OmniThreadLibrary 2.0 sneak preview [2]

Futures in the OTL were not planned – they just happened. In fact, they are so new that you won’t find them in the SVN. (Don’t worry, they’ll be committed soon.)

As a matter of fact, I always believed that futures must be supported by the compiler. That changed few weeks ago when somebody somewhere (sorry, can’t remember the time and place) asked if they can be implemented in the OmniThreadLibrary. That question made me rethink the whole issue and I found out that not only it’s possible to implement them without changing the compiler – the implementation is almost trivial!

In the OTL 2.0 you’ll be able to declare a future …

var
  numPrimes: IOmniFuture<integer>;

… start the evaluation …

  numPrimes := TOmniFuture<integer>.Create(a delegate returning integer);

… and wait on the result.

  numPrimes.Value

As simple as that. Declare the IOmniFuture<T>, create TOmniFuture<T> and retrieve the result by calling Value: T.

As a real-world example, the code below creates a future that calculates number of primes from 1 to CPrimesHigh and displays this value.

var
  numPrimes: IOmniFuture<integer>;
begin
  numPrimes := TOmniFuture<integer>.Create(function: integer
    var
      i: integer;
    begin
      Result := 0;
      for i := 1 to CPrimesHigh do
        if IsPrime(i) then
          Inc(Result);
    end
  );
  // do something else
  lbLog.Items.Add(Format('%d primes from 1 to %d', 
    [numPrimes.Value, CPrimesHigh]));
end;

As a general rule, I would recommend against putting too much of the code inside the future’s constructor. A following approach is more readable and easier to maintain.

function CountPrimesToHigh(high: integer): integer;
var
  i: integer;
begin
  Result := 0;
  for i := 1 to CPrimesHigh do
    if IsPrime(i) then
      Inc(Result);
end;

var
  numPrimes: IOmniFuture<integer>;
begin
  numPrimes := TOmniFuture<integer>.Create(function: integer
    begin
      Result := CountPrimesToHigh(CPrimesHigh);
    end
  );
  // do something else
  lbLog.Items.Add(Format('%d primes from 1 to %d', 
    [numPrimes.Value, CPrimesHigh]));
end;

Or you can take another step and create a future factory. That’s especially recommended if you’ll be using futures of the same kind in different places.

function StartCountingPrimesTo(high: integer): TOmniFuture<integer>;
begin
  Result := TOmniFuture<integer>.Create(function: integer
    var
      i: integer;
    begin
      Result := 0;
      for i := 1 to high do
        if IsPrime(i) then
          Inc(Result);
    end
  );
end;

var
  numPrimes: IOmniFuture<integer>;
begin
  numPrimes := StartCountingPrimesTo(CPrimesHigh);
  // do something else
  lbLog.Items.Add(Format('%d primes from 1 to %d', 
    [numPrimes.Value, CPrimesHigh]));
end;

Implementation

Believe it or not, the whole implementation fits in 27 lines (not counting empty lines).

type
  TOmniFutureDelegate<T> = reference to function: T;

  IOmniFuture<T> = interface
    function  Value: T;
  end; { IOmniFuture<T> }

  TOmniFuture<T> = class(TInterfacedObject, IOmniFuture<T>)
  private
    ofResult: T;
    ofTask  : IOmniTaskControl;
  public
    constructor Create(action: TOmniFutureDelegate<T>);
    function  Value: T;
  end; { TOmniFuture<T> }

constructor TOmniFuture<T>.Create(action: TOmniFutureDelegate<T>);
begin
  ofTask := CreateTask(procedure (const task: IOmniTask)
    begin
      ofResult := action();
    end,
     'TOmniFuture action').Run;
end; { TOmniFuture<T>.Create }

function TOmniFuture<T>.Value: T;
begin
  ofTask.Terminate;
  ofTask := nil;
  Result := ofResult;
end; { TOmniFuture<T>.Value }

As you can see, the whole OTL task support is only used to simplify background thread creation. It would be quite simple to implement futures around Delphi’s own TThread. In fact, I think I’ll just go ahead and implement it!