- Haskell-Cafe - Haskell.org

When does ICFP open registration?
by Alexey Raga 22 Apr '19

22 Apr '19

Hello, For bureaucracy reasons, it may be a time in our company to start sorting out ICFP trips/tickets. Does anyone have any information about when the registration may be opening? I couldn't find anything on their website, and I couldn't find any contact details on that website either, so asking here just in case if anyone knows :) Best Regards, Alexey Raga

3 2

Re: [Haskell-cafe] Is there already a lib to parse a line of string with a pattern and generate key-value map or a record?
by Olaf Klinke 19 Apr '19

19 Apr '19

You could emulate this with some custom parser combinators on top of any monadic parser combinator library such as attoparsec or megaparsec. keyed :: (Ord key, Functor parser) => key -> parser a -> parser (Map key a) keyed key = fmap (singleton key) Then, assuming 'char' and 'digitChar' exists in the parser library, you may write integer = some digitChar logline = fmap mconcat $ sequence [ keyed "year" (integer <* char '/'), keyed "month" (integer <* char '/'), keyed "day" (integer <* char ' '), -- etc. ] Olaf

2 2

-fdefer-type-errors
by Bill Hallahan 18 Apr '19

18 Apr '19

Hi, I'm trying to use the -fdefer-type-error flag, discussed in an ICFP paper from 2012: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.366.841&rep=rep1&t… <http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.366.841&rep=rep1&t…> On page 2, the paper has an example, showing that fdefer-type-errors will allow a the first element to be extracted fro a tuple, even if the second element does not correctly typecheck: ghci> let foo = (True, ’a’ && False) Warning: Couldn’t match ‘Bool’ with ‘Char’ ghci> :type foo (Bool, Bool) ghci> fst foo True However, with GHC 8.2.2, I get an error when trying to do this: GHCi, version 8.2.2: http://www.haskell.org/ghc/ :? for help Prelude> let foo = (True, 'a' && False) <interactive>:1:18: warning: [-Wdeferred-type-errors] • Couldn't match expected type ‘Bool’ with actual type ‘Char’ • In the first argument of ‘(&&)’, namely ‘'a'’ In the expression: 'a' && False In the expression: (True, 'a' && False) Prelude> :type foo foo :: (Bool, Bool) Prelude> fst foo *** Exception: <interactive>:1:18: error: • Couldn't match expected type ‘Bool’ with actual type ‘Char’ • In the first argument of ‘(&&)’, namely ‘'a'’ In the expression: 'a' && False In the expression: (True, 'a' && False) (deferred type error) Does anyone know the reason for the change in the behavior? Is there any way to get back to the old behavior, i.e. allow the first element of the tuple to still be extracted? Ideally, I would like to get the errors given by defer-type-errors to be as fine-grained/close-to-the-source as possible. Thanks, Bill Hallahan

4 4

Is there already a lib to parse a line of string with a pattern and generate key-value map or a record?
by Magicloud Magiclouds 18 Apr '19

18 Apr '19

Hi, I saw this function in some web tools, pretty useful when parsing log or so. So I wonder if Haskell has similar libs already. Sample: Input line: "2019/04/17 17:27 User magicloud runs command ls." Pattern: "${year}/${month}/${day} ${hour}:${minute} User ${username} runs command ${command}." Output: toList [ ("year", "2019"), ("month", "04") , etc ] -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

5 8

monad example
by Damien Mattei 17 Apr '19

17 Apr '19

hello, i still seaching a good monad example and after passed the classic tutorial about Maybe monad or State monad which is found in almost 90% of the litterature i try to learn in deep the example of monad that talk about probability ( http://learnyouahaskell.com/for-a-few-monads-more ) for this i try to trace the calls of the inner monad definition like in the file, i give the file in attachment (monade.hs) and the output has a lot of garbage data, not the nicer way to display it,sorry. What i can not figure out is at what stage the program construct the [Tails,Heads] , [Heads,Heads] ... etc i do not know, if someone could explain me in detail the action of a <- coin b <- loadedCoin return [a,b] step by step it would be of great help... Regards, Damien note: here is the ouput: *Main> :load monade.hs [1 of 1] Compiling Main ( monade.hs, interpreted ) Ok, one module loaded. *Main> flipTwoVal Prob {getProb = (Monad Prob >>= flatten (fmap f m) Functor Prob Functor Prob (f x,1 % 2) flatten multAll map ... p= 1 % 2 (Monad Prob >>= flatten (fmap f m) Functor Prob Functor Prob (f x,1 % 10) flatten multAll map ... p= 1 % 10 Monad Prob return) [ flatten multAll (1 % 10*1 % 1) flatten multAll (1 % 2*1 % 10) ([Heads,Heads],1 % 20) Functor Prob (f x,9 % 10) flatten multAll map ... p= 9 % 10 Monad Prob return) , flatten multAll (9 % 10*1 % 1) flatten multAll (1 % 2*9 % 10) ([Heads,Tails],9 % 20) Functor Prob (f x,1 % 2) flatten multAll map ... p= 1 % 2 (Monad Prob >>= flatten (fmap f m) Functor Prob Functor Prob (f x,1 % 10) flatten multAll map ... p= 1 % 10 Monad Prob return) , flatten multAll (1 % 10*1 % 1) flatten multAll (1 % 2*1 % 10) ([Tails,Heads],1 % 20) Functor Prob (f x,9 % 10) flatten multAll map ... p= 9 % 10 Monad Prob return) , flatten multAll (9 % 10*1 % 1) flatten multAll (1 % 2*9 % 10) ([Tails,Tails],9 % 20)]} *Main> flipTwoVal Prob {getProb = [([Heads,Heads],1 % 20),([Heads,Tails],9 % 20),([Tails,Heads],1 % 20),([Tails,Tails],9 % 20)]}

