April 2018 - Beginners

Join'ing a State Monad, example from LYH
by Olumide 09 May '18

09 May '18

Dear List, I've been stumped for a few months on the following example, from chapter 13 of LYH http://learnyouahaskell.com/for-a-few-monads-more#useful-monadic-functions runState (join (State $ \s -> (push 10,1:2:s))) [0,0,0] I find the following implementation of join in the text is hard to understand or apply join :: (Monad m) => m (m a) -> m a join mm = do m <- mm m In contrast, I find the following definition(?) on Haskell Wikibooks https://en.wikibooks.org/wiki/Haskell/Category_theory#Monads join :: Monad m => m (m a) -> m a join x = x >>= id easier to understand, and although I can apply it to the following Writer Monad example, in the same section of LYH, runWriter $ join (Writer (Writer (1,"aaa"),"bbb")) I cannot apply it to the State Monad example. Regards, - Olumide

2 2

Efficient Binary Multiplication
by Quentin Liu 28 Apr '18

28 Apr '18

Hi all, Suppose I want to multiply two binary numbers whose representation uses lists (e.g. 14 is represented as [1, 1, 1, 0]). Is there any efficient way to do binary multiplication? The way I could come up with involves a lot of intermediate lists that will be discarded eventually and is extremely inefficient. I know one fast algorithm that uses Array but would it be possible to do the multiplication with only lists? Regards, Qingbo Liu

4 4

phantom type
by PICCA Frederic-Emmanuel 18 Apr '18

18 Apr '18

Hello, Here my problem. I extract some information from a database and put them into a type with 50 parameters Type MyData = MyData Int String ... But I need to be able to differenciate two different case. depending on the String content. something like if the String start with "ref-" it is a Caracterization and if not it is a Collect So I end up with this solution data Unchecked data Caracterization data Collect data MyData t = MyData Int String ... now I defined a function getMyDataFromDatabase :: ... -> Mydata Unchecked and isCollect in order to verify that I have a Collect and not something Else isACollect :: MyData Unchecked -> Either Text (MyData Collect). which use coerc in order to avoid copying all the members. It is great. now Here is my problem. I create a method myMethod :: MyData Collect -> IO () but I want my method to work for MyData Collect and Mydata Caracterization but not Unchecked. So It seems to me that I can not just do myMethod :: MyData t -> IO () what is the right way to solve this problem ? thanks for your help. Frederic

3 4

Call for Participation: Summer School on Advanced Functional Programming
by Alejandro Serrano Mena 16 Apr '18

16 Apr '18

# Call for Participation SUMMER SCHOOL ON ADVANCED FUNCTIONAL PROGRAMMING Utrecht, the Netherlands, 27-31 August 2018 http://www.afp.school ## ABOUT The Advanced Functional Programming summer school has been running for more than ten years. We aim to educate aspiring Haskell programmers beyond the basic material covered by many textbooks. The lectures will cover several more advanced topics regarding the theory and practice of Haskell programming, including topics such as: * lambda calculus; * monads and monad transformers; * lazy evaluation; * generalized algebraic data types; * type families and type-level programming; * concurrency and parallelism. The summer school consists of a mix of lectures, labs, and a busy social program. ## LECTURERS Utrecht staff: * Johan Jeuring * Alejandro Serrano Mena * Doaitse Swierstra * Wouter Swierstra Guest lectures: * Manuel Chakravarty * Koen Claessen * Gabriele Keller ## PREREQUISITES We expect students to have a basic familiarity with Haskell already. You should be able to write recursive functions over algebraic data types, such as lists and trees. There is a great deal of material readily available that covers this material. If you’ve already started learning Haskell and are looking to take your functional programming skills to the next level, this is the course for you. ## DATES Registration deadline: 1 August, 2017 School: 27-31 August ## COSTS €1700 - Housing and registration €1500 - Registration only We offer a €1000 discount for students and staff members affiliated with a university. ## SCHOLARSHIPS If you’re struggling to finance your trip to Utrecht, please let us know. We have a limited number of scholarships or discounts available for students that would not be able to attend otherwise, especially for women and under represented minorities. ## FURTHER INFORMATION Further information, including instructions on how to register, is available on our website: http://www.afp.school

