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[Call for Contributions] Haskell Communities and Activities Report, November 2017 edition (33rd edition)
by Mihai Maruseac 06 Oct '17

06 Oct '17

Dear all, We would like to collect contributions for the 33rd edition of the ============================================================ Haskell Communities & Activities Report http://www.haskell.org/haskellwiki/Haskell_Communities_and_Activities_Report Submission deadline: 5 November 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/11-2017/template.tex There is also a LaTeX style file at http://haskell.org/communities/11-2017/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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Coverage percents - BUG?
by Baa 04 Oct '17

04 Oct '17

Hello, all! Currently I'm trying to add coverage report to my application tests, and I see such line in the HPC report: .. 100% boolean coverage (0/0) .. but 0/0 =/= 100% (an undefined limit ;) IMHO better is to be reported as 0. Is it a bug in HPC? === Best regards, Paul

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Lifting Numbers
by PATRICK BROWNE 03 Oct '17

03 Oct '17

Hi, I am trying to compile, run, and understand the following code from [1]. type Moving v = Time -> v class Number a where (+), (-), (*) :: a -> a -> a sqr, sqrt :: a -> a sqr a = a * a instance Number v => Number (Moving v) where (+) a b = \t -> (a t) + (b t) (-) a b = \t -> (a t) - (b t) (*) a b = \t -> (a t) * (b t) sqrt a = \t -> sqrt (a t) I followed the compiler advice to produce the following version which compiles: {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE TypeSynonymInstances #-} module MovingPoint where type Time = Float -- Type synonym assumed, could it be data type?? type Moving v = Time -> v class Number a where (+), (-), (*) :: a -> a -> a sqr :: a -> a sqrt :: a -> a instance (Floating v) => Number (Moving v) where (+) a b = \t -> (a t) Prelude.+ (b t) (-) a b = \t -> (a t) Prelude.- (b t) (*) a b = \t -> (a t) Prelude.* (b t) sqr a = \t -> (a t) Prelude.* (a t) sqrt a = \t -> Prelude.sqrt (a t) I do not know how to invoke any of the operations. In general I do know how to execute lambdas. I do not understand the bracketed pairs e.g. (a t). Any help on understanding and running the program would be appreciated. Thanks, Pat [1] Ontology for Spatio-temporal Databases http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.113.9804&rep=rep1&… -- This email originated from DIT. If you received this email in error, please delete it from your system. Please note that if you are not the named addressee, disclosing, copying, distributing or taking any action based on the contents of this email or attachments is prohibited. www.dit.ie Is ó ITBÁC a tháinig an ríomhphost seo. Má fuair tú an ríomhphost seo trí earráid, scrios de do chóras é le do thoil. Tabhair ar aird, mura tú an seolaí ainmnithe, go bhfuil dianchosc ar aon nochtadh, aon chóipeáil, aon dáileadh nó ar aon ghníomh a dhéanfar bunaithe ar an ábhar atá sa ríomhphost nó sna hiatáin seo. www.dit.ie Tá ITBÁC ag aistriú go Gráinseach Ghormáin – DIT is on the move to Grangegorman <http://www.dit.ie/grangegorman>

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Export of names only for testing
by Baa 02 Oct '17

02 Oct '17

Hello, All! What is the standard Haskell convenience about export of module names for testing? As I understand, I should export all of them which looks like abstraction leak, but without this I can't test them, right? === Best regards, Paul

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Loading a ~200 column CSV file in Haskell into a Record structure
by Guru Devanla 01 Oct '17

01 Oct '17

Hello All, I am in the process of replicating some code in Python in Haskell. In Python, I load a couple of csv files, each file having more than 100 columns into a Pandas' data frame. Panda's data-frame, in short is a tabular structure which lets me performs on bunch of joins, and filter out data. I generated different shapes of reports using these operations. Of course, I would love some type checking to help me with these merge, join operations as I create different reports. I am not looking to replicate the Pandas data-frame functionality in Haskell. First thing I want to do is reach out to the 'record' data structure. Here are some ideas I have: 1. I need to declare all these 100+ columns into multiple record structures. 2. Some of the columns can have NULL/NaN values. Therefore, some of the attributes of the record structure would be 'MayBe' values. Now, I could drop some columns during load and cut down the number of attributes i created per record structure. 3. Create a dictionary of each record structure which will help me index into into them.' I would like some feedback on the first 2 points. Seems like there is a lot of boiler plate code I have to generate for creating 100s of record attributes. Is this the only sane way to do this? What other patterns should I consider while solving such a problem. Also, I do not want to add too many dependencies into the project, but open to suggestions. Any input/advice on this would be very helpful. Thank you for the time!

