- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] What is a safe Haskell data type to store and manipulate Money values?
by David Turner 04 Apr '17

04 Apr '17

On 4 Apr 2017 13:57, "David Turner" <dct25-561bs(a)mythic-beasts.com> wrote: > It's the currency conversions that will provide the most interesting > challenges in that list. If you were only working in a single currency then > something based on `Integer` would, I think, be fine. Perhaps `Int` or > `Int64` if you needed the extra speed. > > There are a number of different ways to implement multiple currencies > depending on your desired semantics and applicable accounting rules and > regulations. Peter Selinger has a good tutorial covering this at > http://www.mathstat.dal.ca/~selinger/accounting/tutorial.html > > One thing to watch out for: do you want to be able to do arithmetic on > amounts from different currencies? As in, do you want £5 + $5 to have a > meaningful result? If you can avoid this, life will be much simpler. > > > > On 4 Apr 2017 12:22, "Saurabh Nanda" <saurabhnanda(a)gmail.com> wrote: > >> Hi Manuel, >> >> Thank you for your reply. Some clarifications below... >> >> >>> It depends on what sort of computation you'll be doing upon those >>> monetary values and on the expectations of your users. It's typical >>> that payment processing applications sacrifice precision and instead >>> prefer to deal in exact quantities >> >> >> Is there a document which explains common use-cases for monetary values >> and best-practices for dealing with them? Any guidelines for which >> **Haskell** data-types to use for what kind of monetary calculations? >> >> Here is what we are doing in our app: >> >> * Powering web-based e-commerce checkout flows >> * Allowing store owners to input tax rates >> * Allowing end-customers to see product prices in different currencies >> (so, currency conversion) >> * Various reports to see total sales, total receivables, and total >> payables (basically a **very** small subset of small-business accounting) >> * Exporting data to full-fledged accounting systems, like Quickbooks and >> Xero. >> >> Based on this use-case, which Haskell data-type would you suggest? >> >> -- Saurabh. >> >> >> _______________________________________________ >> 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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stack ignores my config.yaml?
by Alexey Egorov 03 Apr '17

03 Apr '17

Hello haskellers, I'm trying to use stack scripting capabilities, but it gives me an error (and suggestion to fix it by adding some lines to extra-deps in config.yaml). However, adding this lines to config.yaml doesn't change anything. My script and config are here - https://pastebin.com/PyRMQtqw Any help would be appreciated.

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HSpec & Monads
by Jan von Löwenstein 03 Apr '17

03 Apr '17

Hi, I have got a typeclass (MonadResource) that represents a client side rest resource and encapsulates http interaction with a server. Inside a `runResource` I want to do arbitrary calls to the server. I have a separate implementation running in `runResourceStub` that is a fake implementation allowing for tests that don't need a real server. I am unsure how to (best) integrate that with HSpec. ``` it "can create related backend resources" $ do runResourceStub $ do pod <- createPod "my-pod" ... secret <- createSecret... (getPod secret) `shouldBe` pod ``` That is what I would like to do. How do I do it? What else should I consider doing? Best Jan

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Fwd: Call for Papers: 16th ACM SIGPLAN Erlang Workshop 2017
by Rob Stewart 03 Apr '17

