- Haskell-Cafe - Haskell.org

Why doesn't this consume all the computer's memory?
by Tyson Whitehead 09 Nov '18

09 Nov '18

I would expect the following to consume all the computer's memory and die due to a buildup of lazy pattern matches for the `y` value. ``` import Data.Either main = print x >> print y where (length -> x, length -> y) = paritionEithers $ repeat (Left ()) ``` That is, `partitionEithers` is ``` partitionEithers :: [Either a b] -> ([a],[b]) partitionEithers = foldr go ([],[]) where go (Left x) ~(xs,ys) = (x:xs,ys) go (Right y) ~(xs,ys) = (xs,y:ys) ``` and, in the -ddump-simpl we see the `go Left` branch returns a thunk on both the right and left sides that hold onto the evaluation of (x:xs,ys) as we would expect ``` Left x_aqy -> (GHC.Types.: @ a_a1q8 x_aqy (case ds1_d1rO of { (xs_aqz, ys_aqA) -> xs_aqz }), case ds1_d1rO of { (xs_aqz, ys_aqA) -> ys_aqA }); ``` Our code keeps generating more and more of these thunks as the left-hand side chases down the infinite list of `Left ()` values, and the machine cannot let go of them because, as far as it knows, we are going to reach the end sometime and then need the right-hand side. Thus I expect it would consume all the memory and crash. But it doesn't. It just sits there forever consuming 100% CPU at a constant memory limit. This means my mental model is defective and I'm unable to properly reason about the space usage of my programs. Could someone please enlighten me as to were I'm missing? Is there some sort of optimization going on here? When can it be depend on? Thanks very much! -Tyson I would expect

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Help understanding this type
by Guru Devanla 08 Nov '18

08 Nov '18

Hello Haskell-Cafe, I have been recently studying the XMonad code and some related types available in the X11 bindings library. I came across this type: newtype XEvent = XEvent XEventPtr type XEventPtr = Ptr <http://hackage.haskell.org/package/base-4.10.1.0/docs/Foreign-Ptr.html#t:Ptr> XEvent <http://hackage.haskell.org/package/X11-1.9/docs/Graphics-X11-Xlib-Event.htm…> Available here: http://hackage.haskell.org/package/X11-1.9/docs/Graphics-X11-Xlib-Event.htm… It seems that this type is circular here. how does one use this type? Is it possible to create a value out of this type? What is the use of this type? Please could someone help me wrap my head around this? Thanks

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Looking for feedback on my beginner's Haskell resource
by André Popovitch 06 Nov '18

06 Nov '18

If you follow /r/haskell you may have seen it posted there as well, but I've written a short guide called Wise Man's Haskell <https://andre.tips/wmh/> which I hope people will find useful. However, I'm not extremely knowledgeable about Haskell and I wouldn't say I'm the best teacher, so if anyone is willing to skim it or provide feedback that would be much appreciated!

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Network deprecation
by Evan Laforge 06 Nov '18

06 Nov '18

I'm starting to get deprecation warnings about Network in the network package. I know this has been in the works for a long time, but now that I search I can't find any info about the rationale and suggested replacement (the deprecation message suggests Network.Socket, but this is a completely different lower level api). Does anyone out there have links to discussions, articles, or whatnot? Thanks in advance!

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reading file content with conduit
by Fabien R 05 Nov '18

05 Nov '18

Hello, I have a strange behaviour with my code when reading data from a file saved by sinkFile. I only see the first record of the file, although the file seems to contain several records. Any hints ? -- built in a sandbox with GHC 8.2.2, base 4.10.1.0, binary 0.8.5.1, bytestring 0.8.10.2, conduit 1.3.1 {-# LANGUAGE DeriveGeneric #-} import qualified Data.ByteString as DB import qualified Data.ByteString.Lazy as DBL import qualified Data.Binary as DBI import GHC.Generics (Generic) import Conduit import Data.Int (Int64) data MySubRec = MySubRec { sr1 :: Float, sr2 :: Float } deriving (Generic, Show) data MyRec = MyRec { r1 :: Int64, r2 :: String, r3 :: [MySubRec], r4 :: [MySubRec] } deriving (Generic, Show) instance DBI.Binary MySubRec instance DBI.Binary MyRec es = MySubRec { sr1 =1.0, sr2 =1000.5 } myList = repeat es e1 = MyRec{ r1=1, r2="e1", r3=take 2 myList, r4=take 1 myList} e2 = MyRec{ r1=2, r2="e2", r3=take 2 myList, r4=take 1 myList} myData = concat $ repeat [e1,e2] dataToBs :: Monad m => ConduitT MyRec DB.ByteString m () dataToBs = do d <- await case d of Just bs -> do yield $ DBL.toStrict $ DBI.encode bs dataToBs _ -> return () bsToData :: Monad m => ConduitT DB.ByteString MyRec m () bsToData = do d <- await case d of Just bs -> do yield $ DBI.decode $ DBL.fromStrict bs bsToData _ -> return () main = do runConduitRes $ yieldMany (take 10 myData) .| dataToBs .| sinkFile "/tmp/res.bin" runConduitRes $ sourceFile "/tmp/res.bin" .| bsToData .| mapM_C (liftIO . putStrLn . show)

