- Haskell-Cafe - Haskell.org

What can be UNPACK'ed?
by Tom Ellis 03 Aug '17

03 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 01 Aug '17

01 Aug '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 01 Aug '17

01 Aug '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 01 Aug '17

01 Aug '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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Shout out for Chinese(?) speakers of Haskell
by Anthony Clayden 31 Jul '17

31 Jul '17

Is there a Chinese language(s) equivalent to StackOverflow or reddit or haskell-cafe? I'm asking because someone keeps posting to Trac with beginner/intermediate level questions. They're posting code that doesn't work/doesn't compile, and expecting Trac support to 'fix' it. Clearly they do not speak good English, because they're not explaining what they want the code to do (despite several requests). There have been some comments via Google translate, which isn't well versed in Haskell terminology (no surprise). Their first few tickets had narrative in Chinese characters. Looks like traditional script, so they're maybe a Cantonese speaker; or in Taiwan(?) Thank you AntC

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Re: [Haskell-cafe] [ANNOUNCE] GHC 8.2.1 available
by Wolfgang Jeltsch 30 Jul '17

30 Jul '17

Am Samstag, den 22.07.2017, 23:03 -0400 schrieb Ben Gamari: > In addition, there are a number of new features, > > * A new, more type-safe type reflection mechanism > > * The long-awaited Backpack module system > > * Deriving strategies to disambiguate between GHC's various instance > deriving mechanisms > > * Unboxed sum types, for efficient unpacked representation of sum > data types > > * Compact regions, allowing better control over garbage collection > in the presence of large heaps containing many long-lived objects. > > * Colorful messages and caret diagnostics for more legible errors > > A more thorough list of the changes in this release can be found in > the release notes, > > https://haskell.org/ghc/docs/8.2.1/docs/html/users_guide/8.2.1-notes.html It seems that the release notes mention the new type reflection mechanism und colorful messages only in the “Highlights” section, not in the “Full details” section, and that they do not mention the Backpack module system and unboxed sums at all. All the best, Wolfgang

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Damn you hmatrix
by Yotam Ohad 29 Jul '17

29 Jul '17

Hi cafe, I have a very annoying problem with hmatrix. I have a project which depends on it. I can use `stack build` with no errors but when I try to load it to GHCi(`stack ghci`) I get the following error: sh-4.3$ stack ghci [0mApple-0.1.0.0: configure (lib) [0m [0mConfiguring Apple-0.1.0.0... [0m [0mApple-0.1.0.0: initial-build-steps (lib) [0m [0mConfiguring GHCi with the following packages: Apple [0m GHCi, version 8.0.2: http://www.haskell.org/ghc/ :? for help ghc.EXE: addDLL: libblas.dll (Win32 error 126): The specified module could not be found. ghc.EXE: Could not load `libblas.dll'. Reason: addDLL: could not load DLL ghc.EXE: addDLL: liblapack.dll (Win32 error 126): The specified module could not be found. ghc.EXE: Could not load `liblapack.dll'. Reason: addDLL: could not load DLL ghc.EXE: C:\Users\Yotam\Apple\.stack-work\install\ebd7be2b\lib\x86_64-windows-ghc-8.0.2\hmatrix-0.18.0.0-IA7RzrKn1o84vo5PLhGcwQ\HShmatrix-0.18.0.0-IA7RzrKn1o84vo5PLhGcwQ.o: unknown symbol `dgesvd_' ghc.EXE: unable to load package `hmatrix-0.18.0.0' This is really bizarre, and could not find something online to help me fix this. I'm using stack 1.5.0 with ghc 8.0.2 On a side note, I've seen some discussions about why the C libraries can't be added automatically but did not understand most of it, if someone can explain it would be highly appreciated. Yotam

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GHCi and custom preprocessors
by aditya siram 29 Jul '17

29 Jul '17

Hi all, I have a custom file format and custom preprocessor in my project that converts it to Haskell. GHCi is unable to detect when changes are made to the custom file, it only sees that the Haskell file hasn't changed and doesn't reload anything. Is this even supported in GHCi? Thanks! -deech

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How to execute a function loaded via GHCI.ObjLink?
by Saurabh Nanda 28 Jul '17

28 Jul '17

Hi, I'm playing around with https://www.stackage.org/haddock/lts-9.0/ghci-8.0.2/GHCi-ObjLink.html to see if I can come up with some sort of hot-loading/plugin system in Haskell. I've managed to load a shared object (compiled from a Haskell source via -rdynamic) with the following code: {-# LANGUAGE OverloadedStrings #-} {-# LANGUAGE MagicHash, UnboxedTuples #-} module Main where import GHC.Exts ( addrToAny# ) import GHC.Ptr ( Ptr(..) ) import System.Info ( os, arch ) import Encoding import GHCi.ObjLink import Debug.Trace main :: IO () main = do traceM "before initObjLinker" initObjLinker traceM "before loadObj" loadObj "/Users/saurabhnanda/projects/test-plugins/test-plugins/app/PluginMarkup.o" traceM "after loadObj" -- NOTE: I've hardcoded the symbol name that I obtained from running `symbols PluginMarkup.o` sym <- lookupSymbol "PluginMarkup_foliage_info" traceM "after lookupsymbol" traceM (show sym) I'm getting the following output, which mean that I'm probably getting the Ptr to the function. before initObjLinker before loadObj after loadObj after lookupsymbol Just 0x0000000104be49a8 Question is, how do I run the function in the same Haskell environment/runtime? The underlying function is actually `foliage :: UtcTime -> Html` The code is available at https://github.com/vacationlabs/hint-test/blob/dll/app/Main.hs and the DLL is available at https://github.com/vacationlabs/hint-test/blob/dll/app/PluginMarkup.o -- Saurabh.

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