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>Certain lexical rules regarding qualified identifiers
30 are slightly different in GHC compared to the Haskell
32 <replaceable>module</replaceable><literal>.</literal><replaceable>reservedop</replaceable>,
33 such as <literal>M.\</literal>, GHC will interpret it as a
34 single qualified operator rather than the two lexemes
35 <literal>M</literal> and <literal>.\</literal>.</para>
40 <sect3 id="infelicities-syntax">
41 <title>Context-free syntax</title>
45 <para>GHC is a little less strict about the layout rule when used
46 in <literal>do</literal> expressions. Specifically, the
47 restriction that "a nested context must be indented further to
48 the right than the enclosing context" is relaxed to allow the
49 nested context to be at the same level as the enclosing context,
50 if the enclosing context is a <literal>do</literal>
53 <para>For example, the following code is accepted by GHC:
56 main = do args <- getArgs
57 if null args then return [] else do
58 ps <- mapM process args
59 mapM print ps</programlisting>
65 <para>GHC doesn't do fixity resolution in expressions during
66 parsing. For example, according to the Haskell report, the
67 following expression is legal Haskell:
69 let x = 42 in x == 42 == True</programlisting>
72 (let x = 42 in x == 42) == True</programlisting>
74 because according to the report, the <literal>let</literal>
75 expression <quote>extends as far to the right as
76 possible</quote>. Since it can't extend past the second
77 equals sign without causing a parse error
78 (<literal>==</literal> is non-fix), the
79 <literal>let</literal>-expression must terminate there. GHC
80 simply gobbles up the whole expression, parsing like this:
82 (let x = 42 in x == 42 == True)</programlisting>
84 The Haskell report is arguably wrong here, but nevertheless
85 it's a difference between GHC & Haskell 98.</para>
90 <sect3 id="infelicities-exprs-pats">
91 <title>Expressions and patterns</title>
93 <para>None known.</para>
96 <sect3 id="infelicities-decls">
97 <title>Declarations and bindings</title>
99 <para>GHC's typechecker makes all pattern bindings monomorphic
100 by default; this behaviour can be disabled with
101 <option>-XNoMonoPatBinds</option>. See <xref
102 linkend="options-language" />.</para>
105 <sect3 id="infelicities-Modules">
106 <title>Module system and interface files</title>
108 <para>None known.</para>
111 <sect3 id="infelicities-numbers">
112 <title>Numbers, basic types, and built-in classes</title>
116 <term>Multiply-defined array elements—not checked:</term>
118 <para>This code fragment should
119 elicit a fatal error, but it does not:
122 main = print (array (1,1) [(1,2), (1,3)])</programlisting>
123 GHC's implementation of <literal>array</literal> takes the value of an
124 array slot from the last (index,value) pair in the list, and does no
125 checking for duplicates. The reason for this is efficiency, pure and simple.
133 <sect3 id="infelicities-Prelude">
134 <title>In <literal>Prelude</literal> support</title>
138 <term>Arbitrary-sized tuples</term>
140 <para>Tuples are currently limited to size 100. HOWEVER:
141 standard instances for tuples (<literal>Eq</literal>,
142 <literal>Ord</literal>, <literal>Bounded</literal>,
143 <literal>Ix</literal> <literal>Read</literal>, and
144 <literal>Show</literal>) are available
145 <emphasis>only</emphasis> up to 16-tuples.</para>
147 <para>This limitation is easily subvertible, so please ask
148 if you get stuck on it.</para>
153 <term><literal>Read</literal>ing integers</term>
155 <para>GHC's implementation of the
156 <literal>Read</literal> class for integral types accepts
157 hexadecimal and octal literals (the code in the Haskell
158 98 report doesn't). So, for example,
159 <programlisting>read "0xf00" :: Int</programlisting>
161 <para>A possible reason for this is that <literal>readLitChar</literal> accepts hex and
162 octal escapes, so it seems inconsistent not to do so for integers too.</para>
167 <term><literal>isAlpha</literal></term>
169 <para>The Haskell 98 definition of <literal>isAlpha</literal>
172 <programlisting>isAlpha c = isUpper c || isLower c</programlisting>
174 <para>GHC's implementation diverges from the Haskell 98
175 definition in the sense that Unicode alphabetic characters which
176 are neither upper nor lower case will still be identified as
177 alphabetic by <literal>isAlpha</literal>.</para>
182 <term>Strings treated as ISO-8859-1</term>
185 Various library functions, such as <literal>putStrLn</literal>,
186 treat Strings as if they were ISO-8859-1 rather than UTF-8.
