November 2017 - Haskell-Cafe

ANN: nice-html, an all-around "nice" html templating library, similar but different to lucid, blaze, type-of-html, etc.
by Mike Ledger 08 Nov '17

08 Nov '17

Hi Haskell-Cafe, I've been working on and off on a HTML templating library that is slightly novel for a few months. Here's the good parts: * it's faster than lucid and blaze * it has a valid monad instance * it has another valid monad instance, that is slightly different! * it paramaterises templates by the data they require * the HTML5 combinators don't rely on any highly overloaded/overlapping typeclasses; type errors in nice-html templates tend to be nice monomorphic type mismatches To achieve this, it has a distinct compilation phase (at runtime) for templates. This means the actual rendering function does very little beyond concatenate Text and escape dynamic input. It also severely restricts usage to the built-in combinators available -- at least, for dynamic data -- and can make writing a nice-html template difficult, though littering _ throughout type signatures helps. Check it out! Benchmark results, a README, and an example: https://github.com/TransportEngineering/nice-html Hackage: https://hackage.haskell.org/package/nice-html The only thing on the roadmap right now is to have some nice :-) way to assign JavaScript event handlers to Markup. This is something that I really appreciate when using React.js, so my eventual aim is for nice-html to be like a (more) server-side version of React. Right now, you can keep track of element ids with `Text.Html.Writer.note`, but it's neither a very good nor very useful interface. Cheers, Mike

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Re: [Haskell-cafe] First annual Haskell users survey
by Taylor Fausak 07 Nov '17

07 Nov '17

Today is the last day to fill out the survey! To the over 1,200 of you have already filled it out: thank you! For those of you that haven't, please take 10 minutes today to fill it out. Thanks! https://haskellweekly.news/surveys/2017.html

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[ANNOUNCE] GHC 8.2.2 release candidate 2
by Ben Gamari 06 Nov '17

06 Nov '17

Hello everyone, The GHC team is very pleased to announce the second 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-rc2/ This is the second and hopefully last of two 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. 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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Replace list item with list.
by mirone 06 Nov '17

06 Nov '17

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"] ?

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asking for feedback: "Magical Haskell" book in progress
by J Ho 04 Nov '17

04 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 04 Nov '17

04 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 01 Nov '17

01 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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