October 2016 - Beginners

Re: [Haskell-beginners] Beginners Digest, Vol 100, Issue 14
by Lai Boon Hui 22 Oct '16

22 Oct '16

Hi David, Thanks for chipping in. I found the solution to be to include "directory" in cabal file "build-depends". I was confused because my stack ghci config didn't need :m + System.directory to work. But somehow cabal repl does not work without some kind of import. On Sat, Oct 22, 2016 at 8:00 PM, <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. cabal repl issue (Lai Boon Hui) > 2. Re: cabal repl issue (David McBride) > 3. Re: Monadic functions definitions for free monadic DSL > (Sumit Raja) > 4. Re: Monadic functions definitions for free monadic DSL > (Kim-Ee Yeoh) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Fri, 21 Oct 2016 22:35:19 +0800 > From: Lai Boon Hui <laiboonh(a)gmail.com> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] cabal repl issue > Message-ID: > <CAJdQggnSvw8z7V+SBY=y2TEfhpcK2znLHpkp9i+Edq5QAgXM7w@mail. > gmail.com> > Content-Type: text/plain; charset="utf-8" > > Hi all, > > when i execute cabal repl i get this: > > <interactive>:1:8: error: > > Not in scope: ‘System.Directory.getCurrentDirectory’ > > No module named ‘System.Directory’ is imported. > because my ghci.conf has > :def pwd (\_-> System.Directory.getCurrentDirectory >>= print >> return > "") > > Does anyone know how i can include System.Directory somehow even though its > not used in my cabal project? > > -- > Best Regards, > Boon Hui >

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Re: [Haskell-beginners] Monadic functions definitions for free monadic DSL
by Sumit Raja 22 Oct '16

22 Oct '16

> I would really like to help you, but without your imports, packages, etc, > it is really hard to interpret your program. Like where does decodeUtf8 > come from, or receive, or TCPSocket? If they are functions you wrote, I > don't need their code, the types would be sufficient. > Imports are: import Protolude import Control.Monad.Free import System.Socket import System.Socket.Family.Inet import System.Socket.Type.Stream import System.Socket.Protocol.TCP import Control.Exception ( bracket, catch ) import Data.ByteString as BS (uncons) decodeUtf8 :: ByteString -> Text encodeUtf8 :: Text -> ByteString I'm using the socket library for the actual networking (https://hackage.haskell.org/package/socket-0.6.0.1) type TCPSocket = Socket Inet Stream TCP receive :: Socket f t p -> Int -> MessageFlags -> IO ByteString Source send :: Socket f t p -> ByteString -> MessageFlags -> IO Int accept :: (Family f, Storable (SocketAddress f)) => Socket f t p -> IO (Socket f t p, SocketAddress f) If it helps the full source is at https://bitbucket.org/sumitraja/network-free/src/a4fcbc74c9e178e81d8b10b60d…. Looking forward to your assistance. Thanks Sumit

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Re: [Haskell-beginners] Monadic functions definitions for free monadic DSL
by Sumit Raja 22 Oct '16

22 Oct '16

> I feel like if you can figure out what you actually want Accept to do, it > will become clearer. Here's my attempt. Accept takes a chan, takes a > procedure to loop on, a procedure to accept on, and then returns the server > chan to continue the loop. I don't know if this is entirely right, but it > type checks and hopefully it will give you some ideas. As you've said Accept needs refinement. I tried writing a interpretUDP which doesn't have an accept loop but does have a bind + listen. I suspect that accept needs to move into the interpretTCP somehow and the DSL needs to be Bind or similar. Bind makes more sense as well if I wanted to write a pipes or a chan based interpreter. Thanks for the rewrite and the pointers. -Sumit

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cabal repl issue
by Lai Boon Hui 21 Oct '16

21 Oct '16

Hi all, when i execute cabal repl i get this: <interactive>:1:8: error: Not in scope: ‘System.Directory.getCurrentDirectory’ No module named ‘System.Directory’ is imported. because my ghci.conf has :def pwd (\_-> System.Directory.getCurrentDirectory >>= print >> return "") Does anyone know how i can include System.Directory somehow even though its not used in my cabal project? -- Best Regards, Boon Hui

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[Q] Bulding an Array of Tree Nodes
by Chul-Woong Yang 19 Oct '16

19 Oct '16

Hi, all. I need to make graph/tree structure, the nodes of which are contained in _array_ to facilitate random access to that node. There is no problem in building an array of graph nodes from edge list. > -- Graphs > data Graph a = GNode a [Graph a] > mkGraph :: [(a,[Int])] -> Array Int (Graph a) > mkGraph table = table' > where n = length table > table' = listArray (0,n-1) $ > map(\(x,adjs) -> GNode x (map (table' !) adjs)) table However, I cannot build tree from the same input, if given edge list is undirected one. The leaf node's edge has a link to parent node when edge list is undirected, and I cannot find way to cut that parent edge in building tree. For example, think of following simple tree: A(0) / \ B(1) C(2) It's hard to build tree with following edge list: [('A',[1,2]), ('B',[0]), ('C',[0])] but it's easy when edge list is following: [('A',[1,2]), ('B',[]), ('C',[])] How can I build an array of tree nodes for the former? In other words, How can I build following mkTree function? data Tree a = Node { value :: a , parent :: Tree a , level :: Int , children :: [Tree a] } deriving Show mkTree :: [(a,[Int])] -> Array Int (Tree a) Any comments or helps will be deeply appreciated. Thank you. Chul-Woong Yang

