- 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

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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.

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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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