ghc/docs/users_guide/vs_haskell.sgml

   1 <Sect1 id="vs-Haskell-defn">
   2 <Title>Haskell&nbsp;98 vs.&nbsp;Glasgow Haskell: language non-compliance
   3 </Title>
   4
   5 <Para>
   6 <IndexTerm><Primary>GHC vs the Haskell 98 language</Primary></IndexTerm>
   7 <IndexTerm><Primary>Haskell 98 language vs GHC</Primary></IndexTerm>
   8 </Para>
   9
  10 <Para>
  11 This section lists Glasgow Haskell infelicities in its implementation
  12 of Haskell&nbsp;98.  See also the &ldquo;when things go wrong&rdquo; section
  13 (<XRef LinkEnd="wrong">)
  14 for information about crashes, space leaks, and other undesirable
  15 phenomena.
  16 </Para>
  17
  18 <Para>
  19 The limitations here are listed in Haskell-Report order (roughly).
  20 </Para>
  21
  22 <Sect2 id="infelicities-exprs-pats">
  23 <Title>Expressions and patterns
  24 </Title>
  25
  26 <Para>
  27 <VariableList>
  28
  29 <VarListEntry>
  30 <Term>Very long <Literal>String</Literal> constants:</Term>
  31 <ListItem>
  32 <Para>
  33 May not go through.  If you add a &ldquo;string gap&rdquo; every
  34 few thousand characters, then the strings can be as long
  35 as you like.
  36 </Para>
  37
  38 <Para>
  39 Bear in mind that string gaps and the <Option>-cpp</Option><IndexTerm><Primary>-cpp option</Primary></IndexTerm>
  40 option don't mix very well (see <XRef LinkEnd="c-pre-processor">).
  41 </Para>
  42 </ListItem>
  43 </VarListEntry>
  44 <VarListEntry>
  45 <Term>Single quotes in module names:</Term>
  46 <ListItem>
  47 <Para>
  48 It might work, but it's just begging for trouble.
  49 </Para>
  50 </ListItem>
  51 </VarListEntry>
  52 </VariableList>
  53 </Para>
  54
  55 </Sect2>
  56
  57 <Sect2 id="infelicities-decls">
  58 <Title>Declarations and bindings
  59 </Title>
  60
  61 <Para>
  62 None known.
  63 </Para>
  64
  65 </Sect2>
  66
  67 <Sect2 id="infelicities-Modules">
  68 <Title>Module system and interface files
  69 </Title>
  70
  71 <Para>
  72 <VariableList>
  73
  74 <VarListEntry>
  75 <Term> Namespace pollution </Term>
  76 <ListItem>
  77 <Para>
  78 Several modules internal to GHC are visible in the standard namespace.
  79 All of these modules begin with <Literal>Prel</Literal>, so the rule
  80 is: don't use any modules beginning with <Literal>Prel</Literal> in
  81 your program, or you will be comprehensively screwed.
  82 </Para>
  83 </ListItem>
  84 </VarListEntry>
  85 </VariableList>
  86 </Para>
  87
  88 </Sect2>
  89
  90 <Sect2 id="infelicities-numbers">
  91 <Title>Numbers, basic types, and built-in classes
  92 </Title>
  93
  94 <Para>
  95 <VariableList>
  96
  97 <VarListEntry>
  98 <Term>Unchecked arithmetic:</Term>
  99 <ListItem>
 100 <Para>
 101 Arguably <Emphasis>not</Emphasis> an infelicity, but&hellip; Bear in
 102 mind that operations on <Literal>Int</Literal>,
 103 <Literal>Float</Literal>, and <Literal>Double</Literal> numbers are
 104 <Emphasis>unchecked</Emphasis> for overflow, underflow, and other sad
 105 occurrences.  (note, however that some architectures trap
 106 floating-point overflow and loss-of-precision and report a
 107 floating-point exception, probably terminating the
 108 program)<IndexTerm><Primary>floating-point
 109 exceptions</Primary></IndexTerm>.
 110 </Para>
 111
 112 <Para>
 113 Use <Literal>Integer</Literal>, <Literal>Rational</Literal>, etc.,
 114 numeric types if this stuff keeps you awake at night.
 115 </Para>
 116 </ListItem>
 117 </VarListEntry>
 118 <VarListEntry>
 119 <Term>Multiply-defined array elements&mdash;not checked:</Term>
 120 <ListItem>
 121 <Para>
 122 This code fragment <Emphasis>should</Emphasis> elicit a fatal error, but it does not:
 123
 124 <ProgramListing>
 125 main = print (array (1,1) [(1,2), (1,3)])
 126 </ProgramListing>
 127
 128 </Para>
 129 </ListItem>
 130 </VarListEntry>
 131 </VariableList>
 132 </Para>
 133
 134 </Sect2>
 135
 136 <Sect2 id="infelicities-Prelude">
 137 <Title>In Prelude support
 138 </Title>
 139
 140 <Para>
 141 <VariableList>
 142
 143 <VarListEntry>
 144 <Term>Arbitrary-sized tuples:</Term>
 145 <ListItem>
 146 <Para>
 147 Plain old tuples of arbitrary size <Emphasis>do</Emphasis> work.
 148 </Para>
 149
 150 <Para>
 151 HOWEVER: standard instances for tuples (<Literal>Eq</Literal>, <Literal>Ord</Literal>, <Literal>Bounded</Literal>, <Literal>Ix</Literal>
 152 <Literal>Read</Literal>, and <Literal>Show</Literal>) are available <Emphasis>only</Emphasis> up to 5-tuples.
 153 </Para>
 154
 155 <Para>
 156 These limitations are easily subvertible, so please ask if you get
 157 stuck on them.
 158 </Para>
 159 </ListItem>
 160 </VarListEntry>
 161 </VariableList>
 162 </Para>
 163
 164 </Sect2>
 165
 166 </Sect1>
 167
 168 <!-- Emacs stuff:
 169      ;;; Local Variables: ***
 170      ;;; mode: sgml ***
 171      ;;; sgml-parent-document: ("users_guide.sgml" "book" "chapter" "sect1") ***
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 173  -->