- Haskell-Cafe - Haskell.org

remove Maybe
by Damien Mattei 03 Jan '19

03 Jan '19

i had filtered a [Maybe Text] type to remove Nothing from the list and now i want to put the result in a [Text] type but the compiler complains about the incompatible type : (bd_rows_WDS :: [Only (Maybe Text)]) <- query conn qry_head_WDS (Only (name::String)) -- remove the records having N°BD NULL let fltWDS :: [Only Text] = Prelude.filter (\(Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:282:33: error: • Couldn't match type ‘Maybe Text’ with ‘Text’ Expected type: [Only Text] Actual type: [Only (Maybe Text)] • In the expression: Prelude.filter (\ (Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS In a pattern binding: fltWDS :: [Only Text] = Prelude.filter (\ (Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS In the expression: do conn <- connect defaultConnectInfo {connectHost = "moita", connectUser = "mattei", connectPassword = "sidonie2", connectDatabase = "sidonie"} (rows :: [(Text, Double)]) <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" (names :: [Only Text]) <- query_ conn "SELECT Nom FROM AngularDistance WHERE distance > 0.000278" let resLstNames = Prelude.map fromOnly names .... | 282 | let fltWDS :: [Only Text] = Prelude.filter (\(Only a) -> | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^... Failed, no modules loaded. how can id do the type conversion,now i'm sure there is no more Nothing values?

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MVar considered harmful
by Станислав Черничкин 02 Jan '19

02 Jan '19

Recently I had an interesting discussion on MVars with cats-effect library designers. Cats-effect brings MVar synchronization primitive along with other IO stuff to the Scala programming language. I tried to persuade them to include some Control.Concurrent.MVar’s functions to the library but has failed completely. Moreover, now I think that MVar is a poor choice for basic synchronization primitive. Your may find discussion here https://github.com/typelevel/cats-effect/issues/451 and event try to advocate, tl;dr. Anyway, what is so wrong with MVar? 1. It’s complex. Each MVar has 2 state transitions, each may block. 2. It does not play well in presence of asynchronous exceptions. More specifically, `take` and `put` operations should be balanced (each `take` must be followed by `put`) this force programmer to mask asynchronous exceptions during the MVar acquisition and since `take` function may block, this will delay task cancelation. Well, you may argue what the `takeMVar` function is actually interruptible, but I’m going to show an easier approach which renders interpretability magic unnecessary. What could be the sensible alternative? Guy from the cats-effect suggested me IVar + atomic reference (IORef). This pattern separates concern of blocking (synchronization) from the atomic mutation. So everything can be represented as atomic reference with IVar inside. Just look at this beautiful mutex implementation https://github.com/ovotech/fs2-kafka/blob/master/src/main/scala/fs2/kafka/i… (By ”beautiful” I mean approach itself of course, but not the Scala’s syntax. Scala is one of most ugliest girls after C++ I was forced to date with by my employer for money. Thankfully he didn’t force me to do the same things with her grandma Java). For people who don’t read Scala, the approach is fairly simple. Each thread which want to touch mutex, will create IVar, atomically swap it in the IORef masked (note, that IORef’s operations non-blocking), unmask and wait for previous become available IVar *unmasked*. Then it will either perform it’s operations or fail due to the interruption or exception and trigger newly installed IVar anyway. It just works. Without any «interruptible» magic. So, which benefits can we get? 1. Simpler implementation of basic primitives. Obliviously IORef is fairly simple. IVar is also mind be simpler than MVar, and possibly faster (just “possibly”, I don’t know how it’s implemented, but I guess lesser state transitions implies less logic). 2. Simplified deadlock analysis. Really, we have only IVar with only one state transition and only one potentially blocking operation. 3. Magicless support of interruptions. We don’t need to separate mask/uninterruptibleMask anymore, because all updates are non-blocking, and all waits are unmasked.

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Jan 2019- Membership of the Group
by William Fearon 31 Dec '18

31 Dec '18

1 0

A few questions about unsafe IO
by Qingbo Liu 31 Dec '18

31 Dec '18

Dear Cafe, I am reading an article on HaskellWiki about unsafe IO[1]. It gives the guideline about usage of unsafeDupablePerformIO: "If you need extra speed, and it's acceptable for the action to be performed multiple times, and it's acceptable if this action is canceled halfway through its execution, use unsafeDupablePerformIO.” Inside `ByteStirng` module[2], I noticed that when converting [Char] to ByteString, it uses unsafeDupablePerformIO to allocate space for the ByteString, as the following code shows (important information highlighted): packChars :: [Char] -> ByteString packChars cs = unsafePackLenChars (List.length cs) cs : unsafePackLenBytes :: Int -> [Word8] -> ByteString unsafePackLenBytes len xs0 = unsafeCreate len $ \p -> go p xs0 where go !_ [] = return () go !p (x:xs) = poke p x >> go (p `plusPtr` 1) xs unsafeCreate :: Int -> (Ptr Word8 -> IO ()) -> ByteString unsafeCreate l f = unsafeDupablePerformIO (create l f) -- | A way of creating ByteStrings outside the IO monad. The @Int@ -- argument gives the final size of the ByteString. -- | Create ByteString of size @l@ and use action @f@ to fill it's contents. create :: Int -> (Ptr Word8 -> IO ()) -> IO ByteString create l f = do fp <- mallocByteString l withForeignPtr fp $ \p -> f p return $! PS fp 0 l {-# INLINE create #-} -- | Wrapper of 'mallocForeignPtrBytes' with faster implementation for GHC -- mallocByteString :: Int -> IO (ForeignPtr a) mallocByteString = mallocPlainForeignPtrBytes {-# INLINE mallocByteString #-} The doc of `mallocPlainForeignPtrBytes`, however, explicitly says that no finalizer is added for the allocated memory. So my question is: would not the allocation code in ByteString module cause memory leaks? The doc of `unsafeDupablePerformIO` mentions that it "duplicated IO actions is only run partially, and then interrupted in the middle without an exception being raised”. Thus, it might happen that we have already allocated the memory but then the action is interrupted, without reclaiming the memory. [1] https://wiki.haskell.org/Evaluation_order_and_state_tokens [2] http://hackage.haskell.org/package/bytestring-0.10.8.2/docs/src/Data.ByteSt… Best Regards, Qingbo Liu

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Taking over the udev package
by Leif Warner 30 Dec '18

30 Dec '18

I'm offering to take over the "udev" package on hackage. I have some code which depends on it, as well as a PR to allow it to build on GHC 8.4, among other things: https://github.com/pxqr/udev/pull/5 It looks like the author, Samvel Truzyan, has been inactive on GitHub and Twitter for the past couple of years. Ilya Portnov might also be a good candidate to maintain this package - Ilya has the wacom-daemon package on hackage which depends on udev, and also has previously submitted a PR which has been merged to the udev package. -Leif Warner

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Fwd: IO monad use
by Damien Mattei 29 Dec '18

29 Dec '18

---------- Forwarded message --------- From: Damien Mattei <damien.mattei(a)gmail.com> Date: Sat, Dec 29, 2018 at 9:46 AM Subject: IO monad use To: Damien MATTEI <damien.mattei(a)gmail.com> again an annoying error with my code, i want to apply sort of MAP on list resut of my database IO accessed extracting info with another query, queries works both but i can not MAP or if i can i do not know how to SHOW the result, here is the code: lstNamesBD <- mapM (\(Only name) -> do let nameStr = Text.unpack name bd <- getBD conn nameStr (nameStr,bd)) names it fails to compile with this error: Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:203:33: error: • Couldn't match type ‘(,) String’ with ‘IO’ Expected type: IO (Maybe Float) Actual type: (String, Maybe Float) • In a stmt of a 'do' block: (nameStr, bd) In the expression: do let nameStr = unpack name bd <- getBD conn nameStr (nameStr, bd) In the first argument of ‘mapM’, namely ‘(\ (Only name) -> do let nameStr = ... bd <- getBD conn nameStr (nameStr, bd))’ | 203 | (nameStr,bd)) | ^^^^^^^^^^^^ Failed, no modules loaded. if i code like this show does not know how to display result: let lstNamesBD = Prelude.map (\(Only name) -> do let nameStr = Text.unpack name bd <- getBD conn nameStr res <- (nameStr,bd) res) names putStr "lstNamesBD =" putStrLn $ show lstNamesBD *Main> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:193:16: error: • No instance for (Show (IO (Maybe Float))) arising from a use of ‘show’ • In the second argument of ‘($)’, namely ‘show lstNamesBD’ In a stmt of a 'do' block: putStrLn $ show lstNamesBD In the expression: do conn <- connect defaultConnectInfo {connectHost = "moita", connectUser = "mattei", connectPassword = "sidonie2", connectDatabase = "sidonie"} (rows :: [(Text, Double)]) <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" (names :: [Only Text]) <- query_ conn "SELECT Nom FROM AngularDistance WHERE distance > 0.000278" let resLstNames = Prelude.map fromOnly names .... | 193 | putStrLn $ show lstNamesBD | ^^^^^^^^^^^^^^^ Failed, no modules loaded. help me please

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Re: [Haskell-cafe] About understanding FunBind in haskell-src-exts.
by Magicloud Magiclouds 29 Dec '18

29 Dec '18

Hi, I can see the case of one FunBind contains multiple Matches, like this one. But I cannot see why the "name" (isJust) is in Match, instead of FunBind. Are there cases other than yours that a few functions having different names should be grouped together? PS: FunBind and Match definitions: FunBind l [Match l] Match l (Name l) [Pat l] (Rhs l) (Maybe (Binds l)) On Sat, Dec 29, 2018 at 6:56 PM Alan & Kim Zimmerman <alan.zimm(a)gmail.com> wrote: > > This is for the case when you have a function like > > isJust Nothing = False > isJust (Maybe _) = True > > This is a single FunBind for isJust, but has two Matches > > Alan > > On Sat, 29 Dec 2018 at 12:25, Magicloud Magiclouds <magicloud.magiclouds(a)gmail.com> wrote: >> >> Hi, >> >> Recently I am playing with haskell-src-exts to help understanding >> Haskell syntax more. Then I got this question. >> >> As top level declarations (the type Decl), TypeSig, PatBind are quite >> straight forward, they mean one item in the source. But `FunBind l >> [Match l] >> `, I could not see it maps to "one item" in the source. >> >> Apparently, the list of Match can hold a few non-related functions >> with different names. Then why a FunBind to hold them together? >> >> -- >> 竹密岂妨流水过 >> 山高哪阻野云飞 >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

