2012-06-16 Reduce - 7 higher order functions

2012-06-16 reduce-7-higher-order-functions

# Title: 
# Author: Mikael Lundin
# Link: http://blog.mikaellundin.name/2012/06/16/reduce-7-higher-order-functions.html

Links.cs

// Links
var links = new[]
{
    "http://litemedia.info/map-7-higher-order-functions",
    "http://litemedia.info/fold-7-higher-order-functions",
    "http://litemedia.info/partition-7-higher-order-functions",
};
 
// Map - noop because of yield
var map = links.Map(OutgoingUrls);
 
// Reduce
var outgoingLinks = map.Reduce((links1, links2) =>
{
    // Add all links2 to links1 list
    var result = new List<string>(links1);
    foreach (var link in links2)
    {
        // Filter unique
        if (!result.Contains(link))
        {
            result.Add(link);
        }
    }
 
    return result;
});
 
// DONE: all outgoing links are now in variable 'outgoingLinks'
 
// For url, get outgoing urls
private IEnumerable<string> OutgoingUrls(string url)
{
    var result = new List<string>();
    var request = HttpWebRequest.Create(url);
 
    // Request url content
    using (var response = request.GetResponse())
    using (var reader = new StreamReader(response.GetResponseStream()))
    {
        // Match all href="http://..." not litemedia
        var matches = Regex.Matches(reader.ReadToEnd(), @"href=""(?<url>(?:http\:)?//(?!litemedia).*?)""");
        foreach (Match match in matches)
        {
            result.Add(match.Groups["url"].Value);
        }
    }
 
    return result;
}

Reduce.cs

public static T Reduce<T>(Func<T, T, T> fn, IEnumerable<T> list)
{
    if (list == null)
    {
        throw new ArgumentNullException("list", "Supplied list to Reduce is not allowed to be null");
    }
 
    if (fn == null)
    {
        throw new ArgumentNullException("fn", "Supplied function to Reduce is not allowed to be null");
    }
 
    IEnumerator<T> enumerator = list.GetEnumerator();
    if (!enumerator.MoveNext())
    {
        throw new ArgumentException("Can't run reduce on an empty list", "list");
    }
 
    var result = enumerator.Current;
    while (enumerator.MoveNext())
    {
        result = fn(result, enumerator.Current);
    }
 
    return result;
}
 
public static T Reduce<T>(this IEnumerable<T> list, Func<T, T, T> fn)
{
    return Reduce(fn, list);
}

Sum.cs

public int Sum(int[] numbers)
{
    return numbers.Reduce((a, b) => a + b);
}