1 0

shake and cmd argument dependency
by PICCA Frederic-Emmanuel 17 Apr '19

17 Apr '19

Hello, I am writting a program which execute some scientific task via shake. liftIO $ shake shakeOptions{ shakeFiles=shakeFiles' , shakeReport=["/tmp/shake.html"] , shakeVerbosity=Diagnostic} $ do want [toFilePath uploaded] -- execute xdsme and deal with input dependencies toFilePath xml %> \_out -> do need (map toFilePath is) processXdsMe cwd' cell sg mr mo rdir images -- upload the result into the Databse toFilePath uploaded %> \_out -> actionXml xml b c uploaded processXdsMe :: Path Abs Dir -> Maybe Cell -> Maybe SpaceGroup -> Maybe Resolution -> Maybe Optimize -> Path Rel File -> String -> Action () processXdsMe cwd' mcell msg mr mo rdir images = cmd opts args where opts :: [CmdOption] opts = px1Opts ++ [Cwd . fromAbsDir $ cwd', AutoDeps] args = xdsMePath : catMaybes params params :: [Maybe String] params = [ Just "--brute" , Just "--weak" , Just "--xml" , Just ("-p" ++ fromRelFile rdir) , fmap (\(Cell a b c alpha beta gamma) -> printf "-c%f,%f,%f,%f,%f,%f" a b c alpha beta gamma) mcell , fmap (\sg -> "-s" ++ unpack sg) msg , fmap (\(Resolution r) -> printf "--resolution %f" r) mr , fmap (\o -> printf "--optimize %d" (fromEnum o)) mo , Just images ] actionXml :: Path Abs File -> Beamline -> SomeDataCollection -> Path Abs File -> Action () actionXml xml b c uploaded = do need [toFilePath xml] container <- liftIO . fromFile . toFilePath $ xml -- post processing let attachment = _autoProcProgramAttachment . _autoProcProgramContainer $ container attachment' <- liftIO $ runReaderT (toRuchePath attachment) b _ <- copyAttachment' attachment attachment' let container' = (autoProcProgramContainer . autoProcProgramAttachment .~ attachment') container -- replace attachement -- upload into ISPYB liftIO $ storeAutoProcIntoISPyB c NoAnomalous container' cmd_ ("touch" :: String) (toFilePath uploaded) My users want to change the arguments in the processXdsMe cmd args. How can I teach shake to rebuild a rules when the argument of the cmd change ? thanks for your help. Cheers

2 1

GHC 8.8 and MonadFail proposal
by Kazu Yamamoto 17 Apr '19

17 Apr '19

Hello, I built the GHC 8.8 branch and tried to use it. I was very surprised because many packages cannot be built due to the removal of the "fail" method from the Monad typeclass. GHC 8.6 (with -Wcompat) does not warn even if "fail" is defined. I had no chance to notice this change. Isn't this change too radical? The same thing happened in GHC 8.4 due to Semigroup. I could not notice this change beforehand because I did not know the "-Wcompat" option. In my opinion, we should have included "-Wcampat" in "-Wall". Anyway, if I understand correctly, "-Wcompat" does not help in the MonadFail case. I'm a fan of both proposal. But I feel that no period of warnings is not a good approach. --Kazu

2 1

Compose Conference tickets are now available
by MightyByte 16 Apr '19

16 Apr '19

I'm pleased to announce that ticket sales are now open for both the New York Compose Conference and Unconference happening June 22 - 25. See our website for more details: http://www.composeconference.org/ There is one week left until the talk submission deadline. So, if you would like to submit a talk, make sure you get it in before April 23 at 11:59PM EDT. Part of our mission is to be as open and inclusive as possible. To support this, we have a limited number of volunteer tickets available if you would like to attend but the ticket price is an obstacle for you. Email us at nyc(a)composeconference.org with some information about you, your situation, and what you would like to get out of the conference and we will work with you. Sent via Superhuman ( https://sprh.mn/?vip=mightybyte@gmail.com )

