path: root/docs/users_guide/7.8.1-notes.xml

<?xml version="1.0" encoding="iso-8859-1"?>
<sect1 id="release-7-8-1">
  <title>Release notes for version 7.8.1</title>

  <para>
      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.
  </para>

  <sect2>
    <title>Highlights</title>

    <para>
        The highlights, since the 7.6 branch, are:
    </para>

    <itemizedlist>
        <listitem>
            <para>
                GHC now supports "type holes" with the
                <literal>TypeHoles</literal> extension. When enabled, the
                unbound literal <literal>_</literal> may be used during
                development in place of a regular identifier, and GHC will
                respond with the type necessary to "fill in the hole."

                TODO FIXME: reference.
           </para>
       </listitem>

        <listitem>
            <para>
                It is now possible to declare a 'closed' <literal>type
                family</literal> when using the
                <literal>TypeFamilies</literal> extension. A closed
                <literal>type family</literal> cannot have any
                instances created other than the ones in its
                definition.

                TODO FIXME: reference.
           </para>
       </listitem>

       <listitem>
           <para>
                Use of the <literal>GeneralizedNewtypeDeriving</literal>
                extension is now subject to <emphasis>role checking</emphasis>,
                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.

                TODO FIXME: reference.
           </para>
       </listitem>

        <listitem>
            <para>
                GHC now supports overloading list literals using the new
                <literal>OverloadedLists</literal> extension.

                TODO FIXME: reference.
           </para>
       </listitem>

        <listitem>
            <para>
                There has been significant overhaul of the type inference engine and
                constraint solver.

                TODO FIXME: reference.
           </para>
       </listitem>

        <listitem>
            <para>
                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 <literal>UNPACK</literal> pragmas for
                e.g. strict <literal>Int</literal> fields. This also
                applies to floating-point values.
           </para>
       </listitem>

        <listitem>
            <para>
                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.
           </para>
       </listitem>

        <listitem>
            <para>
                The LLVM backend now supports 128bit SIMD
                operations. This is now exploited in both the
                <literal>vector</literal> and <literal>dph</literal>
                packages, exposing a high level interface.

                TODO FIXME: reference.
           </para>
            <para>
                This is only available with the LLVM backend.
           </para>
       </listitem>

        <listitem>
            <para>
                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.
           </para>

            <para>
                The old code generator has been removed completely.
           </para>
       </listitem>

        <listitem>
            <para>
                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.

                TODO FIXME: reference.
           </para>
       </listitem>

        <listitem>
            <para>
                PrimOps for comparing unboxed values now return
                <literal>Int#</literal> instead of <literal>Bool</literal>.
                New PrimOps' names end with <literal>$#</literal> for operators and
                <literal>I#</literal> for ordinary names, e.g. <literal>==$#</literal>
                compares <literal>Int#</literal>s for equality and
                <literal>eqCharI#</literal> does the same for <literal>Char#</literal>s.
                Old PrimOps have been removed and turned into wrappers. See
                <ulink url="http://ghc.haskell.org/trac/ghc/wiki/NewPrimopsInGHC7.8">
                this GHC wiki page</ulink> for instructions how to update your
                existing code.
           </para>
       </listitem>

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

        <listitem>
            <para>
                TODO: mention dynamic changes
           </para>
       </listitem>

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

        <listitem>
            <para>
                GHC now has a parallel compilation driver. When
                compiling with <literal>--make</literal> (which is on
                by default,) you may also specify
                <literal>-jN</literal> in order to compile
                <replaceable>N</replaceable> modules in parallel.
           </para>
       </listitem>

        <listitem>
            <para>
                GHC now generates warnings when definitions conflict with the
                Applicative-Monad Proposal (AMP).

                TODO FIXME: reference.
           </para>

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

            <para>
                The warnings are enabled by default, and can be controlled
                using the new flag <literal>-f[no-]warn-amp</literal>.
           </para>
       </listitem>
   </itemizedlist>
  </sect2>

  <sect2>
    <title>Full details</title>
    <sect3>
        <title>Language</title>
        <itemizedlist>
            <listitem>
                <para>
                    There is a new extension,
                    <literal>NullaryTypeClasses</literal>, which
                    allows you to declare a type class without any
                    parameters.
               </para>
           </listitem>
       </itemizedlist>

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

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

        <itemizedlist>
            <listitem>
                <para>
                     The <literal>IncoherentInstances</literal>
                     extension has seen a behavioral change, and is
                     now 'liberated' and less conservative during
                     instance resolution. This allows more programs to
                     compile than before.
               </para>
                <para>
                     Now, <literal>IncoherentInstances</literal> will
                     always pick an arbitrary matching instance, if
                     multiple ones exist.
               </para>
           </listitem>
       </itemizedlist>


    </sect3>

    <sect3>
        <title>Compiler</title>
        <itemizedlist>
            <listitem>
                <para>
                    GHC can now build both static and dynamic object
                    files at the same time in a single compilation
                    pass, when given the
                    <literal>-dynamic-too</literal> flag. This will
                    produce both a statically-linkable
                    <literal>.o</literal> object file, and a
                    dynamically-linkable <literal>.dyn_o</literal>
                    file. The output suffix of the dynamic objects can
                    be controlled by the flag
                    <literal>-dynosuf</literal>.
               </para>

                <para>
                    Note that GHC still builds statically by default.
               </para>
           </listitem>
            <listitem>
                <para>
                    GHC now supports a <literal>--show-options</literal> flag,
                    which will dump all of the flags it supports to standard out.
               </para>
           </listitem>
            <listitem>
                <para>
                    GHC now supports warning about overflow of integer
                    literals, enabled by
                    <literal>-fwarn-overflowed-literals</literal>. It
                    is enabled by default
               </para>
           </listitem>
            <listitem>
                <para>
                    It's now possible to switch the system linker on Linux
                    (between GNU gold and GNU ld) at runtime without problem.
               </para>
           </listitem>
            <listitem>
                <para>
                    The <literal>-fwarn-dodgy-imports</literal> flag now warns
                    in the case an <literal>import</literal> statement hides an
                    entity which is not exported.
               </para>
           </listitem>
            <listitem>
                <para>
                    The LLVM backend was overhauled and rewritten, and
                    should hopefully be easier to maintain and work on
                    in the future.
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>GHCi</title>
        <itemizedlist>
            <listitem>
                <para>
                    GHCi now supports a <literal>prompt2</literal>
                    setting, which allows you to customize the
                    continuation prompt of multi-line input.

