- Haskell-Cafe - Haskell.org

Help wanted: Postfix admin guru for Haskell.org
by Gershom B 10 Nov '19

10 Nov '19

We've been getting increasing amounts of bounces and dropped mail for haskell.org emails (things sent hackage trustees, things sent from our wiki and hackage servers, etc). We _mainly_ have kept things working, but it looks like policies have been amped up in terms of requiring various measures. Additionally, there's something about reverse dns and valid hostnames that seems to not be configured correctly, according to mxtoolbox. Along with all that, we'd really like to migrate the mail infrastructure to our new packet servers, away from rackspace. As such, we could use some concentrated help from someone familiar with maintaining and administering postfix servers, if someone is up to volunteer for the task. Thanks! --Gershom

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Type class constraints in rewrite rules
by Janus Troelsen 10 Nov '19

10 Nov '19

Hi, I am trying to speed up some code using the Simplicity library by roconnor. He suggested I apply a rewrite rule to implement the full-adder using Haskell's addition: > fullAdder @(Kleisli m) w > into > Kleisli (\((a,b),c) -> return $ let sum = fromWord w a + fromWord w b + fromWord1 c in (toBit (sum >= 2 ^ wordSize w), toWord w sum)) I turned that description into a rewrite rule by prepending "forall m w.". But that results in "Forall'd variable ‘m’ does not appear on left hand side" and "Not in scope: type variable ‘m’" (which seems contradictory?) So I thought, OK, typically Type Applications are not necessary, I will just remove it. So the RULE becomes: "fullAdderOpt" forall w. fullAdder w = Kleisli (\((a,b),c) -> return $ let sum = fromWord w a + fromWord w b + fromWord1 c in (toBit (sum >= 2 ^ wordSize w), toWord w sum)) but compiling that, I get: > Could not deduce (Monad m) arising from a use of ‘return’". > Possible fix: add (Monad m) to the context of the RULE "fullAdderOpt" OK, that must mean I should prepend "(Monad m) =>". I tried inserting that either before or after the forall, but both variants are parse errors. So I am suspecting a GHC bug, since I was suggested to do something that doesn't parse. But really, I am just wondering how to make a rewrite rule that does what roconnor suggested. The full source code of the final version is here: https://hushfile.it/api/file?fileid=5dc706576e329 Regards, Janus

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ANN: ban-instance-0.1.0.1
by Jack Kelly 08 Nov '19

08 Nov '19

I have just pushed an initial release of ban-instance[1] to Hackage. ban-instance uses template haskell to generate compile errors for typeclass instances that should not exist: -- Declare that Foo should never have a ToJSON instance $(banInstance [t|ToJSON Foo|] "why ToJSON Foo should never be defined") The custom errors then say why an instance is banned: • Attempt to use banned instance (ToJSON Foo) Reason for banning: why ToJSON Foo should never be defined Instance banned at [moduleName] filePath:lineNumber We have found this useful to prevent definition of ToJSON/FromJSON instances on core data types, as this forces programmers to instead place the serialisation instances on newtypes at the serialisation boundary. GitHub issues and PRs are welcome[2]. In particular, there are currently a few limitations: 1. There is currently no support for classes with associated types or associated data types; 2. It would be great to generate haddocks for banned instances, marking them as such; and 3. Type quotations [t|...|] do not support free variables (GHC#5616). Nevertheless, I hope that it is useful. Best, -- Jack [1]: http://hackage.haskell.org/package/ban-instance-0.1.0.1 [2]: https://github.com/qfpl/ban-instance/

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[ANN] Keycloak-hs
by Corentin Dupont 08 Nov '19

08 Nov '19

Hello, I'm very proud to announce the release of Keycloak-hs, a library to access Keycloak: http://hackage.haskell.org/package/keycloak-hs. Keycloak (www.keycloak.org) is a tool allowing to authenticate users and protect your API resources. I included a tutorial in the README file. Keycloak as many many features, so this library will evolve to cover more. Help/PRs are welcome! Cheers! Corentin

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Explicit approach to lazy effects For probability monad?
by Benjamin Redelings 08 Nov '19

08 Nov '19

Hi, I'm wondering if people could point me to some background on explicitly representing lazy effects in Haskell? The only effect I am concerned about at this point is `seq`. If there is a way to do this with monads, that would be great. (I know that a lot of people hate lazy effects. However, I think this is an orthogonal issue to the question of how to *represent* lazy effects explicitly.) As background, I'm using a lazy probability monad where random sampling is done with unsafeInterleaveST/IO (or something similar), so that random variables are only sampled if they are actually used: model = do x <- sample $ normal 0 1 -- the interpreter does `unsafeInterleaveIO` to implement 'sample' cond <- sample $ bernoulli 0.5 let y = if cond == 1 then x else 0 return y Here, x is only sampled if cond==1. This has a lot of benefits in procedures like Markov chain Monte Carlo. I initially thought that I could model lazy effects by mapping a -> (a,state) as in the ST monad. However, it seems like lazy effects basically require allowing forked state threads to join CONDITIONALLY on if a value is used/forced, and I don't see how to do that without a source-to-source transformation of functions. It seems like lazy effects require tracking state within each closure, and not globally within the monad interpreter. It kind of seems like this can't work if >>= has type 'm a -> (a -> m b) -> mb', assuming m a = (a,state). The second argument needs to have type (m a -> m b) instead of (a -> m b), since the computation needs to have access to the state produced by the first "m a" computation, in order to (optionally) depend on the state thread for first computation. Does this make any sense? I don't know the literature in this area. Q1. Is there a nice way of representing lazy effects that someone could point me to? Q2. Alternatively, is there a proof that either (i) there is not a monadic way to represent lazy effects, or (ii) fmap f requires a source-to-source transformation (for example to explicitly join state threads on strict operations like ($) and case). -BenRI P.S. As even more background, I'm using a probability monad that allows you to e.g. generate an infinite list of random variables, and only instantiate the ones that are looked at: model = do n <- sample $ geometric 0.5 ys <- sample $ list (repeat $ normal 0.0 1.0) zs <- take n ys return (sum zs) Here the number of random variables (n) can change over time, and I ultimately want effects to happen when they are instantiated. There is a brief overview at http://www.bali-phy.org/models.php

