Grammar Composition

Grammar Composition

As seen in Using a Grammar, a grammar is a standard Pegged parser with a .parse( ) static method. That means you can call a grammar in another grammar, as you do for the predefined parsers. In fact, a grammar is nothing but a parser with a lot of plumbing inside. That opens whole vistas for modularization and code re-use. If you see in a grammar any functionality in a grammar that could be extracted and stand by itself, make it another grammar.

Have a look for example at the pegged.examples.strings and pegged.examples.numbers modules. The define grammars that recognize respectively double-quote-delimited strings and standard integer and floating point numbers. To use them in your own grammar definitions, just import the modules to make the symbols visible.

module my.grammar;

import std.stdio;
import pegged.grammar;
import pegged.examples.strings, pegged.examples.numbers;

enum g = grammar(`
LOG:
    LogFile <- LogLine+ EOI
    LogLine < String Numbers.Hexa ':' Numbers (',' Numbers)*
`);
mixin(g);

enum log = 
`"File1" 123AC7AF   : 123, 78.265, 0.00
"File2" 31F039DC9BE : 49.45, 42.220, 0.02, -22.3
"File3" D0043869930 : 0
`;

void main()
{
    writeln(LOG.parse(log));
}

Numbers also define a Hexa rule to recognize hexadecimal numbers. It's never invoked directly by Numbers itself, but can be called by qualifiying its name: Numbers.Hexa.parse("A73FEC384CBB") (see Four Levels of Encapsulation). If two grammars define each a rule with the same name, you can also use their qualified names to distinguish them. Pegged allow rule invokation (the rhs in a rule definition) to contain qualified identifiers:

module my.grammar;

import std.stdio;
import pegged.grammar;
import pegged.examples.strings, pegged.examples.numbers;

enum g = grammar(`
LOG:
    LogFile <- LogLine+ EOI
    LogLine < String Numbers.Hexa ':' Numbers (',' Numbers)*
`);
mixin(g);

enum log = 
`"File1" 123AC7AF   : 123, 78.265, 0.00
"File2" 31F039DC9BE : 49.45, 42.220, 0.02, -22.3
"File3" D0043869930 : 0
`;

void main()
{
    writeln(LOG.parse(log));
}

Pegged does not allow rule names (lhs in a definition) to be qualified.