July 2017 - Beginners

Multiline code in GHCi
by Shishir Srivastava 29 Aug '17

29 Aug '17

Hi, I am trying to run this basic multiline code in GHCi. I am not sure if this is even possible without writing it in a file and compiling it in a traditional way. ------- Prelude> :set +m Prelude> let main1 = do Prelude| a <- readLn Prelude| return a Prelude| Prelude> Prelude> main1 <interactive>:75:1: No instance for (Show (IO b0)) arising from a use of `print' In the first argument of `print', namely `it' In a stmt of an interactive GHCi command: print it ------- Could someone please confirm if the function can be defined and called like this in GHCi or point to where am going wrong. Thanks, Shishir Srivastava

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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.

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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

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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

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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

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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

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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>

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How works this `do` example?
by Baa 13 Jul '17

13 Jul '17

Hello, Dear List! Consider, I have: request1 :: A -> Connection -> IO () request2 :: A -> Connection -> IO A How does it work - resp <- getConnection >>= do request1 myA request2 anotherA ?! It is compiled but seems that does not execute `request1`... `request1 myA` gets `Connection` value, good. But it does not return `IO Connection`! It returns `IO ()`. But how does `request2 anotherA` get `Connection` value too? Because this is not compiled sure: resp <- getConnection >>= request1 myA >>= request2 anotherA I tried this: module Main where f1 :: Int -> IO () f1 i = do print "f1!" print i return () f2 :: Int -> IO Int f2 i = do print "f2!" print i return i f0 :: IO Int f0 = pure 10 main :: IO () main = f0 >>= do f1 f2 >> print "end" and I get output: "f2!" 10 "end" which means that `f1` is not executing in `do..`-block, but how does `f2` get 10 as input?! == Cheers, Paul

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Custom partition lists into groups by providing group sizes using foldl
by Apoorv Ingle 11 Jul '17

11 Jul '17

Hi, I am trying to write a partition function where we pass group sizes and the list we want to partition into groups as arguments and get back a list of groups (or list of lists in this case). My first attempt was by using an auxiliary inner function {-# LANGUAGE ScopedTypeVariables #-} module Partition where partition :: [Int] -> [a] -> [[a]] partition ds ps = reverse $ paux ds ps [] where paux :: [Int] -> [a] -> [[a]] -> [[a]] paux [] [] ps' = ps' paux [] ps ps' = [ps] ++ ps’ paux _ [] ps' = ps' paux (d:ds') ps ps' = paux ds' (snd (splitAt d ps)) ([fst (splitAt d ps)] ++ ps') —————— *Partition> partition [2, 3] [1,2,3,4,5] [[1,2],[3,4,5]] *Partition> partition [1, 2] [1,2,3,4,5] [[1],[2,3],[4,5]] *Partition> partition [1, 2, 5] [1,2,3,4,5] [[1],[2,3],[4,5]] I was speculating if we could write the same function using foldl function but haven’t been able to figure it out. I would really appreciate if you can give me pointers on how we can implement it. partition' :: [Int] -> [a] -> [[a]] partition' [] ds = [ds] partition' ps ds = foldl ??? ???' ???'' contrary to my speculation is it even possible to write such a function using foldl if so why not? Regards, Apoorv Ingle Graduate Student, Computer Science apoorv.ingle(a)ku.edu <mailto:apoorv.ingle@ku.edu>

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Fwd: using Shake to compile c++
by David McBride 08 Jul '17

08 Jul '17

Sorry this should have been sent to the list. ---------- Forwarded message ---------- From: David McBride <toad3k(a)gmail.com> Date: Sat, Jul 8, 2017 at 8:49 AM Subject: Re: [Haskell-beginners] using Shake to compile c++ To: Roger Mason <rmason(a)mun.ca> There's three errors here. pkg-config is an IO action and thus in order to use its output, you must use <- instead of a let binding. Then you must choose what you want to save from that command. You could get the exit result, stderr or stdout or both stdout and stderr. We want stdout. And lastly, Stdout is parameterized over a type a, and you have to decide which. You can either do the following Stdout i <- cmd "pkg-config glib-2.0 --cflags" () <- cmd "c++ -c" [c] "-o" [out] "-MMD -MF" [m] [i :: String] Or you can enable ScopedTypeVariables and go Stdout (i :: String) <- cmd "pkg-config glib-2.0 --cflags" I don't really understand why Stdout is parameterized, rather than just being forced to be a string like Stdin, that's a question for the shake authors. On Sat, Jul 8, 2017 at 7:57 AM, Roger Mason <rmason(a)mun.ca> wrote: > David McBride <toad3k(a)gmail.com> writes: > >> Sorry that should have been command, not command_ which is very different. >> >> On Sat, Jul 8, 2017 at 7:37 AM, David McBride <toad3k(a)gmail.com> wrote: >>> The easy option is to just use command instead of cmd. Variadic >>> functions are always a little weird to type check. >>> >>> command_ [] "pkg-config" ["glib-2.0","--cflags"] >>> >>> That will *probably* solve the ambiguity in both lines, but I haven't tested. > > Thank you for your replies. > > This is what I have now: > > "objects//*.o" %> \out -> do > let c = dropDirectory1 $ out -<.> "cxx" > let m = out -<.> "m" > let i = command [] "pkg-config" ["glib-2.0","--cflags"] > () <- cmd "c++ -c" [c] "-o" [out] "-MMD -MF" [m] [i] > needMakefileDependencies m > > That produces: > > Build.hs:29:17: error: > * Ambiguous type variable `r0' arising from a use of `command' > prevents the constraint `(CmdResult r0)' from being solved. > Relevant bindings include i :: Action r0 (bound at Build.hs:29:13) > Probable fix: use a type annotation to specify what `r0' should be. > These potential instances exist: > instance CmdResult CmdLine > -- Defined in `Development.Shake.Command' > instance CmdResult CmdTime > -- Defined in `Development.Shake.Command' > instance CmdResult Exit -- Defined in `Development.Shake.Command' > ...plus 9 others > ...plus two instances involving out-of-scope types > (use -fprint-potential-instances to see them all) > * In the expression: > command [] "pkg-config" ["glib-2.0", "--cflags"] > In an equation for `i': > i = command [] "pkg-config" ["glib-2.0", "--cflags"] > In the expression: > do { let c = dropDirectory1 $ out -<.> "cxx"; > let m = out -<.> "m"; > let i = command ... "pkg-config" ...; > () <- cmd "c++ -c" [c] "-o" [out] "-MMD -MF" [m] [i]; > .... } > > Build.hs:30:15: error: > * No instance for (Development.Shake.Command.Arg [Action r0]) > arising from a use of `cmd' > * In a stmt of a 'do' block: > () <- cmd "c++ -c" [c] "-o" [out] "-MMD -MF" [m] [i] > In the expression: > do { let c = dropDirectory1 $ out -<.> "cxx"; > let m = out -<.> "m"; > let i = command ... "pkg-config" ...; > () <- cmd "c++ -c" [c] "-o" [out] "-MMD -MF" [m] [i]; > .... } > In the second argument of `(%>)', namely > `\ out > -> do { let ...; > let ...; > .... }' > > Thanks again, > Roger

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