July 2017 - Haskell-Cafe

Archival of community.haskell.org
by Mario Rogic 19 May '18

19 May '18

Hi all As announced back in 2015 <http://blog.haskell.org/post/7/the_future_of_community.haskell.org/> we have started archiving community.haskell.org services. We've created https://archives.haskell.org/, and the old projects mailing lists have been the first to be archived. Next on the list is the remainder of projects.haskell.org, and the code.haskell.org site. As always, you can email admin(a)h.o or hop on the #haskell-infrastructure freenode channel to provide feedback / concerns. Thanks - Mario

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Join a transformer
by Michael Snoyman 11 Apr '18

11 Apr '18

I'm wondering if this pattern exists and has a name. We have the concept of joining a Monad: join :: Monad m => m (m a) -> ma How about joining a monad transformer? joinT :: (Monad m, MonadTrans t) => t (t m) a -> t m a I believe implementing this in terms of MonadTransControl[1] might be possible, but I was wondering if there's an already existing idiom for this. Michael [1] http://haddocks.fpcomplete.com/fp/7.4.2/20130301-40/monad-control/Control-M…

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PatternSignatures (rant)
by Johannes Waldmann 06 Apr '18

06 Apr '18

Dear all, (this is a rant - please ignore) why has "PatternSignatures" been renamed to "ScopedTypeVariables" (in ghc-6.10)? show me the alleged "type variable" in this code: do x :: Int <- [ 1 .. 10 ] ; return $ x^2 and why on earth do I need to use a language extension at all to get to the most basic thing of declarative programming: to declare the type of an identifier? This seriously hurts (me, at least) when teaching Haskell. Thanks - J.W.

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[hs-bibutils] intention to take over package
by Václav Haisman 28 Sep '17

28 Sep '17

Hi. I would like to take over the hs-bibutils Hackage package. It has not been updated since 2014, the listed maintainer is not responsive to emails, and listed home page is 404 page. -- VH

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Re: [Haskell-cafe] type (++) = (<>)
by Doug McIlroy 12 Aug '17

12 Aug '17

> What do you think of making (++) the same as (<>) This seems to be a call for returning to the old situation in which (++) was an operator of class MonadPlus. Why was that abolished in Haskell 98? Doug

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What can be UNPACK'ed?
by Tom Ellis 02 Aug '17

02 Aug '17

What types can be UNPACK'ed? From my vague understanding of what UNPACK does, it can only be types whose size is bounded by a known value, so whilst data Foo = Foo {-# UNPACK #-} !Float {-# UNPACK #-} !Int is fine, presumably data Foo = Foo {-# UNPACK #-} ![Float] is not fine. Or is it? Technically it could be possible to reserve enough space in Foo to store either a [] or a (:) Float [Float]. And if that is possible could we also UNPACK polymorphic fields? data Storable a => Foo a = Foo {-# UNPACK #-} a or data Foo a where Foo :: Storable a => {-# UNPACK #-} a -> Foo a if either DatatypeContexts worked or my imaginary GADT UNPACK syntax worked (I'm not sure if either does). What are the limits of what we could reasonably get to UNPACK in GHC? Thanks, Tom

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Problems with Type family and Typeable
by Kai Zhang 01 Aug '17

01 Aug '17

I got an error when compiling the following codes: {-# LANGUAGE TypeOperators #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE ScopedTypeVariables #-} {-# LANGUAGE UndecidableInstances #-} import Data.Type.Bool import Data.Type.Equality import Data.Tagged (Tagged(..)) import Data.Typeable type family Elem x xs :: Bool where Elem _ '[] = 'False Elem a (x ': xs) = a == x || Elem a xs elemTag :: forall a s t ts . Proxy s -> Tagged (t ': ts) a -> Bool elemTag _ _ = if typeOf (Proxy :: Proxy (Elem s (t ': ts))) == typeOf (Proxy :: Proxy 'True) then True else False GHC says: • No instance for (Typeable (Data.Type.Equality.EqStar s t || Elem s ts)) arising from a use of ‘typeOf’ • In the first argument of ‘(==)’, namely ‘typeOf (Proxy :: Proxy (Elem s (t : ts)))’ In the expression: typeOf (Proxy :: Proxy (Elem s (t : ts))) == typeOf (Proxy :: Proxy True) In the expression: if typeOf (Proxy :: Proxy (Elem s (t : ts))) == typeOf (Proxy :: Proxy True) then True else False My question: why is ghc unable to deduce that "Data.Type.Equality.EqStar s t || Elem s ts" resolves to "Bool" which should be an instance of Typeable? How to fix this?

