A Beginner’s Guide to Scala Parser Combinators

Introduction

Scala parser combinators are a powerful library for writing text parsers. This library allows you to create
parsers by combining smaller, simpler parsers using higher-order functions called combinators.

What are Scala Parser Combinators and their API

Scala parser combinators are a parsing library built on the idea of combining simple parsers to create more complex parsers. The main components of the library are:

• Parser: A basic building block that represents a parser for a specific input.
• RegexParsers: A trait that provides the ability to create parsers from regular expressions.
• Parsers: A trait that contains the basic combinators for constructing new parsers.

To use the library, you need to extend the Parsers trait or one of its subclasses (like RegexParsers).
Then, you can define your parsers by combining the basic parsers and combinators provided by the library.

What are Scala Combinators and their API

Combinators are higher-order functions that take parsers as arguments and return new parsers. Some common combinators include:

• ~: Sequences two parsers, returning a tuple with their results.
• ~>: Sequences two parsers, discarding the result of the first parser.
• <~: Sequences two parsers, discarding the result of the second parser.
• |: Tries two parsers in order, returning the result of the first successful parser.
• ^^: Transforms the result of a parser using a given function.
• rep: Repeats a parser zero or more times, returning a list of results.
• repsep: Repeats a parser zero or more times with a separator, returning a list of results.
• opt: Makes a parser optional, returning Some(result) if successful or None if not.

How to Use Scala Parser Combinators

• Add the necessary dependency to your build.sbt file.
• Import the required modules in your Scala file.
• Create a new class extending the RegexParsers trait.
• Define your basic parsers and combinators.
• Implement the main parsing method.

libraryDependencies +=("org.scala-lang.modules" %% "scala-parser-combinators" % "2.2.0")

Example

You can define the following parsers

def number: Parser[Int] = "\\d+".r ^^ (_.toInt)
def letter: Parser[String] = "[a-zA-Z]".r

Now, you want to create a parser that can parse a number followed by a letter.
You can use the ~ combinator like this:

def numberAndLetter: Parser[(Int, String)] = number ~ letter

However, there are situations where you might want to ignore the result of one of the parsers.
In these cases, you can use the ~> or <~ combinators.

• p1 ~> p2: This combinator works like ~, but only keeps the result of p2. The result of p1 is discarded.
• p1 <~ p2: This combinator also works like ~, but only keeps the result of p1. The result of p2 is discarded.

For example, if you want to parse a number followed by a letter but only want to keep the result of the number, you can use the <~ combinator:

def numberBeforeLetter: Parser[Int] = number <~ letter

Conclusion

In this beginner’s guide, we introduced Scala parser combinators and their API, explained how to use combinator. With a solid understanding of the basic concepts and combinators, you can start building your own parsers for various applications. Happy parsing!