Getting Started

We use some small examples to show how you can start using Meerkat parsers. Check the Download and Install page to get the necessary libraries. You can use a text editor such as Sublime Text or an IDE such as Scala IDE or IntelliJ IDEA to run these examples.


The first step is to create a Scala file and add the necessary imports:

import org.meerkat.Syntax._
import org.meerkat.parsers._
import Parsers._

Balanced parenthesis

As the first example, let’s write a parser that recognizes balanced parenthesis. We can directly write the grammar in Scala using the Meerkat library:

val S: Nonterminal = 
  syn ( "(" ~ S ~ ")"
      | epsilon

The syn function defines a nonterminal and assigns it to S. Since the definition of S is recursive, we need to define the return type explicitly, as required by Scala. S has two alternatives: the first alternative defines the balanced parentheses and the second alternative produces an empty string, which effectively terminates the recursion in the first alternative. The ~ combinator defines the sequence and | defines alternation. Note that the ~ combinator also allows arbitrary whitespace and comments between the symbols. The default whitespace and comment (which we refer to as Layout) can be redefined by the user. More information on Layout insertion in the Meerkat library can be found here.

We can parse an input string, say (()), using the parse method. The parse function returns an instance of Either[ParseError, ParseSuccess] which encapsulates either a parse success or a parse error. We can use Scala pattern matching to process the result of parsing:

parse(S, "(())") match {
    case Right(success) => println(success.root)
    case Left(error)    => println(error)

A ParseSuccess contains the root of the created parse tree, accessible via root, and some statistics about parsing, such as the number of nodes in the parse tree and ambiguities. The format of the parse trees Meerkat Parsers produce is explained here in detail. In the next section section, we show how you can traverse the parse tree and pretty print the brackets.

Pretty printing balanced parentheses

import org.meerkat.tree._

val Nt = NonterminalSymbol
val Term = TerminalSymbol
def prettyPrint(t: Tree, i: Int = 0): String = t match {
    // S ::= "(" S ")"
    case RuleNode(Rule(Nt("S"), Sequence(Term("("), Nt("S"), Term(")"))), 
                                List(_, s, _)) => s"""|${" " * i}(
                                                      |${prettyPrint(s, i + 2)}
                                                      |${" " * i})
    // S ::= epsilon                                                   
    case RuleNode(Rule(Nt("S"), Sequence(Term("epsilon"))), _)    => ""