Empty String Rules Fail to Match

[Axiomatic-Mind]

When I match a grammar such as

<main> ::= "foo:" <bar>
 <bar> ::= "bar"

against foo:bar, it works. When I match a grammar such as

<main> ::= "foo:" ""

against foo:, it works. However, when I match

<main> ::= "foo:" <baz>
 <baz> ::= ""

against foo:, it doesn't work.

Replicable using this code:

fn main() {
	let grammar1: bnf::Grammar = "
		<main> ::= \"foo:\" <bar>
		 <bar> ::= \"bar\"
	".parse().unwrap();

	let grammar2: bnf::Grammar = "
		<main> ::= \"foo:\" \"\"
	".parse().unwrap();

	let grammar3: bnf::Grammar = "
		<main> ::= \"foo:\" <baz>
		 <baz> ::= \"\"
	".parse().unwrap();

	assert!(grammar1.parse_input("foo:bar").next().is_some(), "grammar1 failed!");
	assert!(grammar2.parse_input("foo:"   ).next().is_some(), "grammar2 failed!");
	assert!(grammar3.parse_input("foo:"   ).next().is_some(), "grammar3 failed!");
}

Why isn't a parse tree like this being generated?

ParseTree {
	lhs: Term::Nonterminal("main"),
	rhs: [
		ParseTreeNode::Terminal("foo:"),
		ParseTreeNode::Nonterminal(ParseTree {
			lhs: Term::Nonterminal("baz"),
			rhs: [
				ParseTreeNode::Terminal(""),
			],
		}),
	],
}

Is this intentional/explicable behavior, or a bug? Thanks!

[CrockAgile]

since the behavior is varying across those similar grammars, this seems like a bug to me. I have a hunch! so thanks for the great reproduction steps, and I will start investigating 🕵️

[shnewto]

@Axiomatic-Mind I just merged the PR that addresses this, the release will be publised to crates soon. Thanks for raising the issue!

[shnewto]

okay version 0.4.2 published to crates.io!

[Axiomatic-Mind]

Thanks for fixing this!