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Amazonka: How to generate S3:// uri from Network.AWS.S3.Types.Object?
by Cody Goodman 02 Nov '17

02 Nov '17

Question: How can I generate an S3://absolute/url from an Object ( https://hackage.haskell.org/package/amazonka-s3-1.4.5/docs/Network-AWS-S3-T… )? Context: I've been using turtle to call "aws s3 ls" and I'm trying to figure out how to replace that with amazonka. Absolute s3 urls were central to how my program worked. I KNow how to get objects now and filter them, but I don't know how to convert an object to an S3 url to integrate with my existing program. I came across the getFile function ( https://github.com/brendanhay/amazonka/blob/4fdd746346323aac7de6e82752b1660…) and tried downloading a file from s3. Perhaps I had something wrong, but it didn't seem like just the S3 Bucket and S3 Object key were enough to download a given file. If I'm wrong about that I need to double check my configuration. Thanks, Cody Goodman

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First annual Haskell users survey
by Taylor Fausak 01 Nov '17

01 Nov '17

https://haskellweekly.news/surveys/2017.html I am super excited to announce this survey! It is inspired by Rust’s recent surveys and Johan Tibell’s state of Haskell surveys from a few years ago. The goal of this survey is to better understand how Haskell users feel about the language, ecosystem, and community. So please, if you’re reading this: Take 10 minutes and fill out the survey. Thanks!

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[ANNOUNCE] GHC 8.2.2 release candidate 3
by Ben Gamari 31 Oct '17

31 Oct '17

Hello everyone, The GHC team is very pleased to announce the third candidate of the 8.2.2 release of the Glasgow Haskell Compiler. Source and binary distributions are available at https://downloads.haskell.org/~ghc/8.2.2-rc3/ This is the third and last of three release candidates leading up the final 8.2.2 release. This release fixes approximately fifty bugs present in 8.2.1. These most notably include, * A correctness issue resulting in segmentation faults in some FFI-users (#13707, #14346) * A correctness issue resulting in undefined behavior in some programs using STM (#14171) * A bug which may have manifested in segmentation faults in out-of-memory condition (#14329) * clearBit of Natural no longer bottoms (#13203) * A specialisation bug resulting in exponential blowup of compilation time in some specialisation-intensive programs (#14379) * ghc-pkg now works even in environments with misconfigured NFS mounts (#13945) * GHC again supports production of position-independent executables (#13702) * Better error messages around kind mismatches (#11198, #12373, #13530, #13610) See [1] for the full list. Release candidate three contains only two patches not present in -rc2 but fixes a packaging issue which lead to unintentional downgrading the process and Cabal libraries. As always, please report any issues you encounter. Happy testing, - Ben [1] https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.2.2&col=id…

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What Does Graham Hutton Mean by Effect
by Steven Leiva 31 Oct '17

31 Oct '17

Hi Everyone, I am reading the 2nd edition of Graham Hutton's Programming in Haskell. I'm not reading the entire book, just the parts of Haskell that I am still iffy on. Anyway, in Chapter 12, Section 3, Hutton introduces monads. He start off with the following code: first module Expr wheredata Expr = Val Int | Div Expr Expreval :: Expr -> Inteval (Val n) = neval (Div el er) = eval el `div` eval er Not using Mixmax yet? And then he points out that the second clause of eval will raise an error if eval er evaluates to 0. One solution is that, instead of using the div function, we use a safeDiv:: Int -> Int -> Maybe Int function, which evaluate to Nothing if the divisor is 0. This means that expr's type changes from eval :: Eval -> Int to eval :: Eval -> Maybe Int, and this means that implementing eval becomes very verbose: second module Expr wheredata Expr = Val Int | Div Expr Expreval :: Expr -> Maybe Inteval (Val n) = Just neval (Div el er) = case eval el of Nothing -> Nothing Just y -> case eval er of Nothing -> Nothing Just x -> y `safeDiv` xsafeDiv :: Int -> Int -> Maybe IntsafeDiv x y | y == 0 = Nothing | otherwise = Just (x `div` y) Not using Mixmax yet? In order to make eval more concise, we can try the applicative style, where the second clause of the eval function becomes pure safeDiv <*> eval el <*> eval er. Of course, that doesn't work because pure safeDiv has the type Int -> Int -> Maybe Int, and what we need is a function of type Int -> Int -> Int. Anyways, this is all setup / context to what Hutton says next: The conclusion is that the function eval does not fit the pattern of effectful programming that is capture by applicative functors. The applicative style restricts us to applying pure functions to effectful arguments: eval does not fit this pattern because the function safeDiv that is used to process the resulting values is not a pure function, but may itself fail. I am confused by Hutton's use of the word effectful and by his description of safeDiv as "not a pure function". I tried skimming the other sections of the book to see if he provided a definition of this somewhere, but if he did, I couldn't find it. So my question is, in what way does Hutton mean for the reader to understand the words effect / effectful, and why does he describe the function safeDiv as not a pure function? Thank you! Steven Leiva 305.528.6038 leiva.steven(a)gmail.com http://www.linkedin.com/in/stevenleiva

