1.5. Release notes for version 7.8.1

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 7.6 branch.

1.5.1. Highlights

The highlights, since the 7.6 branch, are:

  • OS X Mavericks with XCode 5 is now properly supported by GHC. As a result of this, GHC now uses Clang to preprocess Haskell code by default for Mavericks builds.

    Note that normally, GHC used gcc as the preprocessor for Haskell code (as it was the default everywhere,) which implements -traditional behavior. However, Clang is not 100% compatible with GCC's -traditional as it is rather implementation specified and does not match any specification. Clang is also more strict.

    As a result of this, when using Clang as the preprocessor, some programs which previously used -XCPP and the preprocessor will now fail to compile. Users who wish to retain the previous behavior are better off using cpphs as an external preprocessor for the time being.

    In the future, we hope to fix this by adopting a better preprocessor implementation independent of the C compiler (perhaps cpphs itself,) and ship that instead.

  • By default, GHC has a new warning enabled, -fwarn-typed-holes, which causes the compiler to respond with the types of unbound variables it encounters in the source code. (It is reminiscient of the "holes" feature in languages such as Agda.) For more information, see Section 7.14, “Typed Holes”.

  • GHC can now perform simple evaluation of type-level natural numbers, when using the DataKinds extension. For example, given a type-level constraint such as (x + 3) ~ 5, GHC is able to infer that x is 2. Similarly, GHC can now understand type-level identities such as x + 0 ~ x.

    Note that the solving of these equations is only used to resolve unification variables - it does not generate new facts in the type checker. This is similar to how functional dependencies work.

  • It is now possible to declare a 'closed' type family when using the TypeFamilies extension. A closed type family cannot have any instances created other than the ones in its definition. For more information, see Section, “Closed type families”.

  • Use of the GeneralizedNewtypeDeriving extension is now subject to role checking, to ensure type safety of the derived instances. As this change increases the type safety of GHC, it is possible that some code that previously compiled will no longer work. For more information, see Section 7.25, “Roles ”.

  • GHC now supports overloading list literals using the new OverloadedLists extension. For more information, see Section 7.6.5, “Overloaded lists”.

  • GHC now supports pattern synonyms, enabled by the -XPatternSynonyms extension, allowing you to name and abstract over patterns more easily. For more information, see Section 7.3.8, “Pattern synonyms ”.

    Note: For the GHC 7.8.1 version, this language feature should be regarded as a preview.

  • There has been significant overhaul of the type inference engine and constraint solver, meaning it should be faster and use less memory.

  • By default, GHC will now unbox all "small" strict fields in a data type. A "small" data type is one whose size is equivalent to or smaller than the native word size of the machine. This means you no longer have to specify UNPACK pragmas for e.g. strict Int fields. This also applies to floating-point values.

  • GHC now has a brand-new I/O manager that scales significantly better for larger workloads compared to the previous one. It should scale linearly up to approximately 32 cores.

  • The LLVM backend now supports 128- and 256-bit SIMD operations.

    Note carefully: this is only available with the LLVM backend, and should be considered experimental.

  • The new code generator, after significant work by many individuals over the past several years, is now enabled by default. This is a complete rewrite of the STG to Cmm transformation. In general, your programs may get slightly faster.

    The old code generator has been removed completely.

  • GHC now has substantially better support for cross compilation. In particular, GHC now has all the necessary patches to support cross compilation to Apple iOS, using the LLVM backend.

  • PrimOps for comparing unboxed values now return Int# instead of Bool. This change is backwards incompatible. See this GHC wiki page for instructions how to update your existing code. See here for motivation and discussion of implementation details.

  • New PrimOps for atomic memory operations. The casMutVar# PrimOp was introduced in GHC 7.2 (debugged in 7.4). This release also includes additional PrimOps for compare-and-swap (casArray# and casIntArray#) and one for fetch-and-add (fetchAddIntArray#).

  • On Linux, FreeBSD and Mac OS X, GHCi now uses the system dynamic linker by default, instead of its built in (static) object linker. This is more robust cross-platform, and fixes many long-standing bugs (for example: constructors and destructors, weak symbols, etc work correctly, and several edge cases in the RTS are fixed.)

