[ghc-hetmet.git] / ghc / docs / users_guide / faq.sgml

<chapter id="faq">
  <title>GHC FAQ</title>

  <para>This section has the answers to questions that get asked
  regularly on the GHC mailing lists, in no particular order.  Please
  let us know if you think there's a question/answer that should be
  added here.</para>
    
  <variablelist>
    <varlistentry>
      <term>How do I port GHC to platform X?</term>
      <listitem>
        <para>There are two distinct possibilities: either</para>
        <itemizedlist>
          <listitem>
            <para>The hardware architecture for your system is already
            supported by GHC, but you're running an OS that isn't
            supported (or perhaps has been supported in the past, but
            currently isn't).  This is the easiest type of porting
            job, but it still requires some careful
            bootstrapping.</para>
          </listitem>

          <listitem>
            <para>Your system's hardware architecture isn't supported
            by GHC.  This will be a more difficult port (though by
            comparison perhaps not as difficult as porting
            gcc).</para>
          </listitem>
        </itemizedlist>

        <para>Both ways require you to bootrap from intermediate
        <literal>HC</literal> files: these are the stylised C files
        generated by GHC when it compiles Haskell source.  Basically
        the idea is to take the HC files for GHC itself to the target
        machine and compile them with <literal>gcc</literal> to get a
        working GHC, and go from there.</para>

        <para>The <ulink
        url="http://www.haskell.org/ghc/latest/building/building-guide.html">Building
        Guide</ulink> has all the details on how to bootstrap GHC on a
        new platform.</para>

      </listitem>
    </varlistentry>
    
    <varlistentry>
      <term>Do I have to recompile all my code if I upgrade
      GHC?</term>
      <listitem>
        <para>Yes.  There are two reasons for this:</para>
        <itemizedlist>
          <listitem>
            <para>GHC does a lot of cross-module optimisation, so
            compiled code will include parts of the libraries it was
            compiled against (including the Prelude), so will be
            deeply tied to the actual version of those libraries it
            was compiled against.  When you upgrade GHC, the libraries
            may change; even if the external interface of the
            libraries doesn't change, sometimes internal details may
            change because GHC optimised the code in the library
            differently.</para>
          </listitem>
          <listitem>
            <para>We sometimes change the ABI (application binary
            interface) between versions of GHC.  Code compiled with
            one version of GHC is not necessarily compatible with code
            compiled by a different version, even if you arrange to
            keep the same libraries.</para>
          </listitem>
        </itemizedlist>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>Why doesn't GHC use shared libraries?</term>
      <listitem>
        <para>The subject of shared libraries has come up several
        times in the past &mdash; take a look through the mailing-list
        archives for some of the previous discussions. The upshot is
        that shared libraries wouldn't really buy much unless you
        really need to save the disk space: in all other
        considerations, static linking comes out better.</para>
          
        <para>Unfortunately GHC-compiled libraries are very tightly
        coupled, which means it's unlikely you'd be able to swap out a
        shared library for a newer version unless it was compiled with
        <emphasis>exactly</emphasis> the same compiler and set of
        libraries as the old version.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>I can't get string gaps to work</term>
      <listitem>
        <para>If you're also using CPP, beware of the known pitfall
        with string gaps mentioned in <xref
        linkend="cpp-string-gaps">.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>GHCi complains about missing symbols like
      <literal>CC_LIST</literal> when loading a previously compiled .o
      file.</term>
      <listitem>
        <para> This probably means the .o files in question were
        compiled for profiling (with <option>-prof</option>).  Workaround:
        recompile them without profiling.  We really ought to detect
        this situation and give a proper error message.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>Linking a program causes the following error on Linux:
      <literal>/usr/bin/ld: cannot open -lgmp: No such file or
      directory</literal></term>
      <listitem>
        <para>The problem is that your system doesn't have the GMP
        library installed.  If this is a RedHat distribution, install
        the RedHat-supplied <literal>gmp-devel</literal> package, and
        the <literal>gmp</literal> package if you don't already have
        it.  There have been reports that installing the RedHat
        packages also works for SuSE (SuSE don't supply a shared gmp
        library).</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>I Can't run GHCi on Linux, because it complains about a
      missing <literal>libreadline.so.3</literal>.</term>
      <listitem>
        <para>The "correct" fix for this problem is to install the
        correct RPM for the particular flavour of Linux on your
        machine.  If this isn't an option, however, there is a hack
        that might work: make a symbolic link from
        <filename>libreadline.so.4</filename> to
        <filename>libreadline.so.3</filename> in
        <literal>/usr/lib</literal>.  We tried this on a SuSE 7.1 box
        and it seemed to work, but YMMV.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>Solaris users may sometimes get link errors due to
      libraries needed by GNU Readline.</term>
      <listitem>
        <para>We suggest you try linking in some combination of the
        <literal>termcap</literal>, <literal>curses</literal> and
        <literal>ncurses</literal> libraries, by giving
        <literal>-ltermcap</literal>, <literal>-lcurses</literal> and
        <literal>-lncurses</literal> respectively.  If you encounter
        this problem, we would appreciate feedback on it, since we
        don't fully understand what's going on here.</para>
      </listitem>
    </varlistentry>
    