03 Apr '17

Please consider submitting papers to the Erlang Workshop, co-located with ICFP in Oxford, UK, on 8th September. The call for papers is below. ---------- Forwarded message --------- From: Natalia Chechina <Natalia.Chechina(a)glasgow.ac.uk> Date: Mon, 13 Mar 2017 at 13:00 Subject: [Spls] Call for Papers: 16th ACM SIGPLAN Erlang Workshop 2017 To: spls(a)dcs.gla.ac.uk <spls(a)dcs.gla.ac.uk> CALL FOR PAPERS =============== Sixteenth ACM SIGPLAN Erlang Workshop http://erlang.org/workshop/2017/ ----------------------------------------------------------- Oxford, United Kingdom, 8 September 2017 Satellite event of the 22nd ACM SIGPLAN International Conference on Functional Programming (ICFP 2017) 3 - 9 September, 2017 The Erlang Workshop aims to bring together the open source, academic, and industrial communities of Erlang, to discuss technologies and languages related to Erlang. The Erlang model of concurrent programming has been widely emulated, for example by Akka in Scala, and even new programming languages were designed atop of the Erlang VM, such as Elixir. Therefore we would like to broaden the scope of the workshop to include systems like those mentioned above. The workshop will enable participants to familiarize themselves with recent developments on new techniques and tools, novel applications, draw lessons from users' experiences and identify research problems and common areas relevant to the practice of Erlang, Erlang-like languages, functional programming, distribution, concurrency etc. We invite three types of submissions. 1. Technical papers describing language extensions, critical discussions of the status quo, formal semantics of language constructs, program analysis and transformation, virtual machine extensions and compilation techniques, implementations and interfaces of Erlang in/with other languages, and new tools (profilers, tracers, debuggers, testing frameworks, etc.). The maximum length for technical papers is restricted to 12 pages. 2. Practice and application papers describing uses of Erlang in the "real-world", Erlang libraries for specific tasks, experiences from using Erlang in specific application domains, reusable programming idioms and elegant new ways of using Erlang to approach or solve a particular problem. The maximum length for the practice and application papers is restricted to 12 pages. Note that this is a maximum length; we welcome shorter papers also, and the program committee will evaluate all papers on an equal basis independent of their lengths. 3. Poster presentations describing topics related to the workshop goals. Each includes a maximum of 2 pages of the abstract and summary. Presentations in this category will be given an hour of shared simultaneous demonstration time. Workshop Co-Chairs ------------------ Scott Lystig Fritchie, VMware, USA Natalia Chechina, Glasgow University, UK Program Committee ----------------------------- (Note: the Workshop Co-Chairs are also committee members) Clara Benac Earle, Universidad Politecnica de Madrid, Spain Richard Carlsson, Klarna, Sweden Laura M. Castro, University of A Coruna, Spain Viktoria Fördős, Erlang Solutions, Hungary James S. Larson, Google, USA Important Dates ----------------------- Submission deadline: Fri May 26, 2017 Author notification: Fri June 23, 2017 Final submission for the publisher: Sat July 15, 2017 Workshop date: Fri September 8, 2017 Instructions to authors -------------------------------- Papers must be submitted online via EasyChair (via the "Erlang2017" event). The submission page is https://www.easychair.org/conferences/?conf=erlang2017 Submitted papers should be in portable document format (PDF), formatted using the ACM SIGPLAN style guidelines. Each submission must adhere to SIGPLAN's republication policy. Violation risks summary rejection of the offending submission. Accepted papers will be published by the ACM and will appear in the ACM Digital Library. The proceedings will be freely available for download from the ACM Digital Library from one week before the start of the conference until two weeks after the conference. Paper submissions will be considered for poster submission in the case they are not accepted as full papers. Venue & Registration Details ------------------------------------------ For registration, please see the ICFP 2017 web site at: http://icfp17.sigplan.org/ Related Links -------------------- ICFP 2017 web site: http://icfp17.sigplan.org/ Past ACM SIGPLAN Erlang workshops: http://www.erlang.org/workshop/ Open Source Erlang: http://www.erlang.org/ EasyChair submission site: https://www.easychair.org/conferences/?conf=erlang2017 Author Information for SIGPLAN Conferences: http://www.sigplan.org/authorInformation.htm Atendee Information for SIGPLAN Events: http://www.sigplan.org/Resources/Policies/CodeOfConduct/ -- You received this message because you are subscribed to the Google Groups "erlang-workshop-sc" group. To unsubscribe from this group and stop receiving emails from it, send an email to erlang-workshop-sc%2Bunsubscribe(a)googlegroups.com<mailto: erlang-workshop-sc%2Bunsubscribe(a)googlegroups.com>. To post to this group, send email to erlang-workshop-sc(a)googlegroups.com <mailto:erlang-workshop-sc@googlegroups.com>. Visit this group at https://groups.google.com/group/erlang-workshop-sc. For more options, visit https://groups.google.com/d/optout. _______________________________________________ SPLS mailing list SPLS(a)mailhost.dcs.gla.ac.uk https://mr1.dcs.gla.ac.uk/mailman/listinfo/spls

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Getting containers updated in Stackage
by David Feuer 03 Apr '17