1 0

PPDP'19 Final CFP
by Ekaterina Komendantskaya 12 Apr '19

12 Apr '19

FINAL CALL FOR PAPERS -- PPDP 2019 21st International Symposium on Principles and Practice of Declarative Programming 7–9 October 2019, Porto, Portugal Collocated with FM'19 http://ppdp2019.macs.hw.ac.uk ====================================================================== Important Dates --------------- Title and abstract registration 26 April 2019 (AoE) Paper submission 3 May 2019 (AoE) Rebuttal period (48 hours) 3 June 2019 (AoE) Author notification 14 June 2019 Final paper version 15 July 2019 Conference 7–9 October 2019 About PPDP ---------- The PPDP 2019 symposium brings together researchers from the declarative programming communities, including those working in the functional, logic, answer-set, and constraint handling programming paradigms. The goal is to stimulate research in the use of logical formalisms and methods for analyzing, performing, specifying, and reasoning about computations, including mechanisms for concurrency, security, static analysis, and verification. Invited Speakers ---------------- Amal Ahmed Northeastern University, USA Title: TBA Naoki Kobayashi The University of Tokyo, Japan Title: 10 Years of the Higher-Order Model Checking Project Scope ----- Submissions are invited on all topics related to declarative programming, from principles to practice, from foundations to applications. Topics of interest include, but are not limited to - Language Design: domain-specific languages; interoperability; concurrency, parallelism and distribution; modules; probabilistic languages; functional languages; reactive languages; database languages; knowledge representation languages; languages with objects; language extensions for tabulation; metaprogramming. - Implementations: abstract machines; interpreters; compilation; compile-time and run-time optimization; memory management. - Foundations: types; logical frameworks; monads and effects; semantics. - Analysis and Transformation: partial evaluation; abstract interpretation; control flow; data flow; information flow; termination analysis; resource analysis; type inference and type checking; verification; validation; debugging; testing. - Tools and Applications: programming and proof environments; verification tools; case studies in proof assistants or interactive theorem provers; certification; novel applications of declarative programming inside and outside of CS; declarative programming pearls; practical experience reports and industrial application; education. For further information, please visit: http://ppdp2019.macs.hw.ac.uk

1 0

Newtype, Data.Text and instance declarations...
by Guy-Laurent Subri 12 Apr '19

12 Apr '19

Hi all, There are a few things that I don't understand involving Data.Text and instance declarations... I'm building a small web app to practise Haskell and am using the SQLite.Simple library. I have defined a few types and functions like so: ''' data BookmarkDB = BookmarkDB Connection Table newtype Table = Table Text instance ToField Table where toField = SQLText initDB :: FilePath -> Table -> IO BookmarkDB initDB fp table = do let query = Query ("CREATE TABLE IF NOT EXISTS :table " <> " (url TEXT, tags TEXT, date INTEGER)") conn <- open fp executeNamed conn query [":table" := table] return $ BookmarkDB conn table ''' 1. My first question is about the instance declaration. Why would ''' instance ToField Table where toField = SQLText ''' return this error ? ''' Couldn't match type ‘Table’ with ‘Text’ Expected type: Table -> SQLData Actual type: Text -> SQLData • In the expression: SQLText In an equation for ‘toField’: toField = SQLText In the instance declaration for ‘ToField Table’ ''' I've noticed that if I write ''' instance ToField Table where toField (Table t) = SQLText t ''' I have no error and it compiles. I don't get it, what is 'toField' supposed to receive except a Table? 2. My second question is concerning the initDB function. If I write it that way, the code compiles, but I get an error during execution: bkmrk: SQLite3 returned ErrorError while attempting to perform prepare "CREATE TABLE IF NOT EXISTS :table (url TEXT, tags TEXT, date INTEGER)": near ":table": syntax error I don't understand what is wrong with the code and I doubt that my query is wrong because everything works with the last code snippet I wrote (see question #3). 3. Trying to get around the error I've noticed a few things. If I try to write this instead: ''' initDB :: FilePath -> Table -> IO BookmarkDB initDB fp table = do let query = Query ("CREATE TABLE IF NOT EXISTS " <> table <> " (url TEXT, tags TEXT, date INTEGER)") conn <- open fp execute_ conn query return $ BookmarkDB conn table ''' it doesn't compile because table is of type Table and not Data.Text.Internal.Text, which I get, but I don't know what is the Haskell way of resolving this. I've also noticed that this will help: ''' initDB :: FilePath -> Table -> IO BookmarkDB initDB fp t@(Table table) = do let query = Query ("CREATE TABLE IF NOT EXISTS " <> table <> " (url TEXT, tags TEXT, date INTEGER)") conn <- open fp execute_ conn query return $ BookmarkDB conn t ''' But I *really* don't get why! The most confusing part for me is that the last initDB function will do exactly what I want it to do, but I am unable to understand why it works...Why am I able to concatenate the 'table' now and not before and why is 't' of a different type than 'table'? All of this is very confusing and I find myself unable to reason about it. Sorry if this is a big/messy question. I hope everything is clear enough to answer... Thank you all!

2 2