-<Chapter id="introduction-GHC">
-<Title>Introduction to GHC
-</Title>
-
-<Para>
-This is a guide to using the Glasgow Haskell compilation (GHC) system. It is
-a batch compiler for the Haskell 98 language, with support for various
-Glasgow-only extensions. In this document, we assume that GHC has been
-installed at your site as <Literal>ghc</Literal>. A separate document,
-“Building and Installing the Glasgow Functional Programming Tools Suite”,
-describes how to install <Literal>ghc</Literal>.
-</Para>
-
-<Para>
-Many people will use GHC very simply: compile some
-modules—<Literal>ghc -c -O Foo.hs Bar.hs</Literal>; and link them—
-<Literal>ghc -o wiggle -O Foo.o Bar.o</Literal>.
-</Para>
-
-<Para>
-But if you need to do something more complicated, GHC can do that,
-too:
-
-<Screen>
-ghc -c -O -fno-foldr-build -dcore-lint -fvia-C -ddump-simpl Foo.lhs
-</Screen>
-
-Stay tuned—all will be revealed!
-</Para>
-
-<Para>
-The rest of this section provide some tutorial information
-on batch-style compilation; if you're familiar with these concepts
-already, then feel free to skip to the next section.
-</Para>
-
-<Sect1 id="batch-system-parts">
-<Title>The (batch) compilation system components</Title>
-
-<Para>
-The Glorious Haskell Compilation System, as with most UNIX (batch)
-compilation systems, has several interacting parts:
-
-<OrderedList>
-<ListItem>
-<Para>
-A <Emphasis>driver</Emphasis><IndexTerm><Primary>driver program</Primary></IndexTerm>
-<Literal>ghc</Literal><IndexTerm><Primary>ghc</Primary></IndexTerm>—which
-you usually think of as “the compiler”—is a program that merely
-invokes/glues-together the other pieces of the system (listed below),
-passing the right options to each, slurping in the right libraries, etc.
-</Para>
-</ListItem>
-
-<ListItem>
-<Para>
-A <Emphasis>literate pre-processor</Emphasis>
-<IndexTerm><Primary>literate pre-processor</Primary></IndexTerm>
-<IndexTerm><Primary>pre-processor, literate</Primary></IndexTerm>
-<Literal>unlit</Literal><IndexTerm><Primary>unlit</Primary></IndexTerm> that extracts Haskell
-code from a literate script; used if you believe in that sort of
-thing.
-</Para>
-</ListItem>
-
-<ListItem>
-<Para>
-The <Emphasis>Haskellised C pre-processor</Emphasis>
-<IndexTerm><Primary>Haskellised C pre-processor</Primary></IndexTerm>
-<IndexTerm><Primary>C pre-processor, Haskellised</Primary></IndexTerm>
-<IndexTerm><Primary>pre-processor, Haskellised C</Primary></IndexTerm>
-<Literal>hscpp</Literal>,<IndexTerm><Primary>hscpp</Primary></IndexTerm> only needed by people requiring conditional
-compilation, probably for large systems. The “Haskellised” part
-just means that <Literal>#line</Literal> directives in the output have been
-converted into proper Haskell <Literal>{-# LINE ... -}</Literal> pragmas. You must give an explicit <Literal>-cpp</Literal> option
-<IndexTerm><Primary>-cpp option</Primary></IndexTerm> for the C pre-processor to be invoked.
-</Para>
-</ListItem>
-
-<ListItem>
-<Para>
-The <Emphasis>Haskell compiler</Emphasis>
-<IndexTerm><Primary>Haskell compiler</Primary></IndexTerm>
-<IndexTerm><Primary>compiler, Haskell</Primary></IndexTerm>
-<Literal>hsc</Literal>,<IndexTerm><Primary>hsc</Primary></IndexTerm>
-which—in normal use—takes its input from the C pre-processor
-and produces assembly-language output (sometimes: ANSI C output).
