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The 13-line example in Text.Megaparsec.Expr
by Jeffrey Brown 25 Sep '16

25 Sep '16

At the bottom of the Hackage documentation for Text.Megaparsec.Expr [1] is a 13-line demonstration program. It includes no import statements. I added the ones I could deduce, which produced this: import Text.Megaparsec import Text.Megaparsec.Expr import Text.Megaparsec.Lexer (symbol,integer) parens = between (symbol "(") (symbol ")") expr = makeExprParser term table <?> "expression" term = parens expr <|> integer <?> "term" table = [ [ prefix "-" negate , prefix "+" id ] , [ postfix "++" (+1) ] , [ binary "*" (*) , binary "/" div ] , [ binary "+" (+) , binary "-" (-) ] ] binary name f = InfixL (reservedOp name >> return f) prefix name f = Prefix (reservedOp name >> return f) postfix name f = Postfix (reservedOp name >> return f) That still won't compile, because GHC does not know what reservedOp means. Does reservedOp refer to something that no longer exists, or have I just not found it? [1] https://hackage.haskell.org/package/megaparsec-4.4.0/docs/Text-Megaparsec-E… -- Jeffrey Benjamin Brown

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Debugging hanging template Haskell
by Cody Goodman 24 Sep '16

24 Sep '16

Can you use the ghci debugger to do this? I tried adding a piece of template Haskell to the frames library and now compilation never terminates: https://github.com/acowley/Frames/pull/59 One compiler warning warned me of overlapping instances in relation to my ZonedTime lift instance which could totally be wrong. I have a feeling maybe the overlapping instances could be making compilation hang, but I'm not sure. I'm more looking for help on how to move forward and keep debugging this than an answer, but if you only have time for an answer I'll gladly do the reverse engineering work to teach myself. Thanks all!

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Munich Haskell Meeting, 2016-09-26 @ 19:30
by Christian Neukirchen 24 Sep '16

24 Sep '16

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, September 26 at *Max-Emanuel-Brauerei* at 19h30. For details see here: http://muenchen.haskell.bayern/dates.html If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-sep-2016/ *Please add yourself quickly so we can get enough places.* Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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Understanding fgl's gfiltermap
by J. Stutterheim 23 Sep '16

23 Sep '16

Hi all, While playing with the fgl library [1] (git rev. 71b66d6), I encountered a situation where the library behaved different from what I expected. I now wonder whether I should adjust my expectations, or whether fgl has a bug. What I want to do is filter out nodes and edges based on edge labels. For a specific edge label l, if a node has an incoming or outgoing edge l it can stay, otherwise I want to remove it from my graph. Likewise, any edge that doesn't have label l needs to be removed from my graph. I wrote a program to do this using gfiltermap, but I got the wrong graph as a result. During debugging, I found that gfiltermap behaved differently from what I would expect. Take the following program: ```haskell {-# LANGUAGE TupleSections #-} module Test where import Debug.Trace import Data.Graph.Inductive main :: IO () main = prettyPrint test test :: Gr () () test = gfiltermap (Just . traceShowId) testGraph where {- - ASCII art representation of the graph: - - 1 <-4<-- 2 - /\ -/| - \ / - \3\/_ -} testGraph = mkGraph testNodes testEdges where testNodes = map (,()) [1, 2, 3, 4] testEdges = [ (2, 4, ()) , (4, 1, ()) , (2, 3, ()) , (3, 1, ()) , (3, 2, ()) ] ``` This simple program just reconstructs the original graph, but prints the contexts in the process. When it's done, it pretty prints the resulting graph. The debug tracing + pretty printing gives the following output (in that order): ([((),3),((),4)],1,(),[]) ([((),3)],2,(),[((),3),((),4)]) ([],3,(),[]) ([],4,(),[]) 1:()->[] 2:()->[((),3),((),4)] 3:()->[((),1),((),2)] 4:()->[((),1)] The pretty-printed graph correctly shows that there is an edges going from node 4 to node 1. However, the traced output shows node 4 (last line of trace output) with no successors and no predecessors. I would expect nodes 1 and 2 to be included in that tuple as the successor and predecessor in that tuple. Note that these two representations of the same graph _are_ isomorphic. Despite the isomorphism, this makes implementing my graph filter significantly more clumsy than I would like it to be. So my question is: are my expectations from the library wrong, or is this a bug? - Jurriën [1] https://hackage.haskell.org/package/fgl

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The meaning of categories constructed from HASK
by Dimitri DeFigueiredo 23 Sep '16

23 Sep '16

In category theory, there are many ways one can make new categories out of an old one. In particular, given a category C one can construct: 1. The arrows category of C: arrows in C become objects and commutative squares in C become arrows 2. The slice category of C given an object A: arrows into a distinguished object A become objects in the slice commutative triangles become arrows There are also functors going from C to these new categories (and back). Are these constructed categories useful when C = `Hask` (the category of haskell types and functions)? What do they represent in programming terms? In other words, is there intuition for what the arrows category of Hask is? What about the slice category of Hask over a specific type? Do the functors between these match some programming abstractions? Any pointers are much appreciated. Thanks, Dimitri

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MPTCs and extended defaulting
by Tom Murphy 23 Sep '16

