Parser generator

Not only other parser generators for web weren't written here, but they lack a set of features we really need:

• Type-safety: API of generated parser should be typed without any
• AST from grammar: converting untyped trees to AST is unsafe and boring
• TBD CST: pretty-printer has to keep comments /**/, underscores in numbers 1_234 and other features that are nowhere represented in AST.
• Named lexemes: good error messages shouldn't report an identifier as "a-z, A-Z, 0-9, or _".
• TBD Error recovery: programming languages should report more than one error at a time.
• TBD Incremental: reparse shouldn't take time proprtional to size of the file.
• High-order rules A<B>: duplicated code leads to increased chance to make a mistake, and high-order rules are required for duplication.
• TBD No stack overflow on large expressions: nested constructions might lead to stack overflow.
• Space skipping: manually annotating grammar with spaces is error-prone and boring.

Comparison to peggy

pgen mostly follows grammar of peggy with a few notable differences.

• Capitalized rules Foo = ... create AST nodes with { $: 'Foo' }.
• Rules have to end with semicolon ;.
• Inline semantic actions { return 42; } are not supported. We can't infer types of AST when there is some inlined JavaScript code, because JS is untyped.
• High-order rules A<B> = ... were added.
• Space skipping was added. It uses space rule.
• Lexification operator # was added.
• Character classes do not support modifiers [a-z]i.

Syntax reference

• Non-AST rule defintion rule = ...;
• AST rule defintion Rule = .... Returns an object with { $: 'Rule', loc: Loc } with rest of the fields defined with named clauses in right-hand side.
• Display override for error messaging Id "identifier" = ...;
• High-order rule defintion inter<A, B> = ...; and call inter<expression, ",">
• Left-biased choice "A" / "B". Will match the first matching clause.
• Sequence foo bar baz. All clauses should match in sequence.
• Named clauses "if" "(" expr:expression ")" stmts:statements. Sequence operator generates an object, and named clauses become its fields { expr: ..., stmts: ... }.
• Picked clause "if" "(" @expression ")". Sequence operator returns only a single value of picked clause.
• Single clause sequence a = b. Works as a = @b.
• Negative lookahead !x. Fails if x matches. Doesn't consume input.
• Positive lookahead &x. Passes if x matches. Doesn't consume input.
• Stringification $x. Ignores AST computed by x, returns string that x matched.
• Lexification #x. Does not skip spaces inside of x. If x calls some other rules, doesn't skip spaces there either.
• Repeat x*.
• Repeat at least once x+.
• Optional x?.
• String "abc".
• Character class [a-z_]. Supports ranges a-z. Supports negation [^a-z].

Implicit syntax

• Spaces are skipped after every terminal: "string", [a-z]
• Spaces are skipped after lexification operator #x
• Spaces are not skipped inside lexification operator #x.
• Spaces are skipped at the start, before rest of the parsing will happen
• If not the whole input was consumed, error will be emitted