194 <sect2 id="haskell98-undefined">
195 <title>GHC's interpretation of undefined behaviour in
196 Haskell 98</title>
198 <para>This section documents GHC's take on various issues that are
199 left undefined or implementation specific in Haskell 98.</para>
204 The <literal>Char</literal> type
205 <indexterm><primary><literal>Char</literal></primary><secondary>size of</secondary></indexterm>
208 <para>Following the ISO-10646 standard,
209 <literal>maxBound :: Char</literal> in GHC is
210 <literal>0x10FFFF</literal>.</para>
217 <indexterm><primary><literal>Int</literal></primary><secondary>size of</secondary></indexterm>
220 <para>In GHC the <literal>Int</literal> type follows the
221 size of an address on the host architecture; in other words
222 it holds 32 bits on a 32-bit machine, and 64-bits on a
223 64-bit machine.</para>
225 <para>Arithmetic on <literal>Int</literal> is unchecked for
226 overflow<indexterm><primary>overflow</primary><secondary><literal>Int</literal></secondary>
227 </indexterm>, so all operations on <literal>Int</literal> happen
229 2<superscript><replaceable>n</replaceable></superscript>
230 where <replaceable>n</replaceable> is the size in bits of
231 the <literal>Int</literal> type.</para>
233 <para>The <literal>fromInteger</literal><indexterm><primary><literal>fromInteger</literal></primary>
234 </indexterm>function (and hence
235 also <literal>fromIntegral</literal><indexterm><primary><literal>fromIntegral</literal></primary>
236 </indexterm>) is a special case when
237 converting to <literal>Int</literal>. The value of
238 <literal>fromIntegral x :: Int</literal> is given by taking
239 the lower <replaceable>n</replaceable> bits of <literal>(abs
240 x)</literal>, multiplied by the sign of <literal>x</literal>
241 (in 2's complement <replaceable>n</replaceable>-bit
242 arithmetic). This behaviour was chosen so that for example
243 writing <literal>0xffffffff :: Int</literal> preserves the
244 bit-pattern in the resulting <literal>Int</literal>.</para>
247 <para>Negative literals, such as <literal>-3</literal>, are
248 specified by (a careful reading of) the Haskell Report as
249 meaning <literal>Prelude.negate (Prelude.fromInteger 3)</literal>.
250 So <literal>-2147483648</literal> means <literal>negate (fromInteger 2147483648)</literal>.
251 Since <literal>fromInteger</literal> takes the lower 32 bits of the representation,
252 <literal>fromInteger (2147483648::Integer)</literal>, computed at type <literal>Int</literal> is
253 <literal>-2147483648::Int</literal>. The <literal>negate</literal> operation then
254 overflows, but it is unchecked, so <literal>negate (-2147483648::Int)</literal> is just
255 <literal>-2147483648</literal>. In short, one can write <literal>minBound::Int</literal> as
256 a literal with the expected meaning (but that is not in general guaranteed.