1 0

Jan 2018- Membership of the Group
by William Fearon 29 Dec '18

29 Dec '18

1 0

About understanding FunBind in haskell-src-exts.
by Magicloud Magiclouds 29 Dec '18

29 Dec '18

Hi, Recently I am playing with haskell-src-exts to help understanding Haskell syntax more. Then I got this question. As top level declarations (the type Decl), TypeSig, PatBind are quite straight forward, they mean one item in the source. But `FunBind l [Match l] `, I could not see it maps to "one item" in the source. Apparently, the list of Match can hold a few non-related functions with different names. Then why a FunBind to hold them together? -- 竹密岂妨流水过山高哪阻野云飞

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[GSoC 2019] Call for Ideas
by Jasper Van der Jeugt 28 Dec '18

28 Dec '18

Google Summer of Code will take place again in 2019, for the 15th year of the program [1]! We are very proud that Haskell.Org has been able to take part every year except for the initial year (2015) and 2016-2017, when we had to resort to running our own program. Last year, we were fortunate enough to join again, and we think the results greatly benefited the Haskell community [2]. We are hoping to do the same for 2019. As far as we know, a really important part of our application to GSoC is the list of ideas we provide. For that, I would like to count on all of you. If you are the maintainer or the user of a Haskell project, and you have an improvement in mind which a student could work on during the summer, please submit an idea here: https://summer.haskell.org/ideas.html For context, Google Summer of Code is a program where Google sponsors students to work on open-source projects during the summer. Haskell.org has taken part in this program from 2006 until 2015, and again in 2018. Many important improvements to the ecosystem have been the direct or indirect result of Google Summer of Code projects, and it has also connected new people with the existing community. Projects should benefit as many people as possible – e.g. an improvement to GHC will benefit more people than an update to a specific library or tool, but both are definitely valid. New libraries and applications written in Haskell, rather than improvements to existing ones, are also accepted. Projects should be concrete and small enough in scope such that they can be finished by a student in three months. Warm regards On behalf of the Haskell.Org committee Jasper Van der Jeugt [1]: https://opensource.googleblog.com/2018/11/google-summer-of-code-15-years-st… [2]: https://summer.haskell.org/news/2018-09-01-final-results.html

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hackage.org down?
by Gregory Guthrie 28 Dec '18

28 Dec '18

I get a failure response from hackage.org: 502 Bad Gateway nginx/1.10.3 (Ubuntu)

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Re: [Haskell-cafe] Find out which part of code are causing parallelism?
by Johannes Waldmann 28 Dec '18

28 Dec '18

> however I just found out that without inserting > any parallism manually the program is already parallelised, That's not your program, it's the garbage collector. "By default, all of the capabilities participate in parallel garbage collection." https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/runtime_con… - J.W.

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[Haskell][ANN] vabal - the cabal companion
by Francesco Magliocca 28 Dec '18

28 Dec '18

Hello, I'm please to announce that vabal 1.2.0 has been released on Hackage: http://hackage.haskell.org/package/vabal vabal is a software that helps you determine the ghc version needed to compile a cabal package. It reads the cabal file and extracts the constraints imposed on the base package. Then it uses ghcup ( https://github.com/haskell/ghcup ) to obtain a compiler version compatible with the constraints (possibly downloading it from the official ghc mirror) and can be combined with cabal to build the project using the obtained ghc compiler. In this way you can treat the `base` package dependency as any other package dependency and don't have to manually deal with different ghc versions anymore. vabal is meant to be used in collaboration with cabal. It leverages cabal's ability to work with multiple ghc versions, and tries to be the least intrusive possible, so that you can keep using your usual workflow, but won't have to worry about `base` package dependency issues, anymore. You can get started here: https://github.com/Franciman/vabal#quick-start For details please refer to: https://github.com/Franciman/vabal#vabal---the-cabal-companion Ideas, suggestions, opinions and pull requests are very welcome! A big thank you goes to all the contributors that helped me with code, suggestions and ideas: https://github.com/Franciman/vabal#contributors

1 0

Find out which part of code are causing parallelism?
by ducis 28 Dec '18

28 Dec '18

Hi all, I am working on parallelising some program, however I just found out that without inserting any parallism manually the program is already parallelised, presumably by some of the libraries referenced, as long as I enable -threaded and +RTS -N8. Unfortunately this parallism are actually slowing down my program, probably due to being of too fine granularity. For example, with +RTS -N8 I have 47.03user 38.06system 0:21.68elapsed 392%CPU While with +RTS -N1 I have 16.57user 0.45system 0:17.02elapsed 100%CPU In .prof file I can't find whether a function fired a spark or how much 'system' time a function used. Nor can I find any names of functions/modules browsing through eventlog with ghc-events. Is there a way to find out which functions/modules are causing any parallelism at all? I am also trailing the post with the imports I used in case anyone immediately recognises parallelism by default in the package. import Control.DeepSeq import GHC.Generics (Generic, Generic1) import Text.Parsec import Data.List import Data.Vector.Unboxed(fromList, (!)) import qualified Data.Vector as V import qualified Data.Map as M import qualified Data.Set as S import Data.Char import Control.Applicative((*>),(<*)) import Test.QuickCheck.Monadic import Test.QuickCheck import Control.Monad import Text.Regex import Control.Applicative ((<$>), (<$), (<|>)) import GHC.Read (readLitChar) import Data.Char(isPrint) import Text.ParserCombinators.ReadP import Text.Show.Pretty import Data.Functor import Data.Maybe import System.Environment(getArgs) import Text.Read(readMaybe) import Text.Printf import Data.Dynamic import Debug.Trace import Data.Ratio import qualified Data.DList as D import Control.Monad.Trans.Writer.Strict import Control.Monad.Trans.Class import Data.Typeable(Typeable) import qualified Data.HashMap.Lazy as H Best, ducis

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Re: [Haskell-cafe] trace output statements
by Ian Denhardt 27 Dec '18

