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Rank2Types example not typechecking w/ GHC8. Bug or feature?
by Michael Karg 29 May '16

29 May '16

Hi devs, could you please have a look at the following code snippet (modeled after a real-world app of mine)? There's a rank2type involved, and it doesn't type-check anymore when the type is e.g. part of a tuple, whereas everything's fine when it's the "outermost" type. With GHC7.10 both variants type-check. Could anyone shed some light on what's behind this? Is the way the types are used in the snippet considered dispreferred or wrong under GHC8? Thanks for having a look and hopefully pointing me to a page/ticket/... providing insight, Michael -------- {-# LANGUAGE Rank2Types #-} module TestTypes where data State a = State a data Dummy = Dummy type Handler result = forall state . State state -> IO result type Resolver = String -> Handler String eventRouter :: Resolver -> String -> IO () eventRouter resolver event = resolver event state >> return () where state :: State () state = undefined {- -- does type check createResolver :: Resolver createResolver = \event state -> return "result" processor :: IO () processor = getLine >>= eventRouter resolver >> processor where resolver = createResolver -} -- does not type check when the rank 2 type isn't the "outermost" one? createResolver :: (Resolver, Dummy) createResolver = (\event state -> return "result", Dummy) processor :: IO () processor = getLine >>= eventConsumer resolver >> processor where (resolver, _) = createResolver {- • Couldn't match type ‘t’ with ‘Resolver’ ‘t’ is a rigid type variable bound by the inferred type of resolver :: t at TestTypes.hs:41:5 Expected type: (t, Dummy) Actual type: (Resolver, Dummy) -}

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real person believed to be a spambot
by Richard Eisenberg 29 May '16

29 May '16

Hi devs, I'm forwarding the following from a friend (cc'd) who was unable to register for Trac. Can anyone help? ---- Hi. I tried to register with username "j6carey" and email "j6carey(a)gmail.com" to report a bug in GHC 8.0.1, and the system says there's a 97% chance I'm a spammer. It asked me a question first ("What does the G stand for in GHC?"), and I thought I answered correctly, in lowercase as requested, but it still rejected me, and now it won't even ask that question again (for the same browser, at least). Do you know what's going wrong?

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Strictness/laziness warnings
by David Feuer 29 May '16

29 May '16

There are certain patterns of strictness or laziness that signal the need for extra caution. I'm wondering whether it might be possible to offer warnings for different varieties of them, and pragmas suppressing the warnings at the relevant sites. Some function behaviors that suggest extra care: 1. Conditionally strict in an argument. In many cases, making it unconditionally strict will improve performance. 2. Strict in an argument that is or could be a function or a newtype wrapper around a function. This can be caused by adding too much strictness defensively or to plug a leak. 3. Lazy in a primitive argument like an Int. This could lead to unnecessary boxing. Any of these could occur in correct, efficient code. But I'd love to be presented a list of warnings to check over, and a way to check items off the list with pragmas.

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Unpacking single-field, single-strict-constructor GADTs and existentials
by David Feuer 29 May '16

29 May '16

Data.IntMap could be cleaned up some if single-field, single strict constructor GADTs/existentials could be unpacked even when wrapping a sum type. We could then have data Status = E | NE data IntMap' (s :: Status) a where Bin :: ... -> ... -> !(IntMap' NE a) -> !(IntMap' NE a) -> IntMap' NE a Tip :: ... -> a -> IntMap' NE a Nil :: IntMap' E a data IntMap a = forall s . IM {-# UNPACK #-} !(IntMap' s a) The representation would be the same as that of a newtype, but the pattern matching semantics would be strict. In the GADT case, this would essentially allow any fixed concrete datatype to serve directly as a witness for an arbitrary set of type equalities demanded on construction. Is there any hope something like this could happen?

