2012-06-21 Collect - 7 higher order functions

2012-06-21 collect-7-higher-order-functions

# Title: Collect - 7 higher order functions
# Author: Mikael Lundin
# Link: http://blog.mikaellundin.name/2012/06/21/collect-7-higher-order-functions.html

BlogPost.cs

private class BlogPost
{
    public IEnumerable<Comment> Comments { get; private set; }
 
    public static IEnumerable<BlogPost> All()
    {
        // return SELECT blogpost.* FROM db
    }
}
 
var blogPosts = BlogPost.All();
var comments = blogPosts.Collect(post => post.Comments);

Collect.cs

public static IEnumerable<U> Collect<T, U>(Func<T, IEnumerable<U>> fn, IEnumerable<T> list)
{
    if (list == null)
    {
        throw new ArgumentNullException("list", "Supplied list to Collect is not allowed to be null");
    }
 
    if (fn == null)
    {
        throw new ArgumentNullException("fn", "Supplied function to Collect is not allowed to be null");
    }
 
    foreach (var item1 in list)
    foreach (var item2 in fn(item1))
    {
        yield return item2;
    }
}
 
public static IEnumerable<U> Collect<T, U>(this IEnumerable<T> list, Func<T, IEnumerable<U>> fn)
{
    return Collect(fn, list);
}

Numbers.cs

var numbers = new[]
{
    new[] { 1, 2, 3 },
    new[] { 4, 5, 6 },
    new[] { 7, 8, 9 },
};
 
var flat = numbers.Collect(arr => arr);
// 1, 2, 3, 4, 5, 6, 7, 8, 9