- Beginners - Haskell.org

How to make instance of MonoFunctor
by Baa 10 Aug '17

10 Aug '17

Hello, Dear List! I want to make functor of my type like newtype UserName = UserName { unName :: Text } sure, it's impossible, so I will make MonoFunctor instead of (from library mono-traversable). So, I try: instance MonoFunctor UserName where omap fn (UserName n) = UserName $ fn $ n but I get error • Couldn't match expected type ‘Element UserName’ with actual type ‘Text’ • In the second argument of ‘($)’, namely ‘unName an’ In the second argument of ‘($)’, namely ‘fn $ unName an’ In the expression: UserName $ fn $ unName an interesting is that Text "type Element Text" of "type family Element mono" (instance?). So, how to make mono-functor for such `UserName` structure? -- Best regards, Paul

2 2

Grappling with State Monad get
by Olumide 08 Aug '17

08 Aug '17

Ahoy Haskellers, In the section "Getting and Setting State" (http://learnyouahaskell.com/for-a-few-monads-more#state) in LYH get is defined as get = state $ \s -> (s, s) How does does get determine the type s, is considering that it has no argument as per the definition given above? Or is the definition written in some sort of point-free notation where an argument has been dropped? I find the line stackNow <- get in the the function(?) stackyStack confusing for the same reason. I guess my difficulty is that state $ \s ->(s , s) has a generic type (s) whereas the stackyStack has a concrete type. Is the type of s determined from the type of the stateful computation/do notation? Regards, - Olumide

3 6

Password Program Questions
by Casey Newsome 03 Aug '17

03 Aug '17

I am very new to Haskell and I am attempting to make a simple password program as my first program. I have ran into a problem and I am not very sure on how to fix it. I do not know how to make a second section for the passwords. My guessing attempts have failed so I am asking for help. Here is my code main = do putStrLn "Hello, Who are you?" name <- getLine --User Input putStrLn ("Hey " ++ name ++ ", What's the password?") pass <- getLine if pass == "12345" then putStrLn ("Welcome") pw else do putStrLn "That is wrong!" mainpw = do putStrLn "What Password do you need?" Thanks in advance, Casey :D

2 1

Libraries for Playing Audio
by Atrudyjane 02 Aug '17

02 Aug '17

Hello Cafe, Is there a recommended Haskell library for playing/manipulating audio files? There's a long list of sound libraries on Hackage, and was thinking looking into either conduit-audio or SDL-mixer. Thank You, Andrea Sent with [ProtonMail](https://protonmail.com) Secure Email.

3 6

How to "cast" FromJSON instance value
by Baa 01 Aug '17

01 Aug '17

Hello, List! I have, for example, function with signature like fn :: FromJSON a => Map Text a -> BlahBlah and I want to treat this `a` as String (or `Text`), to read it with `readMaybe`. Am I right that it's impossible? I get error about "...a is rigid type and could not be matched with Text...". But `Text` is `FromJSON` instance. Is it possible to do it (cast, convert, unwrap, etc)? The problem's source is that early I worked with `Text` values only (getting from HTML form) but now they become any type: int's, bool's, etc). === Best regards, Paul

2 6

Streams
by mike h 27 Jul '17

27 Jul '17

Hi, I'm looking at this: data Stream a = Cons a (Stream a) and with streamFromSeed :: (a -> a) -> a -> Stream a streamFromSeed f a = Cons a (streamFromSeed f (f a)) I can do, for example, evens :: Stream Integer evens = streamFromSeed (+2) 0 what I'd like to do is take one item at a time from the stream. My first shot was next :: (Stream a) -> (Stream a, a) but that means I need to keep a ref to the stream that is returned in the tuple which makes for messy code for subsequent calls to next. I sense that the State monad needs to be involved but I'm not sure exactly how as my experiments with State still required I keep a ref to the 'new' stream. Conceptually I see this as a 'global' state and next is just next :: a but that's 30 years of imperative programming speaking and is, I think, wrong! Any help would be really appreciated. Thanks Mike

