[DGD] Desperately Wanted

[Par Winzell]

David,

 > I would kill for, give booze / pizza / money / code boring objects for,
 > do just about anything for a code snippet or a deeper explanation
 > of the parse_string() function.

First, a computer science degree helps or an equivalent amount
of immersion in context-free grammars help. Alternately you can
try to give yourself a crash course; I suggest buying a book in
print. The traditional work is "the dragon book",

http://www.amazon.com/exec/obidos/ISBN%3D0201100886/104-5095468-2962302

but it's not cheap. You can get the basic idea from various web
pages, but any non-trivial use of parse_string gets into subtle
stuff pretty quickly (specifically, ambiguous grammars).

My second recommendation is a Google search for

	mud-dev string parsing

where you have to click on 'repeat search with ommitted results
included' at the bottom of the page. This will uncover all the
discussions in late 1997 on DGD's parse_string(), most of it by
Dworkin himself. I believe this is the first node:

http://www.kanga.nu/archives/MUD-Dev-L/1997Q4/msg00164.php


There are also lots of people who have done parse_string work
on this very list. Here is a small grammar that Skotos uses to
parse ascii representations of LPC values. This is overkill,
because a simple recursive descent parser can do the trick... 

... but hell, it works pretty darn well.

=========================================================================
private
string query_grammar() {
   return
      "whitespace = /[ \t\r\n]+/\n" +
      "int = /[0-9]+/\n" +
      "float = /[+-]?([0-9]+\\.|[0-9]*\\.[0-9]+)([eE][+-]?[0-9]+)?/\n" +
      "str = /\"([^\\\\\"]*\\\\.)*[^\\\\\"]*\"/\n" +
      "obj = /<[^>]+>/\n" +

      "Value: 'nil' ? make_nil\n" +
      "Value: int ? parse_int\n" +
      "Value: float ? parse_flt\n" +
      "Value: str ? parse_str\n" +
      "Value: obj ? parse_obj\n" +
      "Value: '{' '}' ? empty_array\n" +
      "Value: '{' ArrList '}' ? pick_middle\n" +
      "Value: '[' ']' ? empty_mapping\n" +
      "Value: '[' MapList ']' ? pick_middle\n" +
      "ArrList: Value ? make_array\n" +
      "ArrList: ArrList ',' Value ? build_array\n" +
      "MapList: Value ':' Value ? make_mapping\n" +
      "MapList: MapList ',' Value ':' Value ? build_mapping\n";
}
=========================================================================

With:

=========================================================================
static
mixed *colour_value(mixed *tree) {
   return ({ paint_value(tree[3], (int) tree[1]) });
}

static
mixed *make_nil(mixed *tree) {
   return ({ nil });
}

static
mixed *parse_int(mixed *tree) {
   return ({ (int) tree[0] });
}

static
mixed *parse_flt(mixed *tree) {
   SysLog(dump_value(tree));
   return ({ (float) tree[0] });
}

      "Value: float {{ return ({ (float) tree[0] }); }}"

static
mixed *parse_str(mixed *tree) {
   return ({
      replace_strings(tree[0][1 .. strlen(tree[0])-2],
		      "\\\"", "\"",
		      "\\\\", "\\")
	 });
}

static
mixed *parse_obj(string *tree) {
   object obj;

   obj = find_object(tree[0][1 .. strlen(tree[0])-2]);
   if (!obj) {
      error("cannot find object " + tree[0]);
   }
   return ({ obj });
}

static
mixed *pick_middle(mixed *tree) {
   return tree[1 .. 1];
}

static
mixed *empty_array(mixed *tree) {
   return ({ ({ }) });
}

static
mixed *make_array(mixed *tree) {
   return ({ tree });
}

static
mixed *build_array(mixed *tree) {
   return ({ tree[0] + ({ tree[2] }) });
}

static
mixed *empty_mapping(mixed *tree) {
   return ({ ([ ]) });
}

static
mixed *make_mapping(mixed *tree) {
   return ({ ([ tree[0] : tree[2] ]) });
}

static
mixed *build_mapping(mixed *tree) {
   return ({ tree[0] + ([ tree[2] : tree[4] ]) });
}
=========================================================================


There are lots more interesting examples out there. Anyone?

Zell
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