-</Para>
-</ListItem>
-
-<ListItem>
-<Para>
-The <Emphasis>ANSI C Haskell high-level assembler :-)</Emphasis>
-<IndexTerm><Primary>ANSI C compiler</Primary></IndexTerm>
-<IndexTerm><Primary>high-level assembler</Primary></IndexTerm>
-<IndexTerm><Primary>assembler, high-level</Primary></IndexTerm>
-compiles <Literal>hsc</Literal>'s C output into assembly language for a particular
-target architecture. In fact, the only C compiler we currently
-support is <Literal>gcc</Literal>, because we make use of certain extensions to the
-C language only supported by gcc. Version 2.x is a must; we recommend
-version 2.7.2.1 for stability (we've heard both good and bad reports
-of later versions).
-</Para>
-</ListItem>
-
-<ListItem>
-<Para>
-The <Emphasis>assembler</Emphasis><IndexTerm><Primary>assembler</Primary></IndexTerm>—a standard UNIX one, probably
-<Literal>as</Literal><IndexTerm><Primary>as</Primary></IndexTerm>.
-</Para>
-</ListItem>
-
-<ListItem>
-<Para>
-The <Emphasis>linker</Emphasis><IndexTerm><Primary>linker</Primary></IndexTerm>—a standard UNIX one, probably
-<Literal>ld</Literal>.<IndexTerm><Primary>ld</Primary></IndexTerm>
-</Para>
-</ListItem>
-
-<ListItem>
-<Para>
-A <Emphasis>runtime system</Emphasis>,<IndexTerm><Primary>runtime system</Primary></IndexTerm> including (most notably)
-a storage manager; the linker links in the code for this.
-</Para>
-</ListItem>
-
-<ListItem>
-<Para>
-The <Emphasis>Haskell standard prelude</Emphasis><IndexTerm><Primary>standard prelude</Primary></IndexTerm>, a
-large library of standard functions, is linked in as well.
-</Para>
-</ListItem>
-
-<ListItem>
-<Para>
-Parts of other <Emphasis>installed libraries</Emphasis> that you have at your site may be linked in also.
-</Para>
-</ListItem>
-
-</OrderedList>
-</Para>
-
-</Sect1>
-
-<Sect1 id="compile-what-really-happens">
-<Title>What really happens when I “compile” a Haskell program?
-</Title>
-
-<Para>
-You invoke the Glasgow Haskell compilation system through the
-driver program <Literal>ghc</Literal>.<IndexTerm><Primary>ghc</Primary></IndexTerm> For example, if you had typed a
-literate “Hello, world!” program into <Literal>hello.lhs</Literal>, and you then
-invoked:
-
-<Screen>
-ghc hello.lhs
-</Screen>
-
-</Para>
-
-<Para>
-the following would happen:
-
-<OrderedList>
-<ListItem>
-
-<Para>
-The file <Literal>hello.lhs</Literal> is run through the literate-program
-code extractor <Literal>unlit</Literal><IndexTerm><Primary>unlit</Primary></IndexTerm>, feeding its output to
-
-</Para>
-</ListItem>
-<ListItem>
-
-<Para>
-The Haskell compiler proper <Literal>hsc</Literal><IndexTerm><Primary>hsc</Primary></IndexTerm>, which produces
-input for
-
-</Para>
-</ListItem>
-<ListItem>
-
-<Para>
-The assembler (or that ubiquitous “high-level assembler,” a C
-compiler), which produces an object file and passes it to
-
-</Para>
-</ListItem>
-<ListItem>
-
-<Para>
-The linker, which links your code with the appropriate libraries
-(including the standard prelude), producing an executable program in
-the default output file named <Literal>a.out</Literal>.
-</Para>
-</ListItem>
-
-</OrderedList>
-
-</Para>
-
-<Para>
-You have considerable control over the compilation process. You feed
-command-line arguments (call them “options,” for short) to the
-driver, <Literal>ghc</Literal>; the “types” of the input files (as encoded in
-their names' suffixes) also matter.
-</Para>
-
-<Para>
-Here's hoping this is enough background so that you can read the rest
-of this guide!