27 Dec '18

(Adding the list back to Cc; you forgot to hit reply all) Quoting Damien Mattei (2018-12-25 16:57:04) > i get in trouble understanding what fmap was doing, fmap (for IO) just applies a function to the result of the action, so: fmap f action is equivalent to: do result <- action return (f result) > and why the same thing to do in main and in a function had so > different implementations... I suspect the modified version of the program has some differences that aren't strictly necessary just to factor out the relevant bits into their own definition; this *shouldn't* be major surgery. Hard for me to point out without having the two full examples handy. Quoting Damien Mattei (2018-12-25 16:57:04) > yes, i understand with your python example, it's like read on a web > page, IO are not a "cake" but IO are "a recipe for the cake"... > functions are pure in haskell, so they can not have side effects... > what put me some trouble is an answer 2 weeks ago, someone gave me > hints that lead to this solution for getDB: > getBD :: Connection -> String -> IO Float > getBD conn name = trace "Entering getBD" noBDfp > � where qry_head_BD_Sidonie = "select `N° BD` from Coordonnées where > Nom = ?" :: Query > � � � � � � � bd_rows :: IO [Only Text] > � � � � � � � bd_rows = query conn qry_head_BD_Sidonie (Only > (name::String)) > � � � � � � � noBDtxt :: IO Text > � � � � � � � noBDtxt = trace "assigning noBDtxt" (fmap (fromOnly . > Prelude.head) bd_rows) > � � � � � � � noBDstr :: IO String > � � � � � � � noBDstr = trace "assigning noBDstr" (fmap Text.unpack > noBDtxt) > � � � � � � � noBDfp :: IO Float > � � � � � � � noBDfp = fmap read noBDstr > which was code different from the code in main,i get in trouble > understanding what fmap was doing , and why the same thing to do in > main and in a function had so different implementations... > > On Tue, Dec 25, 2018 at 10:19 PM Ian Denhardt <[1]ian(a)zenhack.net> > wrote: > > The correct type annotation for getDB3 should be: > � � getDB3 :: Connection -> String -> IO Float > Note the IO at the end. Functions in Haskell are just pure > computation; > they can't have side effects -- so a function returning a Float > can't > possibly talk to a database. > Instead, The type `IO a` represents a description of an action to > perform.� It's still just a value -- calling getDB3 doesn't *do* > anything. You can stitch these together using do-notation or > functions > like >>=, and when the program is run the action defined by 'main' > is > performed. > --- > An analogy: you could imagine instead of IO we could give up, and > write > code that computes a program in some other (imperative) programming > language) that we then hand off to an interpreter. For example, we > could > compute a python program that counts from 1 to 99 like so: > � � printNum :: Int -> String > � � printNum n = "print('" ++ show n ++ "')\n" > � � pythonProgram = concatMap printNum [1..99] > So we've defined a variable fullProgram that is a string with the > source > code to a python program like: > � � print('1') > � � print('2') > � � print('3') > � � ... > � � print('99') > ..but we haven't actually run it. To do that we'd have to pass the > string off to the python interpreter. > This is a good way to think about what IO is -- main is like our > pythonProgram above, in that it is a description of actions to > perform, > but *evaluating* it doesn't have any side effects -- it just > computes > the description. When you run a Haskell program, this is like taking > the description defined by Main and passing it off to an > interpreter. > --- > So your definition of getDB3 is a description of actions to perform > to > get a float from the database, but your type declaration says it's a > function that computes a float (without having to perform any > "actions"). > This is a critical distinction that exists in Haskell but not most > other > languages. > Hope this helps, > -Ian > Quoting Damien Mattei (2018-12-25 15:07:35) > >� � yes i use do notation, but for example i have code that works > in main > >� � and not in a function! > >� � i print the code perheaps someone could help me: > >� � first the function, so you have the import list too: > >� � import Database.MySQL.Simple > >� � import Database.MySQL.Simple.QueryResults > >� � import Database.MySQL.Simple.Result > >� � import Database.MySQL.Simple.QueryParams > >� � import Database.MySQL.Simple.Param > >� � import Control.Monad > >� � import Data.Text as Text > >� � --import Data.Int as Int > >� � --import Data.List > >� � import Debug.Trace > >� � import Data.Maybe as Maybe > >� � -- this function will return th N°BD from Sidonie for a > given name > >� � -- note: names have been standardized between Sidonie and WDS > >� � getBD3 :: Connection -> String -> Float > >� � getBD3 conn name = do > >� � � � � � � � � � � � �� let > qry_head_BD_Sidonie = "select `N° BD` from > >� � Coordonnées where Nom = ?" :: Query > >� � � � � � � � � � � � �� (bd_rows :: > [Only Text]) <- query conn > >� � qry_head_BD_Sidonie (Only (name::String)) > >� � � � � � � � � � � � �� let noBDtxt = > fromOnly (Prelude.head bd_rows) :: > >� � Text > >� � � � � � � � � � � � �� let noBDstr = > Text.unpack noBDtxt :: String > >� � � � � � � � � � � � �� let noBDfp = > read $ noBDstr :: Float > >� � � � � � � � � � � � �� return noBDfp > >� � with this function i have this error: > >� � Prelude> :load UpdateSidonie > >� � [1 of 1] Compiling Main� � � � � � � � � > � � �� ( UpdateSidonie.hs, > >� � interpreted ) > >� � UpdateSidonie.hs:54:13: error: > >� � � � �� Couldn't match expected type �Float� > with actual type �IO > >� � Float� > >� � � � �� In a stmt of a 'do' block: > >� � � � � � � � �� (bd_rows :: [Only Text]) <- > query > >� � � � � � � � � � � � � � � � � > � � � � � � � � � � � � � � � � � > � � � � > >� � � �� conn qry_head_BD_Sidonie (Only (name :: String)) > >� � � � � � �� In the expression: > >� � � � � � � � �� do let qry_head_BD_Sidonie = > ... > >� � � � � � � � � � � �� (bd_rows :: [Only > Text]) <- query > >� � � � � � � � � � � � � � � � � > � � � � � � � � � � � � � � � � � > � � � � > >� � � � � � �� conn qry_head_BD_Sidonie (Only (name :: > String)) > >� � � � � � � � � � � �� let noBDtxt = ... > >� � � � � � � � � � � �� let noBDstr = ... > >� � � � � � � � � � � �� .... > >� � � � � � �� In an equation for �getBD3�: > >� � � � � � � � � � �� getBD3 conn name > >� � � � � � � � � � � � �� = do let > qry_head_BD_Sidonie = ... > >� � � � � � � � � � � � � � � � � > �� (bd_rows :: [Only Text]) <- query > >� � � � � � � � � � � � � � � � � > � � � � � � � � � � � � � � � � � > � � � � > >� � � � � � � � � � � � �� conn > qry_head_BD_Sidonie (Only (name :: String)) > >� � � � � � � � � � � � � � � � � > �� let noBDtxt = ... > >� � � � � � � � � � � � � � � � � > �� .... > >� � � �� | > >� � 54 |� � � � � � � � � � � �� > (bd_rows :: [Only Text]) <- query conn > >� � qry_head_BD_Sidonie (Only (name::String)) > >� � � �� |� � � � � � � � � � � � > >� � > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ > ^^^ > >� � ^^^^^^^^^ > >� � Failed, no modules loaded. > >� � i do not understand the error complaining that i return an IO > >� � float,because i'm sure it's a float in noBDfp > >� � if i put the same lines of code in the main it works !!! : > >� � main :: IO () > >� � main = > >� � �� do > >� � � � �� conn <- connect defaultConnectInfo > >� � � � � � �� { connectHost = "moita", > >� � � � � � � � �� connectUser = "mattei", > >� � � � � � � � �� connectPassword = "sidonie2", > >� � � � � � � � �� connectDatabase = "sidonie" } > >� � � let qry_head_BD_Sidonie = "select `N° BD` from > Coordonnées where > >� � Nom = ?" :: Query > >� � � (bd_rows :: [Only Text]) <- query conn > qry_head_BD_Sidonie (Only > >� � (name::String)) > >� � putStr "bd_rows =" > >� � putStrLn $ show bd_rows > >� � � � �� let noBDtxt = fromOnly (Prelude.head bd_rows) > :: Text > >� � � � �� let noBDstr = Text.unpack noBDtxt :: String > >� � � � �� let noBDfp = read $ noBDstr :: Float > >� � � � �� putStr "noBDfp =" > >� � � � �� (putStrLn (show noBDfp)) > >� � � close conn > >� � it works i have output like this: > >� � *Main> main > >� � bd_rows =[Only {fromOnly = "-04.3982"}] > >� � noBDtxt ="-04.3982" > >� � noBDfp =-4.3982 > >� � noBDfp + 1 = -3.3982 > >� � i'm well getting a float in noBDfp , i even can add 1 to it > :-) ( cool > >� � haskell...) > >� � but i'm just wanting to that in the function getDB3 but it > does not > >� � compile... > >� � ?????? > >� � Damien > > > >� � On Sun, Dec 23, 2018 at 4:54 PM Tom Ellis > >� � <[1][2]tom-lists-haskell-cafe-2017(a)jaguarpaw.co.uk> wrote: > > > >� � � I think forgetting about monads and just using do-notation > will help > >� � � you. > >� � � On Sun, Dec 23, 2018 at 04:44:57PM +0100, Damien Mattei > wrote: > >� � � > i think learning Monads from scratch again will help me > >� � � > > >� � � > On Sun, Dec 23, 2018 at 4:11 PM Tom Ellis < > >� � � > [2][3]tom-lists-haskell-cafe-2017(a)jaguarpaw.co.uk> > wrote: > >� � � > > >� � � > > Yes, exactly! > >� � � > > > >� � � > > On Sun, Dec 23, 2018 at 02:08:57PM +0100, Damien > Mattei wrote: > >� � � > > > lazyness....? > >� � � > > > > >� � � > > > On Sun, Dec 23, 2018 at 8:40 AM Tom Ellis < > >� � � > > > [3][4]tom-lists-haskell-cafe-2017(a)jaguarpaw.co.uk> > wrote: > >� � � > > > > >� � � > > > > On Sat, Dec 22, 2018 at 09:52:18AM +0100, Damien > Mattei > >� � � wrote: > >� � � > > > > > i have inserted trace statement that output > variable > >� � � > > > > > ... i have strange behavior of output: > >� � � > > > > > >� � � > > > > Let's take a simpler example.�� Do you > understand why the > >� � � trace > >� � � > > statments > >� � � > > > > from this small program appear in the order that > they do?� > >� � � (And for > >� � � > > what > >� � � > > > > it's worth I really think you'll be better off > writing > >� � � programs using > >� � � > > do > >� � � > > > > notation). > >� � � > > > > > >� � � > > > > > >� � � > > > > % cat test.hs > >� � � > > > > import Debug.Trace > >� � � > > > > > >� � � > > > > result = > >� � � > > > >�� let a = trace "evaluating a" 2 > >� � � > > > >�� b = trace "evaluating b" 10 > >� � � > > > >�� c = trace "evaluating c" (a + > b) > >� � � > > > >�� in c > >� � � > > > > ~% ghci -e result test.hs > >� � � > > > > evaluating c > >� � � > > > > evaluating b > >� � � > > > > evaluating a > >� � � > > > > 12 > >� � � > > _______________________________________________ > >� � � > > Haskell-Cafe mailing list > >� � � > > To (un)subscribe, modify options or view archives go > to: > >� � � > > > [4][5]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >� � � > > Only members subscribed via the mailman list are > allowed to > >� � � post. > >� � � > _______________________________________________ > >� � � > Haskell-Cafe mailing list > >� � � > To (un)subscribe, modify options or view archives go to: > >� � � > > [5][6]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >� � � > Only members subscribed via the mailman list are allowed > to post. > >� � � _______________________________________________ > >� � � Haskell-Cafe mailing list > >� � � To (un)subscribe, modify options or view archives go to: > >� � � > [6][7]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >� � � Only members subscribed via the mailman list are allowed > to post. > > > > Verweise > > > >� � 1. mailto:[8]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > >� � 2. mailto:[9]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > >� � 3. mailto:[10]tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > >� � 4. > [11]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >� � 5. > [12]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > >� � 6. > [13]http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > > Verweise > > 1. mailto:ian@zenhack.net > 2. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 3. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 4. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 5. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > 6. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > 7. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > 8. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 9. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 10. mailto:tom-lists-haskell-cafe-2017@jaguarpaw.co.uk > 11. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > 12. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > 13. http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe

2 2

Poor performance when using hsc2hs to cross-compile
by Vanessa McHale 26 Dec '18

26 Dec '18

Hi all, I've been trying to cross-compile X11 for ARM. Unfortunately, the build runs in the preprocessor phase for a very very long time when preprocessing Graphics/X11/Types.hsc Upon passing -v to hsc2hs I saw many steps like checking XK_abrevebelowdot `GreaterOrEqual` Unsigned 17039360 executing: /usr/bin/arm-linux-gnueabihf-gcc -c /home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/Graphics/X11/ExtraTypes/XorgDefault_hsc_test37988.c -o /home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/Graphics/X11/ExtraTypes/XorgDefault_hsc_test37988.o -marm -fno-stack-protector '-fuse-ld=gold' -Wl,-z,noexecstack '-D__GLASGOW_HASKELL__=806' '-Dlinux_BUILD_OS=1' '-Dx86_64_BUILD_ARCH=1' '-Dlinux_HOST_OS=1' '-Darm_HOST_ARCH=1' -Iinclude -I/home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/include -I/home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/autogen -I/home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/global-autogen -include /home/vanessa/git-builds/junk/X11-1.9/dist-newstyle/build/arm-linux/ghc-8.6.3/X11-1.9/build/autogen/cabal_macros.h -I/usr/local/lib/arm-linux-gnueabihf-ghc-8.6.3/base-4.12.0.0/include -I/usr/local/lib/arm-linux-gnueabihf-ghc-8.6.3/integer-gmp-1.0.2.0/include -I/usr/local/lib/arm-linux-gnueabihf-ghc-8.6.3/include result: False which I assume are necessary or at least progressing... Is there any way to speed this up? Is it a bug of some sort in hsc2hs or X11? Is there any hope in reporting upstream to the appropriate project (and what is the appropriate project in this case?)? Cheers, Vanessa

1 0

ZuriHac 2019 registration now open
by Jasper Van der Jeugt 26 Dec '18

26 Dec '18

Hello Friends of Haskell, We are happy to announce that registration for ZuriHac 2019 is now open. Participation is free but limited to 450 attendees. You can register at: https://zfoh.ch/zurihac2019/#registration This year, we've taken dogfooding a step further and rolled our own open source registration system in Haskell, so please let me know if you experience any issues. ZuriHac 2019 will take place from Friday the 14th to Sunday the 16th of June 2019. It will be hosted at the Hochschule Rapperswil on the shores of beautiful Lake Zurich. This will mark the 8th anniversary of ZuriHac since our beginnings in 2010. The Zurich Haskell Hackathon is a free, international, grassroots, and collaborative coding festival. Our goal is to connect Haskellers, expand the community, learn things from each other, and to work on Haskell libraries, tools, and infrastructure. This year, we will enjoy keynotes from: - Simon Peyton Jones - Susan Potter - Richard Eisenberg - Ryan Trinkle More keynote speakers will be announced. The event is open to any experience level, from beginners to gurus. We want to make a special effort to ensure that the event is welcoming and accessible to people completely new to Haskell. That is why we are super excited that, Julie Moronuki, co-author of Haskell Programming from first principles [1] and Joy of Haskell [2], has kindly agreed to teach a beginners course in one of the classrooms we have available. Additionally, there will be mentors that you can approach during the whole event with any Haskell-related questions. This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools or even start new ones! You can find all of this information and more on our website: https://zfoh.ch/zurihac2019 We would also like to thank our sponsors Digital Asset [3], DFINITY [4] and HSR [5] for their strong commitment to the Haskell community and for supporting this great event! Looking forward to seeing you there, The Zurich Friends of Haskell association. [1]: https://www.goodreads.com/book/show/25587599-haskell-programming [2]: https://joyofhaskell.com/ [3]: https://digitalasset.com/careers.html [4]: https://dfinity.org/jobs [5]: https://www.hsr.ch/

1 0

Taking over maintainership of syb
by Sergey Vinokurov 25 Dec '18

25 Dec '18

I'd like to become maintainer of the syb package (http://hackage.haskell.org/package/syb) The current maintainer, José Pedro Magalhães, would like to step down and is looking for anyone interested in picking up the package (https://github.com/dreixel/syb/pull/19) Best regards, Sergey

2 1

trace output statements
by Damien Mattei 25 Dec '18

25 Dec '18

i have this function: -- this function will return th N°BD from Sidonie for a given name -- note: names have been standardized between Sidonie and WDS getBD :: Connection -> String -> IO Float getBD conn name = trace "Entering getBD" noBDfp where qry_head = "select `N° BD` from Coordonnées where Nom = ?" :: Query bd_rows :: IO [Only Text] bd_rows = query conn qry_head (Only (name::String)) -- noBDtxt :: [Text] -- noBDtxt = fromOnly (Prelude.head bd_rows) -- noBDtxt :: IO [Text] -- lg = fmap Prelude.length bd_rows -- lg2 = fmap show lg noBDtxt :: IO Text -- noBDtxt = trace "lg " (fmap (fromOnly . Prelude.head) bd_rows) noBDtxt = trace "assigning noBDtxt" (fmap (fromOnly . Prelude.head) bd_rows) -- noBDstr :: String -- noBDstr = Text.unpack noBDtxt noBDstr :: IO String noBDstr = trace "assigning noBDstr" (fmap Text.unpack noBDtxt) -- noBDfp = read $ noBDstr :: Float noBDfp :: IO Float noBDfp = fmap read noBDstr call by : let lstBD = Prelude.map (\elem -> getBD conn (Text.unpack (fst elem))) rows it works ok, in fact at some point it fails due to NULL sql value not again handle correctly , i have inserted trace statement that output variable, i understand it's not really good becaus it breaks the purity of haskell function, perheaps for this reason i have strange behavior of output: ... Entering getBD assigning noBDstr assigning noBDtxt Entering getBD assigning noBDstr assigning noBDtxt Entering getBD assigning noBDstr assigning noBDtxt *** Exception: UnexpectedNull {errSQLType = "VarString", errHaskellType = "Text", errFieldName = "N\194\176 BD", errMessage = "unexpected null in table Coordonn\195\169es of database sidonie"} *Main> you will notice noBDstr seems to be assigned before noBDtxt, but in the code i have the statements in this order: noBDtxt = trace "assigning noBDtxt" (fmap (fromOnly . Prelude.head) bd_rows) noBDstr :: IO String noBDstr = trace "assigning noBDstr" (fmap Text.unpack noBDtxt) i just want to understand wht's happening, this is not critical for code as it works... any idea? Damien

4 10

Re: [Haskell-cafe] Places to write and publish projects that 'explain ideas'
by Doug McIlroy 25 Dec '18

25 Dec '18

The project (or an appropriate-sized coherent piece thereof) seems like a good fit for the "programming pearls" department in the Journal of Functional Programming Doug McIlroy

1 0

Re: [Haskell-cafe] Places to write and publish projects that "explain ideas"?
by Siddharth Bhat 25 Dec '18

25 Dec '18

Regarding (1), as far as I'm aware, the requirements are flexible, but the general requirement is "published a paper at a good venue, either journal or conference". This implementation was a hobby project of mine, but I was hoping to be able to justify spending more time on it by finding a venue to publish it at. As for (2), it's mostly expository. The techniques that I am interesting in showing off have no known "simple" implementations as far as I'm aware -- scalar evolution, polyhedral compilation, some kinds of inter-procedural analyses, SSA, etc. do not have simple implementations. Indeed, the goal is to show how to write *optmising* compilers. Most compilers books teach one how to write *a compiler, *while there's a second course usually on compiler optimisation. I don't know of readable, clean implementations of, say, the SSA construction algorithm, or the scalar evolution analysis. Regarding nanopass: I think it makes a lot of sense as a philosophy as a way to architect compilers. However, there are a lot of nice haskell-isms to write compilers which are scattered throughout the literature, as far as I can tell: Hoopl, trees that grow, much of Matt might's work on abstracting abstract interpreters, Scrap-your-boilerplate style techniques, equality saturation, etc. which are all fantastic, but I've never seen them under the same umbrella. So that's a sketch of the general story I want to tell :) Thanks. ~Siddharth On Tue, Dec 25, 2018 at 7:30 PM Richard O'Keefe <raoknz(a)gmail.com> wrote: > > Question 1: what does your department require? The first person to > consult > is your supervisor. Helping students figure out where to publish is > definitely > part of a supervisor's job (speaking as a former supervisor of PhD > students). > > Question 2: what is the TOPIC of your publication? Since you are merely > (!) > demonstrating known techniques on a toy language, does it really count as > work on compilers? Is it software engineering? Is it computer science > education? Is the aim to develop something more maintainable? What is the > quality metric by which your work should be judged? > > Observation: if you haven't already read the "Nanopass" paper, > https://www.cs.indiana.edu/~dyb/pubs/nano-jfp.pdf , > you should do so now. http://nanopass.org/ is the associated web site. > There is also a mailing list, > https://groups.google.com/forum/#!forum/nanopass-framework > I think your aims are sufficiently close to theirs for you to have > interesting things to say to each other, despite you using Haskell and > them having used Scheme. > > >

1 0

Places to write and publish projects that "explain ideas"?
by Siddharth Bhat 25 Dec '18

25 Dec '18

Hello all, I have somewhat of an unorthodox problem. I've been writing on and off, a series on implementing compilers "beautifully" in haskell, called tiny-optimising-compiler <http://github.com/bollu/tiny-optimising-compiler>. They're literate haskell files, with the aim of explaining the really elegant ideas that exist in compilers literature - data flow analysis, abstract interpretation, SSA, continuations, scalar evolution, and some more slightly out-there / research-y things, like polyhedral compilation, equality saturation. However, I'm also a research student at my university, and am expected to publish before I graduate. I was looking for possible places to publish a project such as this, whose selling point would be "explains things elegantly, and possibly rewords standard things to nice looking haskell". Are there places where one could conceivably publish about such a project? If not, I forsee myself not being able to finish this project for a while longer, and that would make me sad. Thanks, and a merry christmas to all, ~Siddharth

4 4

Happy Haskell Programming for GHC 8.x
by Kazu Yamamoto 25 Dec '18

25 Dec '18

Hello Cafe, I'm happy to announce that Happy Haskell Programming (HHP) version 0.0.0 has been released: http://hackage.haskell.org/package/hhp In short, HHP is a fork from ghc-mod 4.1.6 and re-written for all GHC 8.x. Emacs front end is included. For more information, please read: https://github.com/kazu-yamamoto/hhp Merry Christmas and Happy Haskell Programming on Emacs with GHC 8.x! --Kazu

