1.6. Release notes for version 6.8.1
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## 1.6. Release notes for version 6.8.1

The significant changes to the various parts of the compiler are listed in the following sections.

The major changes in this release are adding Haskell Program Coverage (hpc) support to the compiler, adding a debugger to GHCi, the first phase of the base package split, and pointer tagging in the code generator (which should mean most code improves by 10-15%, and as a result the compiler is also faster).

### 1.6.1. User-visible compiler changes

- The GHC version numbering policy has been changed so 6.8.1 is the first release in this branch; see Section 1.3, “GHC version numbering policy” for details.

- Registerised builds with gcc >= 4.2 now work.

- While older releases did mostly work on Windows Vista, 6.8.1 fixes some issues, e.g. Windows used to think that cabal Setup programs were installers and refused to run them. However, Win9x support has now officially been dropped, as we are not able to test on that platform.

- Haskell Program Coverage Adding hpc tools, as a single program GHC now comes with “Haskell Program Coverage”, a code coverage tool. This is in the form of a new `-fhpc` flag and an `hpc` program. See Section 6.6, “Observing Code Coverage” for more details.

- There is the beginnings of support for “associated types” in GHC. However, this is incomplete and not a supported feature.

- There is the beginnings of support for “nested data parallelism” in GHC. However, this is incomplete and not a supported feature.

- It is now possible to derive class instances with standalone top-level declarations, rather than having to attach a deriving clause to the datatype declaration. The syntax is

      deriving instance Cxt => Head

  For more details see Section 8.5.2, “Stand-alone deriving declarations”.

- GHC now does left-to-right impredicative instantiation, so for example

      runST $ foo

  will now work. Don't rely on this, though, as it is subject to change in the future!

- A new extension `OverloadedStrings` means that string literals have type

      Data.String.IsString t => t

  where `Data.String.IsString` is a new class with a single method

      fromString :: String -> a

- There is a new record field disambiguation extension, enabled by `-fdisambiguate-record-fields`, which allows normally-ambiguous field names to be used when it is clear which is meant due to the use of a constructor. See Section 8.3.7, “Record field disambiguation” for more details.

- There are also new flags `-frecord-puns` and `-frecord-dot-dot` for record punninng, but these do not work correctly yet, and so are not supported.

- GHC now has an `--info` flag which prints information about the settings used to build the compiler. The information is in a machine-parsable (Haskell) format. For example,

      $ ghc --info
       [("Project name","The Glorious Glasgow Haskell Compilation System")
       ,("Project version","6.8.1")
       ,("Booter version","6.6")
       ,("Stage","2")
       ,("Interface file version","6")
       ,("Have interpreter","YES")
       ,("Object splitting","NO")
       ,("Have native code generator","YES")
       ,("Support SMP","YES")
       ,("Unregisterised","NO")
       ,("Tables next to code","YES")
       ,("Win32 DLLs","")
       ,("RTS ways"," debug  thr thr_p thr_debug")
       ,("Leading underscore","NO")
       ]

- The runtime system now has an `--info` flag which prints various information about the way it was compiled. Thus you can give `+RTS --info` flags to any Haskell program, e.g.

      $ darcs +RTS --info
       [("GHC RTS", "Yes")
       ,("GHC version", "6.8.1")
       ,("RTS way", "rts_thr")
       ,("Host platform", "x86_64-unknown-linux")
       ,("Build platform", "x86_64-unknown-linux")
       ,("Target platform", "x86_64-unknown-linux")
       ,("Compiler unregisterised", "NO")
       ,("Tables next to code", "YES")
       ]

- GHC now accepts `-XFoo` and `-XNoFoo` flags, where `Foo` is any extension that can be listed in a `LANGUAGE` pragma. As a result the following flags are now deprecated:

  - `-fth` (use `-XTemplateHaskell`)

  - `-ffi`, `-fffi` (use `-XForeignFunctionInterface`)

  - `-farrows` (use `-XArrows`)

  - `-fgenerics` (use `-XGenerics`)