-</Para>
-
-</Sect1>
-
-<Sect1 id="mailing-lists-GHC">
-<Title>Meta-information: Web sites, mailing lists, etc.
-</Title>
-
-<Para>
-<IndexTerm><Primary>mailing lists, Glasgow Haskell</Primary></IndexTerm>
-<IndexTerm><Primary>Glasgow Haskell mailing lists</Primary></IndexTerm>
-</Para>
-
-<Para>
-On the World-Wide Web, there are several URLs of likely interest:
-</Para>
-
-<Para>
-
-<ItemizedList>
-<ListItem>
-
-<Para>
- <ULink
-URL="http://www.haskell.org/"
->Haskell home page</ULink
->
-</Para>
-</ListItem>
-<ListItem>
-
-<Para>
- <ULink
-URL="http://www.haskell.org/ghc/"
->GHC home page</ULink
->
-</Para>
-</ListItem>
-<ListItem>
-
-<Para>
- <ULink
-URL="http://www.cs.nott.ac.uk/Department/Staff/mpj/faq.html"
->comp.lang.functional FAQ</ULink
->
-</Para>
-</ListItem>
-
-</ItemizedList>
-
-</Para>
-
-<Para>
-We run two mailing lists about Glasgow Haskell. We encourage you to
-join, as you feel is appropriate.
-</Para>
-
-<Para>
-<VariableList>
-
-<VarListEntry>
-<Term>glasgow-haskell-users:</Term>
-<ListItem>
-<Para>
-This list is for GHC users to chat among themselves. Subscribe by
-sending mail to <email>majordomo@haskell.org</email>, with a message
-body (not header) like this:
-</Para>
-
-<Para>
-
-<Screen>
-subscribe glasgow-haskell-users MyName <m.y.self@bigbucks.com>
-</Screen>
-
-</Para>
-
-<Para>
-(The last bit is your all-important e-mail address, of course.)
-</Para>
-
-<Para>
-To communicate with your fellow users, send mail to <email>glasgow-haskell-users@haskell.org</email>.
-</Para>
-
-<Para>
-To contact the list administrator, send mail to
-<email>owner-glasgow-haskell-users@haskell.org</email>. An archive
-of the list is available on the Web at the <ULink
-URL="http://www.mail-archive.com/glasgow-haskell-users@haskell.org">glasgow-haskell-users
-mailing list archive</ULink>.
-</Para>
-</ListItem>
-</VarListEntry>
-<VarListEntry>
-<Term>glasgow-haskell-bugs:</Term>
-<ListItem>
-<Para>
-Send bug reports for GHC to this address! The sad and lonely people
-who subscribe to this list will muse upon what's wrong and what you
-might do about it.
-</Para>
-
-<Para>
-Subscribe via <email>majordomo@haskell.org</email> with:
-</Para>
-
-<Para>
-
-<Screen>
-subscribe glasgow-haskell-bugs My Name <m.y.self@hackers.r.us>
-</Screen>
-
-</Para>
-
-<Para>
-Again, you may contact the list administrator at <email>owner-glasgow-haskell-bugs@haskell.org</email>.
-And, yes, an archive of the list is available on the Web at the <ULink
-URL="http://www.mail-archive.com/glasgow-haskell-bugs@haskell.org">
-glasgow-haskell-bugs mailing list archive</ULink
->
-</Para>
-</ListItem>
-</VarListEntry>
-</VariableList>
-</Para>
-
-<Para>
-There is also the general Haskell mailing list. Subscribe by sending
-email to <email>majordomo@haskell.org</email>, with the usual message body:
-</Para>
-
-<Para>
-
-<Screen>
-subscribe haskell My Name <m.y.self@fp.rules.ok.org>
-</Screen>
-
-</Para>
-
-<Para>
-Some Haskell-related discussion takes place in the Usenet newsgroup
-<Literal>comp.lang.functional</Literal>.
