4. Release notes for version 8.0.2

The significant changes to the various parts of the compiler are listed in the following sections. There have also been numerous bug fixes and performance improvements over the 8.0.1 release.

Warning

Only Cabal versions 1.24 and newer will function properly with this release. (see Trac #11558). Consequently it will likely be necessary to recompile cabal-install before installing new packages.

The reason for this is a change in how packages are identified in GHC 8.0. While previous versions of Cabal identified packages to GHC with a package key (with GHC’s -this-package-key argument), GHC 8.0 and later uses installed package IDs in place of package keys.

Note

Users compiling GHC on Mac OS X with XCode 7.3 will need to tell the build system to use the nm-classic command instead of Apple’s new nm implementation as the latter breaks POSIX compliance (see Trac #11744). This can be done by passing something like --with-nm=$(xcrun --find nm-classic) to configure.

4.1. Highlights

The highlights, since the 8.0.1 release, are:

• Compatibility fixes with macOS Sierra and recent Linux distributions.
• Many, many bug fixes.
• A bug has been fixed that caused standalone derived Ix instances to fail for GADTs with exactly one constructor (Trac #12583).
• Interface files produced by GHC should now be deterministic.

4.2. Full details

4.2.1. Language

• A bug has been fixed that caused derived Show instances to fail in the presence of -XRebindableSyntax and -XOverloadedStrings (Trac #12688).

• GHC is now a bit more strict in typechecking code generated by -XGeneralizedNewtypeDeriving. For example, GHC will now reject this program:

  class C m where
    foo :: C m => m ()
  
  newtype N m a = N (m a)
    deriving C -- This is now an error
  

  This is in contrast to GHC 8.0.1 and earlier, which would accept this code. To fix this code, simply remove the C m constraint from foo, as it is wholly unnecessary:

  class C m where
    foo :: m ()
  

• Some programs using -XDefaultSignatures that incorrectly type-checked in GHC 8.0.1 are now rejected by GHC 8.0.2. Here is a characteristic example:

  class Monad m => MonadSupply m where
    fresh :: m Integer
    default fresh :: (MonadTrans t, MonadSupply m) => t m Integer
    fresh = lift fresh
  
  instance MonadSupply m => MonadSupply (IdentityT m)
  

  Note that the m in the default type signature is being used in a completely different way than the m in the non-default signature! We can fix this (in a backwards-compatible way) like so:

  class Monad m => MonadSupply m where
    fresh :: m Integer
    default fresh :: (MonadTrans t, MonadSupply m', m ~ t m') => m Integer
                     -- Same 'm Integer' after the '=>'
    fresh = lift fresh
  

• Some programs which combine default type class method implementations and overlapping instances may now fail to type-check. Here is an example:

  class Foo a where
    foo :: a -> [a]
    foo _ = []
  
  instance Foo a
  instance Foo Int
  

  The problem is that the overlapping Foo Int instance is not explicitly marked as overlapping. To fix this, simply add an OVERLAPPING pragma:

  instance {-# OVERLAPPING #-} Foo Int
  

• GHC now adheres more closely to the Haskell 2010 Report with respect to defaulting rules. As a result, GHC will now reject some defaulting rules which GHC 8.0.1 and earlier would accept. For example, this is now rejected

  module Foo where
  default (Bool)
  

  because when the -XExtendedDefaultRules extension is not enabled, defaulting rules only work for the Num class, of which Bool is not an instance. To make GHC accept the above program, simply enable the -XExtendedDefaultRules extension.

4.2.2. Compiler

• A compiler bug present in 8.0.1 resulting in undefined reference errors while compiling some packages has been fixed. (see Trac #12076).

• A code generator bug which resulted in segmentation faults in compiled programs has been fixed (see Trac #12757).

• GHC now supports systems whose C compiler produces position-independent executables by default. (see Trac #12579).

• GHC can now be built on systems which use the gold linker by default (see Trac #12816).

• GHC now reliably runs on macOS Sierra systems. Sierra introduced a linker limitation which GHC occassionally surpassed when compiling programs with many package dependencies. (see Trac #12479).

• The -Wredundant-constraints flag has been removed from the -Wall flag set (see Trac #10635).

• Added -fdefer-out-of-scope-variables, which converts out-of-scope variable errors into warnings.

• The RTS -xb now reads the base heap address in any base, defaulting to decimal, hexadecimal if the address starts with 0x, and octal if the address starts with 0.

• Due to an oversight in GHC 8.0.1, the value of the preprocessor macro __GLASGOW_HASKELL_LLVM__, which exposes the LLVM version used by GHC, was no longer an integer. This value is now turned into an integer again, but the formatting is changed to be in line with __GLASGOW_HASKELL__ (Trac #12628).

• Parallel programs should be significantly more reliable on platforms with weak memory consistency guarantees (Trac #12469)

• Interface files should now be bit-wise identical for a given build. (Trac #4012)

• Nearly two-hundred more bugs. See `Trac

  <https://ghc.haskell.org/trac/ghc/query?status=closed&milestone=8.0.2&col=id&col=summary&col=status&col=type&col=priority&col=milestone&col=component&order=priority>`_ for a complete list.

4.2.3. Runtime system

• The Runtime linker on Windows is once again recognizing POSIX functions under their “deprecated” name. e.g. “strdup” will now be recognized and internally forwarded to “_strdup”. If you have existing code already using the correct names (e.g. _strdup) then this will just continue to work and no change is needed. For more information about how the forwarding is done please see MSDN . This should now introduce the same behavior both compiled and interpreted. (see Trac #12497).
• Profiles from the cost-center profiler now provide source span information. (see Trac #11543).
• The number of threads used for garbage collection is now configurable independently from the number of capabilities with the new -qn flag.
• The runtime system should now wake-up less often with large capability counts
• The runtime system is now a more efficient in handling programs with many bound threads. (Trac #12419)
• A number of runtime system bugs which could result in crashes (see Trac #12728, Trac #10860, Trac #12019, Trac #11978, Trac #12038, Trac #12208)

4.2.4. Template Haskell

• addModFinalizer now exposes the local typing environment at the splice point. This allows reify to see local and top-level definitions in the current declaration group when used as in

  f x = $(addModFinalizer (reify 'x >>= runIO . print) >> [| x |])
  

4.2.5. ghc library

• Accessors are now exposed for ErrUtils.ErrMsg and ErrUtils.ErrDoc.
• There is now a createIservProcessHook to allow API users to redirect the stdout and stderr handles.