03 Apr '17

It's come to my attention that Stackage doesn't have the latest and greatest versions of containers. Stackage nightly is on 0.5.7.1, and I just released 0.5.10.2. It'd be nice to get that fixed, but the containers maintainers do not have the time to take responsibility for Stackage curation. Could whoever's responsible please take care of it? Thanks, David

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[Call for Contributions] Haskell Communities and Activities Report, May 2017 edition (32nd edition)
by Mihai Maruseac 03 Apr '17

03 Apr '17

Dear all, We would like to collect contributions for the 32nd edition (10000th edition) of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 30 April 2017 (please send your contributions to hcar at haskell.org, in plain text or LaTeX format, both are equally accepted) ============================================================ This is the short story: * If you are working on any project that is in some way related to Haskell, please write a short entry and submit it. Even if the project is very small or unfinished or you think it is not important enough --- please reconsider and submit an entry anyway! * If you are interested in an existing project related to Haskell that has not previously been mentioned in the HCAR, please tell us, so that we can contact the project leaders and ask them to submit an entry. * If you are working on a project that is looking for contributors, please write a short entry and submit it, mentioning that your are looking for contributors. * Feel free to pass on this call for contributions to others that might be interested. More detailed information: The Haskell Communities & Activities Report is a bi-annual overview of the state of Haskell as well as Haskell-related projects over the last, and possibly the upcoming six months. If you have only recently been exposed to Haskell, it might be a good idea to browse the previous edition --- you will find interesting projects described as well as several starting points and links that may provide answers to many questions. Contributions will be collected until the submission deadline. They will then be compiled into a coherent report that is published online as soon as it is ready. As always, this is a great opportunity to update your webpages, make new releases, announce or even start new projects, or to talk about developments you want every Haskeller to know about! Looking forward to your contributions, Mihai Maruseac FAQ: Q: What format should I write in? A: The usual format is a LaTeX source file, adhering to the template that is available at: http://haskell.org/communities/05-2017/template.tex There is also a LaTeX style file at http://haskell.org/communities/05-2017/hcar.sty that you can use to preview your entry. If you do not know LaTeX or don't want to use it or don't have time to translate your entry into it, then please use plain text, it is better to have an entry in plain-text which we will translate than not have it at all. If you modify an old entry that you have written for an earlier edition of the report, you should soon receive your old entry as a template (provided we have your valid email address). Please modify that template, rather than using your own version of the old entry as a template. Q: Can I include Haskell code? A: Yes. Please use lhs2tex syntax (http://www.andres-loeh.de/lhs2tex/) The report is compiled in mode polycode.fmt. Q: Can I include images? A: Yes, you are even encouraged to do so. Please use .jpg or .png format, then. PNG is preferred for simplicity. Q: Should I send files in .zip archives or similar? A: No, plain file attachments are the way. Q: How much should I write? A: Authors are asked to limit entries to about one column of text. A general introduction is helpful. Apart from that, you should focus on recent or upcoming developments. Pointers to online content can be given for more comprehensive or "historic" overviews of a project. Images do not count towards the length limit, so you may want to use this opportunity to pep up entries. There is no minimum length of an entry! The report aims for being as complete as possible, so please consider writing an entry, even if it is only a few lines long. Q: Which topics are relevant? A: All topics which are related to Haskell in some way are relevant. We usually had reports from users of Haskell (private, academic, or commercial), from authors or contributors to projects related to Haskell, from people working on the Haskell language, libraries, on language extensions or variants. We also like reports about distributions of Haskell software, Haskell infrastructure, books and tutorials on Haskell. Reports on past and upcoming events related to Haskell are also relevant. Finally, there might be new topics we do not even think about. As a rule of thumb: if in doubt, then it probably is relevant and has a place in the HCAR. You can also simply ask us. Q: Is unfinished work relevant? Are ideas for projects relevant? A: Yes! You can use the HCAR to talk about projects you are currently working on. You can use it to look for other developers that might help you. You can use HCAR to ask for more contributors to your project, it is a good way to gain visibility and traction. Q: If I do not update my entry, but want to keep it in the report, what should I do? A: Tell us that there are no changes. The old entry will typically be reused in this case, but it might be dropped if it is older than a year, to give more room and more attention to projects that change a lot. Do not resend complete entries if you have not changed them. Q: Will I get confirmation if I send an entry? How do I know whether my email has even reached its destination, and not ended up in a spam folder? A: Prior to publication of the final report, we will send a draft to all contributors, for possible corrections. So if you do not hear from us within two weeks after the deadline, it is safer to send another mail and check whether your first one was received. -- Mihai Maruseac (MM) "If you can't solve a problem, then there's an easier problem you can solve: find it." -- George Polya