  - `-fimplicit-prelude` (use `-XImplicitPrelude`)

  - `-fbang-patterns` (use `-XBangPatterns`)

  - `-fmonomorphism-restriction` (use `-XMonomorphismRestriction`)

  - `-fmono-pat-binds` (use `-XMonoPatBinds`)

  - `-fextended-default-rules` (use `-XExtendedDefaultRules`)

  - `-fimplicit-params` (use `-XImplicitParams`)

  - `-fscoped-type-variables` (use `-XScopedTypeVariables`)

  - `-fparr` (use `-XPArr`)

  - `-fallow-overlapping-instances` (use `-XOverlappingInstances`)

  - `-fallow-undecidable-instances` (use `-XUndecidableInstances`)

  - `-fallow-incoherent-instances` (use `-XIncoherentInstances`)

- GHC now has a `--supported-languages` flag which lists the strings it understands as a `-X` flag and in a `LANGUAGE` pragma. You can prepend `NO` to any of the strings. `LANGUAGE` pragmas also only turn on exactly what you asked for now; many of them used to just enable `-fglasgow-exts` before.

- GHC now understands `GENERATED` pragmas, which are used to indicate to tools like HPC where some tool-generated code came from. They look like

      {-# GENERATED "SourceFile" 12:3-14:8 #-} expr

- GHC now has a `--print-docdir` flag which prints to stdout where GHC believes its documentation is located.

- The `-O` no longer enables `-fvia-C`.

- There is a new flag `-fobject-code`, which uses the default of `-fasm` or `-fvia-C`, and `-fbyte-code`, which only works in GHCi and sets byte-code as the target.

- The warning for importing `T(..)` when `T` is an abstract type used to be enabled only by `-W` or `-Wall`. It no has its own flag `-fwarn-dodgy-imports`.

- A new flag `-fwarn-tabs` warns if there are tab characters in your source files.

- A new flag `-fwarn-implicit-prelude` warns if your module implicitly imports the `Prelude`.

- A new flag `-fwarn-monomorphism-restriction` warns if the monomorphism restriction applies to your code.

- The “missing type-signature” warning now tells you what type GHC inferred for the function.

- The `-w` flag now turns off all options, not just those that are enabled by `-Wall`.

- A new flag `-Wwarn` makes warnings just warn (the default). It can be used to override `-Werror`.

- The `-keep-hc-file`, `-keep-s-file` and `-keep-raw-s-file` flags can now have an `s` appended to their name (which reads better when using `--make`). None of them, nor `-keep-tmp-files`, can have any other suffix.

- There is a new flag `-fspec-threshold` which subsumes the now-deprecated `-fliberate-case-threshold`.

- A new flag `-fprint-explicit-foralls` controls whether or not GHC prints explicit `forall`s when printing types.

- A new flag `-ddump-to-file` allows you to specify that the output of the various ddump flags should be put in a given file.

- A new flag `-ddump-rule-firings` makes GHC print out when its `RULE`s fire.

- A new flag `-ddump-mod-cycles`, used with the `-M` mode, make GHC print out any module import loops.

- The `-fasm` flag no longer takes an argument (which it always used to ignore anyway).

- The new `-fhardwire-lib-paths` flag means the path to libraries are baked in, using the `-rpath` linker option.

- A new RTS flag `+RTS --install-signal-handlers=>yes|no<` tells the RTS whether or not to install signal handlers (default is yes). When making a DLL you will probably want to set this to no.

- When using runghc you can now leave a space after the `-f` argument telling it where to find GHC. You can now use `--` to separate the `-f` argument from the GHC arguments, and the GHC arguments from the program to run.

- ghc-pkg has a new command `check` which prints a list of all packages that are broken and which dependencies they are missing. Additionally, the `list` command now puts braces around broken packages.

- ghc-pkg now has a new flag `--names-only`, used in conjunction with `list --simple-output`, which prints package names only, i.e. without their version numbers.

- Some parts of the documentation, notably the building guide, have been moved to the wiki.

- Linear implicit parameter support has been removed.