1 0

Lazyness and forM_
by Marc Busqué 13 Apr '18

13 Apr '18

Hi there! I'm using [selda](https://github.com/valderman/selda) package to work with databases. I'm trying to write a function to generate several tables at once. In the following examples, `categories` and `expenses` are two selda `Table`. The other functions in use and their respective imports should be self-evident: If I do: ``` migrate :: IO () migrate = do dir <- dBDir createDirectoryIfMissing True dir forM_ (categories, expenses) $ withDB . createTable ``` tables are not actually created. However, if I do: ``` migrate :: IO () migrate = do dir <- dBDir createDirectoryIfMissing True dir withDB . createTable $ categories withDB . createTable $ expenses ``` Both tables are actually created. So it seems like the thugs created with `formM_` are actually never executed. Is it so? I'm new with Haskell and it seems strange for me. If it is so, doesn't make it useless `forM_` for IO? Furthermore, I can't reproduce it in ghci. Doing this: ``` ["a", "b"] `forM_` print ``` Actually prints both `"a"` and `"b"`. Thanks for any enlightment. Marc Busqué http://waiting-for-dev.github.io/about/

2 2

EitherT
by mike h 13 Apr '18

13 Apr '18

Hi, I’m trying to write EitherT from first principles and I’m stuck at the first hurdle - Functor. I’m looking for a hint rather than a complete answer :) This is what I have newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)} instance Monad m => Functor (EitherT m a) where ---- fmap :: (a -> b) -> f a -> f b fmap f m = EitherT $ do mv <- runEitherT m case mv of Left _ -> return mv Right rv -> return $ Right (f rv) and here is the compilers view Phrase.hs:31:25: error: • Couldn't match type ‘b’ with ‘a1’ ‘b’ is a rigid type variable bound by the type signature for: fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b at Phrase.hs:27:5-8 ‘a1’ is a rigid type variable bound by the type signature for: fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b at Phrase.hs:27:5-8 Expected type: m (Either a a1) Actual type: m (Either a b) • In the expression: return $ Right (f rv) In a case alternative: Right rv -> return $ Right (f rv) In a stmt of a 'do' block: case mv of Left _ -> return mv Right rv -> return $ Right (f rv) • Relevant bindings include rv :: a1 (bound at Phrase.hs:31:19) mv :: Either a a1 (bound at Phrase.hs:28:9) m :: EitherT m a a1 (bound at Phrase.hs:27:12) f :: a1 -> b (bound at Phrase.hs:27:10) fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b (bound at Phrase.hs:27:5) what I think I need to do is fmap over the right value after pulling if out of the monad m by doing mv <- runEitherT m these lines from the compiler are particularly confusing Expected type: m (Either a a1) Actual type: m (Either a b) as I believe f is f:: a1->b So just hints please and I expect I’ll have another duh moment. Thanks Mike