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Journal of Functional Programming - Call for PhD Abstracts
by Graham Hutton 05 Nov '18

05 Nov '18

Dear all, If you or one of your students recently completed a PhD in the area of functional programming, please submit the dissertation abstract for publication in JFP: simple process, no refereeing, open access, deadline 30th November 2018. Please share! Best wishes, Graham Hutton ============================================================ CALL FOR PHD ABSTRACTS Journal of Functional Programming Deadline: 30th November 2018 http://tinyurl.com/jfp-phd-abstracts ============================================================ PREAMBLE: Many students complete PhDs in functional programming each year. As a service to the community, the Journal of Functional Programming publishes the abstracts from PhD dissertations completed during the previous year. The abstracts are made freely available on the JFP website, i.e. not behind any paywall. They do not require any transfer of copyright, merely a license from the author. A dissertation is eligible for inclusion if parts of it have or could have appeared in JFP, that is, if it is in the general area of functional programming. The abstracts are not reviewed. Please submit dissertation abstracts according to the instructions below. We welcome submissions from both the PhD student and PhD advisor/supervisor although we encourage them to coordinate. ============================================================ SUBMISSION: Please submit the following information to Graham Hutton <graham.hutton(a)nottingham.ac.uk> by 30th November 2018. o Dissertation title: (including any subtitle) o Student: (full name) o Awarding institution: (full name and country) o Date of PhD award: (month and year; depending on the institution, this may be the date of the viva, corrections being approved, graduation ceremony, or otherwise) o Advisor/supervisor: (full names) o Dissertation URL: (please provide a permanently accessible link to the dissertation if you have one, such as to an institutional repository or other public archive; links to personal web pages should be considered a last resort) o Dissertation abstract: (plain text, maximum 350 words; you may use \emph{...} for emphasis, but we prefer no other markup or formatting; if your original abstract exceeds the word limit, please submit an abridged version within the limit) Please do not submit a copy of the dissertation itself, as this is not required. JFP reserves the right to decline to publish abstracts that are not deemed appropriate. ============================================================ PHD ABSTRACT EDITOR: Graham Hutton School of Computer Science University of Nottingham Nottingham NG8 1BB United Kingdom ============================================================ This message and any attachment are intended solely for the addressee and may contain confidential information. If you have received this message in error, please contact the sender and delete the email and attachment. Any views or opinions expressed by the author of this email do not necessarily reflect the views of the University of Nottingham. Email communications with the University of Nottingham may be monitored where permitted by law.

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Re: [Haskell-cafe] Looking for feedback on my beginner's Haskell resource
by Olaf Klinke 04 Nov '18

04 Nov '18

This guide would be a perfect fit for literate Haskell [1]. It is a file format where you mix markup (e.g. LaTeX) and Haskell, so the file can be interpreted either way. In a nutshell, your html content becomes Haskell comments or the Haskell becomes <code> blocks in html. In that way, your readers could just load your document into ghci and play with it. Not sure whether html is supported out of the box, though. You might need to pass the .lhs file through pandoc [2] or another tool. Can anyone clarify/help on this? Cheers, Olaf [1] https://wiki.haskell.org/Literate_programming [2] https://groups.google.com/forum/#!topic/hakyll/lFfHAk2nxbs

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Get number of fields for non-record ADTs using Data type class?
by Markus Läll 04 Nov '18

04 Nov '18

Dear list, Is it possible te get the number of fields for data constructors for a plain ADT, i.e something with no record fields? E.g for data T = A Int Double | B String (Maybe String) it would give 2 for both `A` and `B`. For a record it's possible using the `constrFields` function from Data.Data. I was trying to follow this tutorial by Christopher Done https://chrisdone.com/posts/data-typeable, and I feel that it must be possible somehow to get these numbers with the gmap*/gfold* functions, but the use of them is over my head at the moment. Best, -- Markus Läll

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Re: [Haskell-cafe] 2018 State of Haskell Survey
by Doug McIlroy 03 Nov '18

03 Nov '18

The survey didn't work for me in either Internet Explorer or Microsoft Edge: In IE an attempt to scroll a pull-down "Add an option" menu would dismiss the menu. In ME the same menus needed a scroll bar, but the bar was missing. Doug McIlroy

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2018 State of Haskell Survey
by Jasper Van der Jeugt 02 Nov '18

02 Nov '18

Hi all, (apologies if you receive multiple copies of this email) We are running a 2018 State of Haskell Survey here: https://airtable.com/shr8G4RBPD9T6tnDf If you have some time to spare, please submit a response. This goes a long way in helping the community. The survey closes on the 15th of November. This survey is co-sponsored by Haskell.org and Haskell Weekly, but I'd like to give credit where credit is due, so I'll add that Taylor Fausak did almost all of the hard work -- thanks a lot! Kind regards Jasper Van der Jeugt on behalf of the Haskell.org committee

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