    As a result of this, GHCi (and Template Haskell) must now load dynamic object files, not static ones. To assist this, there is a new compilation flag, -dynamic-too, which when used during compilation causes GHC to emit both static and dynamic object files at the same time. GHC itself still defaults to static linking.

    Note that Cabal will correctly handle -dynamic-too for you automatically, especially when -XTemplateHaskell is needed - but you must tell Cabal you are using the TemplateHaskell extension.

    Note that you must be using Cabal and Cabal-install 1.18 for it to correctly build dynamic shared libraries for you.

    Currently, Dynamic GHCi and -dynamic-too are not supported on Windows (32bit or 64bit.)

  • Typeable is now poly-kinded, making Typeable1, Typeable2, etc., obsolete, deprecated, and relegated to Data.OldTypeable. Furthermore, user-written instances of Typeable are now disallowed: use deriving or the new extension -XAutoDeriveTypeable, which will create Typeable instances for every datatype declared in the module.

  • GHC now has a parallel compilation driver. When compiling with --make (which is on by default,) you may also specify -jN in order to compile N modules in parallel. (Note: this will automatically scale on multicore machines without specifying +RTS -N to the compiler.)

  • GHC now has support for a new pragma, {-# MINIMAL #-}, allowing you to explicitly declare the minimal complete definition of a class. Should an instance not provide the minimal required definitions, a warning will be emitted. See Section 7.20.5, “MINIMAL pragma” for details.

  • In GHC 7.10, Applicative will become a superclass of Monad, potentially breaking a lot of user code. To ease this transition, GHC now generates warnings when definitions conflict with the Applicative-Monad Proposal (AMP).

    A warning is emitted if a type is an instance of Monad but not of Applicative, MonadPlus but not Alternative, and when a local function named join, <*> or pure is defined.

    The warnings are enabled by default, and can be controlled using the new flag -f[no-]warn-amp.

  • Using the new InterruptibleFFI extension, it's possible to now declare a foreign import as interruptible, as opposed to only safe or unsafe. An interruptible foreign call is the same as a safe call, but may be interrupted by asynchronous Haskell exceptions, such as those generated by throwTo or timeout.

    For more information (including the exact details on how the foreign thread is interrupted,) see Section 8.1.4, “Interruptible foreign calls”.

  • GHC's internal compiler pipeline is now exposed through a Hooks module inside the GHC API. These hooks allow you to control most of the internal compiler phase machinery, including compiling expressions, phase control, and linking.

    Note: this interface will likely see continuous refinement and API changes in future releases, so it should be considered a preview.

  • The LLVM code generator has been fixed to support dynamic linking. This enables runtime-linking (e.g. GHCi) support for architectures without support in GHC's own runtime linker (e.g. ARM).

    Note: Tables-next-to-code is disabled when building on ARM with binutil's ld due to a bug in ld.

  • GHC now uses Unicode left/right single quotation marks (i.e. U+2018 and U+2019) in compiler messages if the current locale supports Unicode characters.

1.5.2. Full details Language

  • There is a new extension, NullaryTypeClasses, which allows you to declare a type class without any parameters.

  • There is a new extension, NumDecimals, which allows you to specify an integer using compact "floating literal" syntax. This lets you say things like 1.2e6 :: Integer instead of 1200000

  • There is a new extension, NegativeLiterals, which will cause GHC to interpret the expression -123 as fromIntegral (-123). Haskell 98 and Haskell 2010 both specify that it should instead desugar to negate (fromIntegral 123)

  • There is a new extension, EmptyCase, which allows to write a case expression with no alternatives case ... of {}.

  • The IncoherentInstances extension has seen a behavioral change, and is now 'liberated' and less conservative during instance resolution. This allows more programs to compile than before.

    Now, IncoherentInstances will always pick an arbitrary matching instance, if multiple ones exist.

  • A new built-in function coerce is provided that allows to safely coerce values between types that have the same run-time-presentation, such as newtypes, but also newtypes inside containers. See the haddock documentation of coerce and of the class Coercible for more details.

    This feature is included in this release as a technology preview, and may change its syntax and/or semantics in the next release.

  • The new pragma, {-# MINIMAL #-}, allows to explicitly declare the minimal complete definition of a class. Should an instance not provide the minimal required definitions, a warning will be emitted.