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can't build with release settings
by Ömer Sinan Ağacan 28 May '16

28 May '16

Is anyone else having this problem when building with default settings (no build.mk): "inplace/bin/ghc-stage2" -hisuf dyn_hi -osuf dyn_o -hcsuf dyn_hc -fPIC -dynamic -H32m -O -Wall -hide-all-packages -i -iutils/haddock/driver -iutils/haddock/haddock-api/src -iutils/haddock/haddock-library/vendor/attoparsec-0.12.1.1 -iutils/haddock/haddock-library/src -iutils/haddock/dist/build -iutils/haddock/dist/build/autogen -Iutils/haddock/dist/build -Iutils/haddock/dist/build/autogen -optP-DIN_GHC_TREE -optP-include -optPutils/haddock/dist/build/autogen/cabal_macros.h -package-id Cabal-1.25.0.0 -package-id array-0.5.1.1 -package-id base-4.9.0.0 -package-id bytestring-0.10.8.1 -package-id containers-0.5.7.1 -package-id deepseq-1.4.2.0 -package-id directory-1.2.6.2 -package-id filepath-1.4.1.0 -package-id ghc-8.1 -package-id ghc-boot-8.1 -package-id transformers-0.5.2.0 -package-id xhtml-3000.2.1 -funbox-strict-fields -Wall -fwarn-tabs -O2 -threaded -XHaskell2010 -no-user-package-db -rtsopts -Wno-unused-imports -Wno-deprecations -Wnoncanonical-monad-instances -odir utils/haddock/dist/build -hidir utils/haddock/dist/build -stubdir utils/haddock/dist/build -c utils/haddock/haddock-library/src/Documentation/Haddock/Types.hs -o utils/haddock/dist/build/Documentation/Haddock/Types.dyn_o utils/haddock/haddock-library/src/Documentation/Haddock/Types.hs:15:8: error: Failed to load interface for ‘Prelude’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/haddock-library/src/Documentation/Haddock/Types.hs:17:1: error: Failed to load interface for ‘Data.Foldable’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/haddock-library/src/Documentation/Haddock/Types.hs:18:1: error: Failed to load interface for ‘Data.Traversable’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/driver/ResponseFile.hs:2:8: error: Failed to load interface for ‘Prelude’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/driver/ResponseFile.hs:8:1: error: Failed to load interface for ‘Control.Exception’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/driver/ResponseFile.hs:9:1: error: Failed to load interface for ‘Data.Char’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/driver/ResponseFile.hs:10:1: error: Failed to load interface for ‘Data.Foldable’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/driver/ResponseFile.hs:11:1: error: Failed to load interface for ‘System.Exit’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/driver/ResponseFile.hs:12:1: error: Failed to load interface for ‘System.IO’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:16:8: error:utils/haddock/haddock-api/src/Haddock/Backends/Hyperlinker/Types.hs:1:8: error: Failed to load interface for ‘Prelude’ It is a member of the hidden package Failed to load interface for ‘base-4.9.0.0’. Use -v to see a list of the files searched for. ‘Prelude’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:19:1: error: Failed to load interface for ‘Control.Arrow’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/Backends/Hyperlinker/Types.hs:4:1: error:utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:20:1: error: Failed to load interface for Failed to load interface for ‘GHC’ ‘Data.Function’ It is a member of the hidden package It is a member of the hidden package ‘base-4.9.0.0’. ‘ghc-8.1’. Use -v to see a list of the files searched for. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:22:1: error: Failed to load interface for ‘Exception’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:23:1: error: Failed to load interface for ‘Outputable’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:24:1: error: Failed to load interface for ‘Name’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:25:1: error: Failed to load interface for ‘Lexeme’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:26:1: error: Failed to load interface for ‘Module’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:27:1: error: Failed to load interface for ‘RdrName’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:28:1: error: Failed to load interface for ‘GhcMonad’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:29:1: error: Failed to load interface for ‘HscTypes’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:30:1: error: Failed to load interface for ‘UniqFM’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:31:1: error: Failed to load interface for ‘GHC’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/haddock-api/src/Haddock/GhcUtils.hs:32:1: error: Failed to load interface for ‘Class’ It is a member of the hidden package ‘ghc-8.1’. Use -v to see a list of the files searched for. utils/haddock/ghc.mk:21: recipe for target 'utils/haddock/dist/build/ResponseFile.dyn_o' failed make[1]: *** [utils/haddock/dist/build/ResponseFile.dyn_o] Error 1 make[1]: *** Waiting for unfinished jobs.... utils/haddock/ghc.mk:21: recipe for target 'utils/haddock/dist/build/Documentation/Haddock/Types.dyn_o' failed make[1]: *** [utils/haddock/dist/build/Documentation/Haddock/Types.dyn_o] Error 1 utils/haddock/ghc.mk:21: recipe for target 'utils/haddock/dist/build/Haddock/Backends/Hyperlinker/Types.dyn_o' failed make[1]: *** [utils/haddock/dist/build/Haddock/Backends/Hyperlinker/Types.dyn_o] Error 1 utils/haddock/ghc.mk:21: recipe for target 'utils/haddock/dist/build/Haddock/GhcUtils.dyn_o' failed make[1]: *** [utils/haddock/dist/build/Haddock/GhcUtils.dyn_o] Error 1 Makefile:129: recipe for target 'all' failed make: *** [all] Error 2 ( make distclean && ./boot && ./configure && make -j4; ) 2>&1 6382.95s user 1069.99s system 336% cpu 36:57.03 total tee build_output 0.04s user 0.60s system 0% cpu 36:57.03 total This is d40682e + one commit that I'm sure wouldn't break the build this way. I can't compile anything with stage2 compiler becuase of this: ➜ strict_cmm ghc-stage2 Main.hs [1 of 1] Compiling Lib ( Main.hs, Main.o ) [Prelude changed] Main.hs:4:1: error: Failed to load interface for ‘Prelude’ It is a member of the hidden package ‘base-4.9.0.0’. Use -v to see a list of the files searched for. -v output: ➜ strict_cmm ghc-stage2 Main.hs -v Glasgow Haskell Compiler, Version 8.1.20160528, stage 2 booted by GHC version 8.0.1 Using binary package database: /home/omer/haskell/ghc_3/inplace/lib/package.conf.d/package.cache loading package database /home/omer/haskell/ghc_3/inplace/lib/package.conf.d wired-in package ghc-prim mapped to ghc-prim-0.5.0.0 wired-in package integer-gmp mapped to integer-gmp-1.0.0.1 wired-in package base mapped to base-4.9.0.0 wired-in package rts mapped to rts wired-in package template-haskell mapped to template-haskell-2.11.0.0 wired-in package ghc mapped to ghc-8.1 wired-in package dph-seq not found. wired-in package dph-par not found. Hsc static flags: loading package database /home/omer/haskell/ghc_3/inplace/lib/package.conf.d wired-in package ghc-prim mapped to ghc-prim-0.5.0.0 wired-in package integer-gmp mapped to integer-gmp-1.0.0.1 wired-in package base mapped to base-4.9.0.0 wired-in package rts mapped to rts-1.0 wired-in package template-haskell mapped to template-haskell-2.11.0.0 wired-in package ghc mapped to ghc-8.1 wired-in package dph-seq not found. wired-in package dph-par not found. *** Chasing dependencies: Chasing modules from: *Main.hs !!! Chasing dependencies: finished in 0.44 milliseconds, allocated 0.417 megabytes Stable obj: [Lib] Stable BCO: [] Ready for upsweep [NONREC ModSummary { ms_hs_date = 2016-05-28 13:53:06.703533428 UTC ms_mod = Lib, ms_textual_imps = [(Nothing, Prelude)] ms_srcimps = [] }] *** Deleting temp files: Deleting: compile: input file Main.hs *** Checking old interface for Lib: [1 of 1] Compiling Lib ( Main.hs, Main.o ) [Prelude changed] *** Parser [Lib]: !!! Parser [Lib]: finished in 0.42 milliseconds, allocated 0.466 megabytes *** Renamer/typechecker [Lib]: !!! Renamer/typechecker [Lib]: finished in 0.05 milliseconds, allocated 0.004 megabytes Main.hs:4:1: error: Failed to load interface for ‘Prelude’ It is a member of the hidden package ‘base-4.9.0.0’. Locations searched: Prelude.hs Prelude.lhs Prelude.hsig Prelude.lhsig Upsweep partially successful. *** Deleting temp files: Deleting: link(batch): upsweep (partially) failed OR Main.main not exported; not linking. *** Deleting temp files: Deleting: *** Deleting temp dirs: Deleting: Any ideas?