- A new RTS flag `+RTS -xbXXXXX` hints to the OS that we'd like the heap to start at address 0xXXXXX. This is mainly of use to people debugging GHC.

### 1.6.2. Profiling changes

- It is now possible to get basic per-constructor profiling information from programs not compiled with `-prof`, by using the `+RTS -hT` flag. This is possible even when running code in GHCi.

- There is now support in the run-time system for using PAPI to do CPU performance counter measurements. This is controlled via the `+RTS -aX` options. Note that it must be enabled when the compiler is built, and is still very experimental.

- Ticky-ticky profiling, mainly for use by GHC developers, has been revived. See Section 6.7, “Using “ticky-ticky” profiling (for implementors)” for more information.

- It is now possible to have the key on a different page in a graph generated by hp2ps, meaning the graph can be larger and the cost centre names longer. The `+RTS -L` flag can be used to specify the maximum length of a cost-centre stack name.

- Uses of `_scc_` now give a deprecated warning. Use `SCC` pragmas instead.

### 1.6.3. GHCi changes

- GHCi now includes an interactive debugger. See Section 3.5, “The GHCi Debugger” for more information.

- The GHCi startup banner has been replaced with a single line. Avid fans of the banner can add

      putStrLn "   ___         ___ _"
      putStrLn "  / _ \\ /\\  /\\/ __(_)"
      putStrLn " / /_\\// /_/ / /  | |   GHC Interactive, for Haskell 98."
      putStrLn "/ /_\\\\/ __  / /___| |   http://www.haskell.org/ghc/"
      putStrLn "\\____/\\/ /_/\\____/|_|   Type :? for help."
      putStrLn ""

  to their `~/.ghci`.

- GHCi now has `()` as the first type in its defaulting list. See Section 3.4.5, “Type defaulting in GHCi” for more information.

- GHCi now treats `import M` at its prompt the same as `:module +M`.

- As well as `:set -fno<foo>`, you can now also say `:unset -f<foo>`.

### 1.6.4. Boot Libraries

These used to be called “core libraries”, but we now call them “boot libraries” (or, more precisely, “GHC boot libraries”) instead.

We now build the libraries with Cabal, rather than our own build system, so it should no longer be possible for differences to creep in between the libraries as built with GHC and as built with Cabal.

The following new boot libraries have been added:

- hpc

The following boot libraries have been removed:

- parsec

- regex-base

- regex-compat

- regex-posix

- stm

The following boot libraries are new, but composed of modules that used to be in the base library:

- array (`Data.Array*`)

- bytestring (`Data.ByteString*`)

- containers (`Data.Graph`, `Data.IntMap`, `Data.IntSet`, `Data.Map`, `Data.Sequence`, `Data.Set`, `Data.Tree`)

- directory (`System.Directory`)

- old-locale (`System.Locale`)

- old-time (`System.Time`)

- packedstring (`Data.PackedString`)

- pretty (`Text.PrettyPrint*`)

- process (`System.Cmd`, `System.Process*`)

- random (`System.Random`)

Additionally, `Control.Parallel*` have been moved to a new package “parallel” which is not a boot library.

#### 1.6.4.1. array

- Version number 0.1 (new library, split off from base).

- The `newArray_` method returns a deterministic result in the `ST` monad, and behaves as before in other contexts. The old `newArray_` is now called `unsafeNewArray_`; the `MArray` class therefore has one more method than before.

- In order to avoid problems where unusual `Ix` instances could cause the array code to access memory it shouldn't, a number of changes have been made:

  - A method

        numElements :: Ix i => a i e -> Int

    has been added to the `IArray` class.

  - `Data.Array.Base` has a new function

        safeRangeSize :: Ix i => (i, i) -> Int

    which always returns a non-negative result.

  - `Data.Array.Base` has a new function

        safeIndex :: Ix i => (i, i) -> Int -> i -> Int

    which only returns a result that is non-negative and not less than its second argument.

  - The `unsafeForeignPtrToStorableArray` function now has an `Ix i` constraint.

#### 1.6.4.2. base

- Version number 3.0 (was 2.1.1).

