Lambda returning another lambda

Question

Hello, is there any way how to return lambda from another lambda recursively?

All I want to do is finite state machine, implemented as lambda, which returns lambda implementing another state (or null).

nesting Func<> won't work as I want.

C#, .NET 3.5

Example:

machine, 3 states, pseudolanguage

private Lambda State1()
{  
    if (SomeConditionIsMet)
        return State2;
    else
        return State1;
}

private Lambda State2()
{  
    while (SomeConditionIsMet)
        return State2;
    else
        return State3;
}

private Lambda State3()
{  
    LogEnd();
    return NULL;
}

public void FSM()
{
    Lambda _currentState = State1;

    while(_currentState != NULL)
    {
        _currentState = _currentState();
    }
}

I know, that I can workaround this using enum+switch, for example, but I'm just curious if I can do this.

Answer 1

You can have a method which builds and returns an expression tree:

public Expression GetExpression()
{

}

Also building expression trees in .NET 4.0 has been greatly enhanced.

Answer 2

Sure, you can return a lambda from another lambda:

Func<int, Func<int, int>> makeAdder = x => y => x + y;
Func<int, int> addTen = makeAdder(10);
Console.WriteLine(addTen(20)); // 30

What aspect of the syntax are you having trouble with? I am interested to know how people get this sort of thing wrong because that helps us design the language and documentation better next time.

UPDATE:

well, but you cannot return lambda returning lambda

Sure you can.

Func<int, Func<int, int>> GetAdderMaker()
{
    return x => y => x + y;
}

Here we are returning a lambda that returns a lambda. Why do you believe this is impossible?

UPDATE:

Aha, I understand. You believe that the word "lambda" means "delegate". It does not. A lambda is a kind of expression that is convertible to a delegate.

If you want a delegate that returns a delegate then just declare that. That's perfectly legal. For example, here's a delegate called a "combinator" -- a combinator is a delegate which takes itself and returns itself:

delegate D D(D d);

That's a delegate named D which takes a D and returns a D.

You can make a lambda expression that is compatible with this delegate type. For example:

D I = x=>x;

is the Identity combinator. Or

D M = x=>x(x);

is the Mockingbird combinator in Raymond Smullyan's whimsical characterization of combinators.

As you correctly note, there's no way to make a generic Func that is this kind of combinator. I wrote an article about this fact back in 2006:

http://blogs.msdn.com/ericlippert/archive/2006/06/23/standard-generic-delegate-types-part-two.aspx

Answer 3

I believe you can declare a delegate type: public delegate Lambda Lambda() which returns a delegate of its own type. It does compile, anyway.