    <varlistentry>
      <term>When I try to start ghci (probably one I compiled myself)
      it says <literal>ghc-5.02: not built for interactive
      use</literal></term>
      <listitem>
        <para>To build a working ghci, you need to build GHC 5.02 with
        itself; the above message appears if you build it with 4.08.X,
        for example.  It'll still work fine for batch-mode
        compilation, though.  Note that you really must build with
        exactly the same version of the compiler.  Building 5.02 with
        5.00.2, for example, may or may not give a working interactive
        system; it probably won't, and certainly isn't supported.
        Note also that you can build 5.02 with any older compiler,
        back to 4.08.1, if you don't want a working interactive
        system; that's OK, and supported.</para>
      </listitem>
    </varlistentry>
   
    <varlistentry>
      <term>When I use a foreign function that takes or returns a
      float, it gives the wrong answer, or crashes.</term>
      <listitem>
        <para>You should use the <option>-#include</option> option to
        bring the correct prototype into scope (see <xref
        linkend="options-C-compiler">).</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>My program that uses a really large heap crashes on
      Windows.</term>
      <listitem>
        <para>For utterly horrible reasons, programs that use more
        than 128Mb of heap won't work when compiled dynamically on
        Windows (they should be fine statically compiled).</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>GHC doesn't like filenames containing
      <literal>+</literal>.</term>
      <listitem>
        <para>Indeed not. You could change <literal>+</literal> to
        <literal>p</literal> or <literal>plus</literal>.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>When I open a FIFO (named pipe) and try to read from it, I
      get EOF immediately.</term>
      <listitem>
        <para>This is a consequence of the fact that GHC opens the
        FIFO in non-blocking mode.  The behaviour varies from OS to
        OS: on Linux and Solaris you can wait for a writer by doing an
        explicit <literal>threadWaitRead</literal> on the file
        descriptor (gotten from <literal>Posix.handleToFd</literal>)
        before the first read, but this doesn't work on FreeBSD
        (although rumour has it that recent versions of FreeBSD
        changed the behavour to match other OSs).  A workaround for
        all systems is to open the FIFO for writing yourself, before
        (or at the same time as) opening it for reading.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>When I <literal>foreign import</literal> a function that
      returns <literal>char</literal> or <literal>short</literal>, I
      get garbage back.</term>
      <listitem>
        <para>This is a known bug in GHC versions prior to 5.02.2.
        GHC doesn't mask out the more significant bits of the result.
        It doesn't manifest with gcc 2.95, but apparently shows up
        with g++ and gcc 3.0.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>My program is failing with <literal>head []</literal>, or
      an array bounds error, or some other random error, and I have no
      idea how to find the bug.  Can you help?</term>

      <listitem>
        <para>Compile your program with <literal>-prof
-auto-all</literal> (make sure you have the profiling libraries
installed), and run it with <literal>+RTS -xc -RTS</literal> to get a
&ldquo;stack trace&rdquo; at the point at which the exception was
raised.  See <xref linkend="rts-options-debugging"> for more
details.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>How do I increase the heap size permanently for a given
      binary?</term>
      <listitem>
        <para>See <xref linkend="rts-hooks">.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>I'm trying to compile my program for parallel execution
      with the <option>-parallel</option>, and GHC complains with an
      error like &ldquo;failed to load interface file for
      Prelude&rdquo;.</term>
      <listitem>
        <para>GHC doesn't ship with support for parallel execution,
        that support is provided separately by the <ulink
                                                          url="http://www.macs.hw.ac.uk/~dsg/gph/">GPH</ulink> project.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>When is it safe to use
      <literal>unsafePerformIO</literal>?</term>
      <listitem>
        <para>We'll give two answers to this question, each of which
        may be helpful.  These criteria are not rigorous in any real
        sense (you'd need a formal semantics for Haskell in order to
        give a proper answer to this question), but should give you a
        feel for the kind of things you can and cannot do with
        <literal>unsafePerformIO</literal>.</para>
        
        <itemizedlist>
          <listitem>
            <para>It is safe to implement a function or API using
            <literal>unsafePerformIO</literal> if you could imagine
            also implementing the same function or API in Haskell
            without using <literal>unsafePerformIO</literal> (forget
            about efficiency, just consider the semantics).</para>
          </listitem>

          <listitem>
            <para>In pure Haskell, the value of a function depends
            only on the values of its arguments (and free variables,
            if it has any).  If you can implement the function using
            <literal>unsafePerformIO</literal> and still retain this
            invariant, then you're probably using
            <literal>unsafePerformIO</literal> in a safe way.  Note
            that you need only consider the
            <emphasis>observable</emphasis> values of the arguments
            and result.</para>
          </listitem>
        </itemizedlist>
        
