Invoking methods with Out and Ref !!!

Straight to code.....

int SomeMethod(string arg1,
    string arg2,
    ref DayOfWeek arg3)
{
    // Wildest implementation!
}

The above method had to be executed on its dispatcher thread. So let unravel a bit of the wildest implementation above.

int SomeMethod(string arg1,
    string arg2,
    ref DayOfWeek arg3)
{
    if (Disptacher.CheckAccess())
    {
        var funcDelegate = (Func<string, string, DayOfWeek, int>)SomeMthod;

        return Dispatcher.Invoke(funcDelegate,
            arg1,
            arg2,
            ref arg3);
    }

    // Wilder implementation!!
}

Before you say anything, yes, the compiler spat the following errors:-

Error 1  No overload for 'SomeMethod' matches delegate 'System.Func<string,string,DayOfWeek,int>

Error 2 The best overloaded method match for 'System.Windows.Threading.Dispatcher.Invoke(System.Delegate, params object[])' has some invalid arguments

Error 3  Argument '4': cannot convert from 'ref System.DayOfWeek' to 'object'

I had some experience with Error 3 earlier, and I had written about it. That came in handy to resolve Error 3. So code is getting better...

int SomeMethod(string arg1,
    string arg2,
    ref DayOfWeek arg3)
{
    if (Disptacher.CheckAccess())
    {
        var funcDelegate = (Func<string, string, DayOfWeek, int>)SomeMethod;

        var args = new object[] {
            arg1,
            arg2,
            arg3
        };

        return Dispatcher.Invoke(funcDelegate, args);
    }

    // Wilder implementation!!
}

Then I was left with one Error 1. I felt I needed a hammer to bang my head to get rid of the error. And it was all nothing but stupidity, or I should have been sleepy. I have been using Func so much that I had got the feeling that it is a keyword (for lack of a better word). To realize that Func delegate had no overload of the type/sort (I was using) took an hour or so.

We can declare a delegate with same name Func as follows:-

public delegate TResult Func<T1, T2, T3, TResult>(T1 arg1,
    T2 arg2,
    ref T3 arg3);

But I declared with a different name to avoid confusion and clear up my misconception. Now the wildness of the implementation was almost unraveled except the method was not working as expected. Let us see the code cleared of all compiler errors:-

int SomeMethod(string arg1,
    string arg2,
    ref DayOfWeek arg3)
{
    if (Disptacher.CheckAccess())
    {
        var funcDelegate = (Func<string, string, DayOfWeek, int>)SomeMthod;

        var args = new object[] {
            arg1,
            arg2,
            arg3
        };

        return Dispatcher.Invoke(funcDelegate, args);
    }

    // No more implementation
    arg3 = DayOfWeek.Friday;

    return 1234;
}

What is that expected behavior that is not experienced? And what is the reason? I will write about in a short while cuz my beer stole the preference.

Hint: Why does not the client code (Code calling SomeMethod) get the correct value in DayOfWeek argument? In our case, DayOfWeek.Friday.

Thinking Currying !!!

Currying is a mathematical concept based on lambda calculus. It is a technique of operating on a function (taking multiple arguments) by splitting and capable of chaining into a series of single argument functions. It is very similar to what a human would attempt to do on paper. For example, if you have to add numbers 1 through 10, what would you do? Class II mathematics....zero in hand, one in the mind, add 0 and 1, so 1 in the mind, then 2 in the hand, ...up to 10. So we compute the addition with one argument at a time.

In the programming world, it is realized by transforming a n-arguments function into a (n-1) arguments function, which takes the remaining one argument. This transformation when applied recursively on each of the single argument functions is the chaining of single argument functions that I discussed earlier. Needless to say, currying is a gift of the functional programming world. In simple words, functional programming is about building functions from other functions, and so functions are treated as first class citizens (like data).

If currying is just transforming a n arguments function into a single argument function, then extension method too is an example of currying.

public static bool Replace<T>(this IList<T> srcList, int position, T item) 
{ 
   // Imagine an implementation here.... 
}

So you would be using the Replace<T> above without explicitly passing the source list to the method; one argument less.

list.Replace(2, newItem);
// instead of Replace(ilistObj, 2, newItem) if extension method was not invented.

Isn't that right? Yes, but that is not the currying from the conventional standpoint of functional programming.

For samples, we will not use the devil (any functional programming language) itself; instead we will use C#, which provides functional programming facilities. Alright, let us curry.

int Add(int x, int y) 
{ 
   return x + y; 
} 

Or to be 'functional', we may (re)write: Func<int, int, int> adderFunc = (x, y) => x + y;

To curry out an increment function, we would write as follows:-

Func<int, Func<int, int>> Incrementer() 
{ 
   return num => Add(num, 1); 
} 

and we use it is as follows:-

var inc = Incrementer(); 
Console.WriteLine(inc(5)); 
Console.WriteLine(inc(12)); 

Is that not better and wise than writing Sum(5, 1)?

If currying is just transforming a n-arguments function into a single argument function, then we should be able to write a generic currying function.

Func<T1, Func<T2, TResult>> Curry<T1, T2, TResult>(Func<T1, T2, TResult> fn) 
{ 
   return x => y => fn(x, y); 
} 

Func<int, Func<int, int>> Adder = Curry(Add); 
var Incrementor = Adder(1); 
var i = Incrementor(5); 

when we may not have actually  encountered a compelling situations to use this in the past

But isn’t this all cryptic? So why would we want to do all such cryptic things when we have not encountered any such situation....in the past? Actually we have.

When simple principles are tough for us to understand, it is our grandma who helps us. Our grandma here is C++; although grandma called it binding.

In C++, many STL algorithms require a functor (roughly equivalent to a function definition) with one argument, where as the desired function was a two or more arguments function. Then we use std::bind1st or std::bind2nd to curry it into unary function.

For instance, the count_if algorithm calculates the number of elements in a sequence that satisfy the predicate, which happens to be a unary function. Suppose we want to count the even numbers in a collection of whole numbers (and imagine this to be a tough calculation). If there was a function that could take a number and return true if it was an even number, it could be fed to count_if. But what if we had a function like the one below - a two argument function.

bool IsDivisible(int number, int divisor)
{
   return number % divisor == 0;
}

The functor we need is nothing but an IsDivisible function with the second argument as 2. We could write an IsEven function which in turn calls IsDivisible. But that could be tedious one we were write such wrappers for a function with 10 arguments. In situations like this, we curry. In C++, we (use) bind.

std::bind1st - A helper template function that creates an adaptor to convert a binary function object into a unary function object by binding the first argument of the binary function to a specified value.
std::bind2nd - A helper template function that creates an adaptor to convert a binary function object into a unary function object by binding the second argument of the binary function to a specified value.

So in our case, we will be using bind2nd, as follows:-

std::vector<int> wholeNumbers =  {
   1, 2, 4, 5, 6, 7, 9, 10, 12, 15, 17, 20
};

std::count_if(wholeNumbers.begin(),
   wholeNumbers.end(),
   std::bind2nd(IsDivisible, 2));

Unfortunately, C++ stayed with bind1st and bind2nd, and currying was not that evident or securely possible since C++ did not have required facilities, say something like delegates. So the concept has been in vogue from long time ago. Like every paradigm in programming, functional programming requires (re)aligning our thought (process). Instead of treating functions as just reusable pieces of code (as considered in procedural programming), they have to conceived as the input processors, which may return either data or whole new function; shrinked?

So that is a quick thought on Currying. I will try to share a few other thoughts related to currying, slightly expanding to functional programming but later.