path: root/docs/users_guide/6.14.1-notes.xml

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<sect1 id="release-6-14-1">
  <title>Release notes for version 6.14.1</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 6.12 branch.
  </para>

  <sect2>
    <title>Highlights</title>
    <itemizedlist>
      <listitem>
        <para>
          GHC now defaults to the Haskell 2010 language standard.
        </para>

        <para>
          Libraries are not quite so straightforward.  By default, GHC
          provides access to the <literal>base</literal> package,
          which includes the Haskell 2010 libraries, albeit with a few
          minor differences.  For those who want to write strictly
          standards-conforming code we also provide
          the <literal>haskell2010</literal> package which provides
          the precise APIs specified by Haskell 2010, but because the
          module names in this package overlap with those in
          the <literal>base</literal> package it is not possible to
          use both <literal>haskell2010</literal>
          and <literal>base</literal> at the same time (this also
          applies to the <literal>array</literal> package).  Hence to use
          the Haskell 2010 libraries you should hide
          the <literal>base</literal> and <literal>array</literal>
          packages, for example with GHCi:
<screen>
$ ghci -package haskell2010 -hide-package base -hide-package array
</screen>
          If you are using Cabal it isn't necessary to
          hide <literal>base</literal> and <literal>array</literal>
          explicitly, just don't include them in your <literal>build-depends</literal>.
        </para>
      </listitem>

      <listitem>
        <para>
          On POSIX platforms, there is a new I/O manager based on
          epoll/kqueue/poll, which allows multithreaded I/O code to
          scale to a much larger number (100k+) of threads.
        </para>
      </listitem>

      <listitem>
        <para>
          The inliner has been overhauled, which should in general
          give better performance while reducing unnecessary code-size
          explosion.
        </para>
      </listitem>

      <listitem>
        <para>
          Large parts of the runtime system have been overhauled, in
          particular the machinery related to blocking and wakeup of
          threads and exception throwing (<literal>throwTo</literal>).
          Several instances of pathological performance have been
          fixed, especially where large numbers of threads are
          involved.
        </para>
      </listitem>

      <listitem>
        <para>
          Due to changes in the runtime system, if you are
          using <literal>Control.Parallel.Strategies</literal> from
          the <literal>parallel</literal> package, please upgrade to
          at least version 2 (preferably version 3).  The
          implementation of Strategies
          in <literal>parallel-1.x</literal> will lose parallelism
          with GHC 6.14.1.
        </para>
      </listitem>

      <listitem>
        <para>
          The full Haskell <literal>import</literal> syntax can now been
          used to bring modules into scope in GHCi, e.g.
        </para>
<programlisting>
Prelude> import Data.List as L
Prelude Data.List> L.length "foo"
3
</programlisting>
      </listitem>

      <listitem>
        <para>
          GHC now comes with a more recent mingw bundled on Windows,
          which includes a fix for windres on Windows 7.
        </para>
      </listitem>
    </itemizedlist>
  </sect2>

  <sect2>
    <title>Language changes</title>
    <itemizedlist>
      <listitem>
        <para>
          GHC now understands the <literal>Haskell98</literal> and
          <literal>Haskell2010</literal> languages.
        </para>

        <para>
          These get processed before the language extension pragmas,
          and define the default sets of extensions that are enabled.
          If neither is specified, then the default is
          <literal>Haskell2010</literal> plus the
          <literal>MonoPatBinds</literal> extension.
        </para>
      </listitem>

      <listitem>
        <para>
          GHC now supports the <literal>DoAndIfThenElse</literal>
          extension, which is part of the Haskell 2010 standard.
        </para>
      </listitem>

      <listitem>
        <para>
          Datatype contexts, such as the <literal>Eq a</literal> in
        </para>
<programlisting>
data Eq a => Set a = NilSet | ConsSet a (Set a)
</programlisting>
        <para>
          are now treated as an extension
          <literal>DatatypeContexts</literal> (on by default) by GHC.
        </para>
      </listitem>

