docs/latex/wx/texpr.tex

   1 \section{wxExpr overview}\label{exproverview}
   2
   3 wxExpr is a C++ class reading and writing a subset of Prolog-like syntax,
   4 supporting objects attribute/value pairs.
   5
   6 wxExpr can be used to develop programs with readable and
   7 robust data files. Within wxWindows itself, it is used to parse
   8 the {\tt .wxr} dialog resource files.
   9
  10 {\bf History of wxExpr}
  11
  12 During the development of the tool Hardy within the AIAI, a need arose
  13 for a data file format for C++ that was easy for both humans and
  14 programs to read, was robust in the face of fast-moving software
  15 development, and that provided some compatibility with AI languages
  16 such as Prolog and LISP.
  17
  18 The result was the wxExpr library (formerly called PrologIO), which is able to read and write a
  19 Prolog-like attribute-value syntax, and is additionally capable of
  20 writing LISP syntax for no extra programming effort.  The advantages of
  21 such a library are as follows:
  22
  23 \begin{enumerate}\itemsep=0pt
  24 \item The data files are readable by humans;
  25 \item I/O routines are easier to write and debug compared with using binary files;
  26 \item the files are robust: unrecognised data will just be ignored by the application
  27 \item Inbuilt hashing gives a random access capability, useful for when linking
  28 up C++ objects as data is read in;
  29 \item Prolog and LISP programs can load the files using a single command.
  30 \end{enumerate}
  31
  32 The library was extended to use the ability to read and write
  33 Prolog-like structures for remote procedure call (RPC) communication.
  34 The next two sections outline the two main ways the library can be used.
  35
  36 \subsection{wxExpr for data file manipulation}\itemsep=0pt
  37
  38 The fact that the output is in Prolog syntax is irrelevant for most
  39 programmers, who just need a reasonable I/O facility.  Typical output
  40 looks like this:
  41
  42 \begin{verbatim}
  43 diagram_definition(type = "Spirit Belief Network").
  44
  45 node_definition(type = "Model",
  46   image_type = "Diamond",
  47   attribute_for_label = "name",
  48   attribute_for_status_line = "label",
  49   colour = "CYAN",
  50   default_width = 120,
  51   default_height = 80,
  52   text_size = 10,
  53   can_resize = 1,
  54   has_hypertext_item = 1,
  55   attributes = ["name", "combining_function", "level_of_belief"]).
  56
  57 arc_definition(type = "Potentially Confirming",
  58   image_type = "Spline",
  59   arrow_type = "End",
  60   line_style = "Solid",
  61   width = 1,
  62   segmentable = 0,
  63   attribute_for_label = "label",
  64   attribute_for_status_line = "label",
  65   colour = "BLACK",
  66   text_size = 10,
  67   has_hypertext_item = 1,
  68   can_connect_to = ["Evidence", "Cluster", "Model", "Evidence", "Evidence", "Cluster"],
  69   can_connect_from = ["Data", "Evidence", "Cluster", "Evidence", "Data", "Cluster"]).
  70 \end{verbatim}
  71
  72 This is substantially easier to read and debug than a series of numbers and
  73 strings.
  74
  75 Note the object-oriented style: a file comprises a series of {\it clauses}.
  76 Each clause is an object with a {\it functor}\/ or object name, followed
  77 by a list of attribute-value pairs enclosed in parentheses, and finished
  78 with a full stop.  Each attribute value may be a string, a word (no quotes),
  79 an integer, a real number, or a list with potentially recursive elements.
  80
  81 The way that the facility is used by an application to read in a file is
  82 as follows:
  83
  84 \begin{enumerate}\itemsep=0pt
  85 \item The application creates a wxExprDatabase instance.
  86 \item The application tells the database to read in the entire file.
  87 \item The application searches the database for objects it requires,
  88 decomposing the objects using the wxExpr API. The database may be hashed,
  89 allowing rapid linking-up of application data.
  90 \item The application deletes or clears the wxExprDatabase.
  91 \end{enumerate}
  92
  93 Writing a file is just as easy:
  94
  95 \begin{enumerate}\itemsep=0pt
  96 \item The application creates a wxExprDatabase instance.
  97 \item The application adds objects to the database using the API.
  98 \item The application tells the database to write out the entire database,
  99 in Prolog or LISP notation.
 100 \item The application deletes or clears the wxExprDatabase.
