2012-06-06 Fold - 7 higher order functions

2012-06-06 fold-7-higher-order-functions

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

Cats.cs

var cats = new[]
{
    new Cat(name : "Miss Marple", age : 4),
    new Cat(name : "Mr. Quincy", age : 12),
    new Cat(name : "Darcy", age : 8)
};
 
cats.Fold((acc, cat) => acc + cat.Age, 0);
// => 24

Fold.cs

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

GrowArray.cs

private T[] GrowArray<T>(T[] acc, T item)
{
    var newArray = new T[acc.Length + 1];
    acc.CopyTo(newArray, 0);
    newArray[acc.Length] = item;
 
    return newArray;
}
 
var list = new List<int> { 1, 2, 3 };
list.Fold(GrowArray, new int[0]);
// => [1, 2, 3]