Ohm: JavaScript Parser Creates a Language in 200 Lines

10 min read Michael Carroll on Aug 30, 2016
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Parsers are an incredibly useful software libraries. While conceptually simple, they can be challenging to implement and are often considered a dark art in computer science. In this blog series, I'll ...

Parsers are an incredibly useful software libraries. While conceptually simple, they can be challenging to implement and are often considered a dark art in computer science. In this blog series, I’ll show you why you don’t need to be Harry Potter to master parsers. But bring your wand just in case.

We’ll explore a new open source Javascript library called Ohm that makes building parsers easy and easier to reuse. In this series, we use Ohm to recognize numbers, build a calculator, and more. By the end of this series you will have created a complete programming language in under 200 lines of code. This powerful tool will let you do things that you might have thought impossible otherwise.

Why Parsers are Hard

Parsers are useful. There are lots of times you might need a parser. A new file format might come along that you need to process and no one else has written a library for it yet. Or maybe you find files in an old file format and the existing parsers aren’t built for the platform you need. I’ve seen this happen over and over. Code will come and go but data is forever.

Fundamentally parsers are simple: just transform one data structure into another. So why does it feel like you need to be Dumbledore to figure them out?

The challenge that parsers have historically been surprisingly difficult to write, and most of the existing tools are old and assume a fair amount of arcane computer science knowledge. If you took a compilers class in college the textbook may well have techniques from the 1970s. Fortunately, parser technology has improved a great deal since then.

Typically a parser is created by defining what you want to parse using a special syntax called a formal grammar. Then you feed this into several tools like Bison and Yacc which generate a bunch of C code that you the need to modify or link into whatever programming language you are actually writing in. The other option is to manually write a parser in your preferred language, which is slow and error prone. That’s a lot of extra work before you get to actually use the parser.

Imagine if your description of the thing you wanted to parse, the grammar, was also the parser? What if you could just run the grammar directly, then add hooks only where you want? That’s what Ohm does.

Introducing Ohm

Ohm is a new kind of parsing system. While it resembles the grammars you may have seen in text books it’s a lot more powerful and easier to use. With Ohm you write your format definition in a very flexible syntax in a .ohm file, then attach semantic meaning to it using your host language. For this blog we will use JavaScript as the host language.

Ohm is based on years of research into making parsers easier and more flexible. VPRI’s STEPS program (pdf) created many custom languages for specific tasks (like a fully parallelizable graphics renderer in 400 lines of code!) using Ohm’s precursor OMeta.

Ohm has many interesting features and notations, but rather than explain them all I think we should just dive in and build something.

Parsing Integers

Let’s parse some numbers. It seems like this would be easy. Just look for adjacent digits in a string of text; but let’s try to handle all forms of numbers: Integers and floating point. Hex and octal. Scientific notation. A leading negative digit. Parsing numbers is easy. Doing it right is hard

To build this code by hand would be difficult and buggy, with lots of special cases which sometimes conflict with each other. A regular expression could probably do it, but would be ugly and hard to maintain. Let’s do it with Ohm instead.

Every parser in Ohm involves three parts: the grammar, the semantics, and the tests. I usually pick part of the problem and write tests for it, then build enough of the grammar and semantics to make the tests pass. Then I pick another part of the problem, add more tests, update the grammar and semantics, while making sure all of the tests continue to pass. Even with our new powerful tool, writing parsers is still conceptually complicated. Tests are the only way to build parsers in a reasonable manner. Now let’s dig in.

We’ll start with an integer number. An integer is composed of a sequences of digits next to each other. Let’s put this into a file called grammar.ohm:

CoolNums {
   // just a basic integer
   Number = digit+

This creates a single rule called Number which matches one or more digits. The + means one or more, just like in a regular expression. This rule will match if there is one digit or more than one digit. It won’t match if there are zero digits or something something other than a digit. A digit is defined as the characters for the numbers 0 to 9. digit is also a rule like Number is, but it’s one of Ohm’s built in rules so we don’t have to define it ourselves. We could override if it we wanted to but that wouldn’t make sense in this case. After all we don’t plan to invent a new form of number (yet!)

