Improve documentation of INLINE pragmas
[ghc.git] / docs / users_guide / bugs.xml
1 <?xml version="1.0" encoding="iso-8859-1"?>
2 <chapter id="bugs-and-infelicities">
3 <title>Known bugs and infelicities</title>
4
5 <sect1 id="vs-Haskell-defn">
6 <title>Haskell&nbsp;98 vs.&nbsp;Glasgow Haskell: language non-compliance
7 </title>
8
9 <indexterm><primary>GHC vs the Haskell 98 language</primary></indexterm>
10 <indexterm><primary>Haskell 98 language vs GHC</primary></indexterm>
11
12 <para>This section lists Glasgow Haskell infelicities in its
13 implementation of Haskell&nbsp;98. See also the &ldquo;when things
14 go wrong&rdquo; section (<xref linkend="wrong"/>) for information
15 about crashes, space leaks, and other undesirable phenomena.</para>
16
17 <para>The limitations here are listed in Haskell Report order
18 (roughly).</para>
19
20 <sect2 id="haskell98-divergence">
21 <title>Divergence from Haskell&nbsp;98</title>
22
23
24 <sect3 id="infelicities-lexical">
25 <title>Lexical syntax</title>
26
27 <itemizedlist>
28 <listitem>
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>
32 </listitem>
33
34 <listitem>
35 <para>Certain lexical rules regarding qualified identifiers
36 are slightly different in GHC compared to the Haskell
37 report. When you have
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>
42 </listitem>
43 </itemizedlist>
44 </sect3>
45
46 <sect3 id="infelicities-syntax">
47 <title>Context-free syntax</title>
48
49 <itemizedlist>
50 <listitem>
51 <para>GHC is a little less strict about the layout rule when used
52 in <literal>do</literal> expressions. Specifically, the
53 restriction that "a nested context must be indented further to
54 the right than the enclosing context" is relaxed to allow the
55 nested context to be at the same level as the enclosing context,
56 if the enclosing context is a <literal>do</literal>
57 expression.</para>
58
59 <para>For example, the following code is accepted by GHC:
60
61 <programlisting>
62 main = do args &lt;- getArgs
63 if null args then return [] else do
64 ps &lt;- mapM process args
65 mapM print ps</programlisting>
66
67 </para>
68 </listitem>
69
70 <listitem>
71 <para>GHC doesn't do fixity resolution in expressions during
72 parsing. For example, according to the Haskell report, the
73 following expression is legal Haskell:
74 <programlisting>
75 let x = 42 in x == 42 == True</programlisting>
76 and parses as:
77 <programlisting>
78 (let x = 42 in x == 42) == True</programlisting>
79
80 because according to the report, the <literal>let</literal>
81 expression <quote>extends as far to the right as
82 possible</quote>. Since it can't extend past the second
83 equals sign without causing a parse error
84 (<literal>==</literal> is non-fix), the
85 <literal>let</literal>-expression must terminate there. GHC
86 simply gobbles up the whole expression, parsing like this:
87 <programlisting>
88 (let x = 42 in x == 42 == True)</programlisting>
89
90 The Haskell report is arguably wrong here, but nevertheless
91 it's a difference between GHC &amp; Haskell 98.</para>
92 </listitem>
93 </itemizedlist>
94 </sect3>
95
96 <sect3 id="infelicities-exprs-pats">
97 <title>Expressions and patterns</title>
98
99 <para>None known.</para>
100 </sect3>
101
102 <sect3 id="infelicities-decls">
103 <title>Declarations and bindings</title>
104
105 <para>None known.</para>
106 </sect3>
107
108 <sect3 id="infelicities-Modules">
109 <title>Module system and interface files</title>
110
111 <para>None known.</para>
112 </sect3>
113
114 <sect3 id="infelicities-numbers">
115 <title>Numbers, basic types, and built-in classes</title>
116
117 <variablelist>
118 <varlistentry>
119 <term>Multiply-defined array elements&mdash;not checked:</term>
120 <listitem>
121 <para>This code fragment should
122 elicit a fatal error, but it does not:
123
124 <programlisting>
125 main = print (array (1,1) [(1,2), (1,3)])</programlisting>
126 GHC's implementation of <literal>array</literal> takes the value of an
127 array slot from the last (index,value) pair in the list, and does no
128 checking for duplicates. The reason for this is efficiency, pure and simple.