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Call for Sponsorships: ICFP 2018
by Lindsey Kuper 31 Oct '17

31 Oct '17

ICFP 2018 The 23rd ACM SIGPLAN International Conference on Functional Programming https://icfp18.sigplan.org Call for Sponsorships Web version of this call for sponsorships: https://icfp18.sigplan.org/attending/supporting-icfp ## Why Sponsor ICFP 2018? ICFP is the premier conference on functional programming languages, covering all aspects of theory, implementation, and application. Every year, we bring together over 500 world-leading researchers, practitioners, and students to discuss the latest findings, collaborate on new ideas, and meet new people. By sponsoring ICFP, your organization can demonstrate its commitment to supporting high quality research and to developing the next generation of functional programming experts. Most of our sponsorship funds are used to help students from around the world afford to attend the conference and get the most out of their experience. We believe that this commitment will pay dividends for our students, our sponsors, and the public for years to come. If you're interested in becoming a sponsor, we'd love to hear from you: get in touch with our sponsorship team at sponsorship-2018(a)icfpconference.org ## Sponsorship Opportunities and Benefits ### Bronze - $750 * Your logo on the ICFP 2018 website * Your name listed in the proceedings ### Silver - $3,000 * All of the benefits of Bronze sponsorship * One complimentary 3-day ICFP registration * A table at the industrial reception * Your logo in the proceedings * Your logo on publicity materials such as banners and posters ### Gold - $6,000 * All of the benefits of Silver sponsorship * One additional complimentary 3-day ICFP registration (2 in total) * A named supporter of the industrial reception * An opportunity to include branded merchandise in participants' swag bag ### Platinum - $10,000 * All the benefits of Gold sponsorship * One additional complimentary 3-day ICFP registration (3 in total) * A named supporter of ICFP 2018 * An opportunity to speak to the audience at the industrial reception * A table/booth-like space in the coffee break areas ## Additional Sponsorship Opportunities We offer some additional sponsorship options to sponsors at the silver level or above. ### Lanyard Sponsor - $4,000 You provide the lanyards that every attendee will wear around their neck. ### Video Sponsor - $4,000 ICFP makes videos available for free to non-attendees following the conference. As a video sponsor, you support the recording and release of these videos. In exchange, your logo will be displayed as part of every ICFP video. ### Banner - $1,000 Post a free-standing banner (up to 2m high and 1m wide, provided by you) on the ICFP main stage throughout the conference.

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newtype and access control
by Steven Leiva 30 Oct '17

30 Oct '17

Hello everyone, I am watching a talk on YouTube calledWrangling Monad Transformer Stacks. The talk has the following code: code{-# LANGUAGE GeneralizedNewtypeDeriving -#} module DBTrans where newtype WithTrans a = WithTrans (WithDBConn a) deriving (Functor, Applicative, Monad) inDBTrans :: WithTrans a -> WithConn a inDBTrans = ... NOT USING MIXMAX YET? The speaker mentions that by using a newtype, we can perform "access control". The functioninDBTrans is "aware" that the type constructorWithTrans is simply a wrapper forWithDBConn, and that functions outside of the module are not privy to that information. Out of necessity, the speaker has to assume a certain level of Haskell knowledge that I don't have, because I am confused as to what language principles would allow forinDBTrans to be used for access control. I understand that a newtype declaration does create a new type from a "type safey" point of view; that a newtype declaration is required to have a single data constructor with a single (not named) field; and that, once the type-checking is done, the compiler can strip out the newtype wrapper and leave the underlying type (informally speaking, of course), but that still doesn't get me toinDBTrans can do access control. Thank you for any help! Steven Leiva 305.528.6038 leiva.steven(a)gmail.com http://www.linkedin.com/in/stevenleiva

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ANN: parameterized, indexed monads with a single "parameter" type variable
by Louis Pan 29 Oct '17

29 Oct '17

Hi Haskell-Cafe, I've been experimenting with indexed/parameterized monads with a single "parameter" type variable. The difference from my variation from Conor McBride's IMonad (which also uses one "parameter") is that the "parameter" doesn't have postcondition semanatics. (FYI, a good intro to indexed monads is https://stackoverflow.com/questions/28690448/what-is-indexed-monad) Instead, in the instance of PApplicative, you have to specify the rule of how to combine the "parameter" `t` and `u` into the final parameter `u`. That way, I can create "parameterized" versions of Semigroup, Monoid, as well as Applicative, Alternative, Monad. It also allows for create different flavours of "parameterized" kinds for ReaderT. For example, I've created OverlappingWhichReader, DistinctWhichReader, and ManyReader newtype wrappers around ReaderT. OverlappingWhichReader and DistinctWhichReader can combine reader that read `Which '[Int, Bool]` (polymorphic variant, analogous to 'Either Int Bool') with another reader that reader `Which '[Bool, String]` into a reader that can read `Which '[Int, Bool, String]` ManyReader can combine reader that read `Many '[Int, Bool]` (heterogenous record, analogous to tuple (Int, Bool)) with another reader that reader `Many '[Bool, String]` into a reader that can read `Many '[Int, Bool, String]` See https://github.com/louispan/parameterized/blob/master/src/Parameterized/Con… https://github.com/louispan/parameterized/blob/master/test/Parameterized/Co… ----- The single "parameter" still allows for the traditional changing state type for StateT. For example, I've created ChangingState newtype wrappers around StateT, which changes the state parameter. I've also created ManyState, which can combine StateT that modifies `Many '[Int, Bool]` with another StateT that modifes `Many '[Bool, String]` into a StateT that can modify `Many '[Int, Bool, String]` See https://github.com/louispan/parameterized/blob/master/src/Parameterized/Con… https://github.com/louispan/parameterized/blob/master/test/Parameterized/Co… --- I'd would appreciate if I could get any feedback on this approach. Cheers, Louis