259 <para>The <literal>fromIntegral</literal> function also
260 preserves bit-patterns when converting between the sized
261 integral types (<literal>Int8</literal>,
262 <literal>Int16</literal>, <literal>Int32</literal>,
263 <literal>Int64</literal> and the unsigned
264 <literal>Word</literal> variants), see the modules
265 <literal>Data.Int</literal> and <literal>Data.Word</literal>
266 in the library documentation.</para>
271 <term>Unchecked float arithmetic</term>
273 <para>Operations on <literal>Float</literal> and
274 <literal>Double</literal> numbers are
275 <emphasis>unchecked</emphasis> for overflow, underflow, and
276 other sad occurrences. (note, however that some
277 architectures trap floating-point overflow and
278 loss-of-precision and report a floating-point exception,
279 probably terminating the
280 program)<indexterm><primary>floating-point
281 exceptions</primary></indexterm>.</para>
291 <title>Known bugs or infelicities</title>
293 <para>The bug tracker lists bugs that have been reported in GHC but not
294 yet fixed: see the <ulink url="http://sourceforge.net/projects/ghc/">SourceForge GHC
295 page</ulink>. In addition to those, GHC also has the following known bugs
296 or infelicities. These bugs are more permanent; it is unlikely that
297 any of them will be fixed in the short term.</para>
299 <sect2 id="bugs-ghc">
300 <title>Bugs in GHC</title>
304 <para> GHC can warn about non-exhaustive or overlapping
305 patterns (see <xref linkend="options-sanity"/>), and usually
306 does so correctly. But not always. It gets confused by
307 string patterns, and by guards, and can then emit bogus
308 warnings. The entire overlap-check code needs an overhaul
313 <para>GHC does not allow you to have a data type with a context
314 that mentions type variables that are not data type parameters.
317 data C a b => T a = MkT a
319 so that <literal>MkT</literal>'s type is
321 MkT :: forall a b. C a b => a -> T a
323 In principle, with a suitable class declaration with a functional dependency,
324 it's possible that this type is not ambiguous; but GHC nevertheless rejects
325 it. The type variables mentioned in the context of the data type declaration must
326 be among the type parameters of the data type.</para>
330 <para>GHC's inliner can be persuaded into non-termination
331 using the standard way to encode recursion via a data type:</para>
333 data U = MkU (U -> Bool)
336 russel u@(MkU p) = not $ p u
339 x = russel (MkU russel)
342 <para>We have never found another class of programs, other
343 than this contrived one, that makes GHC diverge, and fixing
344 the problem would impose an extra overhead on every
345 compilation. So the bug remains un-fixed. There is more
347 url="http://research.microsoft.com/~simonpj/Papers/inlining/">
348 Secrets of the GHC inliner</ulink>.</para>
352 <para>GHC does not keep careful track of
353 what instance declarations are 'in scope' if they come from other packages.
354 Instead, all instance declarations that GHC has seen in other
355 packages are all in scope everywhere, whether or not the
356 module from that package is used by the command-line
357 expression. This bug affects only the <option>--make</option> mode and
364 <sect2 id="bugs-ghci">
365 <title>Bugs in GHCi (the interactive GHC)</title>
368 <para>GHCi does not respect the <literal>default</literal>
369 declaration in the module whose scope you are in. Instead,
370 for expressions typed at the command line, you always get the
371 default default-type behaviour; that is,
372 <literal>default(Int,Double)</literal>.</para>
374 <para>It would be better for GHCi to record what the default
375 settings in each module are, and use those of the 'current'
376 module (whatever that is).</para>
380 <para>On Windows, there's a GNU ld/BFD bug
381 whereby it emits bogus PE object files that have more than
382 0xffff relocations. When GHCi tries to load a package affected by this
383 bug, you get an error message of the form
385 Loading package javavm ... linking ... WARNING: Overflown relocation field (# relocs found: 30765)
387 The last time we looked, this bug still
388 wasn't fixed in the BFD codebase, and there wasn't any
389 noticeable interest in fixing it when we reported the bug
392 <para>The workaround is to split up the .o files that make up
393 your package into two or more .o's, along the lines of
394 how the "base" package does it.</para>
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