1 0

Reminder Dutch FP-Day 2019
by Bas Lijnse 24 Dec '18

24 Dec '18

Hi Everybody, Just a quick reminder. You can still register for the "FP Dag" 2019 on January 11 in Breda. The program and additional details are available at https://wiki.clean.cs.ru.nl/NL-FP_dag_2019 Registration closes on January 4. Participation is free, but registration is mandatory. To register simply reply to this e-mail with your name, affiliation (optional) and whether you intend to join us for dinner afterwards. Happy Holidays! Bas and Jan Martin

1 0

Re: [Haskell-cafe] Upload new version of indexed-extras or let me take over
by Yuji Yamamoto 24 Dec '18

24 Dec '18

Confirmed! Thanks! 2018年12月24日(月) 16:04 Alexis Williams <alexis(a)typedr.at>: > It's done. > > Alexis Williams > Sent from Polymail > <https://polymail.io/?utm_source=polymail&utm_medium=referral&utm_campaign=s…> > > On Sun, Dec 23rd, 2018 at 11:1 PM, Yuji Yamamoto < > whosekiteneverfly(a)gmail.com> wrote: > >> Thank you Reiner, and Alexis. >> >> But I'm afraid the new version doesn't look uploaded yet... >> http://hackage.haskell.org/package/indexed-extras >> If you're okay, I'd like you to add me to the GitHub Project and the >> maintainer on Hackage. >> Here're my accounts: >> >> http://hackage.haskell.org/user/igrep >> https://github.com/igrep/ >> >> 2018年12月20日(木) 6:49 Reiner Pope <reiner.pope(a)gmail.com>: >> >> (haskell-cafe to bcc) >> >> Hi Yuji, thanks for the ping and sorry for the delay. >> >> Alexis (cc'd) took over ownership of indexed and indexed-extras at the >> beginning of this year, but apparently her permissions to upload packages >> to Hackage got reset by a Hackage reset. I've now fixed that, so she can >> reupload now (she was generally much more prompt than me). >> >> I'll upload a new version tonight. >> >> Not required, but if you like I'm also happy to add you on the GitHub >> project and the Hackage packages if you'd like to give me your details. >> >> Reiner >> >> On Sat, Dec 15, 2018 at 1:42 AM Yuji Yamamoto < >> whosekiteneverfly(a)gmail.com> wrote: >> >> Hello, this is a reminder of the message I sent (only to Reiner) about >> three weeks ago. >> >> I'm developing some libraries in Haskell with your package indexed-extras >> <http://hackage.haskell.org/package/indexed-extras>. >> I found the package can't be built with the latest bifunctor package due >> to its version constraint. >> I sent a pull request <https://github.com/reinerp/indexed-extras/pull/8> >> to fix the problem and it has already been merged. >> Can you release the new version on Hackage? >> It's now the biggest obstacle developing my libraries depending on it. >> >> If you can't maintain the package anymore, I'm willing to take over. >> Thanks in advance. >> >> >> -- >> 山本悠滋 >> twitter: @igrep >> GitHub: https://github.com/igrep >> GitLab: https://gitlab.com/igrep >> Facebook: http://www.facebook.com/igrep >> >> >> >> -- >> 山本悠滋 >> twitter: @igrep >> GitHub: https://github.com/igrep >> GitLab: https://gitlab.com/igrep >> Facebook: http://www.facebook.com/igrep >> > > -- 山本悠滋 twitter: @igrep GitHub: https://github.com/igrep GitLab: https://gitlab.com/igrep Facebook: http://www.facebook.com/igrep

1 0

Upload new version of indexed-extras or let me take over
by Yuji Yamamoto 24 Dec '18

24 Dec '18

Hello, this is a reminder of the message I sent (only to Reiner) about three weeks ago. I'm developing some libraries in Haskell with your package indexed-extras <http://hackage.haskell.org/package/indexed-extras>. I found the package can't be built with the latest bifunctor package due to its version constraint. I sent a pull request <https://github.com/reinerp/indexed-extras/pull/8> to fix the problem and it has already been merged. Can you release the new version on Hackage? It's now the biggest obstacle developing my libraries depending on it. If you can't maintain the package anymore, I'm willing to take over. Thanks in advance. -- 山本悠滋 twitter: @igrep GitHub: https://github.com/igrep GitLab: https://gitlab.com/igrep Facebook: http://www.facebook.com/igrep

2 2

handling NULL value in database query with Maybe (or other ...)
by Damien Mattei 20 Dec '18

20 Dec '18

Hi, i have this query in SQL used by my Haskell code: let name = "A 20" -- return the list of N°BD from WDS for a given name let qry_head_WDS = "select DNUM from WDS where DISC = ?" :: Query putStrLn "before query WDS" bd_rows_WDS <- query conn qry_head_WDS (Only (name::String)) putStrLn "after query WDS" forM_ bd_rows_WDS $ \(Only a) -> putStrLn $ Text.unpack a works well if there is no NULL values in a database, but if NULL value for field N°BD i got this exception at runtime: before query WDS *** Exception: UnexpectedNull {errSQLType = "String", errHaskellType = "Text", errFieldName = "DNUM", errMessage = "unexpected null in table WDS of database sidonie"} *Main> i need some code ,perheaps uing Maybe to handle the NULL value in the field N°BD i think Haskell have special feature to program this , i read the article: https://lukeplant.me.uk/blog/posts/null-pointers-vs-none-vs-maybe/ but for now i have no concrete solution.... Regards, Damien -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

9 20

Re: [Haskell-cafe] handling NULL value in database query with Maybe (or other ...)
by Damien Mattei 19 Dec '18

19 Dec '18

thank tou for your C/HASKELL "rosetta stone" about the Monad it helps a lot understanding the thing... i always thougt Monads are powerful but they are not easy sot manipulates syntactically for a beginner in Haskell. yes in fact what i secretly expected was a way to discard NULL fields automagically and it seems really possible with Haskell. Le 19/12/2018 11:27, Mario Lang a écrit : > Damien Mattei <mattei(a)oca.eu> writes: > >> using Nothing and Just compared to other language, it's the same than >> if i had written in C something like : >> if (x == NULL) printf("NULL\n"); >> else printf("%s",*x); >> >> i expected more from Haskell... > > Haskell actually offers more. > > 1. The type of "Maybe a" prevents you from ignoring the fact that "a" might > be null. In C, you can accidentally write > > printf("%s",*x); > > and risking a segfault. The compiler will not care. IN Haskell, the > types dont even allow you to write something like that. You would be > forced to check for Just or Nothing in one of several ways. > > Now, "Maybe a" doesn't only limit your abilities, it also offers you the > functions from the Functor, Applicative and Monad typeclasses. > > 2. Functor allows you to apply a function to the "a", as long as there is > some "a" inside the "Maybe a". So, "fmap f x" is roughly equivalent to: > > typeOfA f(typeOfA); > typeOfA *x func(typeOfA *x) { > if (x) { > typeOfA *val = malloc(sizeof(typeOfA)); > *val = f(*x); > return val; > } > return NULL; > } > func(x); > > (note that pure Haskell doesn't have assignment, so the translation to C > is a little bit more verbose then it might be if you were using values > on the stack.) i tried this: let resFMAP = fmap (\(Only a) -> (putStrLn ("a =" ++ Text.unpack a))) bd_rows_WDS but compile complains now that i use a Maybe variable instead of a plain one,strange because this exactly what i expected from Maybe use it to automatically discard the Nothing values (avoiding to putStrln them): Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:293:75: error: • Couldn't match type ‘Maybe Text’ with ‘Text’ Expected type: [Only Text] Actual type: [Only (Maybe Text)] • In the second argument of ‘fmap’, namely ‘bd_rows_WDS’ In the expression: fmap (\ (Only a) -> (putStrLn ("a =" ++ unpack a))) bd_rows_WDS In an equation for ‘resFMAP’: resFMAP = fmap (\ (Only a) -> (putStrLn ("a =" ++ unpack a))) bd_rows_WDS | 293 | let resFMAP = fmap (\(Only a) -> (putStrLn ("a =" ++ Text.unpack a))) bd_rows_WDS | ^^^^^^^^^^^ Failed, no modules loaded. bd_rows_WDS is defined like this with a Maybe: (bd_rows_WDS :: [Only (Maybe Text)]) <- query conn qry_head_WDS (Only (name::String)) anyway i know enought in haskell now to remove myself the Nothing (NULL) values of the list doing that (it worked): -- remove the records having N°BD NULL let fltWDS = Prelude.filter (\(Only a) -> case a of Nothing -> False Just a -> True) bd_rows_WDS > > So you do not need to write an explicit if in Haskell. The Functor > instance of Maybe "knows" how to deal will null cases. > In particular, if it encounters a Nothing, it returns a Nothing, > ignoring the provided function. > > 3. Applicative allows you to apply a function to the "a"s of several "Maybe a" > values, returning a Maybe wrapping the result of that function. > So "f <$> x <*> y" is equivalent to > > typeOfA f(TypeOfA); > typeOfA *func(typeOfA *x, typeOfA *y) { > if (x && y) { > typeOfA *val = malloc(sizeof(typeOfA)); > *val = f(*x, *y); > return val; > } > return NULL; > } > func(x, y); > > The amount of boilerplate eliminated grows. > > 3. Monad allows you to chain several null-checks without having to write > out the nesting. This time, f, g and h return a "Maybe a", not just an "a": > > do > a <- f x > b <- g a > h b > > or > > f x >>= \a -> g a >>= \b -> h b > > is equvalent to something like > > typeOfA *f(typeOfA); > typeOfA *g(typeOfA); > typeOfA *h(typeOfA); > typeOfA *func(typeOfA x) { > if (x) { > type *a = f(*x); > if (a) { > type *b = g(*a); > if (b) { > return f(*b); > } > } > } > return NULL; > } > > or, in Haskell: > > case f x of > Just a -> case g a of > Just b -> h b > Nothing -> Nothing > Nothing -> Nothing > > The Monad typeclass really pays off, as it saves you from doing the null > checks and dereferencing. The nested "if null then null" stuff goes all > away, and you can chain several functions together that return a > possible null value. > >> ok reading twice the article in link, i admit there should be some >> adavantage using Nothing instead of null, such as type, and genericity >> and not being spcecific to alloated memory as in C ... > > Exactly. As I see it, the Functor, Applicative and Monad typeclasses > of the Maybe type give you the tools > necessary to reduce the boilerplate that results from forcing you to > deal with each and every null case in your code. > -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

