README.md

Sprache Parser Combinator Lab

Demonstration of the Sprache parser combinator for .NET.

Parsing Text is Simple

Parser combinators are a clever way to express the grammar of the input language directly as higher-order functions in the host language, eliminating the need for our old friends, Lex and Yacc.

With Sprache the parsing is defined in Linq expressions.

For example, consider the following EBNF production:

StringLiteral ::= QUOTE-CHAR ( NONQUOTE-CHAR )* QUOTE-CHAR ;

Here the terminals QUOTE-CHAR is the double-quote ("), and NONQUOTE-CHAR is any other character. Translated to Sprache, it looks like this:

        public static Parser<StringLiteral> StringLiteral =
            (from openQuote in Parse.Char('"')
             from text in Parse.CharExcept('"').Many().Text()
             from closeQuote in Parse.Char('"')
             select new StringLiteral(text));

Many denotes zero or more, Text folds the resulting sequence of chars from Many of Parse.CharExcept into a single string. StringLiteral is the class we have defined to represent a StringLiteral in our Abstract Syntax Tree, so the result of the above code is mapping directly from text input to objects.

That's pretty cool for a few lines of code.

For a larger example see ParserTest.cs and the files defining the parser (Grammar.cs) and the Abstract Syntax Tree types (AstTypes.cs).

Parser Non-Textual Input is Possible

With a twist of the arm, Sprache also parse object graphs not just text input. The key is to define an IResult, the return type of the parser. For example, if we want to parse Literal subclasses we can define an instance like so:

    internal class NonStringInput : IResult<Literal>
    {
        public NonStringInput(Literal value)
        {
            Value = value;
            WasSuccessful = true;
            Message = "NonStringInput";
            Expectations = new string[]{};
            Remainder = null;
        }

        public Literal Value { get; private set; }
        public bool WasSuccessful { get; private set; }
        public string Message { get; private set; }
        public IEnumerable<string> Expectations { get; private set; }
        public Input Remainder { get; private set; }
    }

The NonStringParsingTest.cs shows and example of how we can specify a grammar that allows us to use Sprache to navigate the object graph defined by this IResult.

It could be useful for defining for example input validation or data mappings on the boundary of an application.

Consider Using Formal Parsers More Often

It is my observation that writing a compiler is something that only graduates fresh out of university can do. After a couple of years, many programmers degrade to a daily routine of building smal hand-written, ad hoc parsers, possibly because they remember parser generators and compiler construction as complex activities.

Parser combinators change all that.

Now it is easy to express for example protocol or data exchange formats in a formal grammar, and parse them accordingly.

This is a good way to reduce security holes or bugs as you can rely on the parser library and grammar to verify that you only accept correct input.

With a formal grammar you also have a clear contract, and it has the added benefit that you get better error messages for invalid input, which may also be useful.

Whether you need to convert a bunch of old scripts to the new version of your ERP system, or migrate data from a legacy system with poor data quality, or just decode some control messages, formal parsers should be one of the first options to consider.

Happy parsing! Martin

License

This lab was created by Martin Jul and released under the MIT license, see the file [LICENSE.txt].

Sprache has the following license:

The MIT License

Copyright (c) 2011 Nicholas Blumhardt

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