129 </para>
130 </listitem>
131 </varlistentry>
132 </variablelist>
133
134 </sect3>
135
136 <sect3 id="infelicities-Prelude">
137 <title>In <literal>Prelude</literal> support</title>
138
139 <variablelist>
140 <varlistentry>
141 <term>Arbitrary-sized tuples</term>
142 <listitem>
143 <para>Tuples are currently limited to size 100. HOWEVER:
144 standard instances for tuples (<literal>Eq</literal>,
145 <literal>Ord</literal>, <literal>Bounded</literal>,
146 <literal>Ix</literal> <literal>Read</literal>, and
147 <literal>Show</literal>) are available
148 <emphasis>only</emphasis> up to 16-tuples.</para>
149
150 <para>This limitation is easily subvertible, so please ask
151 if you get stuck on it.</para>
152 </listitem>
153 </varlistentry>
154
155 <varlistentry>
156 <term><literal>Read</literal>ing integers</term>
157 <listitem>
158 <para>GHC's implementation of the
159 <literal>Read</literal> class for integral types accepts
160 hexadecimal and octal literals (the code in the Haskell
161 98 report doesn't). So, for example,
162 <programlisting>read "0xf00" :: Int</programlisting>
163 works in GHC.</para>
164 <para>A possible reason for this is that <literal>readLitChar</literal> accepts hex and
165 octal escapes, so it seems inconsistent not to do so for integers too.</para>
166 </listitem>
167 </varlistentry>
168
169 <varlistentry>
170 <term><literal>isAlpha</literal></term>
171 <listitem>
172 <para>The Haskell 98 definition of <literal>isAlpha</literal>
173 is:</para>
174
175 <programlisting>isAlpha c = isUpper c || isLower c</programlisting>
176
177 <para>GHC's implementation diverges from the Haskell 98
178 definition in the sense that Unicode alphabetic characters which
179 are neither upper nor lower case will still be identified as
180 alphabetic by <literal>isAlpha</literal>.</para>
181 </listitem>
182 </varlistentry>
183 </variablelist>
184 </sect3>
185 </sect2>
186
187 <sect2 id="haskell98-undefined">
188 <title>GHC's interpretation of undefined behaviour in
189 Haskell&nbsp;98</title>
190
191 <para>This section documents GHC's take on various issues that are
192 left undefined or implementation specific in Haskell 98.</para>
193
194 <variablelist>
195 <varlistentry>
196 <term>
197 The <literal>Char</literal> type
198 <indexterm><primary><literal>Char</literal></primary><secondary>size of</secondary></indexterm>
199 </term>
200 <listitem>
201 <para>Following the ISO-10646 standard,
202 <literal>maxBound :: Char</literal> in GHC is
203 <literal>0x10FFFF</literal>.</para>
204 </listitem>
205 </varlistentry>
206
207 <varlistentry>
208 <term>
209 Sized integral types
210 <indexterm><primary><literal>Int</literal></primary><secondary>size of</secondary></indexterm>
211 </term>
212 <listitem>
213 <para>In GHC the <literal>Int</literal> type follows the
214 size of an address on the host architecture; in other words
215 it holds 32 bits on a 32-bit machine, and 64-bits on a
216 64-bit machine.</para>
217
218 <para>Arithmetic on <literal>Int</literal> is unchecked for
219 overflow<indexterm><primary>overflow</primary><secondary><literal>Int</literal></secondary>
220 </indexterm>, so all operations on <literal>Int</literal> happen
221 modulo
222 2<superscript><replaceable>n</replaceable></superscript>
223 where <replaceable>n</replaceable> is the size in bits of
224 the <literal>Int</literal> type.</para>
225
226 <para>The <literal>fromInteger</literal><indexterm><primary><literal>fromInteger</literal></primary>
227 </indexterm>function (and hence
228 also <literal>fromIntegral</literal><indexterm><primary><literal>fromIntegral</literal></primary>
229 </indexterm>) is a special case when
230 converting to <literal>Int</literal>. The value of
231 <literal>fromIntegral x :: Int</literal> is given by taking
232 the lower <replaceable>n</replaceable> bits of <literal>(abs
233 x)</literal>, multiplied by the sign of <literal>x</literal>
234 (in 2's complement <replaceable>n</replaceable>-bit
235 arithmetic). This behaviour was chosen so that for example
236 writing <literal>0xffffffff :: Int</literal> preserves the
237 bit-pattern in the resulting <literal>Int</literal>.</para>
238
239
240 <para>Negative literals, such as <literal>-3</literal>, are
241 specified by (a careful reading of) the Haskell Report as
242 meaning <literal>Prelude.negate (Prelude.fromInteger 3)</literal>.
243 So <literal>-2147483648</literal> means <literal>negate (fromInteger 2147483648)</literal>.
244 Since <literal>fromInteger</literal> takes the lower 32 bits of the representation,
245 <literal>fromInteger (2147483648::Integer)</literal>, computed at type <literal>Int</literal> is
246 <literal>-2147483648::Int</literal>. The <literal>negate</literal> operation then
247 overflows, but it is unchecked, so <literal>negate (-2147483648::Int)</literal> is just
248 <literal>-2147483648</literal>. In short, one can write <literal>minBound::Int</literal> as
249 a literal with the expected meaning (but that is not in general guaranteed.