-</Para>
-
-</Sect1>
+<chapter id="introduction-GHC">
+ <title>Introduction to GHC</title>
+
+ <para>This is a guide to using the Glasgow Haskell Compiler (GHC):
+ an interactive and batch compilation system for the <ulink
+ url="http://www.haskell.org/">Haskell 98</ulink>
+ language.</para>
+
+ <para>GHC has two main components: an interactive Haskell
+ interpreter (also known as GHCi), described in <xref
+ linkend="ghci">, and a batch compiler, described throughout <xref
+ linkend="using-ghc">. In fact, GHC consists of a single program
+ which is just run with different options to provide either the
+ interactive or the batch system.</para>
+
+ <para>The batch compiler can be used alongside GHCi: compiled
+ modules can be loaded into an interactive session and used in the
+ same way as interpreted code, and in fact when using GHCi most of
+ the library code will be pre-compiled. This means you get the best
+ of both worlds: fast pre-compiled library code, and fast compile
+ turnaround for the parts of your program being actively
+ developed.</para>
+
+ <para>GHC supports numerous language extensions, including
+ concurrency, a foreign function interface, exceptions, type system
+ extensions such as multi-parameter type classes, local universal and
+ existential quantification, functional dependencies, scoped type
+ variables and explicit unboxed types. These are all described in
+ <xref linkend="ghc-language-features">.</para>
+
+ <para>GHC has a comprehensive optimiser, so when you want to Really
+ Go For It (and you've got time to spare) GHC can produce pretty fast
+ code. Alternatively, the default option is to compile as fast as
+ possible while not making too much effort to optimise the generated
+ code (although GHC probably isn't what you'd describe as a fast
+ compiler :-).</para>
+
+ <para>GHC's profiling system supports “cost centre
+ stacks”: a way of seeing the profile of a Haskell program in a
+ call-graph like structure. See <xref linkend="profiling"> for more
+ details.</para>
+
+ <para>GHC comes with a large collection of libraries, with
+ everything from parser combinators to networking. These are all
+ described in <xref linkend="book-hslibs">.</para>
+
+ <Sect1 id="mailing-lists-GHC">
+ <Title>Meta-information: Web sites, mailing lists, etc.</Title>
+
+ <IndexTerm><Primary>mailing lists, Glasgow Haskell</Primary></IndexTerm>
+ <IndexTerm><Primary>Glasgow Haskell mailing lists</Primary></IndexTerm>
+
+ <para>On the World-Wide Web, there are several URLs of likely
+ interest:</para>
+
+ <itemizedlist>
+ <listitem>
+ <para><ulink URL="http://www.haskell.org/" >Haskell home
+ page</ulink></para>
+ </listitem>
+
+ <listitem>
+ <para><ulink URL="http://www.haskell.org/ghc/">GHC home
+ page</ulink></para>
+ </listitem>
+
+ <listitem>
+ <para><ulink
+ URL="http://www.cs.nott.ac.uk/Department/Staff/mpj/faq.html">comp.lang.functional
+ FAQ</ulink></para>
+ </listitem>
+
+ </itemizedlist>
+
+ <para>We run the following mailing lists about Glasgow Haskell.
