# 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.

## 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 .

- 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 .

- 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 .

- GHC now supports overloading list literals using the new `OverloadedLists` extension.

  For more information, see .

- GHC now supports pattern synonyms, enabled by the `-XPatternSynonyms` extension, allowing you to name and abstract over patterns more easily.

  For more information, see .

  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 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 .

- 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.

## 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 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 .

- 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 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.

- Template Haskell now supports pattern splices.

### 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 `StablePtr`s and `StableName`s 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.

## Libraries

### array

- Version number 0.5.0.0 (was 0.4.0.1)

### base

- Version number 4.7.0.0 (was 4.6.0.1)

- 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 0.7.1.0 (was 0.5.1.1)

### bytestring

- Version number 0.10.4.0 (was 0.10.0.0)

### Cabal

- Version number 1.18.1.3 (was 1.16.0)

### containers

- Version number 0.5.4.0 (was 0.5.0.0)

### deepseq

- Version number 1.3.0.2 (was 1.3.0.1)

### directory

- Version number 1.2.0.2 (was 1.2.0.1)

- 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 1.3.0.2 (was 1.3.0.1)

### ghc-prim

- Version number 0.3.1.0 (was 0.3.0.0)

- 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 for more details concerning `-fspec-constr`.

### haskell98

- Version number 2.0.0.3 (was 2.0.0.2)

### haskell2010

- Version number 1.1.1.1 (was 1.1.1.0)

### hoopl

- Version number 3.10.0.0 (was 3.9.0.0)

### hpc

- Version number 0.6.0.1 (was 0.6.0.0)

### integer-gmp

- Version number 0.5.1.0 (was 0.5.0.0)

### old-locale

- Version number 1.0.0.6 (was 1.0.0.5)

### old-time

- Version number 1.1.0.2 (was 1.1.0.1)

### process

- Version number 1.2.0.0 (was 1.1.0.2)

- Several bugs have been fixed, including deadlocks in `readProcess` and `readProcessWithExitCode`.

### template-haskell

- Version number 2.9.0.0 (was 2.8.0.0)

- Typed Template Haskell expressions are now supported. See 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 2.7.0.0 (was 2.6.0.0)

- 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 2.3.0.1 (was 2.3.0.0)

## 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
                                                                     dist-bindist/build/tmp-72252/Text/XHtml/Frameset.hs
                                                                     dist-bindist/build/tmp-72252/Text/XHtml/Strict.hs
                                                                     dist-bindist/build/tmp-72252/Text/XHtml/Transitional.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.