vernac
v2.0.0
Published
A simple DSL for creating parsing expression grammars
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Vernac
A simple DSL for creating parsing expression grammars
Usage
Build up parsing expressions using the functions
eg:
let sum = Terminal('\\d').then('\\+').then(Terminal('\\d'));
sum.listen(() => {
console.log('matched');
}
sum.parse("1+2");
// stdout >> 'matched'
API
Terminal
Matches a basic symbol, eg a number is:
let num = new Terminal('\\d')
or more complicated regex:
let ident = new Terminal('[A-Za-z]+\\w*');
let a = new Terminal('a');
a.parse('a'); // succeeds
a.parse('b'); // fails
a.parse('ba'); // succeeds at index 1
Empty
Matches explicitly nothing, a synonymous with new Terminal('');
let e = new Empty();
e.parse(''); // succeeds
e.parse('b'); // succeeds with b still to be parsed
Associated actions
We can add listeners for when a grammar matches.
let a = new Terminal('a');
a.listen((result_obj, input) => {
console.log(result_obj); // Result {matched: true, result: 'a', remaining: ''}
});
a.parse('a');
Ordered Choice
Noted as a | b
. Tries for a first and then b.
Usage
let a = new Terminal('a');
let b = new Terminal('b');
let choice = a.or(b);
choice.parse('a'); // matches a
choice.parse('b'); // matches b
choice.parse('ab'); // matches a
Sequence
Notes as ab
. Needs to find a and then b.
Usage
let a = new Terminal('a');
let b = new Terminal('b');
let seq = a.then(b);
seq.parse('a'); // fails
seq.parse('b'); // fails
seq.parse('ab'); // succeeds
Lookahead
Noted as a&b
. Tries to find an a
followed by a b
. Only remembers the a
.
Similar to sequence but only remembers the first.
Usage
let a = new Terminal('a');
let b = new Terminal('b');
let lookahead = a.before(b);
lookahead.parse('a') // fails
lookahead.parse('b') // fails
lookahead.parse('ab') // succeeds and returns 'a' with 'b' still to be parsed
Negated Lookahead
Noted as a!b
. Tries to find an a
explicitly not followed by a b
. Only
remembers the a
.
Similar to lookahead but inverted.
Usage
let a = new Terminal('a');
let b = new Terminal('b');
let negated = a.notBefore(b);
negated.parse('a'); // succeeds
negated.parse('b'); // fails
negated.parse('ab'); // fails
negated.parse('ac'); // succeeds and returns 'a' with 'c' still to be parsed
0 or more
Noted as a*
. Tries to match a
0 or more times.
Usage
let a = new Terminal('a');
let zero_more = a.times(0);
zero_more.parse('a'); // succeeds and returns 'a'
zero_more.parse('ab'); // succeeds and returns 'a' with 'b' still to be parsed
zero_more.parse('aaac'); // succeeds and returns 'aaa' with 'c' still to be parsed
zero_more.parse(''); // succeeds
1 or more
Noted as a+
. Tries to match a
1 or more times.
Usage
let a = new Terminal('a');
let one_more = a.times(1);
one_more.parse('a'); // succeeds and returns 'a'
one_more.parse('ab'); // succeeds and returns 'a' with 'b' still to be parsed
one_more.parse('aaac'); // succeeds and returns 'aaa' with 'c' still to be parsed
one_more.parse(''); // fails
Optional
Noted as a?
. Tries to match a
0 or 1 times.
Usage
let a = new Terminal('a');
let optional = a.optionally();
optional.parse('a'); // succeeds
optional.parse(''); // succeeds
optional.parse('aa'); // succeeds and returns 'a' with 'a' still to be parsed
optional.parse('ab'); // succeeds and returns 'a' with 'b' still to be parsed
optional.parse('b'); //succeeds and returns '' with 'b' still to be parsed
TODO
- Add tests to verify the result field.
- Add stronger tests for listeners and ast building
- Add semantic error return possibilities for the listeners
- Add syntactic sugar for creating it from a list of tuples