+ We encourage you to join, as you feel is appropriate.</para>
+
+ <variablelist>
+ <varlistentry>
+ <term>glasgow-haskell-users:</term>
+ <listitem>
+ <para>This list is for GHC users to chat among
+ themselves.</para>
+
+ <variablelist>
+ <varlistentry>
+ <term>list email address:</term>
+ <listitem>
+ <para><email>glasgow-haskell-users@haskell.org</email></para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>subscribe at:</term>
+ <listitem>
+ <para><ulink
+ url="http://www.haskell.org/mailman/listinfo/glasgow-haskell-users"><literal>http://www.haskell.org/mailman/listinfo/glasgow-haskell-users</literal></ulink>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>admin email address:</term>
+ <listitem>
+ <para><email>glasgow-haskell-users-admin@haskell.org</email></para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>list archives:</term>
+ <listitem>
+ <para><ulink
+ url="http://www.haskell.org/pipermail/glasgow-haskell-users/"><literal>http://www.haskell.org/pipermail/glasgow-haskell-users/</literal></ulink></para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>glasgow-haskell-bugs:</term>
+ <listitem>
+ <para>Send bug reports for GHC to this address! The sad and
+ lonely people who subscribe to this list will muse upon
+ what's wrong and what you might do about it.</para>
+
+ <variablelist>
+ <varlistentry>
+ <term>list email address:</term>
+ <listitem>
+ <para><email>glasgow-haskell-bugs@haskell.org</email></para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>subscribe at:</term>
+ <listitem>
+ <para><ulink
+ url="http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs"><literal>http://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs</literal></ulink>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>admin email address:</term>
+ <listitem>
+ <para><email>glasgow-haskell-bugs-admin@haskell.org</email></para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>list archives:</term>
+ <listitem>
+ <para><ulink
+ url="http://www.haskell.org/pipermail/glasgow-haskell-bugs/"><literal>http://www.haskell.org/pipermail/glasgow-haskell-bugs/</literal></ulink></para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>cvs-ghc:</term>
+ <listitem>
+ <para>The hardcore GHC developers hang out here. This list
+ also gets commit message from the CVS repository. There are
+ several other similar lists for other parts of the CVS
+ repository (eg. <literal>cvs-hslibs</literal>,
+ <literal>cvs-happy</literal>, <literal>cvs-hdirect</literal>
+ etc.)</para>
+
+ <variablelist>
+ <varlistentry>
+ <term>list email address:</term>
+ <listitem>
+ <para><email>cvs-ghc@haskell.org</email></para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>subscribe at:</term>
+ <listitem>
+ <para><ulink
+ url="http://www.haskell.org/mailman/listinfo/cvs-ghc"><literal>http://www.haskell.org/mailman/listinfo/cvs-ghc</literal></ulink>.</para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>admin email address:</term>
+ <listitem>
+ <para><email>cvs-ghc-admin@haskell.org</email></para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>list archives:</term>
+ <listitem>
+ <para><ulink
+ url="http://www.haskell.org/pipermail/cvs-ghc/"><literal>http://www.haskell.org/pipermail/cvs-ghc/</literal></ulink></para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ <para>There are several other haskell and GHC-related mailing
+ lists served by <literal>www.haskell.org</literal>. Go to <ulink
+ url="http://www.haskell.org/mailman/listinfo/"><literal>http://www.haskell.org/mailman/listinfo/</literal></ulink>
+ for the full list.</para>
+
+ <para>Some Haskell-related discussion also takes place in the
+ Usenet newsgroup <literal>comp.lang.functional</literal>.</para>
+
+ </sect1>
+
+ <sect1 id="bug-reporting">
+ <title>Reporting bugs in GHC</title>
+ <indexterm><primary>bugs</primary><secondary>reporting</secondary>
+ </indexterm>
+ <indexterm><primary>reporting bugs</primary>
+ </indexterm>
+
+ <para>Glasgow Haskell is a changing system so there are sure to be
+ bugs in it. </para>
+
+ <para>To repot a bug, either:</para>
+
+ <itemizedlist>
+ <listitem>
+ <para>Go to the <ulink
+ url="http://sourceforge.net/projects/ghc/">SoureForge GHC
+ page</ulink>, go to the <quote>bugs</quote> section, click on
+ <quote>submit</quote>, and enter your bug report. You can also
+ check the outstanding bugs here to make sure it hasn't already
+ been reported. Or:</para>
+ </listitem>
+ <listitem>
+ <para>Email your bug report to
+ <Email>glasgow-haskell-bugs@haskell.org</Email>. </para>
+ </listitem>
+ </itemizedlist>
+
+ <sect2>
+ <title>What to put in a bug report</title>
+ <indexterm><primary>bug reports</primary><secondary>contents</secondary></indexterm>
+
+ <para>The name of the bug-reporting game is: facts, facts,
+ facts. Don't omit them because “Oh, they won't be
+ interested…”</para>
+
+ <orderedlist>
+ <listitem>
+ <para>What kind of machine are you running on, and exactly
+ what version of the operating system are you using?
