gistfile1.txt

namespace MyNamespace;

internal static class Rules
{
    internal static IEnumerable<string> EmptyErrorList { get; } = Array.Empty<string>();
    internal delegate IEnumerable<string> Validator<T>(T value);

    internal static Validator<T> ParentOrChildren<T>(Validator<T> parent, IEnumerable<Validator<T>> children) =>
        value =>
        {
            var p = parent(value).ToArray();
            return p.Any() 
                ? p 
                : children.Aggregate(value);
        };

    internal static Validator<T> ConditionExcludesChildren<T>(Func<T, bool> condition, IEnumerable<Validator<T>> children) =>
        value =>
        {
            return condition(value) ? EmptyErrorList : children.Aggregate(value);
        };

    private static IEnumerable<string> Aggregate<T>(this IEnumerable<Validator<T>> children, T value) =>
        children.Select(c => c(value)).Aggregate(EmptyErrorList, (e1, e2) => e1.Concat(e2));
}

When converting line 20 to statement lambda, I get the following:

internal static Validator<T> ConditionExcludesChildren<T>(Func<T, bool> condition, IEnumerable<Validator<T>> children) =>
   value => condition(value) ? EmptyErrorList : Enumerable.Aggregate(children, value);

which does not even compile because children.Aggregate was replaced by Enumerable.Aggregate