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Ambiguous type error: multiparam class + type alias
by Baa 28 Sep '17

28 Sep '17

Hello, List! The further - the more interesting... For example, I want own `Show` analogue, which will be parameterized with type meaning "context"/"reason". So, to represent title of the value I will call: repr x::TitleRepr or simple `Show`: repr x::ShowRepr or repr x::Repr AsShow etc. I try: {-# LANGUAGE AllowAmbiguousTypes #-} {-# LANGUAGE MultiParamTypeClasses #-} data AsShow data AsTitle type Repr a = String class ReprC a b where repr :: a -> Repr b instance ReprC Int AsTitle where repr n = "a number " ++ show n main = do let n = 5 :: Int print $ (repr n :: Repr AsTitle) and sure I get error: • Ambiguous type variable ‘b0’ arising from a use of ‘repr’ prevents the constraint ‘(ReprC Int b0)’ from being solved. Probable fix: use a type annotation to specify what ‘b0’ should be. These potential instance exist: instance ReprC Int AsTitle -- Defined at .../.stack-work/intero/intero31144sPV.hs:12:10 • In the second argument of ‘($)’, namely ‘(repr n :: Repr AsTitle)’ In a stmt of a 'do' block: print $ (repr n :: Repr AsTitle) In the expression: do { let n = ...; print $ (repr n :: Repr AsTitle) } (intero) Sure, I can use as `Repr` not type-alias but `newtype`. But in this case I will need additional call (show/runRepr/coerce/etc.): coerce $ (repr x::AsTitle) So, what is the reason that GHCI is see that `repr n` is `::Repr AsTitle` (AsTitle!!) and says me that `b0` is ambigous?! It should know what concreate `repr` I mean! :) If I use `newtype` - no problem, so I suppose problem is in the type alias. It's not sterling type for such goal, right? Another question is: how to accomplish such goal, i.e. without to make additional call (coerce/show/runRepr/etc) when `repr` will return `String`, wrapped in newtype? PS. Execuse such silly and training example. Actually, I planned to made such class and to use it instead of `Show`. Sure, it can be splitted to several classed (my current state) but.. example is to learn Haskell and to understand my errors... === Best regards, Paul

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Sequence function
by Jimbo 26 Sep '17

26 Sep '17

Hello everyone, Just trying to understand the sequence function as follows: sequence [Just 1] -- evaluates to Just [1] sequence = foldr mcons (return []) where mcons p q = p >>= \x -> q >>= \y -> return (x:y) -- I'm trying to walk through the code as follows, I understand what is below isn't -- haskell code p >>= \x -> [] q >>= \y -> Just 1 return (x:y) -- [] : Just 1 Am I thinking of sequence correctly here? Best regards, Jim

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Ambogous error in returning value
by Baa 26 Sep '17

26 Sep '17

Hello, everyone. Considering, I have a class: class Flt a where allows :: FltOpts -> [a] denies :: FltOpts -> [a] crit :: a -> [a] -> Bool flt :: FltOpts -> a -> Bool flt opts a = allowed && not denied where allowed = if null $ allows opts then True else a `crit` (allows opts) denied = if null $ denies opts then False else a `crit` (denies opts) I get error here: • Could not deduce (Flt a1) arising from a use of ‘allows’ from the context: Flt a bound by the class declaration for ‘Flt’ at .../.stack-work/intero/intero5319V42.hs:(31,1)-(38,97) The type variable ‘a1’ is ambiguous These potential instance exist: instance Flt MyType -- Defined at ... • In the second argument of ‘($)’, namely ‘allows opts’ .................................................... As I understand, GHC can not deduce type if it's a return's value (contraposition?). OK, but it knows its type: it is `[a]`! What is the problem to keep `flt` method as a generic, i.e. without concreate type, but only `[a]` ? Second, I implemented instance: instance Flt MyType where allows = ... denies = ... flt opts a = allowed && not denied where allowed = if null $ (allows opts::[MyType]) then True else a `crit` (allows opts) denied = if null $ (denies opts::[MyType]) then False else a `crit` (denies opts) and without this explicite type annotation of `allows opts` I get again ambigous error. But why? GHC knows that `allows` returns `[a]` and `a` is `MyType`, so `[a]` is `[MyType]`. Why I need to write it explicitly? May be I need some extension here? === Best regards, Paul

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take until duplicate
by mike h 24 Sep '17

24 Sep '17

I’m looking at how to take from a list until a duplicate is found. e.g. takeUntilDup [1,2,3,4,3] = [1,2,3,4] I found this implementation takeUntilDup = foldr (\x r -> x : takeWhile (/= x) r) [] It works but I can’t see how!!? The accumulated result is built up in r so with input [1,2,3,4,3] then, at the point when r = [1, 2, 3, 4], the fold is about to use the number 3. i.e. it does takeWhile (/=3) [1,2,3,4] which gives [1,2] Please, how does this work? Thanks Mike .

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Writing a polymorphic function in Haskell
by Matt Williams 23 Sep '17

23 Sep '17

Dear All, I am having problems writing a polymorphic function. I have a type that has two constructors: data Arg = IndArg {evi::Evi, t1::Treatment, t2::Treatment, out::Outcome, dir::Direction} | MetaArg {target::EviType} deriving (Show) I have a function checks for conflict: checkConflict :: Arg -> Arg -> Bool checkConflict a b = if t1 a == t1 b && t2 a == t2 b && dir a /= dir b then True else False However, I can't make this work with both types of Argument - if I pass it MetaArg, it raises an error. In another language, I would write something like: checkConflict(ArgA,ArgB):

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