Now we can read in this grammar and process it with the Ohm library.

Put this into test1.js

var ohm = require('ohm-js');
var fs = require('fs');
var assert = require('assert');
var grammar = ohm.grammar(fs.readFileSync('src/blog_numbers/syntax1.ohm').toString());

The ohm.grammar call will read in the file and parse it into a grammar object. Now we can add semantics. Add this to your Javascript file:

var sem = grammar.createSemantics().addOperation('toJS', {
    Number: function(a) {
        return parseInt(this.sourceString,10);

This creates a set of semantics called sem with the operation toJS. The semantics are essentially a bunch of functions matching up to each rule in the grammar. Each function will be called when the corresponding rule in the grammar is parsed. The Number function above will be called when the Number rule in the grammar is parsed. The grammar defines what chunks are in the language. The semantics define what to do with chunks once they’ve been parsed.

Our semantics functions can do anything we want, such as print debugging information, create objects, or recursively call toJS on any sub-nodes. In this case we just want to convert the matched text into a real Javascript integer.

All semantic functions have an implicit this object with some useful properties. The source property represents the part of the input text that matches this node. this.sourceString is the matched input as a string. Calling the built in JavaScript function parseInt turns this string to a number. The 10 argument to parseInt tells JavaScript that we are giving it a number in base ten. If we leave it out then JS will assume it’s base 10 anyway, but I’ve included it because later on we will support base 16 (hex) numbers, so it’s good to be explicit.

Now that we have some semantics, let’s actually parse something to see if our parser works. How do we know our parser works? By testing it. Lots and lots of testing. Every possible edge case needs a test.

With the standard assert API, here is a test function which matches some input then applies our semantics to it to turn it into a number, then compares the number with the expected input.

   function test(input, answer) {
     var match = grammar.match(input);
     if(match.failed()) return console.log("input failed to match " + input + match.message);     
     var result = sem(match).toJS();
     console.log('success = ', result, answer);

That’s it. Now we can write a bunch of tests for different numbers. If the match fails then our script will throw an exception. If not it will print success. Let’s try it out. Add this to the script


Then run the script with node test1.js

Your output should look like this:

success =  123 123
success =  999 999
input failed to match abcLine 1, col 1:
> 1 | abc
Expected a digit

Cool. The first two succeed and the third one fails, as it should. Even better, Ohm automatically gave us a nice error message pointing to the match failure.

Floating Point

Our parser works, but it doesn’t do anything very interesting. Let’s extend it to parse both integers and floating point numbers. Change the grammar.ohm file to look like this:

CoolNums {
  // just a basic integer
  Number = float | int
  int    = digit+
  float  = digit+ "." digit+

This changes the Number rule to point to either a float or an int. The | means or. We read this as “a Number is composed of a float or an int.” Then int is defined as digit+ and float is defined as digit+ followed by a period followed by another digit+. This means there must be at least one digit before the period and at least one after. If there is not a period then it won’t be a float at all, so int will match instead.

Now let’s go look at our semantic actions again. Since we now have new rules we need new action functions: one for int and one for float.

var sem = grammar.createSemantics().addOperation('toJS', {
    Number: function(a) {
        return a.toJS();
    int: function(a) {
        console.log("doing int", this.sourceString);
        return parseInt(this.sourceString,10);
    float: function(a,b,c) {
        console.log("doing float", this.sourceString);
        return parseFloat(this.sourceString);

There’s two things to note here. First, int and float and Number all have matching grammar rules and functions. However, the action for Number no longer does anything interesting. It receives the child node ‘a’ and returns the result of toJS on the child. In other words the Number rule simply returns whatever its child rule matched. Since this is the default behavior of any rule in Ohm we can actually just leave the Number action out. Ohm will do it for us.

