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asking for feedback: "Magical Haskell" book in progress
by J Ho 05 Nov '17

05 Nov '17

Dear Haskellers, I've systematized my notes from the last 3 years, bit the bullet and started working on the visual Haskell teaching book that had been my desire for a long time now :) The goal is to build it on the proper math foundation of type and category theory to structure concepts and functional patterns as they apply to real world problems neatly from the start, without relying on imperative language experience too much, but make it *visual*, easy to grasp, and not *math-technically* heavy. Basically, I was thinking *"how do I teach Haskell to my kids?"* and *"how would people who learned only functional programming from school, without any imperative exposure, think?"* (I don't think such people exist yet but I sure would like to speak to those if / when they appear :) - but the goal is to make it accessible to, hopefully, teens with some reasonable math and / or programming background, and also, in the later chapters, make it useful to adults who better grasp concepts visually (as I do) to have a neat and systematized Haskell concepts building in their brain. Here's the direct link to the first chapter: Wizards, Types and Functions <https://leanpub.com/magicalhaskell/read#leanpub-auto-chapter-1-wizards-type…> Here's the book's page on leanpub <https://leanpub.com/magicalhaskell> that gets gradually updated as usual, and it will of course always stay free. Would very much appreciate your feedback, suggestions and thoughts! If you have another 7 minutes to spare and want more context, here's a more expanded rationale to this approach to teaching haskell - "Haskell Is Not Programming <https://superstrings.io/haskell-is-not-programming-3246779f2ef6>" - to which I received some very valuable feedback (and some understandable imperative camp critique :)) ) on reddit already, which helped me adjust the book itself. Thanks a lot! Best regards, - J.

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approxRatio and Ratio Word
by Vilem-Benjamin Liepelt 05 Nov '17

05 Nov '17

I have been experiencing curious behaviour with approxRatio: Prelude Data.Ratio> let a = 0 :: Word Prelude Data.Ratio> let b = 10 :: Word Prelude Data.Ratio> a%b 0 % 1 Prelude Data.Ratio> approxRational (a%b) 0.01 1 % 1 Prelude Data.Ratio> approxRational (0%1) 0.01 0 % 1 Is this expected? If so, why?

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Re: [Haskell-cafe] 回复： Replace list item with list.
by David Lilue 04 Nov '17

04 Nov '17

There is a function "concatMap" to simplify "concat $ map". Furthermore, you could use inline notation as follow: concatMap (\x -> x == "z" then B else [x]) A On Sat, Nov 4, 2017 at 7:50 AM, Денис Мирзоев <nolane16(a)gmail.com> wrote: > If you sure "z" is the last element of A you can use > > init A ++ B > > if it's somwhere in the middle consider > > cocat $ map f A > where > f x > | x == "z" = B > | otherwise = [x] > > On Sat, Nov 4, 2017 at 2:41 PM, Денис Мирзоев <nolane16(a)gmail.com> wrote: > >> If you sure "z" is the last element of A you can use >> >> init A ++ B >> >> if it's somwhere in the middle consider >> >> cocat $ map f A >> where >> f x >> | x == "z" = B >> | otherwise = [x] >> >> On Sat, Nov 4, 2017 at 1:57 PM, FlyingSheep <flyingsheep(a)qq.com> wrote: >> >>> (take 2 A) ++ B >>> >>> ------------------ >>> 李鹏 >>> 四川大学计算机学院 >>> >>> >>> >>> ------------------ 原始邮件 ------------------ >>> *发件人:* "mirone"<saul-mirone(a)qq.com>; >>> *发送时间:* 2017年11月4日(星期六) 晚上6:48 >>> *收件人:* "haskell-cafe"<haskell-cafe(a)haskell.org>; >>> *主题:* [Haskell-cafe] Replace list item with list. >>> >>> Hello everyone! >>> If I have a list A : ["x", "y", "z"] >>> and a list B: ["f", "g"]. >>> Here is my question: >>> What's the simplest way to replace "z" in the list A by list B, and get >>> ["x", "y", "f", "g"] ? >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> To (un)subscribe, modify options or view archives go to: >>> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >>> Only members subscribed via the mailman list are allowed to post. >>> >> >> > > _______________________________________________ > Haskell-Cafe mailing list > To (un)subscribe, modify options or view archives go to: > http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe > Only members subscribed via the mailman list are allowed to post. > ᐧ

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Re: [Haskell-cafe] 回复： Replace list item with list.
by Денис Мирзоев 04 Nov '17

04 Nov '17

If you sure "z" is the last element of A you can use init A ++ B if it's somwhere in the middle consider cocat $ map f A where f x | x == "z" = B | otherwise = [x] On Sat, Nov 4, 2017 at 2:41 PM, Денис Мирзоев <nolane16(a)gmail.com> wrote: > If you sure "z" is the last element of A you can use > > init A ++ B > > if it's somwhere in the middle consider > > cocat $ map f A > where > f x > | x == "z" = B > | otherwise = [x] > > On Sat, Nov 4, 2017 at 1:57 PM, FlyingSheep <flyingsheep(a)qq.com> wrote: > >> (take 2 A) ++ B >> >> ------------------ >> 李鹏 >> 四川大学计算机学院 >> >> >> >> ------------------ 原始邮件 ------------------ >> *发件人:* "mirone"<saul-mirone(a)qq.com>; >> *发送时间:* 2017年11月4日(星期六) 晚上6:48 >> *收件人:* "haskell-cafe"<haskell-cafe(a)haskell.org>; >> *主题:* [Haskell-cafe] Replace list item with list. >> >> Hello everyone! >> If I have a list A : ["x", "y", "z"] >> and a list B: ["f", "g"]. >> Here is my question: >> What's the simplest way to replace "z" in the list A by list B, and get >> ["x", "y", "f", "g"] ? >> >> _______________________________________________ >> Haskell-Cafe mailing list >> To (un)subscribe, modify options or view archives go to: >> http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe >> Only members subscribed via the mailman list are allowed to post. >> > >

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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 01 Nov '17

01 Nov '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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