2 2

Updates in the ST monad, seems slow
by lud.sund＠gmail.com 27 Jul '17

27 Jul '17

Hi all. I'm a Haskell beginner looking for valuable inputs, I don't expect my program to magically fix itself. I've implemented a maximum matching graph algorithm in Haskell. The implementation specification uses some state, some arrays that are indexed and updated throughout the algorithm. At first, I implemented the state using HashMaps. This went well, but the runtime wasn't close to my professors corresponding C program. I profiled my program and found out that adjusting the maps were taking alot of time. Then, I had a look at Data.HashTable.ST.Basic. I thought, getting average constant time lookup and insert would surely fix the problem with the slow state updates. I reimplemented the algorithm using HashTables and ran the program. As it turned out, the runtime didn't decrease using the ST monad, rather increase. And heavily. A program instance that took 100 seconds using maps now took 400 seconds. Looking at a profiling sample, it shows that HashTable lookup and other various procedures related to HashTables are the big time consumers. How can this be possible? Any ideas? The code is available under https://github.com/lsund/edmonds-matching/ where: The 'main' branch is the map implementation, and 'hashtable' branch is the hashtable implementation. The code is quite messy, and since I'm a beginner, probably inefficient. It is the first time I implemented anything using the ST monad. So I'm afraid that I don't understand it perfectly, and made some serious implementation flaw. Best Regards, Ludvig

2 1

Count with Writer asynchronously
by Baa 25 Jul '17

25 Jul '17

Hello, Dear List! There is package `async` (https://hackage.haskell.org/package/async-2.1.1.1/docs/Control-Concurrent-A…) Before, I had: import qualified Control.Concurent.Async as A ... runIt :: ... -> IO () ... sequence [A.async $ runIt ...] >>= mapM_ A.wait But now I want to count something inside `runIt`. I will use `Writer` monad for it (sure, it can be `State` also, not in principle to me). To do it synchronously, I done: module Main where import Control.Monad.Trans.Writer import Control.Monad.IO.Class import Data.Monoid runIt :: (Show a, Num a) => WriterT (Sum a) IO () -> a -> WriterT (Sum a) IO () runIt w x = do censor (+1) w -- emulates conditional count of something liftIO $ print x main = do let l = [1,2,3,4] w = writer ((), 0) :: WriterT (Sum Int) IO () z <- runWriterT $ sequence [runIt w i | i <- l] print $ snd z but now my `runIt` changes it's signature: runIt :: Num a => WriterT (Sum a) IO () -> ... -> WriterT (Sum a) IO () ... sequence [A.async $ runIt ...] >>= mapM_ A.wait ^^^^^^^^^^^ ` ERROR is here! I get the error because `async`::IO () -> IO (A.Async ()) but I'm trying to pass it `WriterT (Sum a) IO ()`! To fix it I added `runWriterT` there: res <- sequence [A.async $ runWriterT (runIt ...) ...] >>= mapM A.wait but now I will get list of counters, not one (res::[((), Sum Int)])! How to solve this problem: to run several actions asyncronously and to count something inside the action with `Writer` monad? === Best regards, Paul

2 4

I can't turn-off conreate warning
by Baa 24 Jul '17

24 Jul '17

Hello, Dear List! I want to turn-off concreate warning (but to have all other, and possible to treat them as errors). This warning is about orphans instances. I tried: -Wall -Werror -Wno-warn-orphans also -Wall -Werror -Wno-warn-warn-orphans But GHC does not understand this option. How to achieve this? -- Best regards, Paul

2 3

How to make this more functional?
by Jeffrey Brown 18 Jul '17

18 Jul '17

I wrote a 10-line program[1] for converting from org-mode format to smsn-mode format. Both formats use indentation to indicate hierarchy. In org, a line at level k (levels are positive integers) starts with k asterisks, followed by a space, followed by the text of the line. In smsn-mode, a line at level k starts with 4*(k-1) spaces, followed by an asterisks, followed by a space. I feel like there ought to be an intermediate step where it converts the data to something other than string -- for instance, data IndentedLine = IndentedLine Int String | BadLine and then generates the output from that. It feels like it needs a parser. But when I resurrect my parser code that I understood many moons ago, it looks like a lot of machinery. Thanks. [1] https://github.com/synchrony/smsn-mode/blob/develop/org-to-smsn-mode.hs -- Jeff Brown | Jeffrey Benjamin Brown Website <https://msu.edu/~brown202/> | Facebook <https://www.facebook.com/mejeff.younotjeff> | LinkedIn <https://www.linkedin.com/in/jeffreybenjaminbrown>(spammy, so I often miss messages here) | Github <https://github.com/jeffreybenjaminbrown>

2 3