C#: Recursive functions with Lambdas

Question

The below does not compile:

Func<int, int> fac = n => (n <= 1) ? 1 : n * fac(n - 1);

Local variable 'fac' might not be initialized before accessing

How can you make a recursive function with lambdas?

[Update]

Here are also two links that I found interesting to read:

1. Eric Lippert's "Why does a recursive lambda cause a definite assignment error?"
2. Anonymous Recursion in C#

Answer 1

This particular style of function is not supported by C# as a single line declaration. You have to separate out the declaration and definition into 2 lines

Func<int, int> fac = null;
fac = n => (n <= 1) ? 1 : n * fac(n - 1);

Answer 2

Well geez, if you'd just typed "why does a recursive lambda cause a definite assignment error?" into some search engine, you'd have found the answer in my article on the subject.

:-)

http://blogs.msdn.com/ericlippert/archive/2006/08/18/why-does-a-recursive-lambda-cause-a-definite-assignment-error.aspx

Answer 3

You'll have to create fac first und assign it later (which is pretty unfunctional because it depends on multiple assignment) or use so called Y-combinators.

Example:

delegate Func<TIn, TOut> FixedPointFunction<TIn, TOut>(Func<TIn, TOut> f);

static Func<T, TRes> Fix<T, TRes>(FixedPointFunction<T, TRes> f) {
    return f(x => Fix(f)(x));
}

static void Main(string[] args) {

    var fact = Fix<int, int>(f => x => (x <= 1) ? x : x * f(x - 1));

    Console.WriteLine(fact(5));            
}

But note that this might be somewhat hard to read/understand.

Answer 4

since c# 7.0 you finally can do this in one line using a local function

int fac(int n) => (n <= 1) ? 1 : n * fac(n - 1);