[DGD] parse string difficulties

[Robert Forshaw]

Despite my best efforts I can't understand how parse string is supposed to 
work. I thought I *might* know what I was doing, when I put it to the test, 
but got some unexpected results.

Right now the reason for me employing parse_string is to interpret 
datafiles. The datafiles are meant to contain three things: operators, a 
prefix-parameter and a suffix-parameter. Operators consist of one symbol 
only and can be: '+', '-', '=', or '.'. A prefix-parameter is optional and 
appears behind the operator, a suffix-parameter is also optional and appears 
in front of it.

So an example data file might look like this:

.food
.chocolate
weight=8
layers+3
fattening=yes
strange-
smell-noxious

The prefix-parameter, operator and suffix-parameter collectively make a 
line, with newlines acting as seperators for these lines.

I want parse_string to interpret the entire datafile in one string, and 
allocate an array in the following format: ({ ({ operator, prefix-parameter, 
suffix-parameter }), ({ operator, prefix-parameter, suffix-parameter }) ... 
}). If a prefix-parameter or suffix parameter is occluded, that respective 
element in the returned array will be nil.


So in the example:

.food
.chocolate
weight=8
layers+3
fattening=yes
strange-
smell-noxious

it would return:


({ ({ ".", nil, "food" }), ({ ".", "nil", "chocolate" }), ({ "=", "weight", 
"8" }), ({ "+", "layers", "3"}),
    ({ "=", "fattening", "yes" }), ({ "-", "strange", nil }), ({ "-", 
"smell", "noxious" }) })

Call me stupid, call me what you like, but I can't figure out how to 
accomplish this using parse_string. I can't even do something much simpler, 
like parseing a single line. Here:

ar = parse_string(
        "whitespace = /[\b\r\n\t ]+/" +
        "word       = /[a-zA-Z]+/" +
        "operator   = /[\.\+\=\-]+/" +
        "SENTENCE   : operator word operator",
        "property+value");

Now this returns nil, but I want it to return ({ "property", "+", "value" 
}).

And for some bizzare reason, changing the operator regexp so that it only 
detects "+":

ar = parse_string(
        "whitespace = /[\b\r\n\t ]+/" +
        "word       = /[a-zA-Z]+/" +
        "operator   = /++/" +
        "SENTENCE   : operator word operator",
        "property+value");

Causes a malformed rule error.

I seriously don't know what I'm doing here. It would be most helpful if 
someone could show me line for line how to write a grammar that interprets 
my datafiles, that would help me relate to what the function is doing. 
Anyway, all help is appreciated...

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