2 1

Timing out a pure evaluation of an expression I did not write myself
by Ryan Reich 19 Dec '18

19 Dec '18

I want to time out a pure computation. My experience, and that described in various previous questions here and elsewhere (the best of which is https://mail.haskell.org/pipermail/haskell-cafe/2011-February/088820.html) is that this doesn't always work: for instance, >>> timeout 1 $ evaluate $ let x = 0 : x in last x does not time out because, apparently, the fact that the expression evaluates in constant space (i.e. never allocates) means that it never yields to the timeout monitor thread that would kill it. The solution that is described in the other iterations is to embed checkpoints in the expression that do allocate, giving the RTS a chance to switch contexts. However, in my application, the expression is /arbitrary/ and I do not have the freedom to inject alterations into it. (Don't argue this point, please. The expression is arbitrary.) How can I time out a tight loop like the above? Clearly, it can be done, because I can, say, alt-tab over to another terminal and kill the process, which exploits the operating system's more aggressively pre-emptive scheduling. Is there a solution using bound threads, say 'forkOS' instead of 'forkIO' in the implementation of 'timeout'? Unix signals? Some FFI-based workaround? Etc. Keep in mind that notwithstanding that comment, I don't actually want to kill the whole process, but just the one evaluation. Thanks in advance, Ryan Reich

8 16

Diagnosing build problems when cross-compiling hslua
by Vanessa McHale 18 Dec '18

18 Dec '18

Hi all, I recently ran into some troubles cross-compiling hslua for Raspberry Pi, viz. |cabal new-build --with-ghc arm-linux-gnueabihf-ghc --with-ghc-pkg arm-linux-gnueabihf-ghc-pkg Build profile: -w ghc-8.6.3 -O1 In order, the following will be built (use -v for more details): - hslua-1.0.1 (lib) (configuration changed) Configuring library for hslua-1.0.1.. Preprocessing library for hslua-1.0.1.. Types.hsc: In function ‘_hsc2hs_test1’: Types.hsc:59:12: error: ‘LUA_INTEGER’ undeclared (first use in this function) Types.hsc:59:12: note: each undeclared identifier is reported only once for each function it appears in Types.hsc:59:24: error: expected ‘;’ before numeric constant compilation failed| Any hints as to what's failing? Is this something wrong with hslua, hsc2hs, or cabal? And how would I go about fixing it? Cheers, Vanessa McHale

1 0

How to unify these three types with identical structures into one definition?
by ducis 16 Dec '18

16 Dec '18

Hello, Say I have a certain type for parse trees: data Expr = Var String | Enclosed String Expr String | Prefix Expr Expr | Ternary Expr String Expr String Expr deriving (Show,Eq,Ord,Generic)--,Typeable) Then I want a new type where every occurence of "Expr" definition changed to "[Expr]": data ExprL = VarL String | EnclosedL String [Expr] String | PrefixL [Expr] [Expr] | TernaryL [Expr] String [Expr] String [Expr] deriving (Show,Eq,Ord,Generic) Sometimes, I also want to use a DList instead of list. data ExprD = VarD String | EnclosedD String (DList Expr) String | PrefixD (DList Expr) (DList Expr) | TernaryD (DList Expr) String (DList Expr) String (DList Expr) deriving (Show,Eq,Ord,Generic) They have exactly the same structure, is there a way to unify the three definitions into one? Furthurmore, is it possible to generalise the latter two for all Functors ? Thanks, ducis

5 5

fmap use
by Damien Mattei 14 Dec '18

14 Dec '18

hello again, another question with Haskell, again with my database but that is again related to Monads: i have this code in main: rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" let lg = Prelude.length rows let tst = if lg > 1 then "some" else "one or zero" it compiles and i assume it works, and i have a new (for those that have follow my previous posts) function like this one: -- this function will return the list of N°BD from WDS for a given name -- note: there could be multiple result for a given name in WDS due to components getBD_WDS :: Connection -> String -> IO Float getBD_WDS conn name = trace "Entering getBD_WDS" noBDfp where qry_head = "select DNUM from WDS where DISC = ?" :: Query bd_rows :: IO [Only Text] bd_rows = query conn qry_head (Only (name::String)) lg = fmap Prelude.length bd_rows -- tst = if (fmap lg) > 1 then "some" else "one or zero" noBDtxt :: IO Text -- noBDtxt = trace "lg " (fmap (fromOnly . Prelude.head) bd_rows) noBDtxt = (fmap (fromOnly . Prelude.head) bd_rows) -- noBDstr :: String -- noBDstr = Text.unpack noBDtxt noBDstr :: IO String noBDstr = fmap Text.unpack noBDtxt -- noBDfp = read $ noBDstr :: Float noBDfp :: IO Float noBDfp = trace "Exiting getBD" (fmap read noBDstr) the problem is that first i noticed that in a function to make working the same code than in Main i had to had almost always fmap before the line, i admit ,even if i'm teacher in functional programming in Scheme at University, i begin in Haskell and i don't always understand what i'm using, i try to learn it by instinct even if sometimes i read pages about Functor and Monads i have no time to get in the Mathematics theoretical knowledge behind Haskell, i try to learn by getting some solutions in Haskell-cafe and it works, i know now a lot more about Haskell than 2 weeks ago,so.... if someon could explain me a few more about fmap or just give what i must place in my function instead of let lg = Prelude.length rows let tst = if lg > 1 then "some" else "one or zero" i replace this in the function by: lg = fmap Prelude.length bd_rows OK for compilation, but then tst = if lg > 1 then "some" else "one or zero" does not compile: *Main> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:80:18: error: • No instance for (Ord (IO Int)) arising from a use of ‘>’ • In the expression: lg > 1 In the expression: if lg > 1 then "some" else "one or zero" In an equation for ‘tst’: tst = if lg > 1 then "some" else "one or zero" | 80 | tst = if lg > 1 then "some" else "one or zero" | ^^^^^^ UpdateSidonie.hs:80:23: error: • No instance for (Num (IO Int)) arising from the literal ‘1’ • In the second argument of ‘(>)’, namely ‘1’ In the expression: lg > 1 In the expression: if lg > 1 then "some" else "one or zero" | 80 | tst = if lg > 1 then "some" else "one or zero" | ^ Failed, no modules loaded. and even the "fmap miraculous function" as i call it no more works: tst = if (fmap lg) > 1 then "some" else "one or zero" Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:80:24: error: • Couldn't match expected type ‘a0 -> b0’ with actual type ‘IO Int’ • In the first argument of ‘fmap’, namely ‘lg’ In the first argument of ‘(>)’, namely ‘(fmap lg)’ In the expression: (fmap lg) > 1 | 80 | tst = if (fmap lg) > 1 then "some" else "one or zero" | ^^ Failed, no modules loaded. a solution to this proble i think , will help me a lot, i sticked with problems in haskell i would have already solved in Scheme or Python, that's really discouraging me of using Haskell, i'm obstinating myself because i really think it's a great language that push the programming to high levels of abstractions but for now it's a bit a nightmare, i must admit it... -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS

3 7

MonadThrow, MonadReader and shake
by PICCA Frederic-Emmanuel 14 Dec '18

14 Dec '18

Hello , I would like your opinion, about this thread. http://mail.haskell.org/pipermail/beginners/2018-December/018403.html I do not know how to solve the issue. thanks Frederic

1 0

Announce: Haskell Platform 8.6.3
by Gershom B 13 Dec '18

13 Dec '18

On behalf of the Haskell Platform team, I'm happy to announce the release of Haskell Platform 8.6.3 Now available at https://www.haskell.org/platform/ This includes GHC 8.6.3, cabal-install 2.4.1.0, and stack 1.9.3. This is the first platform released in the 8.6 series, as we have waited until a number of bugfix ghc releases stabilized things across all core platforms (linux, os x, windows). A full list of contents is available at https://www.haskell.org/platform/contents.html The list of GHC changes is available at: https://ghc.haskell.org/trac/ghc/blog/ghc-8.6.1-released https://ghc.haskell.org/trac/ghc/blog/ghc-8.6.2-released https://ghc.haskell.org/trac/ghc/blog/ghc-8.6.3-released A list of cabal changes is available at: https://hackage.haskell.org/package/cabal-install-2.4.1.0/changelog A list of stack changes is available at: https://docs.haskellstack.org/en/stable/ChangeLog/#v193 There are a number of important changes in this release, with more changes planned in the future. First: Win32 builds are working again and provided. Thanks to all the folks (especially Tamar) who helped sort out the build issues on that platform. Second, only core builds are provided, not "full" builds. This release is the first one where cabal-install warns on legacy commands and asks users to either use the v1-prefix for them or the v2/new prefix to move to the new-build system. As such, providing additional global packages outside of the core set now makes even less sense than in the past, where we had been already discouraging it for some time. Finally, there are no linux generic builds provided, and instead we recommend use of the ghcup tool (https://github.com/haskell/ghcup/) in combination with the stack install script. We feel this gives a smoother and better experience than the existing install process, being less invasive (not requiring root) and more flexible (by running ghc's own configure script it can better detect differences in configuration between distros). What does this all mean for the future of the platform? What I would like to move towards is the following. First: replacing the mac installer by ghcup as well in the near future. While a native installer has its advantages, the same reasons that ghcup is recommended on linux hold for mac as well, although with somewhat less force. Because of how Windows works, the difficulties of moving from a native installer are much more real, and we would anticipate keeping a native installer for the time being. Second: with the platform installers (or recommended installers) now really a delivery mechanism for core binaries and nothing else, to move to split the platform into two components. A) a set of recommended install instructions for major platforms (and a native windows installer), and B) a set of recommended and known-compatible packages which cover most "extended standard-lib" bases and which we again grow much more freely, as in the past. This will require some redesign and reconceptualization of the website, and would be a great opportunity for people that want to chip-in to move things forward to get involved. Please reach out if you'd like to lend a hand! Happy Haskell Hacking all, Gershom