+ (on a Unix system, <Command>uname -a</Command> or <Command>cat
+ /etc/motd</Command> will show the desired information.)</para>
+ </listitem>
+
+ <listitem>
+ <para> What version of GCC are you using? <Command>gcc
+ -v</Command> will tell you.</para>
+ </listitem>
+
+ <listitem>
+ <para> Run the sequence of compiles/runs that caused the
+ offending behaviour, capturing all the input/output in a
+ “script” (a UNIX command) or in an Emacs shell
+ window. We'd prefer to see the whole thing.</para>
+ </listitem>
+
+ <listitem>
+ <para> Be sure any Haskell compilations are run with a
+ <Option>-v</Option> (verbose) flag, so we can see exactly
+ what was run, what versions of things you have, etc.</para>
+ </listitem>
+
+ <listitem>
+ <para> What is the program behaviour that is wrong, in your
+ opinion?</para>
+ </listitem>
+
+ <listitem>
+ <para> If practical, please send enough source files for us
+ to duplicate the problem.</para>
+ </listitem>
+
+ <listitem>
+ <para> If you are a Hero and track down the problem in the
+ compilation-system sources, please send us patches relative
+ to a known released version of GHC, or whole files if you
+ prefer.</para>
+ </listitem>
+ </orderedlist>
+ </sect2>
+ </sect1>
+
+ <sect1 id="version-numbering">
+ <title>GHC version numbering policy</title>
+ <indexterm><primary>version, of ghc</primary></indexterm>
+
+ <para>As of GHC version 4.08, we have adopted the following
+ policy for numbering GHC versions:</para>
+
+ <variablelist>
+ <varlistentry>
+ <term>Stable Releases</term>
+ <listitem>
+ <para>These are numbered <literal>x.yy.z</literal>, where
+ <literal>yy</literal> is <emphasis>even</emphasis>, and
+ <literal>z</literal> is the patchlevel number (the trailing
+ <literal>.z</literal> can be omitted if <literal>z</literal>
+ is zero). Patchlevels are bug-fix releases only, and never
+ change the programmer interface to any system-supplied code.
+ However, if you install a new patchlevel over an old one you
+ will need to recompile any code that was compiled against the
+ old libraries.</para>
+
+ <para>The value of <literal>__GLASGOW_HASKELL__</literal>
+ (see <xref linkend="c-pre-processor">) for a major release
+ <literal>x.yy.z</literal> is the integer
+ <literal>xyy</literal>.</para>
+ <indexterm>
+ <primary><literal>__GLASGOW_HASKELL__</literal></primary>
+ </indexterm>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>Snapshots/unstable releases</term>
+ <listitem>
+ <para>We may make snapshot releases of the current
+ development sources from time to time, and the current
+ sources are always available via the CVS repository (see the
+ GHC web site for details).</para>
+
+ <para>Snapshot releases are named
+ <literal>x.yy.YYYYMMDD</literal> where <literal>yy</literal>
+ is <emphasis>odd</emphasis>, and <literal>YYYYMMDD</literal>
+ is the date of the sources from which the snapshot was
+ built. In theory, you can check out the exact same sources
+ from the CVS repository using this date.</para>
+
+ <para>The value of <literal>__GLASGOW_HASKELL__</literal>
+ for a snapshot release is the integer
+ <literal>xyy</literal>. You should never write any
+ conditional code which tests for this value, however: since
+ interfaces change on a day-to-day basis, and we don't have
+ finer granularity in the values of
+ <literal>__GLASGOW_HASKELL__</literal>, you should only
+ conditionally compile using predicates which test whether
+ <literal>__GLASGOW_HASKELL__</literal> is equal to, later
+ than, or earlier than a given major release.</para>
+ <indexterm>
+ <primary><literal>__GLASGOW_HASKELL__</literal></primary>
+ </indexterm>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+
+ <para>The version number of your copy of GHC can be found by
+ invoking <literal>ghc</literal> with the
+ <literal>--version</literal> flag (see <xref
+ linkend="options-help">).</para>
+ </sect1>
+
&relnotes
projects / ghc-hetmet.git / blobdiff