PEG-GML

Parsing Expression Grammars for GML (port of cpp-peglib)

Instead of writing your own parser in GML for whatever it is you want to parse, you can use PEG-GML to create a parser for you. Simply write a PEG describing the syntax of whatever it is you want to parse, then write handler functions for each symbol to describe how to convert it into an actual value.

Example

Parsing arithmetic statements containing +, *, and parentheses:

var parser = peggml_parser_create("
    # Grammar for Calculator...
    Additive    <- Multitive '+' Additive / Multitive
    Multitive   <- Primary '*' Multitive / Primary
    Primary     <- '(' Additive ')' / Number
    Number      <- < [0-9]+ >
    %whitespace <- [ \t]*
")

peggml_parser_set_handler(parser, "Additive", handle_additive)
peggml_parser_set_handler(parser, "Multitive", handle_multitive)
peggml_parser_set_handler(parser, "Number", handle_number)

var result = peggml_parse(parser, "5 + (3 *7)   + 2")

show_debug_message("result is: " + string(result))
// "result is: 28"

// clean up
peggml_parser_destroy(parser)

// handlers ----------------------------------------
#define handle_additive
// add all operands together
var value = 0;
for (var i = 0; i < peggml_parse_elt_get_child_count(); ++i)
{
    value += peggml_parse_elt_get_child_value(i);
}

return value

#define handle_multitive
// multiply all operands together
var value = 1;
for (var i = 0; i < peggml_parse_elt_get_child_count(); ++i)
{
    value *= peggml_parse_elt_get_child_value(i);
}

return value

#define handle_number
// interpret token as number (provided as convenience)
return peggml_parse_elt_get_token_number()
// (could also do real(peggml_parse_elt_get_token_string()))

Installation

Simply add the script peggml.gml to your projects's scripts, and all the datafiles to your datafiles.