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