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formal treatment of module dependencies
by Dimitri DeFigueiredo 02 Apr '17

02 Apr '17

It seems to me that specifying and solving module dependencies is a ubiquitous problem in software development. All the languages that I know try to make the problem go away by using version numbers and maximizing backward compatibility. Does anyone know of references that formalize this problem for Haskell? Thanks, Dimitri -- 2E45 D376 A744 C671 5100 A261 210B 8461 0FB0 CA1F

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ANN: Haskell World News
by Gershom B 02 Apr '17

02 Apr '17

Fellow Haskellers, Over the years, a number of newsletters have been produced on news and developments in the Haskell world. There has been the venerable Haskell Weekly News, under multiple editors. More recently, the Haskell Weekly was started. These have been great publications. Nonetheless, in some ways they have been too ambitious, and in other ways, not ambitious enough. Too ambitious in seeking to publish weekly -- a grueling schedule, doomed to sporadic lapses. Insufficiently ambitious in being just roundups of articles by others. The Haskell world is maturing, and it needs a publication that can reflect that, with in-depth reporting, original research, and feature stories. That takes time to produce, and resources. I'm happy to announce the launch of Haskell World News, a subscription-only PDF newsletter with exclusive content that will break new stories and cover the world of Haskell programming in a manner that befits its emerging commercial success. Why subscription-only, you may ask? Because print media, and subscription media, is serious media. Fly-by-night weblogs report false theorems all the time. The wave of the future is not bite-sized tweets and open-letters on medium. It's shoe leather reporting that gets to the real stories. To give just one example of the sorts of things we hope to cover -- everyone agrees Elvis Presley would have been a Functional Programmer. But would he have used Haskell, or OCaml? It takes time and resources to contract a medium to hold a seance and ask his spirit. And those are the resources that Haskell World News will bring to bear. More details will be forthcoming shortly - but for now here's a sneak preview of the cover of the first issue: http://gbaz.github.io/haskell-world-news-1.jpg Credit to James Deikun, Jason Dagit, and Edward Kmett for suggestions as to some of the topics covered. Regards, The Haskell World News Team "Well Typed News Doesn't Break"

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Suggestion on how to solve a type issue with Numeric.AD
by Frederic Cogny 01 Apr '17