3 4

Re: [Haskell-beginners] EitherT
by 鲍凯文 12 Apr '18

12 Apr '18

Hi, Try to unify the types returned by each of the case alternatives, starting from what the compiler has inferred so far. I’ve made this mistake many times lol. Best, toz On Thu, Apr 12, 2018 at 5:38 AM <beginners-request(a)haskell.org> wrote: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. EitherT (mike h) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Thu, 12 Apr 2018 11:25:57 +0100 > From: mike h <mike_k_houghton(a)yahoo.co.uk> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: [Haskell-beginners] EitherT > Message-ID: <57AFC9F5-3601-4D18-B860-41352D3F2204(a)yahoo.co.uk> > Content-Type: text/plain; charset=utf-8 > > Hi, > I’m trying to write EitherT from first principles and I’m stuck at the > first hurdle - Functor. I’m looking for a hint rather than a complete > answer :) > > > This is what I have > > newtype EitherT m a b = EitherT {runEitherT :: m (Either a b)} > instance Monad m => Functor (EitherT m a) where > ---- fmap :: (a -> b) -> f a -> f b > fmap f m = EitherT $ do > mv <- runEitherT m > case mv of > Left _ -> return mv > Right rv -> return $ Right (f rv) > > > > and here is the compilers view > Phrase.hs:31:25: error: > • Couldn't match type ‘b’ with ‘a1’ > ‘b’ is a rigid type variable bound by > the type signature for: > fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b > at Phrase.hs:27:5-8 > ‘a1’ is a rigid type variable bound by > the type signature for: > fmap :: forall a1 b. (a1 -> b) -> EitherT m a a1 -> EitherT m a b > at Phrase.hs:27:5-8 > Expected type: m (Either a a1) > Actual type: m (Either a b) > • In the expression: return $ Right (f rv) > In a case alternative: Right rv -> return $ Right (f rv) > In a stmt of a 'do' block: > case mv of > Left _ -> return mv > Right rv -> return $ Right (f rv) > • Relevant bindings include > rv :: a1 (bound at Phrase.hs:31:19) > mv :: Either a a1 (bound at Phrase.hs:28:9) > m :: EitherT m a a1 (bound at Phrase.hs:27:12) > f :: a1 -> b (bound at Phrase.hs:27:10) > fmap :: (a1 -> b) -> EitherT m a a1 -> EitherT m a b > (bound at Phrase.hs:27:5) > > > what I think I need to do is fmap over the right value after pulling if > out of the monad m by doing mv <- runEitherT m > > these lines from the compiler are particularly confusing > Expected type: m (Either a a1) > Actual type: m (Either a b) > > as I believe f is f:: a1->b > > So just hints please and I expect I’ll have another duh moment. > > Thanks > > Mike > > > > > > ------------------------------ > > Subject: Digest Footer > > _______________________________________________ > Beginners mailing list > Beginners(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners > > > ------------------------------ > > End of Beginners Digest, Vol 118, Issue 6 > ***************************************** >

1 0

May I ask what is the meaning of ch <= 'z' and 'a' <= ch.
by 清羽 10 Apr '18

10 Apr '18

Hello everyone, I have a simple question here Define a function isLower :: Char -> Bool which returns True if a given character is a lower case letter. You can use the fact that characters are ordered, and for all lower case letters ch we have ′a′ ≤ ch and ch ≤ ′z′. Alternatively, you can use the fact that ['a'..'z'] evaluates to a list containing all lower case letters. And May I ask what is the meaning of ch <= 'z' and 'a' <= ch? I copied this as a condition and solved the problem.here is my work. isLower :: Char -> Bool isLower x = if x ch <= 'z' && 'a' <= ch then True else False. Thanks in advance.

3 2

2018 report
by Karim Salah 10 Apr '18

10 Apr '18

hi everyone, I am wondering how can i submit a contribution in the Haskell report. It is stated up here: http://mail.haskell.org/pipermail/haskell-cafe/2018-March/128829.html that i shall send my contribution hcar at haskell.org which i have no clue how to this i will appreciate any help Thanks in advance

2 2

A question regarding the syntax behind the parenthesis and type class.
by 清羽 04 Apr '18

04 Apr '18

this is the original question: Write a function showAreaOfCircle which, given the radius of a circle, calculates the area of the circle, showAreaOfCircle 12.3 ⇒ "The area of a circle with radius 12.3cm is about 475.2915525615999 cm^2" Use the show function, as well as the predefined value pi :: Floating a => a to write showAreaOfCircle. the type signature works for this function is: showAreaOfCircle :: (Show a, Floating a) => a -> a -> String my question is : I don't quite understand the syntax behind the parenthesis and the type class Floating a. Since pi is not a function, and hence the entire function only takes one argument which belongs to the type class Show, why should i put a Floating a in the type signature in the first place?(i come up with this type signature because of the error message and the suggestion stated above) Thanks

2 1