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Munich Haskell Meeting, 2017-10-30 @ 19:30
by Christian Neukirchen 28 Oct '17

28 Oct '17

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on **Monday, October 30** at Augustiner-Gaststätte Rumpler (Baumstraße 21) at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html Please note the changed date! Everybody is welcome. cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Adding a DB Connection Pool to a Web Application
by Steven Leiva 27 Oct '17

27 Oct '17

Hi Everyone, I am trying to set up a simple web application, but with all the trappings of a production app. (This is strictly for learning purposes). At the moment, I am trying to get a database pool up and running that the rest of the application can use. You can see my current progress at this repository. You may want to look at only the code in the back-end folder, as there is a whole front-end application. So, again, my goal is to get a database pool up and running. At the moment, if I try to build the executable target, I get the following error: Error /Users/stevenleiva/Repos/todo/back-end/app/Main.hs:10:13: error: • Couldn't match type ‘persistent-2.7.0:Database.Persist.Class.PersistStore.BaseBackend backend0’ with ‘Database.Persist.Sql.Types.Internal.SqlBackend’ arising from a use of ‘createPostgresqlPool’ The type variable ‘backend0’ is ambiguous • In a stmt of a 'do' block: pool <- createPostgresqlPool connStr 1 In the expression: do { pool <- createPostgresqlPool connStr 1; startApp } In an equation for ‘main’: main = do { pool <- createPostgresqlPool connStr 1; startApp } Not using Mixmax yet? I tried helping out the compiler by adding pool :: Pool Connection, after importing those type constructors / constants. At that point, I got a different error: Error 2 /Users/stevenleiva/Repos/todo/back-end/app/Main.hs:11:32: error: • No instance for (monad-logger-0.3.25.1:Control.Monad.Logger.MonadLogger IO) arising from a use of ‘createPostgresqlPool’ • In a stmt of a 'do' block: pool :: Pool Connection <- createPostgresqlPool connStr 1 In the expression: do { pool :: Pool Connection <- createPostgresqlPool connStr 1; startApp } In an equation for ‘main’: main = do { pool :: Pool Connection <- createPostgresqlPool connStr 1; startApp } Not using Mixmax yet? I understand that createPostgresqlPool eventually gives me a value of type m (Pool Connection), where there are some constraints on m. I also understand that at the moment I am getting back a value IO (Pool Connection), and that IO does not have an instance of MonadLogger. >From here, I am at a bit of a lose on how to proceed. I thought implementing an instance of MonadLogger for IO that would simply typecheck (and not log), but that would lead to an orphaned instance, correct? What's the easiest next step to make forward progress here? P.S. I am aware that there are a lot of advanced (for me anyway) Haskell that can be done via monad transformers, natural transformations, etc., in order to wire up a DB with Servant, but I am happy with the "next simplest step" until I learn some of those concepts better. (Then Haskell makes refactoring a breeze!). Steven Leiva 305.528.6038 leiva.steven(a)gmail.com http://www.linkedin.com/in/stevenleiva Steven Leiva 305.528.6038 leiva.steven(a)gmail.com http://www.linkedin.com/in/stevenleiva

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Haskell CI with many repositories/packages
by Jeroen Bransen 27 Oct '17

27 Oct '17

Hi cafe, Does anyone know of a good setup for doing continuous integration with a set of Haskell packages, each in its own repository? Just building everything upon every commit is not so hard, but to speed up building times I'd like to build and test only the minimal set of packages. In particular, at a commit for some package A, I would like to build and test A and all packages that depend on A. The problem is that most CI tools use some notion of 'build artefact', which Stack doesn't really seem to give me. Ideally building a package results in some object file, which can then be used by the other packages. When building failed, packages that depend on it can still use the last succesful build. I've tried to look up some Haskell projects, but most of them seem to use some ad hoc setup. Some pointers are appreciated, as we are using Gitlab a gitlab-runner specific option would be great, but I am also open to use Jenkins or other tools. And I guess my main struggle now is on the stack/Haskell side. Regards, Jeroen Bransen

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