        <para>For more information, see <ulink
        url="http://www.haskell.org/pipermail/glasgow-haskell-users/2002-July/003681.html">this
        thread</ulink>.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>Why does linking take so long?</term>
      <listitem>
        <para>Linking a small program should take no more than a few
        seconds.  Larger programs can take longer, but even linking
        GHC itself only takes 3-4 seconds on our development
        machines.</para>
        
        <para>Long link times have been attributed to using Sun's
        linker on Solaris, as compared to GNU <command>ld</command>
        which appears to be much faster.  So if you're on a Sun box,
        try switching to GNU <command>ld</command>.  <ulink
        url="http://www.haskell.org/pipermail/glasgow-haskell-users/2002-November/004477.html">This
        article</ulink> from the mailing list has more
        information.</para>
      </listitem>
    </varlistentry>

    <varlistentry> 
      <term>If I explicitely set the buffering on a Handle to
      "NoBuffering" I'm not able to enter EOF by typing
      "Ctrl-D".</term>

      <listitem>
        <para>This is a consequence of Unixy terminal semantics.  Unix
        does line buffering on terminals in the kernel as part of the
        terminal processing, unless you turn it off.  However, the
        Ctrl-D processing is also part of the terminal processing
        which gets turned off when the kernel line buffering is
        disabled.  So GHC tries its best to get NoBuffering semantics
        by turning off the kernel line buffering, but as a result you
        lose Ctrl-D.  C'est la vie.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>If I print out a string using <literal>putStr</literal>,
      and then attempt to read some input using
      <literal>hGetLine</literal>, I don't see the output from the
      <literal>putStr</literal>.</term>

      <listitem>
        <para>The <literal>stdout</literal> handle is line-buffered by
        default, which means that output sent to the handle is only
        flushed when a newline (<literal>/n</literal>) is output, the
        buffer is full, or <literal>hFlush</literal> is called on the
        Handle.  The right way to make the text appear without sending
        a newline is to use <literal>hFlush</literal>:</para>

<programlisting>
  import System.IO
  main = do
    putStr "how are you today? "
    hFlush stdout
    input <- hGetLine
    ...</programlisting>

        <para>You'll probably find that the behaviour differs when
        using GHCi: the <literal>hFlush</literal> isn't necessary to
        make the text appear.  This is because in GHCi we turn off the
        buffering on <literal>stdout</literal>, because this is
        normally what you want in an interpreter: output appears as it
        is generated.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>I can't get finalizers to work properly.  My program
      sometimes just prints
      <literal>&lt;&lt;loop&gt;&gt;</literal>.</term>
      
      <listitem>
        <para>Chances are that your program is trying to write a
        message to <literal>stdout</literal> or
        <literal>stderr</literal> in the finalizer.  Handles have
        finalizers themselves, and since finalizers don't keep other
        finalized values alive, the <literal>stdout</literal> and
        <literal>stderr</literal> Handles may be finalized before your
        finalizer runs.  If this happens, your finalizer will block on
        the handle, and probably end up receiving a
        <literal>NonTermination</literal> exception (which is printed
        as <literal>&lt;&lt;loop&gt;&gt;</literal>).</para>
      </listitem>
    </varlistentry>

    
    <varlistentry>
      <term>Does GHC implement any kind of extensible records?</term>

      <listitem>
        <para>No, extensible records are not implemented in GHC.
        <ulink url="http://www.haskell.org/hugs/">Hugs</ulink>
        implements TRex, one extensible record variant.  The problem
        is that the record design space is large, and seems to lack
        local optima.  And all reasonable variants break backward
        compatibility.  As a result, nothing much happens.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>Why do I get errors about missing include files when
      compiling with <option>-O</option> or
      <option>-prof</option>?</term>

      <listitem>
        <para>Certain options, such as <option>-O</option>, turn on
        via-C compilation, instead of using the native code generator.
        Include files named by <option>-&num;include</option> options
        or in <literal>foreign import</literal> declarations are only
        used in via-C compilation mode.  See <xref
        linkend="finding-header-files"> for more details.</para>
      </listitem>
    </varlistentry>

    <varlistentry>
      <term>How do I compile my program for profiling without
      overwriting the object files and <literal>hi</literal> files
      I've already built?</term>
      <listitem>
        <para>You can select alternative suffixes for object files and
        interface files, so you can have several builds of the same
        code coexisting in the same directory.  For example, to
        compile with profiling, you might do this:</para>
        
        <screen>ghc --make -prof -o foo-prof -osuf p.o -hisuf p.hi Main</screen>
        
        <para>See <xref linkend="options-output"> for more details on
        the <option>-osuf</option> and <option>-hisuf</option>
        options.</para>
      </listitem>
    </varlistentry>

  </variablelist>

</chapter>



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