Second, int has one argument a while float has three: a, b, and c. This is because of the rule’s arity. Arity means how many arguments a rule has. If we look back at the grammar, the rule for float is

  float  = digit+ "." digit+

The float rule is defined by three parts: the first digit+, the "." and the second digit+. All three of those parts will be passed as parameters to the action function for float. Thus float must have three arguments or else the Ohm library will give us an error. In this case we don’t care about the arguments because we will just grab the input string directly, but we still need the arguments listed to avoid compiler errors. Later on we will actually use some of these parameters.

Now we can add a few more tests for our new floating point number support.


Note that the last test will fail. A floating point number must begin with a digit, even if it’s just zero. .123 is not valid, and in fact the real JavaScript language has the same rule.


So now we have integers and floats but there’s a few other number syntaxes that might be good to support: hexadecimal and scientific notation. Hex numbers are integers in base sixteen. The digits can be from 0 to 9 and A to F. Hex is often used in computer science when working with binary data because you can represent 0 through 255 exactly with only two digits.

In most C derived programming languages (including JavaScript) hex numbers are preceded by 0x to indicate to the compiler that what follows is a hexadecimal number. To support hex numbers in our parser we just need to add another rule.

  Number = hex | float | int
  int    = digit+
  float  = digit+ "." digit+
  hex    = "0x" hexDigit+
  hexDigit := "0".."9" | "a".."f" | "A".."F"

I’ve actually added two rules. hex says that a hex number is the string 0x followed by one or more hexDigits. A hexDigit is any letter from 0 to 9, or a to f, or A to F (covering both upper and lower case). I also modified Number to recognize hex as another possible option. Now we just need another action rule for hex.

    hex: function(a,b) {
        return parseInt(this.sourceString,16);

Notice that in this case we are passing 16 as the radix to parseInt because we want JavaScript to know that this is a hexadecimal number.

I skipped over something important to notice. The rule for hexDigit looks like this.

  hexDigit := "0".."9" | "a".."f" | "A".."F"

Notice that I used := instead of =. In Ohm, the := is used when you are overriding a rule. It turns out Ohm already has a default rule for hexDigit, just as it does for digit, space and a bunch of others. If I had used = then Ohm would have reported an error. This is a check so I can’t override a rule unintentionally. Since our new hexDigit rule is actually the same as Ohm’s built in rule we can just comment it out and let Ohm do it. I left the rule in just so we can see what’s really going on.

Now we can add some more tests and see that our hex digits really work:


Scientific Notation

Finally let’s support scientific notation. This is for very large or small numbers like 1.8 x 10^3 In most programming languages numbers in scientific notation would be written as 1.8e3 for 18000 or 1.8e-3 for .0018. Let’s add another couple of rules to support this exponent notation.

    float  = digit+ "." digit+ exp?
    exp    = "e" "-"? digit+

This adds a the exp rule to the end of the float rule with a question mark. The ? means zero or one, so exp is optional but there can’t be more than one. Adding the exp rule also changes the arity of the float rule, so we need to add another argument to the float action, even if we don’t use it.

    float: function(a,b,c,d) {
        console.log("doing float", this.sourceString);
        return parseFloat(this.sourceString);

And now our new tests can pass:



Ohm is a great tool for building parsers because it’s easy to get started and you can incrementally add to it. It also has other great features that I didn’t cover today, like a debugging visualizer and sub-classing.

So far we have used Ohm to translate character strings into JavaScript numbers, and often Ohm is used for this very purpose: converting one representation to another. However, Ohm can be used for a lot more. By putting in a different set of semantic actions you can use Ohm to actually process and calculate things. This is one of Ohm’s magic features. A single grammar can be used with many different semantics.

In the next article of this series I’ll show you how to not just parse numbers but actually evaluate math expressions like (4.8+5 * (238-68)/2), just like a real calculator.

Bonus challenge: Can you extend the grammar with support for octal numbers? These are numbers in base 8 and can be represented with only the digits 0 to 7, preceded by a zero and the letter o. See if you are right with these test cases. Next time I’ll show you the answer.


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