01 Apr '17

Hello Café *Issue: * I'm having trouble using the AD package presumably due to a wrong use of types (Sorry I know it's a bit vague) Any help or pointer on how to solve this would be greatly appreciated. Thanks in advance *Background: * I've implemented a symbolic expression tree [full code here: https://pastebin.com/FDpSFuRM] -- | Expression Tree data ExprTree a = Const a -- ^ number | ParamNode ParamName -- ^ parameter | VarNode VarName -- ^ variable | UnaryNode MonoOp (ExprTree a) -- ^ operator of arity 1 | BinaryNode DualOp (ExprTree a) (ExprTree a) -- ^ operator of arity 2 | CondNode (Cond a) (ExprTree a) (ExprTree a) -- ^ conditional node deriving (Eq,Show, Generic) An evaluation function on it -- | evaluates an Expression Tree on its Context (Map ParamName a, Map VarName a) evaluate :: (Eq a, Ord a, Floating a) => ExprTree a -> Context a -> a And a few instances on it: instance Num a => Default (ExprTree a) where ... instance Num a => Num (ExprTree a) where ... instance Fractional a => Fractional (ExprTree a) where ... instance Floating a => Floating (ExprTree a) where ... instance (Arbitrary a) => Arbitrary (ExprTree a) where ... This allows me to easily create (using derivation rules) the derivative(s) of such a tree with respect to its variable(s): diff :: (Eq a, Floating a) => ExprTree a -> VarName -> ExprTree a grad :: (Eq a, Floating a) => ExprTree a -> Map VarName (ExprTree a) hessian :: (Eq a, Floating a) => ExprTree a -> Map VarName (Map VarName (ExprTree a)) So far, so good ... Now, to gain assurance in my implementation I wanted to check the derivatives against the Numeric.AD module so I create the two following -- | helper for AD usage exprTreeToListFun :: (RealFloat a) => ExprTree a -- ^ tree -> Map ParamName a -- ^ paramDict -> ([a] -> a) -- fun from var values to their evaluation exprTreeToListFun tree paramDict vals = res where res = evaluate tree (paramDict, varDict) varDict = Map.fromList $ zip (getVarNames tree) vals gradThroughAD :: RealFloat a => ExprTree a -> Context a -> Map VarName a gradThroughAD tree (paramDict, varDict) = res where varNames = Map.keys varDict varVals = Map.elems varDict gradList = AD.grad (exprTreeToListFun tree paramDict) varVals res = Map.fromList $ zip varNames gradList it unfortunately does not type check with message: • Couldn't match type ‘a’ with ‘Numeric.AD.Internal.Reverse.Reverse s a’ ‘a’ is a rigid type variable bound by the type signature for: gradThroughAD :: forall a. RealFloat a => ExprTree a -> Context a -> Map VarName a at src/ SymbolicExpression.hs:452:18 Expected type: [Numeric.AD.Internal.Reverse. Reverse s a] -> Numeric.AD.Internal.Reverse.Reverse s a Actual type: [a] -> a • In the first argument of ‘AD.grad’, namely ‘(exprTreeToListFun tree paramDict)’ In the expression: AD.grad (exprTreeToListFun tree paramDict) varVals In an equation for ‘gradList’: gradList = AD.grad (exprTreeToListFun tree paramDict) varVals • Relevant bindings include gradList :: [a] (bound at src/SymbolicExpression.hs:457:5) varVals :: [a] (bound at src/ SymbolicExpression.hs:456:5) varDict :: Map VarName a (bound at src/SymbolicExpression.hs:453:32) paramDict :: Map ParamName a (bound at src/SymbolicExpression.hs:453:21) tree :: ExprTree a (bound at src/ SymbolicExpression.hs:453:15) gradThroughAD :: ExprTree a -> Context a -> Map VarName a (bound at src/SymbolicExpression.hs:453:1) I was a bit surprised (I guess naively) by this, since to me, 'a' is polymorphic with RealFloat as type constraint and that is "supposed" to work with AD So anyway, I tried to modify as follow: {-# LANGUAGE Rank2Types #-} -- | helper for AD usage exprTreeToListFun :: (RealFloat a) => ExprTree a -- ^ tree -> Map ParamName a -- ^ paramDict -> *(RealFloat b => *[b] -> b) -- fun from var values to their evaluation exprTreeToListFun tree paramDict vals = res where res = *realToFrac* $ evaluate tree (paramDict, varDict) varDict = Map.fromList $ zip (getVarNames tree) *$ map realToFrac *vals This now typechecks and runs but going through AD returns me *all derivatives as zero* as if treating the variables (that passed through *realToFrac*) as constants, so not that much helpful either. Any idea how this can be solved ? Apologies if this is a question that already has an answer somewhere but poking around SO <http://stackoverflow.com/search?q=%5Bhaskell%5D+ad+package> It looks like I'm not the only one having similar issues and unfortunately none of the answers I found really helped me, if anything it confirms the same issue of null derivatives: http://stackoverflow.com/questions/36878083/ad-type-unification-error-with-… Thanks again -- Frederic Cogny +33 7 83 12 61 69

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typeclass for rest resource clients
by Jan von Löwenstein 01 Apr '17

01 Apr '17

Hi, for a project of mine I want to abstract away the rest resources (client side) and provide a stub implementation of the backend that I can use for testing. I thought about a StateT based stub instance, however one of the methods besides the usual crud stuff should be `waitFor` so that I can wait for resources to reach a certain state. Very concretely I want to create kubernetes pods and have my code wait until the pod is `Running`. What would I have to throw into the mix in order to have a stub that creates a pod and only later changes the status to `Running`? Any thoughts, advice or hint? Best Jan

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