1 0

Call for Haskell.org Committee Nominations
by George Wilson 13 Dec '18

13 Dec '18

Dear Haskellers, It is time to put out a call for new nominations (typically but not necessarily self-nominations) to the haskell.org committee. We have one member of our committee due for retirement -- our chair Gershom Bazerman. To nominate yourself, please send an email to committee at haskell.org by the 7th of January, 2019. Retiring members are eligible to re-nominate themselves. Please feel free to include any information about yourself that you think will help us to make a decision. The Haskell.org committee serves as a board of directors for Haskell.org, a 501(c)3 nonprofit which oversees technical and financial resources related to Haskell community infrastructure. Being a member of the committee does not necessarily require a significant amount of time, but committee members should aim to be responsive during discussions when the committee is called upon to make a decision. Strong leadership, communication, and judgement are very important characteristics for committee members. The role is about setting policy, providing direction/guidance for Haskell.org infrastructure, planning for the long term, and being fiscally responsible with the Haskell.org funds (and donations). As overseers for policy regarding the open source side of Haskell, committee members must also be able to set aside personal or business related bias and make decisions with the good of the open source Haskell community in mind. We seek a broad representation from different segments of the Haskell world -- including but not limited to those focused on education, those focused on industrial applications, those with background in organizing users-groups, and those focused directly on our technical infrastructure. More details about the committee's roles and responsibilities are on https://wiki.haskell.org/Haskell.org_committee If you have any questions about the process, please feel free to e-mail us at committee at haskell.org, or contact one of us individually. Regards, George Wilson

1 0

ZuriHac 2019 takes place 14 to 16 June in Zurich, Switzerland
by Jasper Van der Jeugt 12 Dec '18

12 Dec '18

Hello Friends of Haskell, It is our great pleasure to announce that the next ZuriHac will take place from Friday the 14th to Sunday the 16th of June 2019. It will be hosted at the Hochschule Rapperswil on the shores of beautiful Lake Zurich. This will mark the 8th anniversary of ZuriHac since our beginnings in 2010. The Zurich Haskell Hackathon is a free, international, grassroots, and collaborative coding festival. Our goal is to connect Haskellers, expand the community, learn things from each other, and to work on Haskell libraries, tools, and infrastructure. This year, we will enjoy keynotes from: - Simon Peyton Jones - Susan Potter - Richard Eisenberg - Ryan Trinkle More keynote speakers will be announced. The event is open to any experience level, from beginners to gurus. We want to make a special effort to ensure that the event is welcoming and accessible to people completely new to Haskell. That is why we are super excited that, Julie Moronuki, co-author of Haskell Programming from first principles [1] and Joy of Haskell [2], has kindly agreed to teach a beginners course in one of the classrooms we have available. Additionally, there will be mentors that you can approach during the whole event with any Haskell-related questions. This is a great opportunity to meet your fellow Haskellers in real life, find new contributors for your project, improve existing libraries and tools or even start new ones! We will have space for over 400 attendees. Registration is free and will open on 26th of December, as a late holiday present to Haskellers around the globe. You can find this information and more on our website: https://zfoh.ch/zurihac2019 We would also like to thank our sponsors Digital Asset [3], DFINITY [4] and HSR [5] for their strong commitment to the Haskell community and for supporting this great event! Looking forward to seeing you there, The Zurich Friends of Haskell association [6] [1]: https://www.goodreads.com/book/show/25587599-haskell-programming [2]: https://joyofhaskell.com/ [3]: https://digitalasset.com/careers.html [4]: https://dfinity.org/jobs [5]: https://www.hsr.ch/ [6]: https://zfoh.ch/

1 0

Fwd: [Haskell-beginners] monad and variable result
by Damien Mattei 12 Dec '18

12 Dec '18

i'v got no solution from Haskell-beginners... perheaps is there some expert here? -------- Message transféré -------- Sujet : [Haskell-beginners] monad and variable result Date : Mon, 10 Dec 2018 11:32:23 +0100 De : Damien Mattei <mattei(a)oca.eu> Répondre à : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> Pour : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> have some code that works but want to put it in a simple function without sucess: getBD :: Connection -> String -> Float getBD conn name = noBDfp where qry_head = "select `N° BD` from sidonie.Coordonnées where Nom = ?" :: Query bd_rows = do local_bd_rows <- query conn qry_head (Only (name::String)) return local_bd_rows i want the variable local_bd_rows accessible in the 'where' clause how can i do that? note the goal is to do the same thing of my main function that works : main :: IO () --main :: Int main = do conn <- connect defaultConnectInfo { connectHost = "moita", connectUser = "mattei", connectPassword = "******", connectDatabase = "sidonie" } -- we get all the Double Stars that have angular distance superior to a threshold of 1 second = 0.000278 degree rows <- query_ conn "SELECT Nom,distance FROM AngularDistance WHERE distance > 0.000278" forM_ rows $ \(name,distance) -> putStrLn $ Text.unpack name ++ " " ++ show (distance :: Double) -- we will get the Durchmusterung Number BD from Sidonie and WDS and compare them for a given name -- Warning: there could be multiple result in WDS for a given entry name (due to components) -- first we get the N°BD from sidonie let name = "A 20" -- let qry = "select `N° BD` from Coordonnées where Nom = " ++ name let qry_head = "select `N° BD` from sidonie.Coordonnées where Nom = ?" :: Query -- bd_rows <- query_ conn "select `N° BD` from sidonie.Coordonnées where Nom = 'A 20'" bd_rows <- query conn qry_head (Only (name::String)) putStrLn $ show bd_rows putStrLn $ show name let resLst = Prelude.map fromOnly bd_rows let noBDtxt = fromOnly (Prelude.head bd_rows) :: Text -- let noBD2 = _ (Prelude.head bd_rows) putStrLn $ show resLst putStrLn $ show noBDtxt forM_ bd_rows $ \(Only a) -> putStrLn $ Text.unpack a let noBDstr = Text.unpack noBDtxt :: String let noBDfp = read $ noBDstr :: Float putStr "noBDfp =" (putStrLn (show noBDfp)) close conn print "Exit." for now i have errors in the function: Prelude> :load UpdateSidonie [1 of 1] Compiling Main ( UpdateSidonie.hs, interpreted ) UpdateSidonie.hs:47:28: error: • Ambiguous type variable ‘r0’ arising from a use of ‘query’ prevents the constraint ‘(QueryResults r0)’ from being solved. Relevant bindings include bd_rows :: IO [r0] (bound at UpdateSidonie.hs:46:9) Probable fix: use a type annotation to specify what ‘r0’ should be. These potential instances exist: instance Result a => QueryResults (Only a) -- Defined in ‘Database.MySQL.Simple.QueryResults’ instance (Result a, Result b) => QueryResults (a, b) -- Defined in ‘Database.MySQL.Simple.QueryResults’ instance (Result a, Result b, Result c) => QueryResults (a, b, c) -- Defined in ‘Database.MySQL.Simple.QueryResults’ ...plus 21 others (use -fprint-potential-instances to see them all) • In a stmt of a 'do' block: local_bd_rows <- query conn qry_head (Only (name :: String)) In the expression: do local_bd_rows <- query conn qry_head (Only (name :: String)) return local_bd_rows In an equation for ‘bd_rows’: bd_rows = do local_bd_rows <- query conn qry_head (Only (name :: String)) return local_bd_rows | 47 | local_bd_rows <- query conn qry_head (Only (name::String)) | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Failed, no modules loaded. -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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Re: [Haskell-cafe] Faster set intersections?
by Olaf Klinke 11 Dec '18

11 Dec '18

> Note that a concrete set "concretizes" anything it touches. Don't take > unions of these sets, though, it'll just be a mess. > > > Won't a union just be the same as intersection but using || instead of && ? > > > -Jan-Willem Maessen Unions of predicates and concrete sets are easy, thanks to Set.member: union (Pred p) (Concrete s) = Pred (\k -> p k || member k s) What you can not do, of course, is enumerate and fold these sets. There is a set type [1] which supports a litte bit more: Set a = Maybe ((a -> Bool) -> a) It has unions, intersections and a Monad instance and can represent infinite sets. If the base type has an Ord instance, the set can be enumerated. If the base type has an Eq instance, so has (Set a). Some functions usually implemented using Foldable are also possible, e.g. minimum and maximum. Caveat: Performance can be poor, depending on how the function inside the set was defined. Cheers, Olaf [1] http://hackage.haskell.org/package/infinite-search

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Is unfoldr too strict?
by Isaac Elliott 11 Dec '18

11 Dec '18

I was reading this article https://wiki.haskell.org/Correctness_of_short_cut_fusion on the Haskell wiki. It presents an example (2.1.2) where destroy/unfoldr fusion behaves oddly. If I use a lazier definition of unfoldr, then this problem goes away: unfoldr :: (b -> Maybe (a, b)) -> b -> [a] > unfoldr f b = > case f b of > Nothing -> [] > Just z -> fst z : unfoldr f (snd z) Does this mean unfoldr is 'too strict'? Or is there a good reason for not writing it this way (performance, perhaps?)