- A new function

      System.Timeout.timeout :: Int -> IO a -> IO (Maybe a)

  allows a computation to be timed out after a given number of microseconds.

- There is a new module `Data.String`, which defines the new `IsString(fromString)` class. This class is used by the new overloaded strings extension (see Section 8.6.4, “Overloaded string literals ”).

- The `Data.List` module now exports a function

      stripPrefix :: Eq a => [a] -> [a] -> Maybe [a]

  which drops the given prefix from a list.

- The `System.IO.Error` module now exports new functions `ioeGetLocation` and `ioeSetLocation`, for manipulating the “location” of an `IOError`.

- The functions `throwErrnoPath`, `throwErrnoPathIf`, `throwErrnoPathIf_`, `throwErrnoPathIfNull`, `throwErrnoPathIfMinus1`, `throwErrnoPathIfMinus1_` have moved from `unix:System.Posix.Error` to `Foreign.C.Error`.

- The `System.Posix.Signals` module has moved to the unix package.

- The `Text.Printf.printf` function now handles

  - The `Int8`, `Int16`, `Int32`, `Int64`, `Word`, `Word8`, `Word16`, `Word32` and `Word64` types.

  - The `+` flag.

  - The `X`, `E` and `G` formatting characters.

- The hash functions in `Data.HashTable` have been improved.

- The Kleisli composition functions

      (>=>) :: (Monad m) => (a -> m b) -> (b -> m c) -> (a -> m c)
      (<=<) :: (Monad m) => (b -> m c) -> (a -> m b) -> (a -> m c)

  together with the combinator

      forever :: (Monad m) => m a -> m ()

  have been added to `Control.Monad`.

- There is now a `Monoid` instance for the `Maybe` type.

- `Data.Monoid` now includes two types `First` and `Last`, which both newtype `Maybe` and return the first and last `Just` respectively when used a a `Monoid`.

- `Data.Monoid.Dual` now derives `Eq`, `Ord`, `Read`, `Show` and `Bounded`.

- We no longer put `stdin`, `stdout` and `stderr` into non-blocking mode, so piping Haskell program output through programs like tee works properly.

- The type of `Data.Generics.Schemes.synthesize` has been generalised from

      s -> (s -> s -> s) -> GenericQ (s -> s) -> GenericQ s

  to

      s -> (t -> s -> s) -> GenericQ (s -> t) -> GenericQ t

- `GHC.Exts` now exports `uncheckedShiftL64#`, `uncheckedShiftRL64#`, `uncheckedIShiftL64#` and `uncheckedIShiftRA64#`.

- `GHC.Exts` no longer defines the `Splittable(split)` class, which used to be used for linear implicit parameters.

- The new value `GHC.Conc.numCapabilities :: Int` is the argument of the `+RTS -N` flag, i.e. the number of Haskell threads that can run simultaneously.

- The new function `GHC.Environment.getFullArgs` returns the complete arguments that the program was invoked with, including any `+RTS` arguments etc.

- `GHC.Handle.openFd` has been renamed to `GHC.Handle.fdToHandle'` to avoid confusion with `System.Posix.IO.openFd`.

#### 1.6.4.3. bytestring

- Version number 0.9 (new library, split off from base).

- `Data.ByteString.packCStringLen` is no longer a pure function; its signature has changed from

      packCStringLen :: Foreign.C.String.CStringLen -> ByteString

  to

      packCStringLen :: Foreign.C.String.CStringLen -> IO ByteString

- The `Data.ByteString.Base` module has been split in two. The unsafe functions are now in `Data.ByteString.Unsafe` (now part of the stable API), and the others are in `Data.ByteString.Internal` (not part of the stable API.

- The internal lazy bytestring representation type has changed; rather than

      newtype ByteString = LPS [Strict.ByteString]

  it is now

      data ByteString = Empty | Chunk !Strict.ByteString ByteString

#### 1.6.4.4. Cabal

- Version number 1.2.2.0 (was 1.1.6.2).

