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<?xml version="1.0" encoding="iso-8859-1"?>
<chapter id="bugs-and-infelicities">
<title>Known bugs and infelicities</title>
<sect1 id="vs-Haskell-defn">
<title>Haskell&nbsp;standards vs.&nbsp;Glasgow Haskell: language non-compliance
</title>
<indexterm><primary>GHC vs the Haskell standards</primary></indexterm>
<indexterm><primary>Haskell standards vs GHC</primary></indexterm>
<para>
This section lists Glasgow Haskell infelicities in its
implementation of Haskell&nbsp;98 and Haskell&nbsp;2010.
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="haskell-standards-divergence">
<title>Divergence from Haskell&nbsp;98 and Haskell&nbsp;2010</title>
<para>
By default, GHC mainly aims to behave (mostly) like a Haskell&nbsp;2010
compiler, although you can tell it to try to behave like a
particular version of the langauge with the
<literal>-XHaskell98</literal> and
<literal>-XHaskell2010</literal> flags. The known deviations
from the standards are described below. Unless otherwise stated,
the deviation applies in Haskell&nbsp;98, Haskell&nbsp;2010 and
the default modes.
</para>
<sect3 id="infelicities-lexical">
<title>Lexical syntax</title>
<itemizedlist>
<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>In Haskell&nbsp;98 mode and by default (but not in
Haskell&nbsp;2010 mode), GHC is a little less strict about the
layout rule when used
in <literal>do</literal> expressions. Specifically, the
restriction that "a nested context must be indented further to
the right than the enclosing context" is relaxed to allow the
nested context to be at the same level as the enclosing context,
if the enclosing context is a <literal>do</literal>
expression.</para>
<para>For example, the following code is accepted by GHC:
<programlisting>
main = do args &lt;- getArgs
if null args then return [] else do
ps &lt;- mapM process args
mapM print ps</programlisting>
This behaviour is controlled by the
<literal>NondecreasingIndentation</literal> extension.
</para>
</listitem>
<listitem>
<para>GHC doesn't do the fixity resolution in expressions during
parsing as required by Haskell&nbsp;98 (but not by Haskell&nbsp;2010).
For example, according to the Haskell&nbsp;98 report, the
following expression is legal:
<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></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>GHC's typechecker makes all pattern bindings monomorphic
by default; this behaviour can be disabled with
<option>-XNoMonoPatBinds</option>. See <xref
linkend="options-language" />.</para>
<para>In its default mode, GHC does not accept datatype contexts,
as it has been decided to remove them from the next version of the
language standard. This behaviour can be controlled with the
<option>DatatypeContexts</option> extension.
See <xref linkend="datatype-contexts" />.</para>
</sect3>
<sect3 id="infelicities-Modules">
<title>Module system and interface files</title>
<para>GHC requires the use of <literal>hs-boot</literal>
files to cut the recursive loops among mutually recursive modules
as described in <xref linkend="mutual-recursion"/>. This more of an infelicity
than a bug: the Haskell Report says
(<ulink url="http://haskell.org/onlinereport/modules.html#sect5.7">Section 5.7</ulink>) "Depending on the Haskell
implementation used, separate compilation of mutually
recursive modules may require that imported modules contain
additional information so that they may be referenced before
they are compiled. Explicit type signatures for all exported
values may be necessary to deal with mutual recursion. The
precise details of separate compilation are not defined by
this Report."
</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 should
elicit a fatal error, but it does not:
<programlisting>
main = print (array (1,1) [(1,2), (1,3)])</programlisting>
GHC's implementation of <literal>array</literal> takes the value of an
array slot from the last (index,value) pair in the list, and does no
checking for duplicates. The reason for this is efficiency, pure and simple.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect3>
<sect3 id="infelicities-Prelude">
<title>In <literal>Prelude</literal> support</title>
<variablelist>
<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
hexadecimal 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>
<varlistentry>
<term><literal>isAlpha</literal></term>
<listitem>
<para>The Haskell 98 definition of <literal>isAlpha</literal>
is:</para>
<programlisting>isAlpha c = isUpper c || isLower c</programlisting>
<para>GHC's implementation diverges from the Haskell 98
definition in the sense that Unicode alphabetic characters which
are neither upper nor lower case will still be identified as
alphabetic by <literal>isAlpha</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term><literal>hGetContents</literal></term>
<listitem>
<para>
Lazy I/O throws an exception if an error is
encountered, in contrast to the Haskell 98 spec which
requires that errors are discarded (see Section 21.2.2
of the Haskell 98 report). The exception thrown is
the usual IO exception that would be thrown if the
failing IO operation was performed in the IO monad, and can
be caught by <literal>System.IO.Error.catch</literal>
or <literal>Control.Exception.catch</literal>.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect3>
</sect2>
<sect2 id="haskell-98-2010-undefined">
<title>GHC's interpretation of undefined behaviour in
Haskell&nbsp;98 and Haskell&nbsp;2010</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>
The <literal>Char</literal> type
<indexterm><primary><literal>Char</literal></primary><secondary>size of</secondary></indexterm>
</term>
<listitem>
<para>Following the ISO-10646 standard,
<literal>maxBound :: Char</literal> in GHC is
<literal>0x10FFFF</literal>.</para>
</listitem>
</varlistentry>
<varlistentry>
<term>
Sized integral types
<indexterm><primary><literal>Int</literal></primary><secondary>size of</secondary></indexterm>
</term>
<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>
<varlistentry>
<term><literal>hs_init()</literal> not allowed
after <literal>hs_exit()</literal></term>
<listitem>
<para>The FFI spec requires the implementation to support
re-initialising itself after being shut down
with <literal>hs_exit()</literal>, but GHC does not
currently support that.</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>
</sect1>
<sect1 id="bugs">
<title>Known bugs or infelicities</title>
<para>The bug tracker lists bugs that have been reported in GHC but not
yet fixed: see the <ulink url="http://sourceforge.net/projects/ghc/">SourceForge GHC
page</ulink>. In addition to those, GHC also has the following known bugs
or infelicities. These bugs are more permanent; it is unlikely that
any of them will be fixed in the short term.</para>
<sect2 id="bugs-ghc">
<title>Bugs in GHC</title>
<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>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>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>
<listitem>
<para>GHC 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. This bug affects only the <option>--make</option> mode and
GHCi.</para>
</listitem>
</itemizedlist>
</sect2>
<sect2 id="bugs-ghci">
<title>Bugs in GHCi (the interactive GHC)</title>
<itemizedlist>
<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>On Windows, there's a GNU ld/BFD bug
whereby it emits bogus PE object files that have more than
0xffff relocations. When GHCi tries to load a package affected by this
bug, you get an error message of the form
<screen>
Loading package javavm ... linking ... WARNING: Overflown relocation field (# relocs found: 30765)
</screen>
The last time we looked, this bug still
wasn't fixed in the BFD codebase, and there wasn't any
noticeable interest in fixing it when we reported the bug
back in 2001 or so.
</para>
<para>The workaround is to split up the .o files that make up
your package into two or more .o's, along the lines of
how the "base" package does it.</para>
</listitem>
</itemizedlist>
</sect2>
</sect1>
</chapter>
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