                    TODO FIXME: reference.
               </para>
           </listitem>
           <listitem>
                <para>
                    The new <literal>:shows paths</literal> command
                    shows the current working directory and the
                    current search path for Haskell modules.
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>Template Haskell</title>
        <itemizedlist>
            <listitem>
                <para>
                    TODO FIXME
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>Runtime system</title>
        <itemizedlist>
            <listitem>
                <para>
                    The RTS linker can now unload object code at
                    runtime (when using the GHC API
                    <literal>ObjLink</literal> module.) Previously,
                    GHC would not unload the old object file, causing
                    a gradual memory leak as more objects were loaded
                    over time.
               </para>

                <para>
                    Note that this change in unloading behavior
                    <emphasis>only</emphasis> affects statically
                    linked binaries, and not dynamic ones.
               </para>
           </listitem>

            <listitem>
                <para>
                    The performance of <literal>StablePtr</literal>s and
                    <literal>StableName</literal>s has been improved.
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>Build system</title>
        <itemizedlist>
            <listitem>
                <para>
                    GHC >= 7.4 is now required for bootstrapping.
               </para>
           </listitem>
            <listitem>
                <para>
                    GHC can now be built with Clang, and use Clang as the
                    preprocessor for Haskell code. Only Clang version 3.4svn is
                    reliably supported.
               </para>
           </listitem>
       </itemizedlist>
    </sect3>
  </sect2>

  <sect2>
    <title>Libraries</title>

    <para>
        There have been some changes that have effected multiple
        libraries:
    </para>

     <itemizedlist>
         <listitem>
             <para>
                 TODO FIXME
            </para>
        </listitem>
    </itemizedlist>

    <para>
        The following libraries have been removed from the GHC tree:
    </para>

     <itemizedlist>
         <listitem>
             <para>TODO FIXME</para>
        </listitem>
    </itemizedlist>

    <para>
        The following libraries have been added to the GHC tree:
    </para>

     <itemizedlist>
         <listitem>
             <para>TODO FIXME</para>
        </listitem>
    </itemizedlist>

    <sect3>
        <title>array</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>base</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number 4.7.0.0 (was 4.6.0.1)
               </para>
           </listitem>
            <listitem>
                <para>
                    The <literal>Control.Category</literal> module now has the
                    <literal>PolyKinds</literal> extension enabled, meaning
                    that instances of <literal>Category</literal> no longer
                    need be of kind <literal>* -> * -> *</literal>
               </para>
           </listitem>
            <listitem>
                <para>
                    There are now <literal>Foldable</literal> and <literal>Traversable</literal>
                    instances for <literal>Either a</literal> and <literal>(,) a</literal>
               </para>
           </listitem>
            <listitem>
                <para>
                    There is now a <literal>Data</literal> instance for <literal>Data.Version</literal>
               </para>
           </listitem>
             <listitem>
                <para>
                    <literal>Control.Concurrent.MVar</literal> has a new
                    implementation of <literal>readMVar</literal>, which
                    fixes a long-standing bug where
                    <literal>readMVar</literal> is only atomic if there
                    are no other threads running
                    <literal>putMVar</literal>.
                    <literal>readMVar</literal> now is atomic, and is
                    guaranteed to return the value from the first
                    <literal>putMVar</literal>.  There is also a new <literal>tryReadMVar</literal>
                    which is a non-blocking version.
               </para>
           </listitem>
            <listitem>
                <para>
                    There are now byte endian-swapping primitives
                    available in <literal>Data.Word</literal>, which
                    use optimized machine instructions when available.
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>bin-package-db</title>
        <itemizedlist>
            <listitem>
                <para>
                    This is an internal package, and should not be used.
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>binary</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>bytestring</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>Cabal</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
              </para>
          </listitem>
      </itemizedlist>
    </sect3>

    <sect3>
        <title>containers</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>deepseq</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>directory</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>filepath</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>ghc-prim</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>haskell98</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>haskell2010</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>hoopl</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>hpc</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>integer-gmp</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>old-locale</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>old-time</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>process</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>template-haskell</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>time</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>unix</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number 2.7.0.0 (was 2.6.0.0)
               </para>
           </listitem>
            <listitem>
                <para>
                    A crash in <literal>getGroupEntryForID</literal>
                    (and related functions like
                    <literal>getUserEntryForID</literal> and
                    <literal>getUserEntryForName</literal>) in
                    multi-threaded applications has been fixed.
               </para>
           </listitem>
            <listitem>
                <para>
                    The functions <literal>getGroupEntryForID</literal>
                    and <literal>getUserEntryForID</literal> now fail
                    with a <literal>isDoesNotExist</literal> error when
                    the specified ID cannot be found.
               </para>
           </listitem>
       </itemizedlist>
    </sect3>

    <sect3>
        <title>Win32</title>
        <itemizedlist>
            <listitem>
                <para>
                    Version number XXXX (was XXXX)
               </para>
           </listitem>
       </itemizedlist>
    </sect3>
  </sect2>
</sect1>