ceph/src/boost/libs/spirit/doc/lex/lexer_semantic_actions.qbk

   1 [/==============================================================================
   2     Copyright (C) 2001-2011 Joel de Guzman
   3     Copyright (C) 2001-2011 Hartmut Kaiser
   4
   5     Distributed under the Boost Software License, Version 1.0. (See accompanying
   6     file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
   7 ===============================================================================/]
   8
   9 [section:lexer_semantic_actions Lexer Semantic Actions]
  10
  11 The main task of a lexer normally is to recognize tokens in the input.
  12 Traditionally this has been complemented with the possibility to execute
  13 arbitrary code whenever a certain token has been detected. __lex__ has been
  14 designed to support this mode of operation as well. We borrow from the concept
  15 of semantic actions for parsers (__qi__) and generators (__karma__). Lexer
  16 semantic actions may be attached to any token definition. These are C++
  17 functions or function objects that are called whenever a token definition
  18 successfully recognizes a portion of the input. Say you have a token definition
  19 `D`, and a C++ function `f`, you can make the lexer call `f` whenever it matches
  20 an input by attaching `f`:
  21
  22     D[f]
  23
  24 The expression above links `f` to the token definition, `D`. The required
  25 prototype of `f` is:
  26
  27     void f (Iterator& start, Iterator& end, pass_flag& matched, Idtype& id, Context& ctx);
  28
  29 [variablelist where:
  30     [[`Iterator& start`]    [This is the iterator pointing to the begin of the
  31                              matched range in the underlying input sequence. The
  32                              type of the iterator is the same as specified while
  33                              defining the type of the `lexertl::actor_lexer<...>`
  34                              (its first template parameter). The semantic action
  35                              is allowed to change the value of this iterator
  36                              influencing, the matched input sequence.]]
  37     [[`Iterator& end`]      [This is the iterator pointing to the end of the
  38                              matched range in the underlying input sequence. The
  39                              type of the iterator is the same as specified while
  40                              defining the type of the `lexertl::actor_lexer<...>`
  41                              (its first template parameter). The semantic action
  42                              is allowed to change the value of this iterator
  43                              influencing, the matched input sequence.]]
  44     [[`pass_flag& matched`] [This value is pre/initialized to `pass_normal`.
  45                              If the semantic action sets it to `pass_fail` this
  46                              behaves as if the token has not been matched in
  47                              the first place. If the semantic action sets this
  48                              to `pass_ignore` the lexer ignores the current
  49                              token and tries to match a next token from the
  50                              input.]]
  51     [[`Idtype& id`]         [This is the token id of the type Idtype (most of
  52                              the time this will be a `std::size_t`) for the
  53                              matched token. The semantic action is allowed to
  54                              change the value of this token id, influencing the
  55                              if of the created token.]]
  56     [[`Context& ctx`]       [This is a reference to a lexer specific,
  57                              unspecified type, providing the context for the
  58                              current lexer state. It can be used to access
  59                              different internal data items and is needed for
  60                              lexer state control from inside a semantic
  61                              action.]]
  62 ]
  63
  64 When using a C++ function as the semantic action the following prototypes are
  65 allowed as well:
  66
  67     void f (Iterator& start, Iterator& end, pass_flag& matched, Idtype& id);
  68     void f (Iterator& start, Iterator& end, pass_flag& matched);
  69     void f (Iterator& start, Iterator& end);
  70     void f ();
  71
  72 [important In order to use lexer semantic actions you need to use type
  73            `lexertl::actor_lexer<>` as your lexer class (instead of the
  74            type `lexertl::lexer<>` as described in earlier examples).]
  75
  76 [heading The context of a lexer semantic action]
  77
  78 The last parameter passed to any lexer semantic action is a reference to an
  79 unspecified type (see the `Context` type in the table above). This type is
  80 unspecified because it depends on the token type returned by the lexer. It is
  81 implemented in the internals of the iterator type exposed by the lexer.
  82 Nevertheless, any context type is expected to expose a couple of
  83 functions allowing to influence the behavior of the lexer. The following table
  84 gives an overview and a short description of the available functionality.
  85
  86 [table Functions exposed by any context passed to a lexer semantic action
  87     [[Name]   [Description]]
  88     [[`Iterator const& get_eoi() const`]
  89      [The function `get_eoi()` may be used by to access the end iterator of
  90       the input stream the lexer has been initialized with]]
  91     [[`void more()`]
  92      [The function `more()` tells the lexer that the next time it matches a
  93       rule, the corresponding token should be appended onto the current token
  94       value rather than replacing it.]]