    See Section 7.20.5, “MINIMAL pragma” for more details. Compiler

  • GHC can now build both static and dynamic object files at the same time in a single compilation pass, when given the -dynamic-too flag. This will produce both a statically-linkable .o object file, and a dynamically-linkable .dyn_o file. The output suffix of the dynamic objects can be controlled by the flag -dynosuf.

    Note that GHC still builds statically by default.

  • GHC now supports a --show-options flag, which will dump all of the flags it supports to standard out.

  • GHC now supports warning about overflow of integer literals, enabled by -fwarn-overflowed-literals. It is enabled by default.

  • It's now possible to switch the system linker on Linux (between GNU gold and GNU ld) at runtime without problem.

  • The -fwarn-dodgy-imports flag now warns in the case an import statement hides an entity which is not exported.

  • The LLVM backend was overhauled and rewritten, and should hopefully be easier to maintain and work on in the future.

  • GHC now detects annotation changes during recompilation, and correctly persists new annotations.

  • There is a new set of primops for utilizing hardware-based prefetch instructions, to help guide the processor's caching decisions.

    Currently, the primops get translated into the associated hardware supported prefetch instructions only with the LLVM backend and x86/amd64 backends. On all other backends, the prefetch primops are currently erased at code generation time. GHCi

  • The monomorphism restriction is now turned off by default in GHCi.
  • GHCi now supports a prompt2 setting, which allows you to customize the continuation prompt of multi-line input. For more information, see Section 2.7, “GHCi commands”.

  • The new :shows paths command shows the current working directory and the current search path for Haskell modules.

  • On Linux, the static GHCi linker now supports weak symbols.

  • The (static) GHCi linker (except 64-bit Windows) now runs constructors for linked libraries. This means for example that C code using __attribute__((constructor)) can now properly be loaded into GHCi.

    Note: destructors are not supported. Template Haskell

  • Template Haskell now supports Roles.

  • Template Haskell now supports annotation pragmas.

  • Typed Template Haskell expressions are now supported. See Section 7.16, “Template Haskell” for more details.

  • Template Haskell declarations, types, patterns, and untyped expressions are no longer typechecked at all. This is a backwards-compatible change since it allows strictly more programs to be typed. Runtime system

  • The RTS linker can now unload object code at runtime (when using the GHC API ObjLink module.) Previously, GHC would not unload the old object file, causing a gradual memory leak as more objects were loaded over time.

    Note that this change in unloading behavior only affects statically linked binaries, and not dynamic ones.

  • The performance of StablePtrs and StableNames has been improved.

  • The default maximum stack size has increased. Previously, it defaulted to 8m (equivalent to passing +RTS -K8m. Now, GHC will use up-to 80% of the physical memory available at runtime. Build system

  • GHC >= 7.4 is now required for bootstrapping.

  • GHC can now be built with Clang, and use Clang as the preprocessor for Haskell code. Only Clang version 3.4 (or Apple LLVM Clang 5.0) or beyond is reliably supported.

    Note that normally, GHC uses gcc as the preprocessor for Haskell code, which implements -traditional behavior. However, Clang is not 100% compatible with GCC's -traditional as it is rather implementation specified, and is more strict.

    As a result of this, when using Clang as the preprocessor, some programs which previously used -XCPP and the preprocessor will now fail to compile. Users who wish to retain the previous behavior are better off using cpphs.

1.5.3. Libraries array

  • Version number (was base

  • Version number (was

  • The Control.Category module now has the PolyKinds extension enabled, meaning that instances of Category no longer need be of kind * -> * -> *.

  • There are now Foldable and Traversable instances for Either a, Const r, and (,) a.

  • There is now a Monoid instance for Const.

  • There is now a Data instance for Data.Version.

  • There are now Data, Typeable, and Generic instances for the types in Data.Monoid and Control.Applicative

  • There are now Num instances for Data.Monoid.Product and Data.Monoid.Sum

  • There are now Eq, Ord, Show and Read instances for ZipList.

  • There are now Eq, Ord, Show and Read instances for Down.

  • There are now Eq, Ord, Show, Read and Generic instances for types in GHC.Generics (U1, Par1, Rec1, K1, M1, (:+:), (:*:), (:.:)).