      <listitem>
        <para>
          GHC's support for unicode source has been improved, including
          removing support for U+22EF for the <literal>..</literal>
          symbol. See <xref linkend="unicode-syntax" /> for more details.
        </para>
      </listitem>

      <listitem>
        <para>
          Pragmas are now reread after preprocessing. In particular,
          this means that if a pragma is used to turn CPP on, then other
          pragmas can be put in CPP conditionals.
        </para>
      </listitem>

      <listitem>
        <para>
          The <literal>TypeOperators</literal> extension now allows
          instance heads to use infix syntax.
        </para>
      </listitem>

      <listitem>
        <para>
          The <literal>PackageImports</literal> extension now understands
          <literal>this</literal> to mean the current package.
        </para>
      </listitem>

      <listitem>
        <para>
          The <literal>INLINE</literal> and <literal>NOINLINE</literal>
          pragmas can now take a <literal>CONLIKE</literal> modifier,
          which indicates that the right hand side is cheap to compute,
          and can thus be duplicated more freely.
          See <xref linkend="conlike" /> for more details.
        </para>
      </listitem>

      <listitem>
        <para>
          A <literal>ForceSpecConstr</literal> annotation on a type, e.g.
        </para>
<programlisting>
import SpecConstr
{-# ANN type SPEC ForceSpecConstr #-}
</programlisting>
        <para>
          can be used to force GHC to fully specialise argument of that
          type.
        </para>
      </listitem>

      <listitem>
        <para>
          A <literal>NoSpecConstr</literal> annotation on a type, e.g.
        </para>
<programlisting>
import SpecConstr
{-# ANN type T NoSpecConstr #-}
</programlisting>
        <para>
          can be used to prevent SpecConstr from specialising on
          arguments of that type.
        </para>
      </listitem>

      <listitem>
        <para>
          There is are two experimental new extensions
          <literal>AlternativeLayoutRule</literal> and
          <literal>AlternativeLayoutRuleTransitional</literal>,
          which are for exploring alternative layout rules in Haskell'.
          The details are subject to change, so we advise against using
          them in real code for now.
        </para>
      </listitem>

      <listitem>
        <para>
          The <literal>NewQualifiedOperators</literal> extension has
          been deprecated, as it was rejected by the Haskell' committee.
        </para>
      </listitem>
    </itemizedlist>
  </sect2>

  <sect2>
    <title>Warnings</title>
    <itemizedlist>
      <listitem>
        <para>
          There is now a warning for missing import lists, controlled
          by the new <literal>-fwarn-missing-import-lists</literal> flag.
        </para>
      </listitem>

      <listitem>
        <para>
          GHC will now warn about <literal>SPECIALISE</literal> and
          <literal>UNPACK</literal> pragmas that have no effect.
        </para>
      </listitem>
    </itemizedlist>
  </sect2>

  <sect2>
    <title>DLLs</title>
    <itemizedlist>
      <listitem>
        <para>
          Shared libraries are once again supported on Windows.
        </para>
      </listitem>

      <listitem>
        <para>
          Shared libraries are now supported on OS X, both on x86 and on
          PowerPC. The new <literal>-dylib-install-name</literal> GHC
          flag is used to set the location of the dynamic library.
          See <xref linkend="finding-shared-libs" /> for more details.
        </para>
      </listitem>
    </itemizedlist>
  </sect2>

  <sect2>
    <title>Runtime system</title>

    <itemizedlist>
      <listitem>
        <para>
          For security reasons, by default, the only RTS flag that
          programs accept is <literal>+RTS --info</literal>. If you want
          the full range of RTS flags then you need to link with the new
          <literal>-rtsopts</literal> flag. See
          <xref linkend="options-linker" /> for more details.
        </para>
      </listitem>

      <listitem>
        <para>
          The RTS now exports a function <literal>setKeepCAFs</literal>
          which is important when loading Haskell DLLs dynamically, as
          a DLL may refer to CAFs that hae already been GCed.
        </para>
      </listitem>