  95     [[`Iterator const& less(Iterator const& it, int n)`]
  96      [The function `less()` returns an iterator positioned to the nth input
  97       character beyond the current token start iterator (i.e. by passing the
  98       return value to the parameter `end` it is possible to return all but the
  99       first n characters of the current token back to the input stream.]]
 100     [[`bool lookahead(std::size_t id)`]
 101      [The function `lookahead()` can be used to implement lookahead for lexer
 102       engines not supporting constructs like flex' `a/b`
 103       (match `a`, but only when followed by `b`). It invokes the lexer on the
 104       input following the current token without actually moving forward in the
 105       input stream. The function returns whether the lexer was able to match a
 106       token with the given token-id `id`.]]
 107     [[`std::size_t get_state() const` and `void set_state(std::size_t state)`]
 108      [The functions `get_state()` and `set_state()` may be used to introspect
 109       and change the current lexer state.]]
 110     [[`token_value_type get_value() const` and `void set_value(Value const&)`]
 111      [The functions `get_value()` and `set_value()` may be used to introspect
 112       and change the current token value.]]
 113 ]
 114
 115 [heading Lexer Semantic Actions Using Phoenix]
 116
 117 Even if it is possible to write your own function object implementations (i.e.
 118 using Boost.Lambda or Boost.Bind), the preferred way of defining lexer semantic
 119 actions is to use __boost_phoenix__. In this case you can access the parameters
 120 described above by using the predefined __spirit__ placeholders:
 121
 122 [table Predefined Phoenix placeholders for lexer semantic actions
 123     [[Placeholder]    [Description]]
 124     [[`_start`]
 125      [Refers to the iterator pointing to the beginning of the matched input
 126       sequence. Any modifications to this iterator value will be reflected in
 127       the generated token.]]
 128     [[`_end`]
 129      [Refers to the iterator pointing past the end of the matched input
 130       sequence. Any modifications to this iterator value will be reflected in
 131       the generated token.]]
 132     [[`_pass`]
 133      [References the value signaling the outcome of the semantic action. This
 134       is pre-initialized to `lex::pass_flags::pass_normal`. If this is set to
 135       `lex::pass_flags::pass_fail`, the lexer will behave as if no token has
 136       been matched, if is set to `lex::pass_flags::pass_ignore`, the lexer will
 137       ignore the current match and proceed trying to match tokens from the
 138       input.]]
 139     [[`_tokenid`]
 140      [Refers to the token id of the token to be generated. Any modifications
 141       to this value will be reflected in the generated token.]]
 142     [[`_val`]
 143      [Refers to the value the next token will be initialized from. Any
 144       modifications to this value will be reflected in the generated token.]]
 145     [[`_state`]
 146      [Refers to the lexer state the input has been match in. Any modifications
 147       to this value will be reflected in the lexer itself (the next match will
 148       start in the new state). The currently generated token is not affected
 149       by changes to this variable.]]
 150     [[`_eoi`]
 151      [References the end iterator of the overall lexer input. This value
 152       cannot be changed.]]
 153 ]
 154
 155 The context object passed as the last parameter to any lexer semantic action is
 156 not directly accessible while using __boost_phoenix__ expressions. We rather provide
 157 predefined Phoenix functions allowing to invoke the different support functions
 158 as mentioned above. The following table lists the available support functions
 159 and describes their functionality:
 160
 161 [table Support functions usable from Phoenix expressions inside lexer semantic actions
 162     [[Plain function] [Phoenix function] [Description]]
 163     [[`ctx.more()`]
 164      [`more()`]
 165      [The function `more()` tells the lexer that the next time it matches a
 166       rule, the corresponding token should be appended onto the current token
 167       value rather than replacing it.]]
 168     [[`ctx.less()`]
 169      [`less(n)`]
 170      [The function `less()` takes a single integer parameter `n` and returns an
 171       iterator positioned to the nth input character beyond the current token
 172       start iterator (i.e. by assigning the return value to the placeholder
 173       `_end` it is possible to return all but the first `n` characters of the
 174       current token back to the input stream.]]
 175     [[`ctx.lookahead()`]
 176      [`lookahead(std::size_t)` or `lookahead(token_def)`]
 177      [The function `lookahead()` takes a single parameter specifying the token
 178       to match in the input. The function can be used for instance to implement
 179       lookahead for lexer engines not supporting constructs like flex' `a/b`
 180       (match `a`, but only when followed by `b`). It invokes the lexer on the
 181       input following the current token without actually moving forward in the
 182       input stream. The function returns whether the lexer was able to match
 183       the specified token.]]
 184 ]
 185
 186 [endsect]