Chapter 1 - Building the basic parser combinators

In this post, we shall build some basic parsers to understand the functioning of parser combinators.

If you haven’t installed the hammer-parser ruby gem, please go back to the previous post and install it first.

• Fire your irb and include the hammer-parser gem.

require 'hammer-parser'

Basic hammer methods

• The hammer class Hammer::Parser has all the following methods that we need to make use of in our journey. We will see how the class is used in the individual examples below.

• ch() - This parser would be used to recognize a single character. For example,

@hammer = Hammer::Parser
parser = @hammer.ch('h')

The above parser ‘parser’ would be used to recognize the character ‘h’.

• ch_change() - This parser would be used to have characters in specific ranges.

@hammer = Hammer::Parser
parser = @hammer.ch_range('a','z')

The above parser would be used to recognize a single character in the range a-z.

• many() - This parser would be used for 0 or more occurences of the parser included as an argument.

@hammer = Hammer::Parser
character = @hammer.ch_range('a','z')
parser = @hammer.many(character)

• many1() - Same as many() but includes 1 or more occurences instead of 0 or more.

• end_p() - To denote the end of the parser.

• sequence()

• token()

• optional()

• middle()

• choice()

• not()

• int_range()

• middle()

  Recognizing the input with the help of the parsers

We make use of the same parser as in the example of many1().

@hammer = Hammer::Parser
character = @hammer.ch_range('a','z')
parser = @hammer.many1(character)
if !parser.parse("abcd").nil?
   true
else
   false
end

If the input is recognized, then the parse() method would return an object of HParseResult. If the input is not recognized, then nil is returned.