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Monadic functions definitions for free monadic DSL
by Sumit Raja 14 Oct '16

14 Oct '16

Hello, I am trying to get my head around free monads by developing a simple network abstraction DSL. I've made good progress before adding TCP/IP semantics of accepting connections. I'm now stuck with the creation of monadic functions. I've defined the following: data NetworkActivity chan next = Accept chan next (chan -> next) | Send chan ByteString (Bool -> next) | Recv chan (ByteString -> next) | Close chan (() -> next) clse :: a -> Free (NetworkActivity a) Text clse chan = liftF (Close chan (const "Quit")) chatterServer :: a -> Free (NetworkActivity a) Text chatterServer svrchan = Free $ Accept svrchan (chatterServer svrchan) chatterLoop chatterLoop :: a -> Free (NetworkActivity a) Text chatterLoop chan = Free $ Recv chan $ \bs -> case BS.uncons bs of Nothing -> clse chan Just x -> if bs == "Bye" then Free $ Close chan (\_ -> Pure "Quit") else Free (Send chan bs (\_ -> chatterLoop chan)) This works fine with the interpretTCP interpreter below accepting multiple connections: interpretTCP :: Free (NetworkActivity TCPSocket) r -> IO r interpretTCP prg = case prg of Free (Accept serverSock svrLoop acceptProc) -> bracket (return serverSock) (\s-> interpretTCP (clse s)) (\s-> do (ss, sa) <- accept s forkIO $ do _ <- interpretTCP (acceptProc ss) return () interpretTCP svrLoop ) Free (Recv sock g) -> do bs <- receive sock 4096 mempty putStrLn (decodeUtf8 bs) interpretTCP (g bs) Free (Close sock g) -> do close sock putStrLn ("Server bye!" :: Text) interpretTCP (g ()) Pure r -> return r Free (Send sock pl g) -> do sent <- send sock pl mempty interpretTCP (g (sent > 0)) Where I'm stuck is defining the monadic version of accept and I'm beginning to think my original data type defined above may be wrong. As an initial step I've defined the following: recv :: a -> Free (NetworkActivity a) ByteString recv chan = liftF (Recv chan identity) sendit :: a -> ByteString -> Free (NetworkActivity a) Bool sendit chan pl = liftF (Send chan pl identity) mchatterServer :: a -> Free (NetworkActivity a) Text mchatterServer chan = Free $ Accept chan (mchatterServer chan) (\s -> return (identity s) >>= mchatterLoop) mchatterServer works as is, the interpreter accepts multiple connections. Similarly all good with recv and sendit. I am struggling with converting the Accept in mchatterServer into a function to use in the do syntax. The signature I think I should be using is acc :: a -> NetworkActivity a Text -> Free (NetworkActivity a) (NetworkActivity a Text) What I can't figure out is why it can't follow the pattern of recv and sendit above: acc chan next = liftF $ Accept chan next identity Which results in error on identity (using Protolude): Expected type: a -> NetworkActivity a Text Actual type: NetworkActivity a Text -> NetworkActivity a Text I can't really see how to get the types to line up and have now can't see through the type fog. What am I missing in my reasoning about the types? Help much appreciated! Thanks Sumit

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Why QuickCheck's Char value is limited to ASCII characters only?
by Birmjin In 14 Oct '16

14 Oct '16

Hi, I found that the Arbitrary instance for the Char type generates only ASCII values while Char type represent Unicode characters. I can't figure out why it has such a limit. Not knowing this pitfall, one can misjudge the test results. Is this intended thing or just not being implemented yet? Thanks.

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Re: [Haskell-beginners] Monadic functions definitions for free monadic DSL
by 鲍凯文 14 Oct '16

