1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="bugs-and-infelicities">
3 <title>Known bugs and infelicities</title>
5 <sect1 id="vs-Haskell-defn">
6 <title>Haskell 98 vs. Glasgow Haskell: language non-compliance
9 <indexterm><primary>GHC vs the Haskell 98 language</primary></indexterm>
10 <indexterm><primary>Haskell 98 language vs GHC</primary></indexterm>
12 <para>This section lists Glasgow Haskell infelicities in its
13 implementation of Haskell 98. See also the “when things
14 go wrong” section (<xref linkend="wrong"/>) for information
15 about crashes, space leaks, and other undesirable phenomena.</para>
17 <para>The limitations here are listed in Haskell Report order
20 <sect2 id="haskell98-divergence">
21 <title>Divergence from Haskell 98</title>
24 <sect3 id="infelicities-lexical">
25 <title>Lexical syntax</title>
29 <para>The Haskell report specifies that programs may be
30 written using Unicode. GHC only accepts the ISO-8859-1
31 character set at the moment.</para>
35 <para>Certain lexical rules regarding qualified identifiers
36 are slightly different in GHC compared to the Haskell
38 <replaceable>module</replaceable><literal>.</literal><replaceable>reservedop</replaceable>,
39 such as <literal>M.\</literal>, GHC will interpret it as a
40 single qualified operator rather than the two lexemes
41 <literal>M</literal> and <literal>.\</literal>.</para>
46 <sect3 id="infelicities-syntax">
47 <title>Context-free syntax</title>
51 <para>GHC doesn't do fixity resolution in expressions during
52 parsing. For example, according to the Haskell report, the
53 following expression is legal Haskell:
55 let x = 42 in x == 42 == True</programlisting>
58 (let x = 42 in x == 42) == True</programlisting>
60 because according to the report, the <literal>let</literal>
61 expression <quote>extends as far to the right as
62 possible</quote>. Since it can't extend past the second
63 equals sign without causing a parse error
64 (<literal>==</literal> is non-fix), the
65 <literal>let</literal>-expression must terminate there. GHC
66 simply gobbles up the whole expression, parsing like this:
68 (let x = 42 in x == 42 == True)</programlisting>
70 The Haskell report is arguably wrong here, but nevertheless
71 it's a difference between GHC & Haskell 98.</para>
76 <sect3 id="infelicities-exprs-pats">
77 <title>Expressions and patterns</title>
79 <para>None known.</para>
82 <sect3 id="infelicities-decls">
83 <title>Declarations and bindings</title>
85 <para>None known.</para>
88 <sect3 id="infelicities-Modules">
89 <title>Module system and interface files</title>
91 <para>None known.</para>
94 <sect3 id="infelicities-numbers">
95 <title>Numbers, basic types, and built-in classes</title>
99 <term>Multiply-defined array elements—not checked:</term>
101 <para>This code fragment should
102 elicit a fatal error, but it does not:
105 main = print (array (1,1) [(1,2), (1,3)])</programlisting>
106 GHC's implemetation of <literal>array</literal> takes the value of an
107 array slot from the last (index,value) pair in the list, and does no
108 checking for duplicates. The reason for this is efficiency, pure and simple.
116 <sect3 id="infelicities-Prelude">
117 <title>In <literal>Prelude</literal> support</title>
121 <term>Arbitrary-sized tuples</term>
123 <para>Tuples are currently limited to size 100. HOWEVER:
124 standard instances for tuples (<literal>Eq</literal>,
125 <literal>Ord</literal>, <literal>Bounded</literal>,
126 <literal>Ix</literal> <literal>Read</literal>, and
127 <literal>Show</literal>) are available
128 <emphasis>only</emphasis> up to 16-tuples.</para>
130 <para>This limitation is easily subvertible, so please ask
131 if you get stuck on it.</para>
136 <term><literal>Read</literal>ing integers</term>
138 <para>GHC's implementation of the
139 <literal>Read</literal> class for integral types accepts
140 hexadecimal and octal literals (the code in the Haskell
141 98 report doesn't). So, for example,
142 <programlisting>read "0xf00" :: Int</programlisting>
144 <para>A possible reason for this is that <literal>readLitChar</literal> accepts hex and
145 octal escapes, so it seems inconsistent not to do so for integers too.</para>
152 <sect2 id="haskell98-undefined">
153 <title>GHC's interpretation of undefined behaviour in
154 Haskell 98</title>
156 <para>This section documents GHC's take on various issues that are
157 left undefined or implementation specific in Haskell 98.</para>
162 The <literal>Char</literal> type
163 <indexterm><primary><literal>Char</literal></primary><secondary>size of</secondary></indexterm>
166 <para>Following the ISO-10646 standard,
167 <literal>maxBound :: Char</literal> in GHC is
168 <literal>0x10FFFF</literal>.</para>
175 <indexterm><primary><literal>Int</literal></primary><secondary>size of</secondary></indexterm>
178 <para>In GHC the <literal>Int</literal> type follows the
179 size of an address on the host architecture; in other words
180 it holds 32 bits on a 32-bit machine, and 64-bits on a
181 64-bit machine.</para>
183 <para>Arithmetic on <literal>Int</literal> is unchecked for
184 overflow<indexterm><primary>overflow</primary><secondary><literal>Int</literal></secondary>
185 </indexterm>, so all operations on <literal>Int</literal> happen
187 2<superscript><replaceable>n</replaceable></superscript>
188 where <replaceable>n</replaceable> is the size in bits of
189 the <literal>Int</literal> type.</para>
191 <para>The <literal>fromInteger</literal><indexterm><primary><literal>fromInteger</literal></primary>
192 </indexterm>function (and hence
193 also <literal>fromIntegral</literal><indexterm><primary><literal>fromIntegral</literal></primary>
194 </indexterm>) is a special case when
195 converting to <literal>Int</literal>. The value of
196 <literal>fromIntegral x :: Int</literal> is given by taking
197 the lower <replaceable>n</replaceable> bits of <literal>(abs
198 x)</literal>, multiplied by the sign of <literal>x</literal>
199 (in 2's complement <replaceable>n</replaceable>-bit
200 arithmetic). This behaviour was chosen so that for example
201 writing <literal>0xffffffff :: Int</literal> preserves the
202 bit-pattern in the resulting <literal>Int</literal>.</para>
205 <para>Negative literals, such as <literal>-3</literal>, are
206 specified by (a careful reading of) the Haskell Report as
207 meaning <literal>Prelude.negate (Prelude.fromInteger 3)</literal>.
