path: root/ghc/docs/users_guide/bugs.sgml

<Chapter id="bugs-and-infelicities">
  <title>Known bugs and infelicities</title>

  <sect1 id="vs-Haskell-defn">
    <title>Haskell&nbsp;98 vs.&nbsp;Glasgow Haskell: language non-compliance
</title>
    
    <indexterm><primary>GHC vs the Haskell 98 language</primary></indexterm>
    <indexterm><primary>Haskell 98 language vs GHC</primary></indexterm>

  <para>This section lists Glasgow Haskell infelicities in its
  implementation of Haskell&nbsp;98.  See also the &ldquo;when things
  go wrong&rdquo; section (<XRef LinkEnd="wrong">) for information
  about crashes, space leaks, and other undesirable phenomena.</para>

  <para>The limitations here are listed in Haskell Report order
  (roughly).</para>

  <sect2 id="haskell98-divergence">
    <title>Divergence from Haskell&nbsp;98</title>
    
      
    <sect3 id="infelicities-lexical">
      <title>Lexical syntax</title>
      
      <itemizedlist>
	<listitem>
	  <para>The Haskell report specifies that programs may be
	  written using Unicode.  GHC only accepts the ISO-8859-1
	  character set at the moment.</para>
	</listitem>

	<listitem>
	  <para>Certain lexical rules regarding qualified identifiers
	  are slightly different in GHC compared to the Haskell
	  report.  When you have
	  <replaceable>module</replaceable><literal>.</literal><replaceable>reservedop</replaceable>,
	  such as <literal>M.\</literal>, GHC will interpret it as a
	  single qualified operator rather than the two lexemes
	  <literal>M</literal> and <literal>.\</literal>.</para>
	</listitem>
      </itemizedlist>
    </sect3>
      
      <sect3 id="infelicities-syntax">
	<title>Context-free syntax</title>
	
      <itemizedlist>
	<listitem>
	  <para>GHC doesn't do fixity resolution in expressions during
	  parsing.  For example, according to the Haskell report, the
	  following expression is legal Haskell:
<programlisting>
    let x = 42 in x == 42 == True</programlisting>
	and parses as:
<programlisting>
    (let x = 42 in x == 42) == True</programlisting>

          because according to the report, the <literal>let</literal>
	  expression <quote>extends as far to the right as
	  possible</quote>.  Since it can't extend past the second
	  equals sign without causing a parse error
	  (<literal>==</literal> is non-fix), the
	  <literal>let</literal>-expression must terminate there.  GHC
	  simply gobbles up the whole expression, parsing like this:
<programlisting>
    (let x = 42 in x == 42 == True)</programlisting>

          The Haskell report is arguably wrong here, but nevertheless
          it's a difference between GHC & Haskell 98.</para>
	</listitem>
      </itemizedlist>
    </sect3>

  <sect3 id="infelicities-exprs-pats">
      <title>Expressions and patterns</title>

	<para>None known.</para>
    </sect3>

    <sect3 id="infelicities-decls">
      <title>Declarations and bindings</title>

      <para>None known.</para>
    </sect3>
      
      <sect3 id="infelicities-Modules">
	<title>Module system and interface files</title>
	
	<para>None known.</para>
    </sect3>

    <sect3 id="infelicities-numbers">
      <title>Numbers, basic types, and built-in classes</title>

      <variablelist>
	<varlistentry>
	  <term>Multiply-defined array elements&mdash;not checked:</term>
	  <listitem>
	    <para>This code fragment <emphasis>should</emphasis>
	    elicit a fatal error, but it does not:

<programlisting>
main = print (array (1,1) [(1,2), (1,3)])</programlisting>

            </para>
	  </listitem>
	</varlistentry>
      </variablelist>
      
    </sect3>

      <sect3 id="infelicities-Prelude">
	<title>In <literal>Prelude</literal> support</title>

      <variablelist>
	<varlistentry>
	  <term>The <literal>Char</literal> type</term>
	  <indexterm><primary><literal>Char</literal></primary><secondary>size
	  of</secondary></indexterm>
	  <listitem>
	    <para>The Haskell report says that the
	    <literal>Char</literal> type holds 16 bits.  GHC follows
	    the ISO-10646 standard a little more closely:
	    <literal>maxBound :: Char</literal> in GHC is
	    <literal>0x10FFFF</literal>.</para>
	  </listitem>
	</varlistentry>

