While parsing text using Sprache, can I determine the current index within the original string?

Question

I have Sprache set up to parse an Equation that has a number of different possible method calls in it. After it resolves the method, is there a way to determine the index values within the original string? Perhaps the Parse has a "current index" value and "length" value that's somehow accessible?

Example input string:

IndexOf("fred", 2) + IndexOf("bob")

using a parser like this...

Parser<Expression> FunctionCall = from namePart in Parse.Letter.Many().Text()
                       from lparen in Parse.Char('(')
                       from expr in Parameter.DelimitedBy(ListDelimiter)
                       from rparen in Parse.Char(')')
                       select CallMethod(namePart, Enumerable.Repeat(sourceData, 1)
                                                             .Concat(expr)
                                                             .ToArray());

Can anyone think of a "trick" that would allow me to determine that the first CallMethod handles SubString(0, 18), and the second CallMethod handles SubString(21, 14) from the original string?

score 3 · Answer 1 · answered Aug 22 '19 at 07:01

If you use a generic class and extension method you can make a more general approach

public class PositionAware<T> : IPositionAware<PositionAware<T>>
{
    public PositionAware(T value)
    {
        Value = value;
    }

    public T Value { get; }
    public Position Start { get; private set; }
    public int Length { get; private set; }
    public PositionAware<T> SetPos(Position startPos, int length)
    {
        Start = startPos;
        Length = length;
        return this;
    }

}
public static Parser<PositionAware<T>> WithPosition<T>(this Parser<T> value)
{
    return value.Select(x => new PositionAware<T>(x)).Positioned();
}

Using it:

from c in Parse.Char('a').WithPosition()
select (c.Start, c.Value)

from c in Parameter.DelimitedBy(ListDelimiter).WithPosition()
select (c.Start, c.Value)

Thanks Patrick, I'm going to consider that because I like the WithPosition syntax you've proposed; I'll let you know. Meanwhile, since you're so smart with Sprache, do you have any insight in my other question here?... https://stackoverflow.com/questions/56990021/how-to-parse-conditional-ternary-syntax-a-b-a-b-using-sprache — Jarvis, Aug 22 '19 at 12:27

score 1 · Accepted Answer · answered Jul 22 '19 at 18:50

I've managed to answer my own question. It's the Positioned() parser extension call that allows a parser to track the position within the original text.

  Parser<Expression> FunctionCall = (from namePart in Parse.Letter.Many().Text()
                            from lparen in Parse.Char('(')
                            from expr in Parameter.DelimitedBy(ListDelimiter)
                            from rparen in Parse.Char(')')
                            select new MethodPosAware(namePart, expr)).Positioned()
                            .Select(x => CallMethod(x.Value, Enumerable.Repeat(sourceData, 1)
                                        .Concat(x.Params)
                                        .ToArray(),
                                        x.Pos.Pos, x.Length));

I had to make a new MethodPosAware class to keep the position information, that derives from Sprache's IPositionAware:

class MethodPosAware : IPositionAware<MethodPosAware>
{
    public MethodPosAware(string methodName, IEnumerable<Expression> parameters)
    {
        Value = methodName;
        Params = parameters;
    }

    public MethodPosAware SetPos(Position startPos, int length)
    {
        Pos = startPos;
        Length = length;
        return this;
    }

    public Position Pos { get; set; }
    public int Length { get; set; }
    public string Value { get; set; }
    public IEnumerable<Expression> Params { get; set; }
}

I think I'll be extending this further to work with more than just Method names, but this is sufficient to answer my question for now. I hope this helps someone down the road.

While parsing text using Sprache, can I determine the current index within the original string?

2 Answers2