How to grammar good

A lot of people have pointed out to me that writing grammars for nearley is hard. The thing is, writing grammars is, in general, very hard. It doesn’t help that certain grammar-related problems are provably undecidable.

As with all of programming, grammar-writing is guided by instinct and pattern recognition (on the part of the human, in this case!). This guide is meant to walk through some common idioms, ideas, anti-patterns and pitfalls, and hopefully make you a confident grammarian. Treat it more as a styleguide than a tutorial.

It might be worth reading Better Earley than Never first, to understand the algorithm and some basic concepts. From here on out, I’m assuming you know what regular expression, Backus Naur Form, context free grammar, terminal, nonterminal, and ambiguous grammar mean. You should know nearley syntax well enough to understand what something like

int -> [0-9]        {% id %}
    | int [0-9]     {% function(d) {return d[0] + d[1]} %}

means (though I like to pride myself on the readability of nearley grammar…).


Your grammar should be structured from the top down. The first few rules in a file should describe a general outline, and tiny details (terminals for whitespace, literals etc.) should be at the bottom. For example, if you’re writing a parser for Scheme, you would start with

Sourcefile -> (S-expression | Comment):*

and then fill out what s-expression and comment mean. Most of the time, rules defined higher in the file reference rules defined lower down, and rules defined near the bottom rarely ever reference rules near the top.

Nonterminal names

Give your nonterminals useful names–mostly nouns that describe the string it will match. By convention, we use _ for optional whitespace and __ for mandatory whitespace: it makes it easy to write rules like

"(" _ expression _ ")"

You can explicitly mark a nonterminal as optional or repetitive by postfixing its name with ? and + (respectively)–as in statementlist+ or comment?. Note that this has no semantic value: use nearley’s EBNF modifiers (:*, :+, :?) for that.

Don’t roll your own unroller

If you want to match one or more of a nonterminal, use an EBNF modifier. It’s semantically legible, contextually appropriate and (most importantly) easy. It’s very easy to mess up and create an exponentially ambiguous monstrosity such as

lotsofletters -> "a" | lotsofletters lotsofletters

Postprocess or dispose

nearley saves a nested array structure by default, but most of the time that’s not what you want. For things like whitespace, you want to throw away all that useless information for memory efficiency, so use a postprocessor that just returns null. For syntactic sugar and stuff, construct object literals so that the code that processes your AST is relatively independent of your grammar. Constructing object literals also lets you discard junk (parens, etc.).

Remember, charclasses aren’t regexes

The [a-z] syntax only allows you to use regex-style character classes, not actual regular expressions. Nearley fundamentally does not support regexes as terminals. Use EBNF modifiers instead–they do what you want.

Debug with nearley-test

Use the nearley-test script (installs alongside nearleyc) to debug your grammars. It lets you inspect the parse tables, and see all the parsings, or the point of failure. This is invaluable when you have a subtle ambiguity issue.

Don’t shy away from left recursion

You were a good little student and you paid attention to your professors when they told you never to write grammars like:

a -> a "something"

because a naïve recursive-descent parser would bork in an infinite loop. Nearley, of course, is much better, so you don’t have to worry about that. If you’re paranoid about efficiency, you should actually prefer left recursion over right recursion (a -> "something" a) because it’s very slightly faster. In any case, that’s how you deal with left or right associativity for binary operators.

Do shy away from left recursion

…if you’re using it where the EBNF :* or :+ makes more sense!

Operator precedence is not black magic

Here’s how you do it: you start with your lowest-precedence operator and work your way up to your highest precedence ones. Each operator gets its own nonterminal:

math -> sum
sum -> sum ("+"|"-") product | product
product -> product ("*"|"/") exp | exp
exp -> number "^" exp | number # this is right associative!

It should be pretty clear how this works, and how to extend it to different types of operators. The main thing is to be careful with your associativity direction. Be careful not to write op -> op "$" op, because that’s ambiguous.

Introducing non-conflicting unary negation ("5 * -5") is left as a trivial exercise for the enterprising reader.

Comment your grammars

It’s very easy to come back to a grammar a week later and have no idea what it’s doing. Leave comments that explain precisely what each nonterminal matches, since a clear description will help you debug things in the future.

Use whitespace prettily

Align your ->s and your |s and your {% ... %}s. Future retinas will thank you.

Parsing an established language? Cheat!

Almost all standards publish syntax guides with an accompanying diagram in (E)BNF. It’s worth it to Google around for this. With just a bit of common sense, you should be able to transliterate it to a .ne file.

Again, grammar-writing is largely about instinct and experience. The more you write, the more you’ll understand how it goes, and the faster you’ll be able to prototype and design.