	<varlistentry>
	  <term>Arbitrary-sized tuples</term>
	  <listitem>
	    <para>Tuples are currently limited to size 100.  HOWEVER:
            standard instances for tuples (<literal>Eq</literal>,
            <literal>Ord</literal>, <literal>Bounded</literal>,
            <literal>Ix</literal> <literal>Read</literal>, and
            <literal>Show</literal>) are available
            <emphasis>only</emphasis> up to 16-tuples.</para>

	    <para>This limitation is easily subvertible, so please ask
            if you get stuck on it.</para>
	    </listitem>
	  </varlistentry>

	  <varlistentry>
	    <term><literal>Read</literal>ing integers</term>
	    <listitem>
	      <para>GHC's implementation of the
	      <literal>Read</literal> class for integral types accepts
	      hexadeciaml and octal literals (the code in the Haskell
	      98 report doesn't).  So, for example,
<programlisting>read "0xf00" :: Int</programlisting>
              works in GHC.</para>
	      <para>A possible reason for this is that <literal>readLitChar</literal> accepts hex and
		octal escapes, so it seems inconsistent not to do so for integers too.</para>
	    </listitem>
	  </varlistentry>
	</variablelist>
    </sect3>
  </sect2>

  <sect2 id="haskell98-undefined">
    <title>GHC's interpretation of undefined behaviour in
    Haskell&nbsp;98</title>

    <para>This section documents GHC's take on various issues that are
    left undefined or implementation specific in Haskell 98.</para>

    <variablelist>
      <varlistentry>
	<term>Sized integral types</term>
	<indexterm><primary><literal>Int</literal></primary><secondary>size of</secondary>
	</indexterm>
	
	<listitem>
	  <para>In GHC the <literal>Int</literal> type follows the
	  size of an address on the host architecture; in other words
	  it holds 32 bits on a 32-bit machine, and 64-bits on a
	  64-bit machine.</para>

	  <para>Arithmetic on <literal>Int</literal> is unchecked for
	  overflow<indexterm><primary>overflow</primary><secondary><literal>Int</literal></secondary>
	    </indexterm>, so all operations on <literal>Int</literal> happen
	  modulo
	  2<superscript><replaceable>n</replaceable></superscript>
	  where <replaceable>n</replaceable> is the size in bits of
	  the <literal>Int</literal> type.</para>

	  <para>The <literal>fromInteger</literal><indexterm><primary><literal>fromInteger</literal></primary>
	    </indexterm>function (and hence
	  also <literal>fromIntegral</literal><indexterm><primary><literal>fromIntegral</literal></primary>
	    </indexterm>) is a special case when
	  converting to <literal>Int</literal>.  The value of
	  <literal>fromIntegral x :: Int</literal> is given by taking
	  the lower <replaceable>n</replaceable> bits of <literal>(abs
	  x)</literal>, multiplied by the sign of <literal>x</literal>
	  (in 2's complement <replaceable>n</replaceable>-bit
	  arithmetic).  This behaviour was chosen so that for example
	  writing <literal>0xffffffff :: Int</literal> preserves the
	  bit-pattern in the resulting <literal>Int</literal>.</para>


	   <para>Negative literals, such as <literal>-3</literal>, are
             specified by (a careful reading of) the Haskell Report as 
             meaning <literal>Prelude.negate (Prelude.fromInteger 3)</literal>.
	     So <literal>-2147483648</literal> means <literal>negate (fromInteger 2147483648)</literal>.
	     Since <literal>fromInteger</literal> takes the lower 32 bits of the representation,
	     <literal>fromInteger (2147483648::Integer)</literal>, computed at type <literal>Int</literal> is
	     <literal>-2147483648::Int</literal>.  The <literal>negate</literal> operation then
	     overflows, but it is unchecked, so <literal>negate (-2147483648::Int)</literal> is just
	     <literal>-2147483648</literal>.  In short, one can write <literal>minBound::Int</literal> as
	     a literal with the expected meaning (but that is not in general guaranteed.
             </para>

	  <para>The <literal>fromIntegral</literal> function also
	  preserves bit-patterns when converting between the sized
	  integral types (<literal>Int8</literal>,
	  <literal>Int16</literal>, <literal>Int32</literal>,
	  <literal>Int64</literal> and the unsigned
	  <literal>Word</literal> variants), see the modules
	  <literal>Data.Int</literal> and <literal>Data.Word</literal>
	  in the library documentation.</para>
	</listitem>
      </varlistentry>

