> 2. You need to wrap [Matrix a] in a newtype. grad requires you to >> >> provide a function f a -> a, however sc2 _ has the type [Matrix a] -> a >> >> so if you replace [] by f you end up with f (Matrix a) -> a which >> >> results in the error you’re seeing. >> >> >> > >> > I've problems following you here. For example this is legit and works >> > grad (\[x,y] -> x^2y^3) [5,7] >> > >> > That is, grad here is taking a function f [a,a] -> a. That seems to >> work as >> > well with sc1 in my example: I do get the rigth gradient. >> >> That function has type [a] -> a which can be rewritten as [] a -> a and >> if you unify [] and f you have f a >> -> a which is exactly what you need. >> >> However in sc2 you have a function of type [Matrix a] -> a which can be >> rewritten as [] (Matrix a) -> a and if you unify f and [] you end up >> with f (Matrix a) -> a. However Matrix a and a are not the same type so >> you get an error. >> >> > I got confused by the expression "[the] function f a -> a": I though the > word 'function' was referring to the symbol 'f'. > > Yet, I still can't follow things completely...uhm, issuing :type grad in > ghci I get > grad > :: (Num a, Traversable f) => > (forall s. > Data.Reflection.Reifies s Numeric.AD.Internal.Reverse.Tape => > f (Numeric.AD.Internal.Reverse.Reverse s a) > -> Numeric.AD.Internal.Reverse.Reverse s a) > -> f a -> f a > > which I'd say could be simplified to > grad :: Num a => ( f Reverse s a -> Reverse s a) -> f a -> f a > > Now, It's not only that I don't see your "grad expects a 'f a -> a' ", but > I don't even see how something like grad (\[x,y]-> x*y) [2,3] fits...or do > I? The notation 'f' seems was misleading me. I'd say what's important is > its type, name Traversable f. [] seems to fit there, isn't? So the first > argument of grad more or less would be what you were referring to ( f a -> > a) , the second it the point to calculate the gradient at (of course a [a], > or, [] a, hence, f a) and it does yield a "vector" (on paper). > Interestingly, its implementation means that grad's output is the same type > the input of the function we want to take its gradient of. > > I think it starts making sense to me now. > > >> >> >> Miguel A. Santos <miguel.a.santos.l(a)gmail.com> writes: >> >> >> >> > mv _ [] = [] >> >> > mv (M []) _ = [] >> >> > mv ( M m ) v = ( dot (head m) v ) : (mv (M (tail m)) v ) >> >> > --- two matrices >> >> >> >> You need explicit type annotations to make this polymorphic here >> because >> >> of the monomorphism >> >> restriction. >> >> >> >> >> > Just for the sake of learning how to use that jargon in this context: >> > which/what is that monomorphism restriction are you referring to? Also, >> > when you say my mbW1 is polymorphic, do you mean, given my definition of >> > it, ghc cannot adscribe it one _unique_ type signature, but that will >> vary >> > depending on the context? >> >> I’m going to refer you to the haskell report [1] for information on the >> monomorphism restriction. By polymorphic I’m referring to the choice of >> a which is now left open. >> >> Thanks!! > > >> >> mbW1 :: Num a => Matrix a >> >> > mbW1 = M $ [[1,0,0],[-1,5,1],[1,2,-3]] >> >> > mbW2 = M $ [[0,0,0],[1,3,-1],[-2,4,6]] >> >> > --- two different scoring functions >> >> >> >> -- Provide a type signature and map auto over the constants >> >> sc1 :: (Floating b, Mode b) => [Scalar b] -> Matrix b -> b >> >> sc1 v m = foldr (+) 0 $ (phi' . (mv m) ) (map auto v) >> >> >> > >> >> -- Provide a type signature, use auto and use the newtype >> >> newtype MatrixList a = MatrixList [Matrix a] deriving >> >> (Functor,Foldable,Traversable) >> >> >> >> sc2 :: (Floating a,Mode a) => [Scalar a] -> MatrixList a -> a >> >> sc2 v [m1, m2] = foldr (+) 0 $ (phi' . (mv m2) . phi' . (mv m1) ) (map >> >> auto v) >> >> >> >> > strToInt = read :: String -> Double >> >> > strLToIntL = map strToInt--- testing >> >> > main = do >> >> > putStrLn $ "mbW1:" ++ (show mbW1) >> >> > putStrLn $ "mbW2:" ++ (show mbW2) >> >> > rawInput <- readFile "/dev/stdin" >> >> > let xin= strLToIntL $ lines rawInput >> >> > putStrLn "sc xin mbW1" >> >> >> >> -- That needs an explicit type annotation because mbW1 is polymorphic >> >> print $ sc1 xin (mbW1 :: Matrix Double) --- ok. = >> >> >> >> uhm...but it doesn't seem to give any problem, especially not after >> using >> > auto...I've still problems getting an intuition on when ghc does need >> "my >> > help" and when it can walk on it own. >> >> Hm maybe you don’t need it after all. I thought I got an error >> locally. Sorry I don’t have any good tip on how to get an >> intuition. I add them when I’m unsure about the type of something myself >> to help me to figure it out and when I get an error. >> >> >> > putStrLn "grad (sc1 xin) mbW1" >> >> > print $ grad ( sc1 xin) mbW1 -- yields an error: expects >> xin >> >> > [Reverse s Double] instead of [Double] >> >> > putStrLn "grad (sc1 [3,5,7]) mbW1" >> >> > print $ grad ( sc1 [3,5,7]) mbW1 --- ok. = >> >> > putStrLn "sc2 xin [mbW1,mbW2]" >> >> > print $ sc2 xin [mbW1, mbW2] >> >> > putStrLn "grad (sc2 [3,5,7) [mbW1,mbW2]" >> >> >> >> -- Use the newtype defined above >> >> print $ grad ( sc2 [3,5,7]) (MatrixList [mbW1, mbW2]) --- >> Error: >> >> see text >> >> >> >> Also as a general recommendation, write your type signatures >> explicitely >> >> at least for top level definitions. >> >> >> > >> > What do you mean by "top level" definitions? --again, just a side >> comment >> > to fine-tune my jargon registers. >> >> Top level definitions are the definitions in your module that are not in >> a let or where. So basically everything that’s accessible from other >> definitions outside of the current one. >> >> > I still have to learn my way through all types. Sometimes this rule makes > my life a bit too hard, specially > when I just want to quickly test some ideas, like here: logistic expects a > Floating a but if I don't make things > explicit, ghc was finding its way around my use of Num a and this Floating > requirement of the exp function. > > I'll keep this rule in mind. > > Thanks a lot!! > > Regards, > MA > PD: Forgot to reply to the list before... > > Cheers Moritz >> >> [1] https://www.haskell.org/onlinereport/decls.html#sect4.5.5 >> > >