- Configurations have been implemented; these allow things to be set conditionally in `.cabal` files.

- A new `--executable` flag for the `haddock` command also builds documentation for executables.

- A new `--hyperlink-source` flag for the `haddock` command makes the docs link to the source code, rendered by hscolour.

- A new `--html-location` flag for the `haddock` command allows you to say where documentation for packages we depend on is.

- A new field `pkgconfig-depends` allows you to specify C libraries you use. The pkg-config command is used to find what C flags etc are needed to use this library.

- A new field `build-tools` allows you to specify tool dependencies such as cpphs.

- The `includes` field now only specifes which include files are automatically included when compiling the package. A new `install-includes` field determines which include files are also installed.

- There are new flags `--docdir` and `--htmldir` for the `configure` command.

- The `configure` command now has `--with-`prog``, `--`prog`-option` and `--`prog`-options` flags for each program `prog`.

- A new field `cpp-options` is used when preprocessing Haskell modules.

- Many bug fixes.

- The Cabal API is not yet stable. There have been many API changes.

#### 1.6.4.5. containers

- Version number 0.1 (new library, split off from base).

- `Data.Tree.Tree` now has `Applicative` and `Monad` instances.

- In `Data.Map` the old functions

      minView :: Monad m => Map k a -> m (Map k a, (k, a))
      maxView :: Monad m => Map k a -> m (Map k a, (k, a))

  have been replaced with

      minView :: Monad m => Map k a -> m (a, Map k a)
      maxView :: Monad m => Map k a -> m (a, Map k a)
      minViewWithKey :: Monad m => Map k a -> m ((k, a), Map k a)
      maxViewWithKey :: Monad m => Map k a -> m ((k, a), Map k a)

- In `Data.Set` the old functions

      minView :: Monad m => Set a -> m (Set a, a)
      maxView :: Monad m => Set a -> m (Set a, a)

  have had their result orders reversed:

      maxView :: Monad m => Set a -> m (a, Set a)
      minView :: Monad m => Set a -> m (a, Set a)

- The following are new functions in `Data.IntMap`:

      maxView                 :: Monad m => IntMap a -> m (a, IntMap a)
      minView                 :: Monad m => IntMap a -> m (a, IntMap a)
      findMin                 :: IntMap a -> a
      findMax                 :: IntMap a -> a
      deleteMin               :: IntMap a -> IntMap a
      deleteMax               :: IntMap a -> IntMap a
      deleteFindMin           :: IntMap a -> (a, IntMap a)
      deleteFindMax           :: IntMap a -> (a, IntMap a)
      updateMin               :: (a -> a) -> IntMap a -> IntMap a
      updateMax               :: (a -> a) -> IntMap a -> IntMap a
      updateMinWithKey        :: (Key -> a -> a) -> IntMap a -> IntMap a
      updateMaxWithKey        :: (Key -> a -> a) -> IntMap a -> IntMap a
      minViewWithKey          :: (Monad m) => IntMap a -> m ((Key, a), IntMap a)
      maxViewWithKey          :: (Monad m) => IntMap a -> m ((Key, a), IntMap a)

- The following are new functions in `Data.IntSet`:

      findMin       :: IntSet -> Int
      findMax       :: IntSet -> Int
      deleteMin     :: IntSet -> IntSet
      deleteMax     :: IntSet -> IntSet
      deleteFindMin :: IntSet -> (Int, IntSet)
      deleteFindMax :: IntSet -> (Int, IntSet)
      maxView       :: Monad m => IntSet -> m (Int, IntSet)
      minView       :: Monad m => IntSet -> m (Int, IntSet)

#### 1.6.4.6. directory

- Version number 1.0 (new library, split off from base).

- The function

      makeRelativeToCurrentDirectory :: FilePath -> IO FilePath

  has been moved from the filepath library to `System.Directory`.

#### 1.6.4.7. filepath

- Version number 1.1 (was 1.0).

- Removed `makeRelativeToCurrentDirectory` (it has moved to `directory:System.Directory`).

