June 2015 - Haskell-Cafe

To "instance C D", GHCI responds that C "is applied to too many type arguments".
by Jeffrey Brown 13 Jun '15

13 Jun '15

jeff@jeffLenovUbu:~$ ghci -XNullaryTypeClasses GHCi, version 7.8.4: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Prelude> class C Prelude> data D = DConstructor Prelude> instance C D <interactive>:4:10: ‘C’ is applied to too many type arguments In the instance declaration for ‘C D’ Prelude>

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Lazy patterns
by Jon Schneider 13 Jun '15

13 Jun '15

Good day, I am trying to understand the lazy patterns as explained at https://www.haskell.org/tutorial/patterns.html If I type in the example without putting a ~ round the pattern sure enough I do not get an answer as the text says. What I completely fail to understand is why there is no CPU usage, increasing memory usage, stack overflow or other runtime error before I use ^C to stop it. What is going on under the hood ? Jon

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Replacing cpp?
by Mike Meyer 12 Jun '15

12 Jun '15

I know the subject of cpp not working well with Haskell comes up every so often, usually with the conclusion that we need a preprocessor that understands Haskell code. If someone is working on that, this should probably be ignored. Maybe gpp could be used instead of having to write one from scratch? it allows you to define the syntax it uses for macros - both user and "meta". Possibly it's flexible enough to do what's needed?

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Proposal: Shorter Import Syntax
by Anthony Cowley 12 Jun '15

12 Jun '15

Hi Everyone, I didn't think this would see any resistance as it doesn't break anything, but this is the internet, so, if you haven't already, take a look at this GHC feature request <https://ghc.haskell.org/trac/ghc/ticket/10478> The idea is that rather than writing, import Data.Map (Map) import qualified Data.Map as M you could instead write, import Data.Map (Map) as M The Map identifier would imported unqualified, while M would be introduced as an alias for the qualified import of Data.Map. Note that, currently, following a parenthesized group with the "as" keyword is a parse error. So allowing this syntax would not affect any currently working programs. I've mentioned this proposal before and gotten good response, so I finally wrote it up last night after people on IRC responded positively to it. As well as IRC and trac, I put the link up on Twitter to get it in front of a large audience, and here's what we have after a bit over 12 hours (not counting the handful of supporters in IRC): +20 -2 You can see the tweet at <https://twitter.com/a_cowley/status/605909841148702720> I'll try to summarize the two negative votes: 1) This isn't that big a burden, and we should instead focus on controlled export of class instances; and 2) We should instead focus on exporting name spaces, and that the ordering of parentheses and the "as" keyword is too close to the existing "import Foo as F(foo)" syntax. Since there are these negative votes, I think it best if as many people as possible at least see the proposal so the GHC developers can get a better sense for the overall response. Anthony

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GHC.Generics: how to derive a Generic instance for an existential type?
by Andreas Abel 12 Jun '15

12 Jun '15

I wonder whether GHC.Generics supports existential types yet... {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE ExistentialQuantification #-} {-# LANGUAGE StandaloneDeriving #-} import GHC.Generics data U = forall a. (Generic a) => U a -- deriving (Generic) -- Can't make a derived instance of ‘Generic U’: -- Constructor ‘U’ has existentials or constraints in its type -- Possible fix: use a standalone deriving declaration instead -- deriving instance Generic U -- Can't make a derived instance of ‘Generic U’: -- U must be a vanilla data constructor -- In the stand-alone deriving instance for ‘Generic U’ data D1Ser data C1_0Ser instance Generic U where type Rep U = D D1Ser (C1 C1_0Ser (S1 NoSelector (Rep a))) -- Not in scope: type variable ‘a’ -- How to bring the existential type `a' into scope? -- Andreas Abel <>< Du bist der geliebte Mensch. Department of Computer Science and Engineering Chalmers and Gothenburg University, Sweden andreas.abel(a)gu.se http://www2.tcs.ifi.lmu.de/~abel/

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Lecturer position in Programming Languages / Security at Kent
by Simon Thompson 12 Jun '15

12 Jun '15

We’re looking for excellent PL/security people to recruit as faculty to join us in the School of Computing at Kent. Do apply if you’re interested, or mail me if you want to find out more. Simon Simon Thompson | Professor of Logic and Computation School of Computing | University of Kent | Canterbury, CT2 7NF, UK s.j.thompson(a)kent.ac.uk | M +44 7986 085754 | W www.cs.kent.ac.uk/~sjt