      <listitem>
        <para>
          The garbage collector no longer allows you to specify a number
          of steps; there are now always 2. The <literal>-T</literal>
          RTS flag has thus been removed.
        </para>
      </listitem>

      <listitem>
        <para>
          A new RTS flag <literal>-H</literal> causes the RTS to use a
          larger nursery, but without exceeding the amount of memory
          that the application is already using. It makes some programs
          go slower, but others go faster.
        </para>
      </listitem>

      <listitem>
        <para>
          GHC now returns memory to the OS, if memory usage peaks and
          then drops again. This is mainly useful for long running
          processes which normally use very little memory, but
          occasionally need a lot of memory for a short period of time.
        </para>
      </listitem>

      <listitem>
        <para>
          On OS X, eventLog events are now available as DTrace probes.
        </para>
      </listitem>

      <listitem>
        <para>
          The PAPI support has been improved. The new RTS flag
          <literal>-a#0x40000000</literal> can be used to tell the RTS
          to collect the native PAPI event <literal>0x40000000</literal>.
        </para>
      </listitem>
    </itemizedlist>
  </sect2>

  <sect2>
    <title>Compiler</title>
    <itemizedlist>
      <listitem>
        <para>
          GHC now defaults to <literal>--make</literal> mode, i.e. GHC
          will chase dependencies for you automatically by default.
        </para>
      </listitem>

      <listitem>
        <para>
          GHC now includes an LLVM code generator.
        </para>
        <para>
          This includes a number of new flags:
          a flag to tell GHC to use LLVM, <literal>-fllvm</literal>;
          a flag to dump the LLVM input ,<literal>-ddump-llvm</literal>;
          flags to keep the LLVM intermediate files,
          <literal>-keep-llvm-file</literal> and
          <literal>-keep-llvm-files</literal>;
          flags to set the location and options for the LLVM assembler,
          optimiser and compiler,
          <literal>-pgmla</literal>,
          <literal>-pgmlo</literal>,
          <literal>-pgmlc</literal>,
          <literal>-optla</literal>,
          <literal>-optlo</literal> and
          <literal>-optlc</literal>.
        </para>
      </listitem>

      <listitem>
        <para>
          It is now possible to use <literal>-fno-code</literal> with
          <literal>--make</literal>.
        </para>
      </listitem>

      <listitem>
        <para>
          The new flag <literal>-dsuppress-coercions</literal> controls
          whether GHC prints coercions in core dumps.
        </para>
      </listitem>

      <listitem>
        <para>
          The new flag <literal>-dsuppress-module-prefixes</literal>
          controls whether GHC prints module qualification prefixes
          in core dumps.
        </para>
      </listitem>

      <listitem>
        <para>
          The inliner has been overhauled. The most significant
          user-visible change is that only saturated functions are
          inlined, e.g.
        </para>
<programlisting>
(.) f g x = f (g x)
</programlisting>
        <para>
          would only be inlined if <literal>(.)</literal> is applied to 3
          arguments, while
        </para>
<programlisting>
(.) f g = \x -> f (g x)
</programlisting>
        <para>
          will be inlined if only applied to 2 arguments.
        </para>
      </listitem>

      <listitem>
        <para>
          The <literal>-finline-if-enough-args</literal> flag is no
          longer supported.
        </para>
      </listitem>

      <listitem>
        <para>
          Column numbers in warnings and error messages now start at 1,
          as is more standard, rather than 0.
        </para>
      </listitem>

      <listitem>
        <para>
          GHCi now understands most linker scripts. In particular, this
          means that GHCi is able to load the C pthread library.
        </para>
      </listitem>

      <listitem>
        <para>
          The <literal>ghc --info</literal> output has been updated:
        </para>
        <para>
          It now includes the
          location of the global package database, in the
          <literal>Global Package DB</literal> field.
        </para>
        <para>
          It now includes the build, host and target platforms, in the
          <literal>Build platform</literal>,
          <literal>Host platform</literal> and
          <literal>Target platform</literal> fields.
        </para>
        <para>
          It now includes a <literal>Have llvm code generator</literal>
          field.
        </para>
        <para>
          The <literal>Win32 DLLs</literal> field has been removed.
        </para>
      </listitem>