  • A zero-width unboxed poly-kinded Proxy# was added to GHC.Prim. It can be used to make it so that there is no the operational overhead for passing around proxy arguments to model type application.

  • Control.Concurrent.MVar has a new implementation of readMVar, which fixes a long-standing bug where readMVar is only atomic if there are no other threads running putMVar. readMVar now is atomic, and is guaranteed to return the value from the first putMVar. There is also a new tryReadMVar which is a non-blocking version.

  • There are now byte endian-swapping primitives available in Data.Word, which use optimized machine instructions when available.

  • Data.Bool now exports bool :: a -> a -> Bool -> a, analogously to maybe and either in their respective modules.

  • Rewrote portions of Text.Printf, and made changes to Numeric (added Numeric.showFFloatAlt and Numeric.showGFloatAlt) and GHC.Float (added formatRealFloatAlt) to support it. The rewritten version is extensible to user types, adds a "generic" format specifier "%v", extends the printf spec to support much of C's printf(3) functionality, and fixes the spurious warnings about using Text.Printf.printf at (IO a) while ignoring the return value. These changes were contributed by Bart Massey.

  • The minimal complete definitions for all type-classes with cyclic default implementations have been explicitly annotated with the new {-# MINIMAL #-} pragma.

  • Control.Applicative.WrappedMonad, which can be used to convert a Monad to an Applicative, has now a Monad m => Monad (WrappedMonad m) instance. bin-package-db

  • This is an internal package, and should not be used. binary

  • Version number (was bytestring

  • Version number (was Cabal

  • Version number (was 1.16.0) containers

  • Version number (was deepseq

  • Version number (was directory

  • Version number (was

  • The function findExecutables now correctly checks to see if the execute bit is set on Linux, rather than just looking in $PATH.

  • There are several new functions for finding files, including findFiles and findFilesWith, which allow you to search for a file given a set of filepaths, and run a predicate over them. filepath

  • Version number (was ghc-prim

  • Version number (was

  • The type-classes Eq and Ord have been annotated with the new {-# MINIMAL #-} pragma.

  • There is a new type exposed by GHC.Types, called SPEC, which can be used to inform GHC to perform call-pattern specialisation extremely aggressively. See Section 4.10, “Optimisation (code improvement)” for more details concerning -fspec-constr. haskell98

  • Version number (was haskell2010

  • Version number (was hoopl

  • Version number (was hpc

  • Version number (was integer-gmp

  • Version number (was old-locale

  • Version number (was old-time

  • Version number (was process

  • Version number (was

  • Several bugs have been fixed, including deadlocks in readProcess and readProcessWithExitCode. template-haskell

  • Version number (was

  • Typed Template Haskell expressions are now supported. See Section 7.16, “Template Haskell” for more details.

  • There is now support for roles.

  • There is now support for annotation pragmas. time

  • Version number 1.4.1 (was 1.4.1) unix

  • Version number (was

  • A crash in getGroupEntryForID (and related functions like getUserEntryForID and getUserEntryForName) in multi-threaded applications has been fixed.

  • The functions getGroupEntryForID and getUserEntryForID now fail with a isDoesNotExist error when the specified ID cannot be found. Win32

  • Version number (was

1.5.4. Known bugs

  • On OS X Mavericks, when using Clang as the C preprocessor, Haddock has a bug that causes it to fail to generate documentation, with an error similar to the following:

    <no location info>:
        module 'xhtml-3000.2.1:Main' is defined in multiple files: dist-bindist/build/tmp-72252/Text/XHtml.hs

    This only affects certain packages. This is due to a bad interaction with Clang, which we hope to resolve soon.

    Note that when using cabal-install, this only effects the package documentation, not installation or building.

  • On OS X 10.7 and beyond, with default build settings, the runtime system currently suffers from a fairly large (approx. 30%) performance regression in the parallel garbage collector when using -threaded.

    This is due to the fact that the OS X 10.7+ toolchain does not (by default) support register variables, or a fast __thread implementation. Note that this can be worked around by building GHC using GCC instead on OS X platforms, but the binary distribution then requires GCC later.

  • On Windows, -dynamic-too is unsupported.

  • On Windows, we currently don't ship dynamic libraries or use a dynamic GHCi, unlike Linux, FreeBSD or OS X.