April 2017 - Haskell-Cafe

Jane Street Question
by Michael Litchard 15 Apr '17

15 Apr '17

Could those of you who have worked at Jane Street comment on how often a hiring round occurs? Once a quarter? Twice a year?

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Re: [Haskell-cafe] Downsides of the Prompt Monad
by Oleg 15 Apr '17

15 Apr '17

David Menendez wrote > It's worth noting that Prompt p is the free monad for a type constructor p, > just as Free f is the free monad for a Functor f. As such, when p is a > Functor, they are isomorphic. > data Free f a = Var a | Wrap (f (Free f a)) > newtype Prompt p a = > Prompt { unPrompt :: forall b. (forall i. p i -> (i -> b) -> b) -> (a -> b) -> b } > -- equivalently, Prompt p a = Done a | forall i. Prompt (p i) (i -> Prompt > p a) Indeed. The Freer monad paper, Sec 2.4, describes this correspondence in general: type FFree p = Free (Lan p) (FFree p is the alternative Prompt p in David's message) where Lan p is a (simple version) of the left Kan extension that turns any p :: * -> * into a functor. Once (Lan p) is a Functor, the standard free monad construction applies. The Freer monad paper then goes on to say that since the continuation is made explicit, it can be represented in a better way than just a function.

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Haskell plugin dynamic linking issue
by Lana Black 14 Apr '17

14 Apr '17

Hello, I'm working on a Haskell plugin (a shared library) for a C program. When I load the plugin, I get the following error: > ./open dlopen() failed: /usr/lib64/ghc-8.0.2/ghc-prim-0.5.0.0/libHSghc-prim-0.5.0.0-ghc8.0.2.so: undefined symbol: stg_thawArrayzh open is just a testing wrapper around dlopen(). I found out that stg_thawArrayzh symbol is exported by the rts, and my library isn't linked to it, and neither is ghc-prim. How do I fix that? Adding -lHSrts-ghc8.0.2 to ld-options doesn't seem to have any effect. Thanks.

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ANN: Nomyx 1.0, the only game where you can change the rules
by Corentin Dupont 14 Apr '17

14 Apr '17

After a loooooong wait... I am finally proud to announce Nomyx <http://www.nomyx.net> V1.0! Nomyx is a unique game where you can change the rules of the game itself while playing it. The major novelty in this version is that you can play even without knowing how to program. There is a library of rules that are ready to be proposed. Let's start a new game! You can login here: http://www.nomyx.net:8000/Nomyx The games are discussed on the mailing list: nomyx-game(a)googlegroups.com Here is a video introduction of the game: http://vimeo.com/58265498 I created a tutorial to learn how to play: http:// corentindupont.info/blog/posts/Programming/2014-09-23-Nomyx-Language.html Some background: this is an implementation of a Nomic [1] game in Haskell (I believe the first complete implementation of a Nomic game on a computer). At the beginning, the initial rules are describing: - how to add new rules and change existing ones. For example a unanimity vote is necessary to have a new rule accepted. - how to win the game. For example you win the game if you have 5 rules accepted. But of course even that can be changed! Cheers, Corentin [1] www.nomic.net

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looking for a file mentioned on haskell.org
by Michael Litchard 12 Apr '17

12 Apr '17

Can someone post/send a copy of the file p83.dat mentioned in the below link? https://wiki.haskell.org/99_questions/80_to_89

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RFC: Handling marginally bad data via logging "soft-errors"
by Saurabh Nanda 12 Apr '17

12 Apr '17

Cross-posted from https://www.reddit.com/r/haskell/comments/64ymup/rfc_handling_marginally_ba… Hi Everyone, I would like to hear some perspectives from people who have more experience in handling production code in Haskell. SCENARIO: You're loading data from a DB (or any other external source) and are expecting certain fields to be present. You end-up loading a record/row which has all the strictly necessary fields, but a not-so-necessary field is missing. Example: loading a full name, stored as the following HSTORE in PG: {'title' => '...', 'first_name' => '...', 'last_name' => '...'} For whatever legacy reasons, there is a possibility that some records may not have the title field, or may not have the last_name field. Now, the skies are not going to fall if you display the name with some components missing, so, you don't want to throw an error and terminate the entire operation. It's a "soft error" which you want to log, and proceed with a default value. OPTION 1: Use unsafePerformIO to avoid making an otherwise pure function, "impure", i.e. parseName :: HStore -> FullName -- uses unsafePerformIO to log soft-errors OPTION 2: Put the data transformation function in IO or the app-wide monad AppM because it will call the logging function, i.e. parseName :: HStore -> AppM FullName OPTION 3: Use a wrapper over Writer monad, called ErrorCollector, that forces the calling function to log while accessing the underlying value, i.e. parseName :: HStore -> ErrorCollector FullName logErrorAndExtract extraLoggingContext (parseName hstoreVal) THOUGHTS: -- The downside of Option 1 is that (a) it's unsafe, and (b) the log won't have surrounding context, for example it won't have the primary key of the row that was being loaded within which this error occurred. -- The downside of Option 2 is that it introduces AppM into the function signature, and even that log won't have enough surrounding context to help in debugging. -- The downside of Option 3 is that it pushes some logging responsibility to the calling functions, but gives the ability to put extra context when a soft-error occurs. How do others deal with this problem? -- Saurabh.