      <varlistentry>
	<term>Unchecked float arithmetic</term>
	<listitem>
	  <para>Operations on <literal>Float</literal> and
          <literal>Double</literal> numbers are
          <emphasis>unchecked</emphasis> for overflow, underflow, and
          other sad occurrences.  (note, however that some
          architectures trap floating-point overflow and
          loss-of-precision and report a floating-point exception,
          probably terminating the
          program)<indexterm><primary>floating-point
          exceptions</primary></indexterm>.</para>
	</listitem>
      </varlistentry>
    </variablelist>
      
    </sect2>
  </sect1>


  <sect1 id="bugs">
    <title>Known bugs or infelicities</title>

    <para>In addition to the divergences from the Haskell 98 standard
    listed above, GHC has the following known bugs or
    infelicities.</para>

    <itemizedlist>
      <listitem>
	<para> GHC can warn about non-exhaustive or overlapping
        patterns (see <xref linkend="options-sanity">, and usually
        does so correctly.  But not always.  It gets confused by
        string patterns, and by guards, and can then emit bogus
        warnings.  The entire overlap-check code needs an overhaul
        really.</para>
      </listitem>

      <listitem>
	<para>Dangers with multiple <literal>Main</literal>
	modules.</para>

	<para>GHC does not insist that module <literal>Main</literal>
	lives in a file called <filename>Main.hs</filename>.  This is
	useful if you want multiple versions of
	<literal>Main</literal>.  But there's a danger: when compiling
	module <literal>Main</literal> (regardless of what file it
	comes from), GHC looks for the interface
	<filename>Main.hi</filename>; it uses this to get version
	information from the last time it recompiled
	<literal>Main</literal>.  The trouble is that this
	<filename>Main.hi</filename> may not correspond to the source
	file being compiled.</para>

	<para>Solution: remove <filename>Main.hi</filename> first.  A
	better solution would be for GHC to record the source-file
	filename in the interface file, or even an MD5 checksum.
	</para>
      </listitem>
    
      <listitem>
	<para>GHC does not allow you to have a data type with a context 
	   that mentions type variables that are not data type parameters.
	  For example:
<programlisting>
  data C a b => T a = MkT a
</programlisting>
	  so that <literal>MkT</literal>'s type is
<programlisting>
  MkT :: forall a b. C a b => a -> T a
</programlisting>
        In principle, with a suitable class declaration with a functional dependency,
	 it's possible that this type is not ambiguous; but GHC nevertheless rejects
	  it.  The type variables mentioned in the context of the data type declaration must
	be among the type parameters of the data type.</para>
      </listitem>

      <listitem>
	<para>GHCi does not respect the <literal>default</literal>
        declaration in the module whose scope you are in.  Instead,
        for expressions typed at the command line, you always get the
        default default-type behaviour; that is,
        <literal>default(Int,Double)</literal>.</para>

	<para>It would be better for GHCi to record what the default
        settings in each module are, and use those of the 'current'
        module (whatever that is).</para>
      </listitem>

      <listitem>
	<para>GHCi does not keep careful track of what instance
        declarations are 'in scope' if they come from other packages.
        Instead, all instance declarations that GHC has seen in other
        packages are all in scope everywhere, whether or not the
        module from that package is used by the command-line
        expression.</para>
      </listitem>

      <listitem>
	<para>GHC's inliner can be persuaded into non-termination
        using the standard way to encode recursion via a data type:</para>
<programlisting>
  data U = MkU (U -> Bool)
       
  russel :: U -> Bool
  russel u@(MkU p) = not $ p u
  
  x :: Bool
  x = russel (MkU russel)
</programlisting>

        <para>We have never found another class of programs, other
        than this contrived one, that makes GHC diverge, and fixing
        the problem would impose an extra overhead on every
        compilation.  So the bug remains un-fixed.  There is more
        background in <ulink
        url="http://research.microsoft.com/~simonpj/Papers/inlining">
        Secrets of the GHC inliner</ulink>.</para>
      </listitem>
    </itemizedlist>
  </sect1>

</chapter>

<!-- Emacs stuff:
     ;;; Local Variables: ***
     ;;; mode: sgml ***
     ;;; sgml-parent-document: ("users_guide.sgml" "book" "chapter" "sect1") ***
     ;;; End: ***
 -->