Error "Cannot read property 'format' of undefined "

[rogerdielrton]

The following JS piece of code, on execution, fails:

window.setTimeout(function () {
    var strict = Optional(Terminal("strict"));
    var graph = Choice(Terminal("graph"), Terminal("digraph"));
    var id = Optional(NonTerminal("ID"));
    var stmtList = NonTerminal("stmt_list");
    var seq = Sequence(strict, graph, id, Terminal("{"), stmtList, Terminal("}"));  
    var o = Diagram(seq);
    o.addTo(window.document.body);
}, 1000);

And, this other one (is an equivalent of the preceding) also:

Diagram(
    Sequence(
        Optional(Terminal("strict")),
        Choice(Terminal("graph"), Terminal("digraph")),
        Optional(NonTerminal("ID")),
        Terminal("{"),
        NonTerminal("stmt_list"),
        Terminal("}")))
.addTo();   

Error details:

Message:
    Uncaught TypeError: Cannot read property 'format' of undefined 
File:
    railroad-diagrams.js
Line:
    291

[tabatkins]

The first argument of Choice() has to be the index of which option is the "normal" option. I've added some checking and better error-messages, so your mistake gets caught upon construction and tells you what's wrong, rather than just throwing a mysterious-looking error message during .format().

[rogerdielrton]

Excuse me for the lost of time. I do't put attention to the example of:
http://www.xanthir.com/etc/railroad-diagrams/example.html

Finally, as general reflexion, I must say that coding in JS is not tented to make validation, checking, etc. Principally because it adds more verbosity and is tedious. But, it save a lot of time when the users (pity users like me) of that library use it in his code.

[tabatkins]

No problem, the error was showing up at a bad and confusing place; I'm happy to make it simpler to find this common authoring error.