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Many packages blocked on Stackage nightly for GHC 8.2.1
by Dan Burton 31 Jul '17

31 Jul '17

Please be advised that many packages were dropped from the Stackage nightly build plan in order to arrive at a successful build with the new ghc. (Note that this does not affect the build plans for LTS 9.) This is to be expected whenever ghc gets a major version bump. For details, see: https://github.com/fpco/stackage/blob/master/MAINTAINERS.md We have attempted to contact package authors whose packages were dropped. However, comprehensive notification is not fully automated, and there may be many who have not been directly contacted. You can check your section of build-constraints.yaml, as well as stackage.org/nightly, to see which of your packages are currently absent from the build plan. You may restore your packages to the nightly build plan (once they are buildable with ghc 8.2.1) in the same way they were added: by opening a PR, uncommenting your packages in build-constraints.yaml. Thank you for your efforts to keep the Haskell ecosystem up to date.

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DSLDI 2017: Second Call for Talk Proposals
by Lindsey Kuper 31 Jul '17

31 Jul '17

********************************************************************* SECOND CALL FOR TALK PROPOSALS DSLDI 2017 Fifth Workshop on Domain-Specific Language Design and Implementation October 22, 2017 Vancouver, Canada Co-located with SPLASH http://2017.splashcon.org/track/dsldi-2017 https://twitter.com/wsdsldi ********************************************************************* Deadline for talk proposals: 7th of August, 2017 Well-designed and implemented domain-specific languages (DSLs) can achieve both usability and performance benefits over general-purpose programming languages. By raising the level of abstraction and exploiting domain knowledge, DSLs can make programming more accessible, increase programmer productivity, and support domain-specific optimizations. ## Workshop Goal Domain-Specific Language Design and Implementation (DSLDI) is a workshop intended to bring together researchers and practitioners interested in discussing how DSLs should be designed, implemented, supported by tools, and applied in realistic contexts. The focus of the workshop is on all aspects of this process, from soliciting domain knowledge from experts, through the design and implementation of the language, to evaluating whether and how a DSL is successful. More generally, we are interested in continuing to build a community that can drive forward the development of modern DSLs. ## Workshop Format DSLDI is a single-day workshop and will consist of an invited speaker followed by moderated audience discussions structured around a series of short talks. The role of the talks is to facilitate interesting and substantive discussion. Therefore, we welcome and encourage talks that express strong opinions, describe open problems, propose new research directions, and report on early research in progress. Proposed talks should be on topics within DSLDI’s area of interest, which include but are not limited to: * solicitation and representation of domain knowledge * DSL design principles and processes * DSL implementation techniques and language workbenches * domain-specific optimizations * human factors of DSLs * tool support for DSL users * community and educational support for DSL users * applications of DSLs to existing and emerging domains * studies of usability, performance, or other benefits of DSLs * experience reports of DSLs deployed in practice ## Call for Talk Proposals We solicit talk proposals in the form of short abstracts (max. 2 pages). A good talk proposal describes an interesting position, open problem, demonstration, or early achievement. The submissions will be reviewed on relevance and clarity, and used to plan the mostly interactive sessions of the workshop day. Publication of accepted abstracts and slides on the website is voluntary. * Deadline for talk proposals: August 7th, 2017 * Notification: September 11th, 2017 * Workshop: October 22nd, 2017 * Submission website: https://dsldi17.hotcrp.com/ ## Workshop Organization Co-chairs: * Lindsey Kuper (lindsey(a)composition.al) Intel Labs * Eric Walkingshaw (eric.walkingshaw(a)oregonstate.edu) Oregon State University Follow us on Twitter at https://twitter.com/wsdsldi Program committee: * Nada Amin (EPFL/University of Cambridge) * Eric Holk (Google) * Gabriele Keller (Data61, CSIRO (formerly NICTA) and UNSW) * Rebekah Leslie-Hurd (Intel Labs) * Chris Martens (NCSU) * Lee Pike (Galois) * Jonathan Ragan-Kelley (UC Berkeley) * Jesús Sánchez Cuadrado (Autonomous University of Madrid) * Vincent St-Amour (Northwestern University) * Philip Wadler (University of Edinburgh)

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Re: [Haskell-cafe] What can be UNPACK'ed?
by Olaf Klinke 31 Jul '17

31 Jul '17

How is UNPACK and BangPatterns related to WHNF? I used to think that unpacking sort of forces the constructors up to the level where the bangs reside. Hence unpacking a tuple makes sense, whereas unpacking a linked list does not save much memory. -- Olaf

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