Editing Lazy evaluation (section)

====.NET ====
In the [[.NET]] framework, it is possible to do lazy evaluation using the class <syntaxhighlight inline lang="csharp">System.Lazy<T></syntaxhighlight>.<ref>{{cite web|url=http://msdn.microsoft.com/de-de/library/vstudio/dd642331.aspx|title=Lazy(T) Class (System)|publisher=Microsoft}}</ref> The class can be easily exploited in [[F Sharp (programming language)|F#]] using the <syntaxhighlight inline lang="fsharp">lazy</syntaxhighlight> keyword, while the <syntaxhighlight inline lang="fsharp">force</syntaxhighlight> method will force the evaluation. There are also specialized collections like <syntaxhighlight inline lang="csharp">Microsoft.FSharp.Collections.Seq</syntaxhighlight> that provide built-in support for lazy evaluation. 
<syntaxhighlight lang="fsharp">
let fibonacci = Seq.unfold (fun (x, y) -> Some(x, (y, x + y))) (0I,1I)
fibonacci |> Seq.nth 1000
</syntaxhighlight>
In C# and VB.NET, the class <syntaxhighlight inline lang="csharp">System.Lazy<T></syntaxhighlight> is directly used. 
<syntaxhighlight lang="csharp">
public int Sum()
{
    int a = 0;
    int b = 0; 
    Lazy<int> x = new Lazy<int>(() => a + b);
    a = 3;
    b = 5;
    return x.Value; // returns 8
}
</syntaxhighlight>
Or with a more practical example: 
<syntaxhighlight lang="csharp">
// recursive calculation of the n'th fibonacci number
public int Fib(int n)
{
   return (n == 1)? 1 : (n == 2)? 1 : Fib(n-1) + Fib(n-2);
}

public void Main()
{
    Console.WriteLine("Which Fibonacci number do you want to calculate?");
    int n = Int32.Parse(Console.ReadLine()); 
    Lazy<int> fib = new Lazy<int>(() => Fib(n)); // function is prepared, but not executed
    bool execute; 
    if (n > 100)
    {
        Console.WriteLine("This can take some time. Do you really want to calculate this large number? [y/n]");
        execute = (Console.ReadLine() == "y"); 
    }
    else execute = true;
    
    if (execute) Console.WriteLine(fib.Value); // number is only calculated if needed
}
</syntaxhighlight>
Another way is to use the <syntaxhighlight inline lang="csharp">yield</syntaxhighlight> keyword: 
<syntaxhighlight lang="csharp">
// eager evaluation 
public IEnumerable<int> Fibonacci(int x)
{
    IList<int> fibs = new List<int>();

    int prev = -1;
    int next = 1;
    for (int i = 0; i < x; i++)
    {
        int sum = prev + next;
        prev = next;
        next = sum;
        fibs.Add(sum); 
    }
    return fibs;
}

// lazy evaluation 
public IEnumerable<int> LazyFibonacci(int x)
{
    int prev = -1;
    int next = 1;
    for (int i = 0; i < x; i++)
    {
        int sum = prev + next;
        prev = next;
        next = sum;
        yield return sum;
    }
}
</syntaxhighlight>

{{Main|Thunk}}
{{Expand section|date=May 2011}}