ghc/docs/users_guide/vs_haskell.vsgml

   1 %************************************************************************
   2 %*                                                                      *
   3 <sect1>Haskell~1.4 vs.~Glasgow Haskell~3.00: language non-compliance
   4 <label id="vs-Haskell-defn">
   5 <p>
   6 <nidx>GHC vs the Haskell 1.4 language</nidx>
   7 <nidx>Haskell 1.4 language vs GHC</nidx>
   8 %*                                                                      *
   9 %************************************************************************
  10
  11 This section lists Glasgow Haskell infelicities in its implementation
  12 of Haskell~1.4.  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 of @Read@ and @Show@ for infix constructors:</tag>
  60 All the carry-on about derived @readsPrec@ and @showsPrec@ for infix
  61 constructors---we don't do it (yet).  We treat them the same way as
  62 all other constructors.
  63
  64 %-------------------------------------------------------------------
  65 <tag>Derived instances for records:</tag> Hmmm.
  66 </descrip>
  67
  68 %************************************************************************
  69 %*                                                                      *
  70 <sect2>Module system and interface files
  71 <label id="infelicities-Modules">
  72 <p>
  73 %*                                                                      *
  74 %************************************************************************
  75
  76 <descrip>
  77 %-------------------------------------------------------------------
  78 <tag> Namespace pollution </tag>
  79
  80 Several modules internal to GHC are visible in the standard namespace.
  81 All of these modules begin with @Prel@, so the rule is: don't use any
  82 modules beginning with @Prel@ in your programl, or you will be
  83 comprehensively screwed.
  84
  85 %-------------------------------------------------------------------
  86 <tag>Can't export primitive types (e.g., @Int#@):</tag>
  87
  88 Don't even try...
  89 </descrip>
  90
  91 %************************************************************************
  92 %*                                                                      *
  93 <sect2>Numbers, basic types, and built-in classes
  94 <label id="infelicities-numbers">
  95 <p>
  96 %*                                                                      *
  97 %************************************************************************
  98
  99 <descrip>
 100 %-------------------------------------------------------------------
 101 <tag>Very large/small fractional constants:</tag>
 102 (i.e., with a decimal point somewhere) GHC does not check that these
 103 are out of range (e.g., for a @Float@), and bad things will inevitably
 104 follow.  (To be corrected?)
 105
 106 This problem does <em>not</em> exist for integral constants.
 107
 108 For very large/small fractional constants near the limits of your
 109 floating-point precision, things may go wrong.  (It's better than it
 110 used to be.)  Please report any such bugs.
 111
 112 %-------------------------------------------------------------------
 113 <tag>Unchecked arithmetic:</tag>
 114
 115 Arguably <em>not</em> an infelicity, but... Bear in mind that
 116 operations on @Int@, @Float@, and @Double@ numbers are
 117 <em>unchecked</em> for overflow, underflow, and other sad occurrences.
 118 (note, however that some architectures trap floating-point overflow
 119 and loss-of-precision and report a floating-point exception).
 120
 121 Use @Integer@, @Rational@, etc., numeric types if this stuff
 122 keeps you awake at night.
 123
 124 %-------------------------------------------------------------------
 125 <tag>Multiply-defined array elements---not checked:</tag>
 126 This code fragment <em>should</em> elicit a fatal error, but it does not:
 127 <tscreen><verb>
 128 main = print (array (1,1) [ 1:=2, 1:=3 ])
 129 </verb></tscreen>
 130 </descrip>
 131
 132 %************************************************************************
 133 %*                                                                      *
 134 <sect2>In Prelude support
 135 <label id="infelicities-Prelude">
 136 <p>
 137 %*                                                                      *
 138 %************************************************************************
 139
 140 <descrip>
 141 %-------------------------------------------------------------------
 142 <tag>Arbitrary-sized tuples:</tag>
 143 Plain old tuples of arbitrary size <em>do</em> work.  Note that lots of
 144 overloading can give rise to large tuples ``under the hood'' of your
 145 program.
 146
 147 HOWEVER: standard instances for tuples (@Eq@, @Ord@, @Bounded@, @Ix@
 148 @Read@, and @Show@) are available <em>only</em> up to 5-tuples.
 149
 150 These limitations are easily subvertible, so please ask if you get
 151 stuck on them.
 152
 153 %-------------------------------------------------------------------
 154 <tag>Unicode character set:</tag>
 155 Haskell~1.4 embraces the Unicode character set, but GHC~3.00 doesn't
 156 handle it. Yet.
 157
 158 </descrip>