250 </para>
251
252 <para>The <literal>fromIntegral</literal> function also
253 preserves bit-patterns when converting between the sized
254 integral types (<literal>Int8</literal>,
255 <literal>Int16</literal>, <literal>Int32</literal>,
256 <literal>Int64</literal> and the unsigned
257 <literal>Word</literal> variants), see the modules
258 <literal>Data.Int</literal> and <literal>Data.Word</literal>
259 in the library documentation.</para>
260 </listitem>
261 </varlistentry>
262
263 <varlistentry>
264 <term>Unchecked float arithmetic</term>
265 <listitem>
266 <para>Operations on <literal>Float</literal> and
267 <literal>Double</literal> numbers are
268 <emphasis>unchecked</emphasis> for overflow, underflow, and
269 other sad occurrences. (note, however that some
270 architectures trap floating-point overflow and
271 loss-of-precision and report a floating-point exception,
272 probably terminating the
273 program)<indexterm><primary>floating-point
274 exceptions</primary></indexterm>.</para>
275 </listitem>
276 </varlistentry>
277 </variablelist>
278
279 </sect2>
280 </sect1>
281
282
283 <sect1 id="bugs">
284 <title>Known bugs or infelicities</title>
285
286 <para>The bug tracker lists bugs that have been reported in GHC but not
287 yet fixed: see the <ulink url="http://sourceforge.net/projects/ghc/">SourceForge GHC
288 page</ulink>. In addition to those, GHC also has the following known bugs
289 or infelicities. These bugs are more permanent; it is unlikely that
290 any of them will be fixed in the short term.</para>
291
292 <sect2 id="bugs-ghc">
293 <title>Bugs in GHC</title>
294
295 <itemizedlist>
296 <listitem>
297 <para> GHC can warn about non-exhaustive or overlapping
298 patterns (see <xref linkend="options-sanity"/>), and usually
299 does so correctly. But not always. It gets confused by
300 string patterns, and by guards, and can then emit bogus
301 warnings. The entire overlap-check code needs an overhaul
302 really.</para>
303 </listitem>
304
305 <listitem>
306 <para>GHC does not allow you to have a data type with a context
307 that mentions type variables that are not data type parameters.
308 For example:
309 <programlisting>
310 data C a b => T a = MkT a
311 </programlisting>
312 so that <literal>MkT</literal>'s type is
313 <programlisting>
314 MkT :: forall a b. C a b => a -> T a
315 </programlisting>
316 In principle, with a suitable class declaration with a functional dependency,
317 it's possible that this type is not ambiguous; but GHC nevertheless rejects
318 it. The type variables mentioned in the context of the data type declaration must
319 be among the type parameters of the data type.</para>
320 </listitem>
321
322 <listitem>
323 <para>GHC's inliner can be persuaded into non-termination
324 using the standard way to encode recursion via a data type:</para>
325 <programlisting>
326 data U = MkU (U -> Bool)
327
328 russel :: U -> Bool
329 russel u@(MkU p) = not $ p u
330
331 x :: Bool
332 x = russel (MkU russel)
333 </programlisting>
334
335 <para>We have never found another class of programs, other
336 than this contrived one, that makes GHC diverge, and fixing
337 the problem would impose an extra overhead on every
338 compilation. So the bug remains un-fixed. There is more
339 background in <ulink
340 url="http://research.microsoft.com/~simonpj/Papers/inlining">
341 Secrets of the GHC inliner</ulink>.</para>
342 </listitem>
343
344 <listitem>
345 <para>GHC does not keep careful track of
346 what instance declarations are 'in scope' if they come from other packages.
347 Instead, all instance declarations that GHC has seen in other
348 packages are all in scope everywhere, whether or not the
349 module from that package is used by the command-line
350 expression. This bug affects only the <option>--make</option> mode and
351 GHCi.</para>
352 </listitem>
353
354 </itemizedlist>
355 </sect2>
356
357 <sect2 id="bugs-ghci">
358 <title>Bugs in GHCi (the interactive GHC)</title>
359 <itemizedlist>
360 <listitem>
361 <para>GHCi does not respect the <literal>default</literal>
362 declaration in the module whose scope you are in. Instead,
363 for expressions typed at the command line, you always get the
364 default default-type behaviour; that is,
365 <literal>default(Int,Double)</literal>.</para>
366
367 <para>It would be better for GHCi to record what the default
368 settings in each module are, and use those of the 'current'
369 module (whatever that is).</para>
370 </listitem>
371
372 <listitem>
373 <para>On Windows, there's a GNU ld/BFD bug
374 whereby it emits bogus PE object files that have more than
375 0xffff relocations. When GHCi tries to load a package affected by this
376 bug, you get an error message of the form
377 <screen>
378 Loading package javavm ... linking ... WARNING: Overflown relocation field (# relocs found: 30765)
379 </screen>
380 The last time we looked, this bug still
381 wasn't fixed in the BFD codebase, and there wasn't any
382 noticeable interest in fixing it when we reported the bug
383 back in 2001 or so.
384 </para>
385 <para>The workaround is to split up the .o files that make up
386 your package into two or more .o's, along the lines of
387 how the "base" package does it.</para>
388 </listitem>
389 </itemizedlist>
390 </sect2>
391 </sect1>
392
393 </chapter>
394
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