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Summer of Haskell Mentors
by Edward Kmett 10 May '16

10 May '16

If you are interesting in helping out as a possible mentor for this year's Summer of Haskell, please email me, and include MENTOR in the title. Thank you! -Edward

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origin of my fibonacci test
by Michael Litchard 10 May '16

10 May '16

I have some code that checks to see if a number is a fibonacci number. It's not mine, I got it from somewhere and I need to credit it. I am pretty sure I got it from Stack Exchange but my search for it went nowhere. If this looks familiar to you, or you can tell me better ways to search, please let me know. isFib :: Integer -> Bool isFib n = n == a where (_, a, _) = unFib (1, 1) n unFib :: (Integer, Integer) -> Integer -> (Integer,Integer,Integer) unFib (a, b) n | n < a = (0, 0, 1) | n < e = (2*k, c, d) | otherwise = (2*k + 1, e, f) where (k, c, d) = unFib (fibPlus (a, b) (a, b)) n (e, f) = fibPlus (a, b) (c, d) fibPlus :: (Integer, Integer) -> (Integer, Integer) -> (Integer,Integer) fibPlus (a, b) (c, d) = (bd - (b - a)*(d - c), a*c + bd) where bd = b*d

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Myrtle Software is looking for Haskell developers for exciting graphics/compilers/hardware work
by Joe Hermaszewski 10 May '16

10 May '16

https://www.myrtlesoftware.com/vacancies/ We're a small software company in Cambridge, UK. We are working on some cool tech for lowering image processing algorithms to efficient hardware designs with a focus on putting these in autonomous vehicles. Our compiler is written completely in Haskell as well as some other tools we've written in house. We are looking to hire some more Haskell talent to help drive things forward. Please ask if you have any questions!

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