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References for topological arguments of programs?
by Siddharth Bhat 11 Dec '18

11 Dec '18

Hello, I was recently intrigued by this style of argument on haskell cafe: One can write a function Eq a => ((a -> Bool) -> a) -> [a] that enumerates the elements of the set. Because we have universal quantification, this list can not be infinite. Which makes sense, topologically: These so-called searchable sets are topologically compact, and the Eq constraint means the space is discrete. Compact subsets of a discrete space are finite. ------- I've seen arguments like these "in the wild" during Scott topology construction and in some other weird places (hyperfunctions), but I've never seen a systematic treatment of this. I'd love to have a reference (papers / textbook preferred) to self learn this stuff! Thanks Siddharth -- Sending this from my phone, please excuse any typos!

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version sorting?
by Javran Cheng 11 Dec '18

11 Dec '18

Hi Cafe, I'm wondering if there's any existing packages that do "version sorting": as an example, for a list like: ["foo-10.20", "foo-1.2", "foo-2.100", "foo-1.12"] I'd like the result to be: ["foo-1.2", "foo-1.12", "foo-2.100", "foo-10.20"] I could think of an implementation that turns String elements of a list into [Either Int String] by grouping consecutive chunks using span and isDigit and then do a sortOn: versionSort :: [String] -> [String] versionSort = sortOn brkND where tr f g (x,y) = f x : g y -- ND for non-digits brkND = tr Right brkD . span (not . isDigit) brkD = tr (Left . read @Int) brkND . span isDigit (side question: does the helper function tr I defined above have a commonly known name?) just wondering if there are more sophisticated solutions available. Best, Javran

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Re: [Haskell-cafe] Literate Haskell to HTML?
by David Banas 10 Dec '18

10 Dec '18

Hi Olaf, Yes, I routinely generate HTML from *.lhs files, using Pandoc. -db > On Dec 10, 2018, at 3:03 PM, haskell-cafe-request(a)haskell.org wrote: > > Does anyone know whether literate haskell can be used to generate html?

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Coercion Quantification-related Meetup Event in Hong Kong (December 13, 2018)
by Tomas Tauber 10 Dec '18

10 Dec '18

FYI: https://www.meetup.com/HK-Functional-programming/events/256404067/ <https://www.meetup.com/HK-Functional-programming/events/256404067/>

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Re: [Haskell-cafe] Faster set intersections?
by Doug McIlroy 10 Dec '18

10 Dec '18

> The following expressions both cause GHCI to hang: > > S.intersection (S.fromList [1,2]) (S.fromList [1..]) > S.intersection (S.fromList [1..]) (S.fromList [1,2]) > > Is there a smarter way to take the intersection of sets when at least one > of them is small (but I don't know which one that is)? "Small" is not enough. If all you know is that the lists represent sets of integers, then S.intersection (S.fromList [-1,-2]) (S.fromList [1..]) must diverge. A sufficient condition for success in such examples is that the sets come from a well-ordered domain and are presented in order. Then it's easy to write a merge-like algorithm. Both sets can be infinite; every finite prefix of the intersection will eventually appear. (Note that the integers are not well-ordered, though the positive integers are.) Doug

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Faster set intersections?
by Jeffrey Brown 09 Dec '18

09 Dec '18

The following expressions both cause GHCI to hang: > S.intersection (S.fromList [1,2]) (S.fromList [1..]) fromList ^CInterrupted. > S.intersection (S.fromList [1..]) (S.fromList [1,2]) fromList ^CInterrupted. > Is there a smarter way to take the intersection of sets when at least one of them is small (but I don't know which one that is)? -- 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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Stack data structure
by daniel huebleitner 08 Dec '18

08 Dec '18

I am a graduate student in theoretical computer science playing around with haskell for the past year or so. I wrote a Stack Data structure today and would like to get some feedback from more experienced haskellers if possible. I know its a really simple thing to write. But the implementation I found on hackage wasn't what I was looking for. I haven't wrote unit tests yet, but it should behave as expected. Thanks in advance. Kind regards. Daniel

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Re: [Haskell-cafe] [Haskell] Guidelines for respectful communication
by Dan Burton 07 Dec '18

07 Dec '18

Thank you, Simon. My experience with the Haskell community has been quite positive over the years. I am continually pleased to see community leaders make an intentional effort to maintain the Haskell community's reputation of kindness, helpfulness, and inclusiveness. Say what you will about design-by-committee processes, but the growing Haskell community -- with the increasing diversity that comes with that -- is refining, extending, and supporting the language in new and interesting ways. By intentionally and explicitly embracing these patterns of respectful communication, you facilitate the continued growth and improvement of the language. Thank you again for the careful thought you have put into the topic of communication. Your effort is greatly appreciated! -- Dan Burton On Thu, Dec 6, 2018 at 5:35 AM Simon Peyton Jones via Haskell < haskell(a)haskell.org> wrote: > Friends > As many of you will know, I have been concerned for several years about > the standards of discourse in the Haskell community. I think things have > improved since the period that drove me to write my Respect email< > https://mail.haskell.org/pipermail/haskell/2016-September/024995.html>, > but it's far from secure. > We discussed this at a meeting of the GHC Steering Committee< > https://github.com/ghc-proposals/ghc-proposals> at ICFP in September, and > many of us have had related discussions since. Arising out of that > conversation, the GHC Steering Committee has decided to adopt these > Guidelines for respectful communication< > https://github.com/ghc-proposals/ghc-proposals/blob/master/GRC.rst> > > We are not trying to impose these guidelines on members of the Haskell > community generally. Rather, we are adopting them for ourselves, as a > signal that we seek high standards of discourse in the Haskell community, > and are willing to publicly hold ourselves to that standard, in the hope > that others may choose to follow suit. > We are calling them "guidelines for respectful communication" rather than > a "code of conduct", because we want to encourage good communication, > rather than focus on bad behaviour. Richard Stallman's recent post< > https://lwn.net/Articles/769167/> about the new GNU Kind Communication > Guidelines<https://gnu.org/philosophy/kind-communication.html> expresses > the same idea. > Meanwhile, the Stack community is taking a similar approach< > https://www.snoyman.com/blog/2018/11/proposal-stack-coc>. > Our guidelines are not set in stone; you can comment here< > https://github.com/ghc-proposals/ghc-proposals/commit/373044b5a78519071b9a2…>. > Perhaps they can evolve so that other Haskell committees (or even > individuals) feel able to adopt them. > The Haskell community is such a rich collection of intelligent, > passionate, and committed people. Thank you -- I love you all! > Simon > > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell >

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Fwd: [Fp-nl] FP-Day 2019
by Bas Lijnse 06 Dec '18

06 Dec '18

Hi Haskell-Cafe This is probably only relevant for (but not exclusively for) the Haskellers from the Netherlands. I just wanted to make you aware of the upcoming Dutch FP-Day on January 11, 2019. Best, Bas -------- Forwarded Message -------- Subject: [Fp-nl] FP-Day 2019 Date: Mon, 3 Dec 2018 20:24:37 +0100 From: Bas Lijnse via Fp-nl <fp-nl(a)lists.science.uu.nl> Reply-To: Bas Lijnse <bas(a)baslijnse.nl> To: fp-nl(a)lists.science.uu.nl Dear Fellow Functional Programmers, As the end of 2018 is rapidly approaching, you may be wondering when and where the Dutch FP-Day 2019 will be. It is traditionally organized one of the first Fridays of the new year after all. We have the pleasure to announce that it will be on *Friday Jan 11, 2019* in *Breda*. It will be hosted by the Netherlands Defence Academy at the "Trip van Zoutlandt Kazerne". More information about registration, the program and other details can be found at: https://wiki.clean.cs.ru.nl/NL-FP_dag_2019 You can register, and/or propose a talk by e-mailing us directly (because of the venue, registration is mandatory). Best regards, Bas Lijnse and Jan Martin Jansen b.lijnse(a)mindef.nl / b.lijnse(a)cs.ru.nl jm.jansen.04(a)mindef.nl / jan.martin.jansen(a)gmail.com PS. Apologies if you receive this e-mail twice. It came to our attention that an earlier attempt of sending this e-mail to this list failed somehow. _______________________________________________ Fp-nl mailing list Fp-nl(a)lists.science.uu.nl https://mailman.science.uu.nl/mailman/listinfo/fp-nl

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ghc install from source fails
by Branimir Maksimovic 06 Dec '18

06 Dec '18

I have this problem when installin ghc from git: nstalling library in /home/bmaxa/ghc/lib/ghc-8.7.20181205/ghc-prim-0.5.3 dist-install/build/HSghc-prim-0.5.3.p_o: copyFile: does not exist (No such file or directory) p_o files are missing, and install fails. google does not helps. Can't find any option to suppress installation of p_o files or to build them. Thanks, Branimir.

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Fwd: [Haskell-beginners] Database simple-mysql
by Damien Mattei 05 Dec '18

05 Dec '18

-------- Message transféré -------- Sujet : [Haskell-beginners] Database simple-mysql Date : Wed, 5 Dec 2018 11:29:30 +0100 De : Damien Mattei <mattei(a)oca.eu> Répondre à : The Haskell-Beginners Mailing List - Discussion of primarily beginner-level topics related to Haskell <beginners(a)haskell.org> Pour : beginners(a)haskell.org why does this works: let name = "'A 20'" bd_rows <- query_ conn "select `N° BD` from sidonie.Coordonnées where Nom = 'A 20'" putStrLn $ show bd_rows putStrLn $ show name i got: [Only {fromOnly = "-04.3982"}] "'A 20'" -04.3982 but not with this: bd_rows <- query conn "select `N° BD` from sidonie.Coordonnées where Nom = ?" (Only (name::String)) i got an empty result: [] ... ??? -- Damien.Mattei(a)unice.fr, Damien.Mattei(a)oca.eu, UNS / OCA / CNRS _______________________________________________ Beginners mailing list Beginners(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners

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