2012-06-19 Expand - 7 higher order functions

2012-06-19 expand-7-higher-order-functions.html

# Title: Expand - 7 higher order functions
# Author: Mikael Lundin
# Link: http://blog.mikaellundin.name/2012/06/19/expand-7-higher-order-functions.html

Employees.cs

var employees = new List<Employee>
{
    new Employee(name : "John Smith", salary : 45000.0),
    new Employee(name : "Carla Sewer", salary : 38000.0),
    new Employee(name : "Sam Shipwright", salary : 55000.0)                   
};
 
const double Raise = 1.03;
 
var table = employees.Map(e => new
{
    e.Name,
    Salaries = e.Salary.Expand(amount => amount * Raise)
});
 
// Print
var format = "{0,14} | {1,7} | {2,7} | {3,7}";
Console.WriteLine(format, string.Empty, "Year 1", "Year 2", "Year 3");
foreach (var line in table)
{
    var salaries = line.Salaries.Take(3).ToArray();
    Console.WriteLine(format, line.Name, salaries[0], salaries[1], salaries[2]);
}
//                |  Year 1 |  Year 2 |  Year 3
//     John Smith |   45000 |   46350 | 47740,5
//    Carla Sewer |   38000 |   39140 | 40314,2
// Sam Shipwright |   55000 |   56650 | 58349,5

Expand.cs

public static IEnumerable<T> Expand<T>(Func<T, T> yield_fn, Predicate<T> while_fn, T init)
{
    if (yield_fn == null)
    {
        throw new ArgumentNullException("yield_fn", "Supplied yield_fn argument to Expand is not allowed to be null");
    }
 
    if (while_fn == null)
    {
        throw new ArgumentNullException("while_fn", "Supplied while_fn argument to Expand is not allowed to be null");
    }
 
    var current = init;
    yield return current; // yield initial value
    while (while_fn(current))
    {
        // yield each calculated new value
        yield return current = yield_fn(current);
    }
}
 
public static IEnumerable<T> Expand<T>(this T init, Func<T, T> yield_fn,  Predicate<T> while_fn)
{
    return Expand(yield_fn, while_fn, init);
}
 
public static IEnumerable<T> Expand<T>(this T init, Func<T, T> yield_fn)
{
    return Expand(yield_fn, i => true, init);
}

Fibonacci.cs

var previous = 0;
var fibonacci = 1.Expand(current =>
{
    var next = previous + current;
    previous = current;
    return next;
});
// fibonacci : [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89..]

Numbers.cs

var numbers = 1.Expand(i => i + 1);
// numbers : [1, 2, 3, 4, 5..]