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[ANNOUNCE] GHC 8.0.1 is available!
by Ben Gamari 27 May '16

27 May '16

=============================================== The Glasgow Haskell Compiler -- version 8.0.1 =============================================== The GHC developers are very pleased to announce the release of the first new super-major version of our Haskell compiler in six years, GHC 8.0.1. This release features dozens of exciting developments including, * A more refined interface for implicit call-stacks, allowing libraries to provide more helpful runtime error messages to users * The introduction of the DuplicateRecordFields language extension, allowing multiple record types to declare fields of the same name * Significant improvements in error message readability and content, including facilities for libraries to provide custom error messages, more aggressive warnings for fragile rewrite rules, and more helpful errors for missing imports * A rewritten and substantially more thorough pattern match checker, providing more precise exhaustiveness checking in GADT pattern matches * More reliable debugging information including experimental backtrace support, allowing better integration with traditional debugging tools * Support for desugaring do-notation to use Applicative combinators, allowing the intuitive do notation to be used in settings which previously required the direct use of Applicative combinators * The introduction of Strict and StrictData language extensions, allowing modules to be compiled with strict-by-default evaluation of bindings * Great improvements in portability, including more reliable linking on Windows, a new PPC64 code generator, support for the AIX operating system, unregisterised m68k support, and significant stabilization on ARM targets * A greatly improved user's guide, with beautiful and modern PDF and HTML output * Introduction of type application syntax, reducing the need for proxies * More complete support for pattern synonyms, including record pattern synonyms and the ability to export patterns "bundled" with a type, as you would a data constructor * Support for injective type families and recursive superclass relationships * An improved generics representation leveraging GHC's support for type-level literals * The TypeInType extension, which unifies types and kinds, allowing GHC to reason about kind equality and enabling promotion of more constructs to the type level * ...and more! A more thorough list of the changes included in this release can be found in the release notes, https://downloads.haskell.org/~ghc/8.0.1/docs/html/users_guide/8.0.1-notes.… As always, we have collected various points of interest for users of previous GHC releases on the GHC 8.0 migration page, https://ghc.haskell.org/trac/ghc/wiki/Migration/8.0 Please let us know if you encounter anything missing or unclear on this page. This release is the culmination of nearly eighteen months of effort by over one hundred contributors. We'd like to thank everyone who has contributed code, bug reports, and feedback over the past year. It's only because of their efforts that GHC continues to evolve. How to get it ~~~~~~~~~~~~~ Both the source tarball and binary distributions for a wide variety of platforms are available at, http://www.haskell.org/ghc/ Background ~~~~~~~~~~ Haskell is a standardized lazy functional programming language. The Glasgow Haskell Compiler (GHC) is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating efficient code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces. GHC is distributed under a BSD-style open source license. Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms we support, and the people responsible for them, can be found on the GHC wiki http://ghc.haskell.org/trac/ghc/wiki/Platforms Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: http://ghc.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ We welcome new contributors. Instructions on getting started with hacking on GHC are available from GHC's developer site, http://ghc.haskell.org/trac/ghc/ Community Resources ~~~~~~~~~~~~~~~~~~~ There are mailing lists for GHC users, develpoers, and monitoring bug tracker activity; to subscribe, use the web interfaces at http://mail.haskell.org/cgi-bin/mailman/listinfo/glasgow-haskell-users http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-tickets There are several other Haskell and GHC-related mailing lists on www.haskell.org; for the full list, see https://mail.haskell.org/cgi-bin/mailman/listinfo Some GHC developers hang out on the #ghc and #haskell of the Freenode IRC network, too: http://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: http://www.haskell.org/ghc/reportabug

