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// A simpleton parser, one that doesn't understand the . (period) and that will fail if it encounters one.
public abstract class SimpleSentenceParser: CharParsers
{
    // Defines the set of all whitespace characters
	private Parser _whitespaceChars;
    public virtual Parser WhitespaceChars
    {
        get
        {
            return _whitespaceChars ??
                   (_whitespaceChars = Char(' ').OR(Char('\t').OR(Char('\n')).OR(Char('\r'))));
        }
    }
 
    // A whitespace is 0 or more repetitions of a valid Whitespace character
	private Parser _whitespace;
    public virtual Parser Whitespace
    {
        get
        {
            return _whitespace ??
                   (_whitespace = Rep(WhitespaceChars));
        }
    }
 
    // A word is multiple (alphabetic) characters that are not whitespaces
	private Parser _word;
    public virtual Parser Word
    {
        get
        {
            return _word ??
                   (_word = from w in Whitespace
                            from c in Char(char.IsLetter)
                            from word in Rep(Char(char.IsLetter))
                            select new WordTerm(word.Aggregate(new string(new[] {c}), (acc, ch) => acc + ch)));
        }
    }
 
    // A sentence is a set of words - simplistic, but will work for single sentences
	private Parser _sentence;
    public virtual Parser Sentence
    {
        get
        {
            return _sentence ??
                   (_sentence = from words in Rep(Word)
                                select new SentenceTerm(words));
        }
    }
 
    // Set of all sentences in the input (start symbol for this grammar)
	private Parser _all;
    public virtual Parser All
    {
        get
        {
            return _all ??
                   (_all = from sentence in Sentence
                           select (Term) sentence);
        }
    }
}
 
// A concrete implementation of the above parser that can accept string input
public class SimpleSentenceParser: SimpleSentenceParser
{
    public override Parser AnyChar // Pick one character at a time
    {
        get
        {
            return input => input.Length > 0 ? 
				new Result(input[0], input.Substring(1)) : 
				null;
        }
    }
}

// A slightly more intelligent parser, it can identify sentences properly
// Note that this parser does NOT redefine what a word means. It only redefines a sentence.
public abstract class BetterSentenceParser: SimpleSentenceParser
{
    private Parser _whitespaceChars;
    public override Parser WhitespaceChars // Adds to existing whitespace characters
    {
        get
        {
            return _whitespaceChars ??
                (_whitespaceChars = base.WhitespaceChars.OR(Char(',')).OR(Char(';')));
        }
    }
 
    // New, defines sentence terminators to allow this parser to understand them
	private Parser _sentenceTerminators;
    public virtual Parser SentenceTerminators
    {
        get
        {
            return _sentenceTerminators ??
                   (_sentenceTerminators = Char('.'));
        }
    }
 
    // Redefines what a sentence means and takes sentence terminators
	// into account
	private Parser _sentence;
    public override Parser Sentence
    {
        get
        {
            return _sentence ??
                   (_sentence = from words in Rep(Word)
                                from terminator in SentenceTerminators
                                select new SentenceTerm(words));
        }
    }
 
    // This now returns multiple sentences
	private Parser _all;
    public override Parser All // Actually selects multiple sentences
    {
        get
        {
            return _all ??
                   (_all = from sentences in Rep(Sentence)
                           select (Term)new ParagraphTerm(sentences));
        }
    }
}
 
// A concrete implementation of a parser that can accept string input
public class BetterSentenceParser : BetterSentenceParser
{
    public override Parser AnyChar
    {
        get
        {
            return input => input.Length > 0 ? new Result(input[0], input.Substring(1)) : null;
        }
    }
}

internal class Program
{
    private const string SimpleText = @"Lorem ipsum dolor sit amet";
 
    private const string ComplexText =
        @"Lorem ipsum dolor sit amet, consectetur adipiscing elit. Phasellus enim lorem," +
        @" tincidunt ac sollicitudin fringilla, vulputate a dolor. Sed eu viverra ligula.";
 
    private static void Main(string[] args)
    {
        // Will parse SimpleText into multiple words and one sentence
		{
			var parser = new SimpleSentenceParser();
			var result = parser.All (SimpleText);
		}
 
        // Will parse ComplexText partially, fail when it encounters the '.'
		{
			var parser = new SimpleSentenceParser();
			var result = parser.All (ComplexText);
		}
 
        // Will parse ComplexText into multiple words and multiple sentences
		{
			var parser = new ComplexSentenceParser();
			var result = parser.All (ComplexText);
		}
    }
}