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Overlapping and incoherent instances

glasgow-user - Tue, 07/29/2014 - 10:11am
Friends One of GHC's more widely-used features is overlapping (and sometimes incoherent) instances. The user-manual documentation is here<>. The use of overlapping/incoherent instances is controlled by LANGUAGE pragmas: OverlappingInstances and IncoherentInstances respectively. However the overlap/incoherent-ness is a property of the *instance declaration* itself, and has been for a long time. Using LANGUAGE OverlappingInstances simply sets the "I am an overlapping instance" flag for every instance declaration in that module. This is a Big Hammer. It give no clue about *which* particular instances the programmer is expecting to be overlapped, nor which are doing the overlapping. It brutally applies to every instance in the module. Moreover, when looking at an instance declaration, there is no nearby clue that it might be overlapped. The clue might be in the command line that compiles that module! Iavor
Categories: Offsite Discussion

Deduce issue.

haskell-cafe - Tue, 07/29/2014 - 7:29am
Hi, I have code like follow. And compiling gave me error as the last part. What should I do? ``` class WidgetClass w data Widget = forall a. WidgetClass a => Widget a instance WidgetClass Widget lookupWin :: (WidgetClass w) => WINDOW -> w -> Maybe w lookupWin xwin widget = if widgetWindowId widget == xwin then Just widget else foldl (\a (Widget b) -> maybe (lookupWin xwin b) Just a ) Nothing $ widgetChildren widget ``` --- ``` Graphics/HUI/Widget.hs:15:23: Could not deduce (w ~ a) from the context (WidgetClass w) bound by the type signature for lookupWin :: WidgetClass w => WINDOW -> w -> Maybe w at Graphics/HUI/Widget.hs:10:14-54 or from (WidgetClass a) bound by a pattern with constructor Widget :: forall a. WidgetClass a => a -> Widget, in a lambda abstraction at Graphics/HUI/Widget.hs:14:19-26 ‘w’ is a rigid type variable bound by the type signature for
Categories: Offsite Discussion

How to update a cabal dependency automatically

haskell-cafe - Tue, 07/29/2014 - 6:52am
I have been working on a haskell scotty project recently. I have just added configuration support for the project, which added an extra dependency of Control.Lens. My question is: is there a way update .cabal file automatically, just like cabal init but without recreate .cabal file? Thanks in advance. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe< at >
Categories: Offsite Discussion

early adopters rejoyce! Haskell Platform 2014.2.0.0Release Candidate 2

haskell-cafe - Tue, 07/29/2014 - 4:31am
The long anticipated Haskell Platform 2014.2 release, including GHC 7.8.3, and numerous updated packages, is almost here! We have created "Release Candidate 2" installers for OS X and Windows, and believe, barring show stopper issues, creepers exploding, or unexpected trips to the nether, these will likely be blessed as the final by the end of the week. If you would like to be an early adopter, please try 'em out... - source tarball: haskell-platform-2014.2.0.0-RC2.tar.gz <> - source repo: haskell/haskell-platform at 2014.2.0.0-RC2 <> - windows 32bit: hskellPlatform-2014.2.0.0-i386-RC2-setup.exe <> - windows 64bit:hskellPlatform-2014.2.0.0-x86_64-RC2-setup.exe <> - os x 64b
Categories: Offsite Discussion

Munich Haskell Meeting

haskell-cafe - Mon, 07/28/2014 - 9:10pm
Dear all, on Wednesday, 30th of July, once again our monthly Haskell Meeting takes place. We meet at 19h30 at Max-Emanuel-Brauerei in Munich. If you plan to join, please go here to find the details and don't forget to click the button: Have a nice evening, Heinrich
Categories: Offsite Discussion

RV 2014: Call for Participation

General haskell list - Mon, 07/28/2014 - 8:16pm
****************************************************************** CALL FOR PARTICIPATION RV 2014: 14th International Conference on Runtime Verification September 22 - September 25, 2014, Toronto, Canada ****************************************************************** ***************** OVERVIEW ***************** Runtime verification is concerned with monitoring and analysis of software and hardware system executions. Runtime verification techniques are important for system correctness, reliability, and robustness; they are complementary to conventional testing, and more practical than exhaustive formal verification. Runtime verification can be used prior to deployment, for testing, verification, and debugging purposes, and after deployment for ensuring reliability, safety, and security and for providing fault containment and recovery as well as online system repair. Topics of interest to the conference include: - specification languages - specification mining - pr
Categories: Incoming News

New gtk2hs 0.12.4 release

gtk2hs - Wed, 11/21/2012 - 12:56pm

Thanks to John Lato and Duncan Coutts for the latest bugfix release! The latest packages should be buildable on GHC 7.6, and the cairo package should behave a bit nicer in ghci on Windows. Thanks to all!


Categories: Incoming News