Parlot

Parlot 是一个快速、轻量级和简单易用的 .NET 解析器组合器。Parlot 提供了一个基于解析器组合器的 fluent API，提供了一个更可读的语法定义。

与所有其他已知的 .NET 解析器组合器相比，Parlot 速度更快，分配更少。

Fluent API 提供了简单的分析器组合器，这些组合器可以用来表达更复杂的表达式。这个API的主要目标是提供和易于阅读的语法。另一个优点是，语法是在运行时建立的，而且可以动态地扩展。

示例（源码）：

public static readonly Parser<Expression> Expression;

static FluentParser()
{
    /*
     * Grammar:
     * expression     => factor ( ( "-" | "+" ) factor )* ;
     * factor         => unary ( ( "/" | "*" ) unary )* ;
     * unary          => ( "-" ) unary
     *                 | primary ;
     * primary        => NUMBER
     *                  | "(" expression ")" ;
    */

    // The Deferred helper creates a parser that can be referenced by others before it is defined
    var expression = Deferred<Expression>();

    var number = Terms.Decimal()
        .Then<Expression>(static d => new Number(d))
        ;

    var divided = Terms.Char('/');
    var times = Terms.Char('*');
    var minus = Terms.Char('-');
    var plus = Terms.Char('+');
    var openParen = Terms.Char('(');
    var closeParen = Terms.Char(')');

    // "(" expression ")"
    var groupExpression = Between(openParen, expression, closeParen);

    // primary => NUMBER | "(" expression ")";
    var primary = number.Or(groupExpression);

    // The Recursive helper allows to create parsers that depend on themselves.
    // ( "-" ) unary | primary;
    var unary = Recursive<Expression>((u) => 
        minus.And(u)
            .Then<Expression>(static x => new NegateExpression(x.Item2))
            .Or(primary));

    // factor => unary ( ( "/" | "*" ) unary )* ;
    var factor = unary.And(ZeroOrMany(divided.Or(times).And(unary)))
        .Then(static x =>
        {
            // unary
            var result = x.Item1;

            // (("/" | "*") unary ) *
            foreach (var op in x.Item2)
            {
                result = op.Item1 switch
                {
                    '/' => new Division(result, op.Item2),
                    '*' => new Multiplication(result, op.Item2),
                    _ => null
                };
            }

            return result;
        });

    // expression => factor ( ( "-" | "+" ) factor )* ;
    expression.Parser = factor.And(ZeroOrMany(plus.Or(minus).And(factor)))
        .Then(static x =>
        {
            // factor
            var result = x.Item1;

            // (("-" | "+") factor ) *
            foreach (var op in x.Item2)
            {
                result = op.Item1 switch
                {
                    '+' => new Addition(result, op.Item2),
                    '-' => new Subtraction(result, op.Item2),
                    _ => null
                };
            }

            return result;
        });            

    Expression = expression;
}