      <listitem>
        <para>
          The registerised via-C backend, and the
          <literal>-fvia-C</literal> flag, have been deprecated. The poor
          floating-point performance in the x86 native code generator
          has now been fixed, so we don't believe there is still any
          reason to use the via-C backend.
        </para>
      </listitem>

      <listitem>
        <para>
          There is now a new flag <literal>--supported-extensions</literal>,
          which currently behaves the same as
          <literal>--supported-languages</literal>.
        </para>
      </listitem>

      <listitem>
        <para>
          GHC progress output such as
        </para>
<programlisting>
[ 1 of 5] Compiling Foo              ( Foo.hs, Foo.o )
</programlisting>
        <para>
          is now sent to stdout rather than stderr.
        </para>
      </listitem>

      <listitem>
        <para>
          The new flag <literal>-fexpose-all-unfoldings</literal>
          makes GHC put unfoldings for <emphasis>everything</emphasis>
          in the interface file.
        </para>
      </listitem>

      <listitem>
        <para>
          There are two new flags, <literal>-fno-specialise</literal>
          and <literal>-fno-float-in</literal>, for disabling the
          specialise and float-in passes.
        </para>
      </listitem>

      <listitem>
        <para>
          The new flag <literal>-fstrictness-before=<replaceable>n</replaceable></literal> tells
          GHC to run an additional strictness analysis pass
          before simplifier phase <replaceable>n</replaceable>.
        </para>
      </listitem>

      <listitem>
        <para>
          There is a new flag
          <literal>-funfolding-dict-discount</literal>
          for tweaking the optimiser's behaviour.
        </para>
      </listitem>

      <listitem>
        <para>
          The <literal>-fspec-inline-join-points</literal> flag has been
          removed.
        </para>
      </listitem>
    </itemizedlist>
  </sect2>

  <sect2>
    <title>GHCi</title>
    <itemizedlist>
      <listitem>
        <para>
          GHCi now understands layout in multi-line commands, so
          this now works:
        </para>
<programlisting>
Prelude> :{
Prelude| let x = 1
Prelude|     y = 2 in x + y
Prelude| :}
3
</programlisting>
      </listitem>
    </itemizedlist>
  </sect2>

  <sect2>
    <title>Template Haskell and Quasi-Quoters</title>
    <itemizedlist>
      <listitem>
        <para>
          It is now possible to quasi-quote patterns with
          <literal>[p| ... |]</literal>.
        </para>
      </listitem>

      <listitem>
        <para>
          It is no longer to use a <literal>$</literal> before the
          name of a quasi-quoter, e.g. one can now say
          <literal>[expr| ... |]</literal> rather than
          <literal>[$expr| ... |]</literal>.
        </para>
      </listitem>

      <listitem>
        <para>
          It is now possible to use a quasi-quoter for types, e.g.
          <literal>f :: [$qq| ... |]</literal>
        </para>
      </listitem>

      <listitem>
        <para>
          It is now possible to quasi-quote existentials and GADTs.
        </para>
      </listitem>
    </itemizedlist>
  </sect2>

  <sect2>
    <title>GHC API</title>
    <itemizedlist>
      <listitem>
        <para>
          There are now <literal>Data</literal> and
          <literal>Typeable</literal> instances for the
          HsSyn typed.
        </para>
      </listitem>

      <listitem>
        <para>
          As language extensions are not applied until after the base
          language (Haskell98, Haskell2010 or the default) has been
          selected, it is now necessary to tell the GHC API the point
          at which the extension flags should be processed. Normally
          this is done by calling
          <literal>DynFlags.flattenExtensionFlags</literal> once all
          the flags and pragmas have been read.
        </para>
      </listitem>
    </itemizedlist>
  </sect2>

<!--
  <sect2>
    <title>Libraries</title>

    <itemizedlist>
    </itemizedlist>
  </sect2>
-->
</sect1>