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ANN: HacFreiburg 2017, Friday 4 August - Sunday 6 August, Freiburg, Germany
by Stefan Wehr 12 Apr '17

12 Apr '17

Dear Haskellers, there is another Haskell Hackathon, taking place in Freiburg, Germany this year! When: Friday 4 August 2017 - Sunday 6 August 2017 Where: Faculty of Engineering, University of Freiburg, Germany (https://goo.gl/maps/ZcHWLpMPhtj) Info page: https://wiki.haskell.org/HacFreiburg2017 Organizers: Peter Thiemann, Alexander Thiemann, Stefan Wehr HacFreiburg is another Haskell Hackathon, where Haskell programmers from all around the world meet in Freiburg, Germany, to discuss, hack together and improve the Haskell infrastructure. We welcome all programmers interested in Haskell, beginners and experts! The Haskell Hackathon in Freiburg is part of the "Sommercampus 2017" (https://sommercampus.fachschaft.tf/en/) organized by the Faculty of Engineering at the University of Freiburg. Hope to see you there, Stefan (with Peter and Alex)

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Semantic Synchrony: some open-source knowledge graph software
by Jeffrey Brown 12 Apr '17

12 Apr '17

Esteemed Haskellers, Semantic Synchrony is an open-source knowledge graph server with an Emacs front end. It integrates with Chrome and Git. Git allows a graph to be selectively shared, as a collection of repositories. Subgraphs can be created and edited using indented plain text or Markdown; the latter allows nodes to serve double-duty as edge labels, by linking to them in section or subsection headings. The front end and server communicate using simple JSON. We are open to taking it in new directions. For hacking it, check out our invitation to coders[1]. For learning more about it, check out the wiki[2], which is thorough and friendly -- it even including videos. [1] github.com/synchrony/smsn-why/blob/master/invitations/to-coders.md [2] github.com/synchrony/smsn/wiki -- 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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install problems
by Gregory Guthrie 11 Apr '17

11 Apr '17

I tried to install HSBC and HSQL-sqlite3, Bot fail, And the error report is: >cabal install HDBC Resolving dependencies... Configuring old-locale-1.0.0.7... Configuring utf8-string-1.0.1.1... Building old-locale-1.0.0.7... Building utf8-string-1.0.1.1... Installed old-locale-1.0.0.7 Configuring old-time-1.1.0.3... Installed utf8-string-1.0.1.1 Failed to install old-time-1.1.0.3 Build log ( C:\Users\guthrie\AppData\Roaming\cabal\logs\old-time-1.1.0.3.log ): Configuring old-time-1.1.0.3... configure: WARNING: unrecognized options: --with-compiler checking for gcc... e:\Plang\Haskell Platform\8.0.2\mingw\bin\gcc.exe checking whether the C compiler works... no configure: error: in `/tmp/cabal-tmp-21488/old-time-1.1.0.3': configure: error: C compiler cannot create executables See `config.log' for more details But the path to gcc is correct, and gc works fine. I had a similar problem once, and the issue was that the autoconf script didn't allow paths with spaces, Odd since Haskell installs into "Haskell Package" on windows. That time I had to reconfigure my whole installation (eliminating the space character), and then install it, and then restore the paths to what they were. Is this an ongoing issue? Should I reconfigure Haskell to install differently?

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Help with hspec-wai
by Ricardo Boccato Alves 11 Apr '17

11 Apr '17

Hello, I have been struggling with hspec-wai with what seems to me to be a simple problem: I want to create an Application using a parameter and access that parameter inside the test. The closer I found was this article: https://begriffs.com/posts/2014-10-19-warp-server-controller-test.html The part I am struggling is to access the connection inside the test, this gist is the closer I got. https://gist.github.com/boccato/e151d3a4e4f5d925b904126f80d8cb65 I know I am doing something very stupid and / or didn't understand something very basic, but I just can't see it by myself. Tks, Ricardo Boccato Alves.

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