23 Sep '16

I've got a case in a library I'm working on where having -XExtendedDefaultRules with MPTCs would be very, very helpful. Is it possible? I.e., we can now write: ``` {-# LANGUAGE ExtendedDefaultRules #-} data AB = A | B Double deriving (Show) class Foo x where foo :: x -> AB instance Foo Double where foo = B main = print $ foo 5 ``` And -XExtendedDefaultRules makes sure we don't need to write "5 :: Double" If, though, I want 'Foo' to take another parameter (here, a :: [Symbol]), it falls apart: ``` {-# LANGUAGE DataKinds #-} {-# LANGUAGE ExtendedDefaultRules #-} {-# LANGUAGE KindSignatures #-} {-# LANGUAGE MultiParamTypeClasses #-} import GHC.TypeLits data AB = A | B Double deriving (Show) class Foo x (a :: [Symbol]) where foo :: x -> AB instance Foo Double a where foo = B main = print $ foo 5 ``` Is there a reason MPTCs can't support ExtendedDefaultRules? Thanks! Tom

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PLDI 2017 call for papers
by Tobias Grosser 21 Sep '16

21 Sep '16

*Call for Contributions* ==================== 2017 ACM Conference on Programming Language Design and Implementation (PLDI) June 19-23, 2017 in Barcelona, Spain http://conf.researchr.org/home/pldi-2017 PLDI is the premier forum in the field of programming languages and programming systems research, covering the areas of design implementation, theory, applications, and performance. PLDI welcomes outstanding research which clearly advances the field and has the potential to make a lasting contribution. *Important Dates* =============== Research paper submissions due 15 Nov 2016 Author response period 26-28 Jan 2017 Author notification 13 Feb 2017 *Author Instructions* ================== http://conf.researchr.org/track/pldi-2017/pldi-2017-papers Submission site: https://pldi17.hotcrp.com/ *Organizing Committee* ==================== General Chair: Albert Cohen, INRIA, France Program Chair: Martin Vechev, ETH Zurich, Switzerland Workshops & Tutorials Chair: Aaron Smith, University of Edinburgh Publicity Chairs: Adrian Sampson, Cornell, USA Tobias Grosser, ETH Zurich, Switzerland http://conf.researchr.org/committee/pldi-2017/pldi-2017-organizing-committee -- Tobias Grosser

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Standard package file format
by Harendra Kumar 21 Sep '16

21 Sep '16

I am starting a new thread for the package file format related discussion. >From a developer's perspective, the major benefit of a standard and widely adopted format and is that people can utilize their knowledge acquired from elsewhere, they do not have to go through and learn differently looking and incomplete documentation of different tools. The benefit of a common config specification is that developers can choose tools freely without worrying about learning the same concepts presented in different ways. Multiple formats flying around also create a psychological impression of complexity in the ecosystem for newcomers. If we have consistency there are better chances of attracting more people to the language ecosystem. I gather the following from the discussion till now: * We have cabal, YAML and TOML as potential candidates for a common package format which can additionally incorporate the concept of snapshots/package collections and potentially more extensions useful across build tools. * cabal has the benefit of incumbency and backward compatibility, it has shortcomings which are being addressed but it is still a format which is very specific to Haskell ecosystem. It is not a standard and not going to become one. We have to always deal with it ourselves and everyone coming to Haskell will have to learn it. * YAML (http://yaml.org/spec/1.2/spec.html) is standard and popular. A significant chunk of developer community is already familiar with it. It is being used by stack and by hpack as an alternative to cabal format. The complaint against it is that the specification/implementation is overly complex. * TOML (https://github.com/toml-lang/toml) is promising, simpler than YAML and is being used by a few important projects but is still evolving and is not completely stable. On a first glance it looks pretty simple and a lot of other tools use a similar config format. It is aiming to become a standard and aiming for a wider adoption. As a next step we can perhaps do an hpack like experiment using the TOML format. That way we will have some experience with that as well and get to know if there are any potential problems expressing the existing cabal files. More thoughts, opinions on the topic will help create a better understanding about it. -harendra

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implement SOAP-based standard
by Olaf Klinke 18 Sep '16

18 Sep '16

Dear Haskellers, I am considering implementing the OPC XML DA standard [1] in Haskell. OPC XML DA is based on SOAP and is used for communicating with servers controlling hardware such as power plants. Currently the only open implementation known to me is PyOPC [2], which is based on an outdated Python libraries. However, Python seems not the nicest language for writing code that is devoid of bugs showing at runtime only. The OPC XML DA standard comes with a WSDL document (600 lines). There is a wsdl package on hackage, but it is incomplete. Can anyone share experience in implementing a webservice specified as WSDL? Actually, the client side would be enough for my purposes. That is, compose well-formed messages and parse the responses. Thanks, Olaf [1] https://opcfoundation.org/developer-tools/specifications-classic/xml-data-a… [2] https://libraries.io/pypi/PyOPC

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New maintainer for HsOpenSSL
by Vladimir Shabanov 18 Sep '16

18 Sep '16

Hello, I would like to become a new maintainer for HsOpenSSL package. Package was last updated on January 6, 2015 and no new commits at https://github.com/phonohawk/HsOpenSSL since then. My pull request https://github.com/phonohawk/HsOpenSSL/pull/40 is waiting here for more than a year. And there are ten more ignored pull requests. I've mailed current maintainer https://github.com/phonohawk/ pho(a)cielonegro.org four months ago and received no answer. Since there are no other maintainers I want to become the one. Who am I? Developer of BazQux Reader which is a commercial Haskell application that uses HsOpenSSL 24x7 in its crawler. Will be glad to hear any suggestions or objections. If not, I'm planning to wait for a few weeks and ask Hackage admins for maintenance rights.

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