208 So <literal>-2147483648</literal> means <literal>negate (fromInteger 2147483648)</literal>.
209 Since <literal>fromInteger</literal> takes the lower 32 bits of the representation,
210 <literal>fromInteger (2147483648::Integer)</literal>, computed at type <literal>Int</literal> is
211 <literal>-2147483648::Int</literal>. The <literal>negate</literal> operation then
212 overflows, but it is unchecked, so <literal>negate (-2147483648::Int)</literal> is just
213 <literal>-2147483648</literal>. In short, one can write <literal>minBound::Int</literal> as
214 a literal with the expected meaning (but that is not in general guaranteed.
217 <para>The <literal>fromIntegral</literal> function also
218 preserves bit-patterns when converting between the sized
219 integral types (<literal>Int8</literal>,
220 <literal>Int16</literal>, <literal>Int32</literal>,
221 <literal>Int64</literal> and the unsigned
222 <literal>Word</literal> variants), see the modules
223 <literal>Data.Int</literal> and <literal>Data.Word</literal>
224 in the library documentation.</para>
229 <term>Unchecked float arithmetic</term>
231 <para>Operations on <literal>Float</literal> and
232 <literal>Double</literal> numbers are
233 <emphasis>unchecked</emphasis> for overflow, underflow, and
234 other sad occurrences. (note, however that some
235 architectures trap floating-point overflow and
236 loss-of-precision and report a floating-point exception,
237 probably terminating the
238 program)<indexterm><primary>floating-point
239 exceptions</primary></indexterm>.</para>
249 <title>Known bugs or infelicities</title>
251 <para>In addition to the divergences from the Haskell 98 standard
252 listed above, GHC has the following known bugs or
255 <sect2 id="bugs-ghc">
256 <title>Bugs in GHC</title>
260 <para> GHC can warn about non-exhaustive or overlapping
261 patterns (see <xref linkend="options-sanity"/>), and usually
262 does so correctly. But not always. It gets confused by
263 string patterns, and by guards, and can then emit bogus
264 warnings. The entire overlap-check code needs an overhaul
269 <para>GHC does not allow you to have a data type with a context
270 that mentions type variables that are not data type parameters.
273 data C a b => T a = MkT a
275 so that <literal>MkT</literal>'s type is
277 MkT :: forall a b. C a b => a -> T a
279 In principle, with a suitable class declaration with a functional dependency,
280 it's possible that this type is not ambiguous; but GHC nevertheless rejects
281 it. The type variables mentioned in the context of the data type declaration must
282 be among the type parameters of the data type.</para>
286 <para>GHC's inliner can be persuaded into non-termination
287 using the standard way to encode recursion via a data type:</para>
289 data U = MkU (U -> Bool)
292 russel u@(MkU p) = not $ p u
295 x = russel (MkU russel)
298 <para>We have never found another class of programs, other
299 than this contrived one, that makes GHC diverge, and fixing
300 the problem would impose an extra overhead on every
301 compilation. So the bug remains un-fixed. There is more
303 url="http://research.microsoft.com/~simonpj/Papers/inlining">
304 Secrets of the GHC inliner</ulink>.</para>
309 <sect2 id="bugs-ghci">
310 <title>Bugs in GHCi (the interactive GHC)</title>
313 <para>GHCi does not respect the <literal>default</literal>
314 declaration in the module whose scope you are in. Instead,
315 for expressions typed at the command line, you always get the
316 default default-type behaviour; that is,
317 <literal>default(Int,Double)</literal>.</para>
319 <para>It would be better for GHCi to record what the default
320 settings in each module are, and use those of the 'current'
321 module (whatever that is).</para>
325 <para>GHCi does not keep careful track of what instance
326 declarations are 'in scope' if they come from other packages.
327 Instead, all instance declarations that GHC has seen in other
328 packages are all in scope everywhere, whether or not the
329 module from that package is used by the command-line
334 <para>On Windows, there's a GNU ld/BFD bug
335 whereby it emits bogus PE object files that have more than
336 0xffff relocations. When GHCi tries to load a package affected by this
337 bug, you get an error message of the form
339 Loading package javavm ... linking ... WARNING: Overflown relocation field (# relocs found: 30765)
341 The last time we looked, this bug still
342 wasn't fixed in the BFD codebase, and there wasn't any
343 noticeable interest in fixing it when we reported the bug
346 <para>The workaround is to split up the .o files that make up
347 your package into two or more .o's, along the lines of
348 how the "base" package does it.</para>
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