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TH to generate imports?
by Corentin Dupont 11 Jun '15

11 Jun '15

Hi guys, I wanted to know if it's possible to use TH on an "import" statement. What I want to do is to generate the names of the packages I need to import. For example: import $(genImport "MyPackage") genImport :: String -> Q TH.Exp genImport s = ... Is that possible? Thanks! Corentin

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Questions about concurrency and laziness
by Tony Garcia 11 Jun '15

11 Jun '15

Hi, First of all, apologies if this is not the right place to ask this question. I'm reading the book "Parallel and concurrent programming in Haskell" and I'm having some issues reasoning about the impact of laziness. Looking at the first example in chapter 8 [1] main = do m1 <- newEmptyMVar m2 <- newEmptyMVar forkIO $ do r <- getURL "http://www.wikipedia.org/wiki/Shovel" putMVar m1 r forkIO $ do r <- getURL "http://www.wikipedia.org/wiki/Spade" putMVar m2 r r1 <- takeMVar m1 r2 <- takeMVar m2 print (B.length r1, B.length r2) I don't understand why this function is not just putting an unevaluated thunk in the MVar. Well, I assume that getURL is an eager function, but looking at its code [2] or at the documentation of Network.Browser [3] I don't see why... Am I looking at this wrong? Is there any rule of thumb to be used in these cases? I have the impression that it's really easy to end up creating thunks in parallel threads and evaluating them in the main one... Thanks, Jose. [1] http://chimera.labs.oreilly.com/books/1230000000929/ch08.html [2] https://github.com/simonmar/parconc-examples/blob/master/GetURL.hs [3] http://hackage.haskell.org/package/HTTP-4000.0.8/docs/Network-Browser.html

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US Haskell Developer Position
by Christopher Lewis 11 Jun '15

11 Jun '15

Karamaan Group, a principal investment firm based in Manhattan, is looking for an outstanding software developer to develop tools for financial analysis and knowledge management. We are a growth oriented firm that values people who take a craftsman's pride in their work. Our ideal candidate is an experienced software developer with strong analytical, organizational, and communication skills. A candidate who demonstrates an intense focus on quality, but has the ability to recognize and make the tradeoffs that are a necessary part of day-to-day software development. Candidates should have at least a degree in a quantitative field and a keen interest in building robust and elegant computer programs. This is a high-impact, high-visibility position where successful candidates will be entrusted with a lot of responsibility for products that have a direct effect on the P&L of the firm and influences our workflow. The ideal candidate will have experience with Haskell, relational database technologies, and Java. All new development is performed in Haskell, but our legacy Java code base occasionally requires maintenance. Unlike most finance companies, our atmosphere is informal and intellectual. We don't require previous experience in finance or business, but knowledge in those areas is a plus. Karamaan Group is situated in Manhattan, we are an investment adviser to hedge funds and family offices. We value innovative thinking, encourage an entrepreneurial atmosphere, and enjoy spirited conversations. We also offer lunch, and every once in a while we go bowling. Telecommuting is an option, but you must be located in the Continental United States and authorized to work in the United States. Please send your CV and cover letter to recruitment at karamaan dot com. Please do not reply to the original poster.

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Benchmarking two versions of the same package?
by Alp Mestanogullari 11 Jun '15

11 Jun '15

Hi -cafe, While we can easily benchmark different functions or libraries easily with criterion, I can't think of a reasonably easy (and accurate!) way of benchmarking two versions of the same package. And not necessarily version as in cabal version -- one of the use cases I have in mind would be running a benchmark suite whenever a PR gets merged to the main branch of a library, so the benchmark would need to compare the performance of the library's-code-before-merging and after. This definitely can't be accomplished with something like criterion because we can't have two different instances of a package in scope for a module, even with -XPackageImports. If we separately build the same program against two instances of the same library and run the benchmarks separately, this might happen far apart enough that the machine running this might be under a different load. This does however seem to be the only actual solution? Run separately at two different commits, diff the numbers, report. I have felt the need for a solution to this for quite some time, but have never needed it bad enough that it became a priority. Thinking about it again today, I thought I should drop an email to the list and see if fellow haskellers have an easy-to-use solution for this? I may very well be overlooking something. -- Alp Mestanogullari

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