Rewriter.Fold.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Sawmill
{
    public static partial class Rewriter
    {
        /// <summary>
        /// Flattens all the nodes in the tree represented by <paramref name="value"/> into a single result,
        /// using an aggregation function to combine each node with the results of folding its children.
        /// </summary>
        /// <typeparam name="T">The rewritable tree type</typeparam>
        /// <typeparam name="U">The type of the result of aggregation</typeparam>
        /// <param name="rewriter">The rewriter</param>
        /// <param name="func">The aggregation function</param>
        /// <param name="value">The value to fold</param>
        /// <returns>The result of aggregating the tree represented by <paramref name="value"/>.</returns>
        public static U Fold<T, U>(this IRewriter<T> rewriter, Func<T, Children<U>, U> func, T value)
        {
            if (rewriter == null)
            {
                throw new ArgumentNullException(nameof(rewriter));
            }
            if (func == null)
            {
                throw new ArgumentNullException(nameof(func));
            }

            Func<T, U> goDelegate = null;
            goDelegate = Go;
            U Go(T x) => func(x, rewriter.GetChildren(x).Select(goDelegate));
            return Go(value);
        }
    }
}