ghc/docs/users_guide/vs_haskell.vsgml

   1 %************************************************************************
   2 %*                                                                      *
   3 <sect1>Haskell~98 vs.~Glasgow Haskell: language non-compliance
   4 <label id="vs-Haskell-defn">
   5 <p>
   6 <nidx>GHC vs the Haskell 98 language</nidx>
   7 <nidx>Haskell 98 language vs GHC</nidx>
   8 %*                                                                      *
   9 %************************************************************************
  10
  11 This section lists Glasgow Haskell infelicities in its implementation
  12 of Haskell~98.  See also the ``when things go wrong'' section
  13 (Section <ref name="What to do when something goes wrong" id="wrong">)
  14 for information about crashes, space leaks, and other undesirable
  15 phenomena.
  16
  17 The limitations here are listed in Haskell-Report order (roughly).
  18
  19 %************************************************************************
  20 %*                                                                      *
  21 <sect2>Expressions and patterns
  22 <label id="infelicities-exprs-pats">
  23 <p>
  24 %*                                                                      *
  25 %************************************************************************
  26
  27 <descrip>
  28
  29 %-------------------------------------------------------------------
  30 <tag>Very long @String@ constants:</tag>
  31 May not go through.  If you add a ``string gap'' every
  32 few thousand characters, then the strings can be as long
  33 as you like.
  34
  35 Bear in mind that string gaps and the @-cpp@<nidx>-cpp option</nidx>
  36 option don't mix very well (see Section <ref id="c-pre-processor"
  37 name="The C pre-processor">).
  38
  39 %-------------------------------------------------------------------
  40 <tag>Very long literal lists:</tag>
  41 These may tickle a ``yacc stack overflow'' error in the parser.
  42 (It depends on the Yacc used to build your parser.)
  43
  44 %-------------------------------------------------------------------
  45 <tag>Single quotes in module names:</tag>
  46 It might work, but it's just begging for trouble.
  47 </descrip>
  48
  49 %************************************************************************
  50 %*                                                                      *
  51 <sect2>Declarations and bindings
  52 <label id="infelicities-decls">
  53 <p>
  54 %*                                                                      *
  55 %************************************************************************
  56
  57 <descrip>
  58 %-------------------------------------------------------------------
  59 <tag>Derived instances for records:</tag> Hmmm.
  60 </descrip>
  61
  62 %************************************************************************
  63 %*                                                                      *
  64 <sect2>Module system and interface files
  65 <label id="infelicities-Modules">
  66 <p>
  67 %*                                                                      *
  68 %************************************************************************
  69
  70 <descrip>
  71 %-------------------------------------------------------------------
  72 <tag> Namespace pollution </tag>
  73
  74 Several modules internal to GHC are visible in the standard namespace.
  75 All of these modules begin with @Prel@, so the rule is: don't use any
  76 modules beginning with @Prel@ in your program, or you will be
  77 comprehensively screwed.
  78
  79 </descrip>
  80
  81 %************************************************************************
  82 %*                                                                      *
  83 <sect2>Numbers, basic types, and built-in classes
  84 <label id="infelicities-numbers">
  85 <p>
  86 %*                                                                      *
  87 %************************************************************************
  88
  89 <descrip>
  90 %-------------------------------------------------------------------
  91 <tag>Unchecked arithmetic:</tag>
  92
  93 Arguably <em>not</em> an infelicity, but... Bear in mind that
  94 operations on @Int@, @Float@, and @Double@ numbers are
  95 <em>unchecked</em> for overflow, underflow, and other sad occurrences.
  96 (note, however that some architectures trap floating-point overflow
  97 and loss-of-precision and report a floating-point
  98 exception)<nidx>floating-point exceptions</nidx>.
  99
 100 Use @Integer@, @Rational@, etc., numeric types if this stuff
 101 keeps you awake at night.
 102
 103 %-------------------------------------------------------------------
 104 <tag>Multiply-defined array elements---not checked:</tag>
 105 This code fragment <em>should</em> elicit a fatal error, but it does not:
 106 <tscreen><verb>
 107 main = print (array (1,1) [ 1:=2, 1:=3 ])
 108 </verb></tscreen>
 109 </descrip>
 110
 111 %************************************************************************
 112 %*                                                                      *
 113 <sect2>In Prelude support
 114 <label id="infelicities-Prelude">
 115 <p>
 116 %*                                                                      *
 117 %************************************************************************
 118
 119 <descrip>
 120 %-------------------------------------------------------------------
 121 <tag>Arbitrary-sized tuples:</tag>
 122 Plain old tuples of arbitrary size <em>do</em> work.
 123
 124 HOWEVER: standard instances for tuples (@Eq@, @Ord@, @Bounded@, @Ix@
 125 @Read@, and @Show@) are available <em>only</em> up to 5-tuples.
 126
 127 These limitations are easily subvertible, so please ask if you get
 128 stuck on them.
 129
 130 %-------------------------------------------------------------------
 131 <tag>Unicode character set:</tag>
 132 Haskell 98 embraces the Unicode character set, but GHC doesn't
 133 handle it. Yet.
 134
 135 </descrip>