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Optimizing "counting" GADTs
by David Feuer 26 May '16

26 May '16

I've started a wiki page, https://ghc.haskell.org/trac/ghc/wiki/OptimizeCountingGADTs , to consider optimizing GADTs that look like natural numbers but that possibly have "heavy zeros". Please take a look.

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Re: instances for closed type families
by Ryan Scott 25 May '16

25 May '16

Simon is right, you cannot use a type family as an instance head. But why do you need to? Typically, if you're deriving a Data instance that involves type families, the type families would be inside another data type. A real-world example is HsBindLR [1]: data HsBindLR idL idR = FunBind { ... bind_fvs :: PostRn idL NameSet, ... } | ... where PostRn is a type family [2]. Now, you can't simply derive Data for HsBindLR, because GHC has no way of knowing what PostRn will evaluate to! But you can use standalone deriving to get what you want: deriving instance (Data (PostRn idL NameSet), ...) => Data (HsBindLR idL idR) And in fact, this is what GHC does [3], using a convenient type synonyms for the long, sprawling context you need [4]. So in your example, while you can't directly create a Data instance for NameOrRdrName itself, you can quite easily create Data instances for anything that might use NameOrRdrName. Does that work for your use cases? Ryan S. ----- [1] http://git.haskell.org/ghc.git/blob/bdc555885b8898684549eca70053c9ce0ec7fa3… [2] http://git.haskell.org/ghc.git/blob/bdc555885b8898684549eca70053c9ce0ec7fa3… [3] http://git.haskell.org/ghc.git/blob/bdc555885b8898684549eca70053c9ce0ec7fa3… [4] http://git.haskell.org/ghc.git/blob/bdc555885b8898684549eca70053c9ce0ec7fa3…

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will close some of the cost-center tickets - how to compare .perf outputs?
by Ömer Sinan Ağacan 25 May '16

25 May '16

I'm trying to write some tests for some profiling-related tickets and I don't understand how do I compare .prof outputs. Can anyone help? I tried generating a .prof.sample and it worked, but somehow if I delete a random line from .prof.sample, my test still succeeds! If I delete the whole file or keep the file but delete the contents, it fails. I have no idea what's going on. Any ideas? Thanks

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instances for closed type families
by Alan & Kim Zimmerman 25 May '16

25 May '16

I am working on https://ghc.haskell.org/trac/ghc/ticket/12105, and have a type to ensure that the `HsMatchContext` has either a `RdrName` or a `Name`, not an `Id`. type family NameOrRdrName id where NameOrRdrName Id = Name NameOrRdrName Name = Name NameOrRdrName RdrName = RdrName Is there any way to declare `Data` and `OutputableBndr` instances for this? Without it I am having to do something like instance (OutputableBndr name, OutputableBndr (NameOrRdrName name)) => Outputable (HsDecl name) where which requires UndecidableInstances. I get compiler/hsSyn/PlaceHolder.hs:114:19: Illegal type synonym family application in instance: NameOrRdrName id In the stand-alone deriving instance for ‘Data (NameOrRdrName id)’ for the parameterised version and compiler/hsSyn/PlaceHolder.hs:115:19: Illegal type synonym family application in instance: NameOrRdrName RdrName In the stand-alone deriving instance for ‘Data (NameOrRdrName RdrName)’ for the one specific to `RdrName`. Regards Alan

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