14 Oct '16

Hi, Although I don't really understand the contents of your code, I think the type error results from the fact that the 3rd field of the Accept constructor has type (chan -> next). In the context of 'acc', (chan :: a) and (next :: NetworkActivity a Text). I'm guessing the type error refers to when you used 'identity' (which I'm hoping is just 'id' from Prelude); its type gets inferred to be (NetworkActivity a Text -> NetworkActivity a text) instead of what it expected (chan -> next, i.e. a ->NetworkActivity a Text). Whether or not acc is the right type for your needs, I don't know. Hope that helps, toz P.S. I don't know if it's good practice, but I usually use type variables in data declarations consistently in other type signatures, e.g. since you declared NetworkActivity using 'chan' and 'next', in 'clse', it'd make more sense (to me) to use (clse :: chan -> Free (NetworkActivity chan) Text) since it seems that 'chan' as a word has some extra connotations as opposed to 'a', which when I read, I think it can be absolutely anything. > > ------------------------------ > > Message: 2 > Date: Thu, 13 Oct 2016 13:15:43 +1100 > From: Sumit Raja <sumitraja(a)gmail.com> > To: beginners(a)haskell.org > Subject: [Haskell-beginners] Monadic functions definitions for free > monadic DSL > Message-ID: > <CAD4nrSc3pZ-K72GBt3=fRDuziWP0UtGsmX9RHGGaEYSB9ewMcQ@mail. > gmail.com> > Content-Type: text/plain; charset=UTF-8 > > Hello, > > I am trying to get my head around free monads by developing a simple > network abstraction DSL. > I've made good progress before adding TCP/IP semantics of accepting > connections. I'm now stuck with the creation of monadic functions. > > I've defined the following: > > data NetworkActivity chan next = Accept chan next (chan -> next) | > Send chan ByteString (Bool -> next) | > Recv chan (ByteString -> next) | > Close chan (() -> next) > > clse :: a -> Free (NetworkActivity a) Text > clse chan = liftF (Close chan (const "Quit")) > > chatterServer :: a -> Free (NetworkActivity a) Text > chatterServer svrchan = Free $ Accept svrchan (chatterServer > svrchan) chatterLoop > > chatterLoop :: a -> Free (NetworkActivity a) Text > chatterLoop chan = Free $ Recv chan $ \bs -> case BS.uncons bs of > Nothing -> clse chan > Just x -> if bs == "Bye" then > Free $ Close chan (\_ -> Pure "Quit") > else > Free (Send chan bs (\_ -> chatterLoop chan)) > > This works fine with the interpretTCP interpreter below accepting > multiple connections: > > interpretTCP :: Free (NetworkActivity TCPSocket) r -> IO r > interpretTCP prg = case prg of > Free (Accept serverSock svrLoop acceptProc) -> bracket (return > serverSock) > (\s-> interpretTCP (clse s)) > (\s-> do > (ss, sa) <- accept s > forkIO $ do > _ <- interpretTCP (acceptProc ss) > return () > interpretTCP svrLoop > ) > Free (Recv sock g) -> do > bs <- receive sock 4096 mempty > putStrLn (decodeUtf8 bs) > interpretTCP (g bs) > Free (Close sock g) -> do > close sock > putStrLn ("Server bye!" :: Text) > interpretTCP (g ()) > Pure r -> return r > Free (Send sock pl g) -> do > sent <- send sock pl mempty > interpretTCP (g (sent > 0)) > > Where I'm stuck is defining the monadic version of accept and I'm > beginning to think my original > data type defined above may be wrong. As an initial step I've defined > the following: > > recv :: a -> Free (NetworkActivity a) ByteString > recv chan = liftF (Recv chan identity) > > sendit :: a -> ByteString -> Free (NetworkActivity a) Bool > sendit chan pl = liftF (Send chan pl identity) > > mchatterServer :: a -> Free (NetworkActivity a) Text > mchatterServer chan = Free $ Accept chan (mchatterServer chan) > (\s > -> return (identity s) >>= mchatterLoop) > > mchatterServer works as is, the interpreter accepts multiple > connections. Similarly all good with recv and sendit. > I am struggling with converting the Accept in mchatterServer into a > function to use in the do syntax. The signature I think I should be > using is > > acc :: a -> NetworkActivity a Text -> Free (NetworkActivity a) > (NetworkActivity a Text) > > What I can't figure out is why it can't follow the pattern of recv and > sendit above: > > acc chan next = liftF $ Accept chan next identity > > Which results in error on identity (using Protolude): > > Expected type: a -> NetworkActivity a Text > Actual type: NetworkActivity a Text -> NetworkActivity a Text > > I can't really see how to get the types to line up and have now can't > see through the type fog. What am I missing in my reasoning about the > types? > > Help much appreciated! > > Thanks > > Sumit >

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Using scanl for Fibonacci sequence
by Lai Boon Hui 12 Oct '16

12 Oct '16

Hi All, i am not very sure how this can work fibs = 1 : scanl (+) 1 fibs Appreciate it if someone can guide me through by showing me a few steps of the function evaluation -- Best Regards, Boon Hui

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Regarding defining cases exhaustively
by Lai Boon Hui 10 Oct '16

10 Oct '16

Hi all, I have this code myMaximumBy :: (a -> a -> Ordering) -> [a] -> a myMaximumBy _ [] = undefined myMaximumBy _ (x:[]) = x myMaximumBy f (x:xs) = case f x maxRestOfList of GT -> x _ -> maxRestOfList where maxRestOfList = myMaximumBy f xs whereby the case myMaximumBy _ [] will never happen because case myMaximumBy _ (x:[]) = x prevents that from happening. However i will still like to define it to keep -Wall warning from complaining. Is using undefined acceptable or is there a better way? -- Best Regards, Boon Hui

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