- A number of driver-related functions have been added:

                  splitDrive :: FilePath -> (FilePath, FilePath)
                  joinDrive  :: FilePath -> FilePath -> FilePath
                  takeDrive  :: FilePath -> FilePath
                  hasDrive   :: FilePath -> Bool
                  dropDrive  :: FilePath -> FilePath
                  isDrive    :: FilePath -> Bool

#### 1.6.4.8. haskell98

- Version number 1.0.1 (was 1.0).

- No other change.

#### 1.6.4.9. hpc

- Version number 0.5 (new library).

#### 1.6.4.10. old-locale

- Version number 1.0 (new library, split off from base).

- No other change.

#### 1.6.4.11. old-time

- Version number 1.0 (new library, split off from base).

- No other change.

#### 1.6.4.12. packedstring

- Version number 0.1 (new library, split off from base).

- There is now a `Data` instance for the `PackedString` type.

#### 1.6.4.13. pretty

- Version number 1.0 (new library, split off from base).

- No other change.

#### 1.6.4.14. process

- Version number 1.0 (new library, split off from base).

- No other change.

#### 1.6.4.15. random

- Version number 1.0 (new library, split off from base).

- No other change.

#### 1.6.4.16. readline

- Version number 1.0.1 (was 1.0).

- No other change.

#### 1.6.4.17. template-haskell

- Version number 2.2 (was 2.1).

- We now derive `Data` and `Typeable` instances for all the TH types.

- There is now a `Functor` instance for `Q`, and `Quasi` instances have a `Functor` constraint.

#### 1.6.4.18. unix

- Version number 2.2 (was 2.1).

- `System.Posix.Unistd` now exports

      nanosleep :: Integer -> IO ()

  (which throws an exception if the OS doesn't provide it).

- `System.Posix.Files` now exports

      fileTypeModes        :: FileMode
      blockSpecialMode     :: FileMode
      characterSpecialMode :: FileMode
      namedPipeMode        :: FileMode
      regularFileMode      :: FileMode
      directoryMode        :: FileMode
      symbolicLinkMode     :: FileMode
      socketMode           :: FileMode

  for use with functions like `createDevice`.

- There is a new module `System.Posix.Semaphore` providing POSIX named semaphore support.

- There is a new module `System.Posix.SharedMem` providing POSIX shared memory support.

- `System.Posix.Terminal` now exports

      openPseudoTerminal :: IO (Fd, Fd)
      getSlaveTerminalName :: Fd -> IO FilePath

- The functions `throwErrnoPath`, `throwErrnoPathIf`, `throwErrnoPathIf_`, `throwErrnoPathIfNull`, `throwErrnoPathIfMinus1`, `throwErrnoPathIfMinus1_` have been removed from `System.Posix.Error`; they are now available from `base:Foreign.C.Error`.

- The `System.Posix.Signals` module has been moved here from the base package.

#### 1.6.4.19. Win32

- Version number 2.2 (was 2.1.1).

- There is a new module `System.Win32.Security` providing POSIX shared memory support.

### 1.6.5. GHC As A Library

- Version number 6.8.1.

- The API should now be much more usable than in previous releases.

- There is a new function

      compileToCore :: Session -> FilePath -> IO (Maybe [CoreBind])

  which compiles a file to core.

- There is a new value `defaultObjectTarget` that gives the `HscTarget` used by default. This name is now preferred to `defaultHscTarget`.

- The old

      data GhcMode
        = BatchCompile
        | Interactive
        | OneShot
        | JustTypecheck
        | MkDepend

  has now been refactored into

      data GhcMode
        = CompManager -- ^ --make, GHCi, etc.
        | OneShot     -- ^ ghc -c Foo.hs
        | MkDepend    -- ^ ghc -M, see Finder for why we need this

  and

      data GhcLink = NoLink | StaticLink

  is now

      data GhcLink = NoLink | LinkBinary | LinkInMemory

- The `HsSyn` types now include all of the haddock comments.

- As the GHC API exposes all the compiler internals there are numerous other changes that is would be impractical to list, but will have little effect on users of the API.

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