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Performance difference parallel 'toString'
by Donald Cordes 07 Nov '19

07 Nov '19

Hi everybody, I'm trying to squeeze more performance out of Haskell, and I'm hoping someone can help me understand why the following two pieces of code have different performance. In particular, I find that the apparently 'more parallel' version of the code performs worse. Quick description of the code: basically it solves the following problem [https://www.spoj.com/problems/PALIN/] The input is a set of huge integer numbers. For each number, print the smallest 'palindrome' that is larger. A palindrome is a number for which when you reverse its digits you end up with the same number. The code uses 'parBuffer' to do the calculations in parallel. Below is high-level description of the steps of each version: Version 1 [palin.parfromstring.hs]: 1. Read the input numbers as strings into a list 2. Perform the following calculation for each number (the steps for this calculation run in sequence, the entire calculation runs in parallel w.r.t. different numbers): 2.1 Convert from string to array of digits 2.2 Calculate the palindrome in-place The result of step 2 is a list of digit-arrays. That concludes the parallel step. 3. For each palindrome, convert it to a string and print it. Version 2 [palin.parfromstringtostring.hs]: 1. Same as above 2.1 Same as above 2.2 Same as above 2.3 Convert the palindrome to a string 3. For each string, print it. I'd expect that version 2 is faster than version 1 because it's doing the conversion back to string also in parallel, but on a set of 50 numbers it's surprisingly ~750ms slower on my computer. I see that the GC is doing a lot more copying for version 2 (1GB more than version 1), but I don't really understand what I'm looking it. :) How can one best think about getting more performance out of parallelism in Haskell? As in, what are some good rules of thumb by which one can decide 'parallelize this, don't parallelize that'? Attached you can find the event logs for ThreadScope and full code for each version. The code is almost identical, so attached is also the diff between the two. Thanks in advance

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Re: [Haskell-cafe] [FP] Postdoc position at University of Bristol in functional programming
by Meng Wang 05 Nov '19

05 Nov '19

Dear Haskellers, The programming languages group at Bristol has an open post doc position in the area of functional programming. Haskell programmers are particularly welcome. Please pass it on to anyone who might be interested. Thanks! Best regards, Meng Meng Wang, PhD (Oxon) Senior Lecturer (Associate Professor) Department of Computer Science University of Bristol Merchant Venturers Building, Woodland Road, Clifton BS8 1UB +44 (0) 117 954 5145 meng.wang(a)bristol.ac.uk We are looking for an enthusiastic, self-motivated individual to contribute to an EPSRC-funded project, which aims to design programming languages that guarantee strong properties, and the application of them. The post holder will be based in the programming languages group at the University of Bristol Computer Science Department, which consists of three academics, two PDRAs, and a number of PhD students. You will also be working with a network of partners from Oxford, Edinburgh, Tohoku Japan, Chalmers Sweden, and industrial partner DFINITY Foundations providing expertise on WebAssembly. You should have a PhD in programming languages, or a closely related field. This post is available immediately and is offered on a full-time based for an initial term of three years. Appointment at a higher salary point than grade I is possible based on relevant experience. Informal enquiries should be addressed to Dr. Meng Wang (meng.wang(a)bristol.ac.uk) For more details about this position, please see: http://www.bristol.ac.uk/jobs/find/details.html?nPostingID=57894&nPostingTa… Deadline: 25 November 2019

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2019 State of Haskell Survey
by Taylor Fausak 04 Nov '19

04 Nov '19

Hi friends! I am excited to announce the third annual State of Haskell Survey. I’d appreciate it if you could take a few minutes to fill it out. Thanks! https://haskellweekly.news/survey/2019.html

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Journal of Functional Programming - Call for PhD Abstracts
by Graham Hutton 04 Nov '19

04 Nov '19

============================================================ CALL FOR PHD ABSTRACTS Journal of Functional Programming Deadline: 30th November 2019 http://tinyurl.com/jfp-phd-abstracts ============================================================ PREAMBLE: Many students complete PhDs in functional programming each year. As a service to the community, twice per year 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 2019: 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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hindent: maintainer needed
by Christopher Done 04 Nov '19

04 Nov '19

Hi all, I'm sunsetting my maintenance of hindent. It basically does what I want and has done for the last four years. I've put enough hours into this project that I'm ready to move onto new things. The approach of using GHC to format code is already being explored by at least two other projects: * brittany <http://hackage.haskell.org/package/brittany> * ormolu <https://github.com/tweag/ormolu> So if you still want hindent to continue being updated, I suggest you take over as maintainer, either as a group or an individual. Feel free to comment here. I'm happy to create an organization and move the project into it. In the absence of anyone taking over, the GitHub project will be archived on Jan 31st. By that point, the issues/PRs will be lost. There is a related GitHub issue here: <https://github.com/chrisdone/hindent/issues/557> Cheers, Chris

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