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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 12 Jul '17

12 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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using Shake to compile c++
by Roger Mason 08 Jul '17

08 Jul '17

Hello, I am trying the Shake build system to compile some c++. I would appreciate some advice on how to use the result of a call to pkg-config in constructing a compiler command. This is what I have currently in Build.hs: import Development.Shake import Development.Shake.Command import Development.Shake.FilePath import Development.Shake.Util main :: IO () main = shakeArgs shakeOptions{shakeFiles="bin"} $ do want ["bin/makelist", "bin/makejpeg" <.> exe] phony "clean" $ do putNormal "Cleaning files in _build" removeFilesAfter "bin" ["//*"] "bin/makelist" <.> exe %> \out -> do cs <- getDirectoryFiles "" ["src/MakeList.cxx"] let os = ["objects" </> c -<.> "o" | c <- cs] need os cmd "c++ -o" [out] os "bin/makejpeg" <.> exe %> \out -> do cs <- getDirectoryFiles "" ["src/MakeJpeg.cxx"] let os = ["objects" </> c -<.> "o" | c <- cs] need os cmd "c++ -o" [out] os "objects//*.o" %> \out -> do let c = dropDirectory1 $ out -<.> "cxx" let m = out -<.> "m" let i = cmd "pkg-config glib-2.0 --cflags" () <- cmd "c++ -c" [c] "-o" [out] "-MMD -MF" [m] [i] needMakefileDependencies m This is the output from 'stack runhaskell Build.sh': Build.hs:29:17: error: * Ambiguous type variable `t0' arising from a use of `cmd' prevents the constraint `(CmdArguments t0)' from being solved. Relevant bindings include i :: t0 (bound at Build.hs:29:13) Probable fix: use a type annotation to specify what `t0' should be. These potential instances exist: instance CmdResult r => CmdArguments (IO r) -- Defined in `Development.Shake.Command' instance CmdResult r => CmdArguments (Action r) -- Defined in `Development.Shake.Command' instance (Development.Shake.Command.Arg a, CmdArguments r) => CmdArguments (a -> r) -- Defined in `Development.Shake.Command' ...plus one other (use -fprint-potential-instances to see them all) * In the expression: cmd "pkg-config glib-2.0 --cflags" In an equation for `i': i = cmd "pkg-config glib-2.0 --cflags" In the expression: do { let c = dropDirectory1 $ out -<.> "cxx"; let m = out -<.> "m"; let i = cmd "pkg-config glib-2.0 --cflags"; () <- cmd "c++ -c" [c] "-o" [out] "-MMD -MF" [m] [i]; .... } Build.hs:30:15: error: * Ambiguous type variable `t0' arising from a use of `cmd' prevents the constraint `(Development.Shake.Command.Arg [t0])' from being solved. Relevant bindings include i :: t0 (bound at Build.hs:29:13) Probable fix: use a type annotation to specify what `t0' should be. These potential instances exist: instance Development.Shake.Command.Arg [CmdOption] -- Defined in `Development.Shake.Command' instance Development.Shake.Command.Arg [String] -- Defined in `Development.Shake.Command' instance Development.Shake.Command.Arg String -- Defined in `Development.Shake.Command' * 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 = cmd "pkg-config glib-2.0 --cflags"; () <- cmd "c++ -c" [c] "-o" [out] "-MMD -MF" [m] [i]; .... } In the second argument of `(%>)', namely `\ out -> do { let ...; let ...; .... }' I would appreciate any help in getting the output of the call to pkg-config into the compiler invocation. Thanks, Roger

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case statement and guarded equations
by PATRICK BROWNE 07 Jul '17

07 Jul '17

In the case statement for the half function I think variable m is bound to the value of (snd (half (n - 1)). With this assumption, I have have written a guarded version called half1. Is it always possible to write case statements as a set of conditional equations? My intuition is that this should be possible and that the guarded version reduces pattern matching? Thanks, Pat data EvenOdd = Even | Odd deriving (Show,Eq) half 0 = (Even,0) half n = case half (n-1) of (Even,m) -> (Odd,m) (Odd,m) -> (Even,m+1) half1 0 = (Even , 0) half1 n | (fst (half (n - 1)) == Even) = (Odd , (snd (half (n - 1)))) half1 n | (fst (half (n - 1)) == Odd) = (Even , ((snd (half (n - 1))) + 1)) -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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Get rid of Maybes in complex types
by Baa 06 Jul '17

06 Jul '17

Hello Dear List! Consider, I retrieve from external source some data. Internally it's represented as some complex type with `Maybe` fields, even more, some of fields are record types and have `Maybe` fields too. They are Maybe's because some information in this data can be missing (user error or it not very valuable and can be skipped): data A = A { a1 :: Maybe B ... } data B = B { b1 :: Maybe C ... } I retrieve it from network, files, i.e. external world, then I validate it, report errors of some missing fields, fix another one (which can be fixed, for example, replace Nothing with `Just default_value` or even I can fix `Just wrong` to `Just right`, etc, etc). After all of this, I know that I have "clean" data, so all my complex types now have `Just right_value` fields. But I need to process them as optional, with possible Nothing case! To avoid it I must create copies of `A`, `B`, etc, where `a1`, `b1` will be `B`, `C`, not `Maybe B`, `Maybe C`. Sure, it's not a case. After processing and filtering, I create, for example, some resulting objects: data Result { a :: A -- not Maybe! ... } And even more: `a::A` in `Result` (I know it, after filtering) will not contain Nothings, only `Just right_values`s. But each function which consumes `A` must do something with possible Nothing values even after filtering and fixing of `A`s. I have, for example, function: createResults :: [A] -> [Result] createResults alst = ... case of (a1 theA) -> Just right_value -> ... Nothing -> logError undefined -- can not happen Fun here is: that it happens (I found bug in my filtering code with this `undefined`). But now I thought about it: what is the idiomatic way to solve such situation? When you need to have: - COMPLEX type WITH Maybes - the same type WITHOUT Maybes Alternative is to keep this Maybes to the very end of processing, what I don't like. Or to have types copies, which is more terrible, sure. PS. I threw IOs away to show only the crux of the problem. --- Cheers, Paul

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Incomplete patterns
by Jona Ekenberg 06 Jul '17

06 Jul '17

Hello, I am currently going through the CIS194 spring 13 course, since it has been recommended by several haskellions. I find the exercises extremely valuable, and the reading material is succinct (in a good way). I just finished the exercises for lesson two, and while everything works, I get this warning: Lektion02.hs:(38,1)-(44,21): warning: [-Wincomplete-patterns] … Pattern match(es) are non-exhaustive In an equation for ‘insert’: Patterns not matched: (LogMessage _ _ _) (Node _ (LogMessage _ _ _) _) I understand that there is a pattern I'm not covering, but I can't really figure out which one. Here is my function: insert :: LogMessage -> MessageTree -> MessageTree insert (Unknown _) mt = mt insert m Leaf = Node Leaf m Leaf insert m@(LogMessage _ t _) (Node left value@(LogMessage _ t' _) right) | t <= t' = Node (insert m left) value right | t > t' = Node left value (insert m right) insert m@(LogMessage _ _ _) (Node left (Unknown _) right) = Node left m right (here's a pastebin with intact indentation: http://lpaste.net/356716) I am not sure on how I should progress to find out which pattern I'm missing. Any help is appreciated. Kind regards, Jona

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