 101 \end{enumerate}
 102
 103 To use the library, include "wxexpr.h".
 104
 105 \subsection{wxExpr compilation}
 106
 107 For UNIX compilation, ensure that YACC and LEX or FLEX are on your system. Check that
 108 the makefile uses the correct programs: a common error is to compile
 109 y\_tab.c with a C++ compiler. Edit the CCLEX variable in make.env
 110 to specify a C compiler. Also, do not attempt to compile lex\_yy.c
 111 since it is included by y\_tab.c.
 112
 113 For DOS compilation, the simplest thing is to copy dosyacc.c to y\_tab.c, and doslex.c to
 114 lex\_yy.c. It is y\_tab.c that must be compiled (lex\_yy.c is included by
 115 y\_tab.c) so if adding source files to a project file, ONLY add y\_tab.c
 116 plus the .cc files. If you wish to alter the parser, you will need YACC
 117 and FLEX on DOS.
 118
 119 The DOS tools are available at the AIAI ftp site, in the tools directory. Note that
 120 for FLEX installation, you need to copy flex.skl into the directory
 121 c:/lib.
 122
 123 If you are using Borland C++ and wish to regenerate lex\_yy.c and y\_tab.c
 124 you need to generate lex\_yy.c with FLEX and then comment out the `malloc' and `free'
 125 prototypes in lex\_yy.c. It will compile with lots of warnings. If you
 126 get an undefined \_PROIO\_YYWRAP symbol when you link, you need to remove
 127 USE\_DEFINE from the makefile and recompile. This is because the parser.y
 128 file has a choice of defining this symbol as a function or as a define,
 129 depending on what the version of FLEX expects. See the bottom of
 130 parser.y, and if necessary edit it to make it compile in the opposite
 131 way to the current compilation.
 132
 133 %To test out wxExpr compile the test program (samples/wxexpr/wxexpr.exe),
 134 %and try loading test.exp into the test
 135 %program. Then save it to another file. If the second is identical to the
 136 %first, wxExpr is in a working state.
 137
 138 \subsection{Bugs}
 139
 140 These are the known bugs:
 141
 142 \begin{enumerate}\itemsep=0pt
 143 \item Functors are permissible only in the main clause (object).
 144 Therefore nesting of structures must be done using lists, not predicates
 145 as in Prolog.
 146 \item There is a limit to the size of strings read in (about 5000 bytes).
 147 \end{enumerate}
 148
 149 \subsection{Using wxExpr}
 150
 151 This section is a brief introduction to using the wxExpr package.
 152
 153 First, some terminology.  A {\it wxExprDatabase}\/ is a list of {\it clauses},
 154 each of which represents an object or record which needs to be saved to a file.
 155 A clause has a {\it functor}\/ (name), and a list of attributes, each of which
 156 has a value.  Attributes may take the following types of value: string, word,
 157 integer, floating point number, and list.  A list can itself contain any
 158 type, allowing for nested data structures.
 159
 160 Consider the following code.
 161
 162 \begin{verbatim}
 163 wxExprDatabase db;
 164
 165 wxExpr *my_clause = new wxExpr("object");
 166 my_clause->AddAttributeValue("id", (long)1);
 167 my_clause->AddAttributeValueString("name", "Julian Smart");
 168 db.Append(my_clause);
 169
 170 ofstream file("my_file");
 171 db.Write(file);
 172 \end{verbatim}
 173
 174 This creates a database, constructs a clause, adds it to the database,
 175 and writes the whole database to a file.  The file it produces looks like
 176 this:
 177
 178 \begin{verbatim}
 179 object(id = 1,
 180   name = "Julian Smart").
 181 \end{verbatim}
 182
 183 To read the database back in, the following will work:
 184
 185 \begin{verbatim}
 186 wxExprDatabase db;
 187 db.Read("my_file");
 188
 189 db.BeginFind();
 190
 191 wxExpr *my_clause = db.FindClauseByFunctor("object");
 192 int id = 0;
 193 wxString name = "None found";
 194
 195 my_clause->GetAttributeValue("id", id);
 196 my_clause->GetAttributeValue("name", name);
 197
 198 cout << "Id is " << id << ", name is " << name << "\n";
 199 \end{verbatim}
 200
 201 Note the setting of defaults before attempting to retrieve attribute values,
 202 since they may not be found.
 203