This series of post will try to explain a complex topic: concurrent and parallel programming, in Dart. I think the only way to deal with that is using the Erlang VM (BEAM), but Clojure and other functional languages are usually doing better job on this part. Unfortunately, to me, most of other languages using OOP don't offer a great abstraction to concurrency and parallelism, but during the last decade, things are changing a bit.

Anyway, before talking about concurrency, parallelism, or even distributed systems, everything starts with synchronous call, the "standard" way to execute a function or a procedure on a computer. In dart, synchronous function call and generators functions need to me studied a little bit.

Firstly, all function without any specific keyword are all defined as synchronous:

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

If the keyword sync was present in the language, the previous snippet would be equivalent to the one below. Don't try to compile it, it will fail.

// could be equivalent to the following
// snippet with the sync keyword. But this
// function will not work, sync keyword is
// invalid during compilation
int addition_sync(int x, int y) sync {
  return x + y;
}

When adding the addition() function inside the main() entry-point, it will directly return the value. In fact, you can't really see this because it's quick, but the computer will block a little bit to wait the result of the computation.

int main() {
  return addition(1, 2);
}

A synchronous function will always wait for any kind of return before continuing the execution of the program. For example, if an infinite loop is created, the function will never return, and the function will block the whole program. The following snippet, when executed, will never return because of the while loop defined in the infinite_loop() function. The only way to close this program is to kill it (for example using Ctrl-C or sending another signal on UNIX systems).

int infinite_loop() {
  while (true) {
    // do something
  }
  return 0;
}

int main() {
  return infinite_loop();
}

Great, we know how to create synchronous functions. Now, let have a look on a specific keyword: sync*. This one is a syntactic sugar to help developers creating generators, a function that return many elements, one at a time. The best example is the List class (or []). This data type - a list - is seen as a generator, and more precisely an Iterable.

A function is a generator if its body is marked with the sync* or async* modifier. [...] Generator functions are a sugar for functions that produce collections in a systematic way, by lazily applying a function that generates individual elements of a collection. Dart provides such a sugar in both the synchronous case, where one returns an iterable, and in the asynchronous case, where one returns a stream.

-- Dart Language Specification, Chapter 9, page 23

Let create our first generator from scratch, let call it int_generator(). It will take 2 integers as arguments and will return the list of elements between them. You can see it as a kind of primitive range or sequence function.

Iterable<int> int_generator(int x, int y) sync* {
  if (y<x) throw "y<x"; 
  while (x <= y) {
    yield x;
    x++;
  }
}

int main() {
  for (var i in int_generator(0, 10)) {
    print(i);
  }
  return 0;
}

In the first place, instead of returning a simple int this function is returning an Iterable<int>. Why? Because we are using yield, but we will explain that a bit later. Next, the function definition looks like a classic one, but the sync* keyword is added just after. It means this function will be a generator.

Then, the body of the function where a first step is to avoid creating an infinite loop by creating a simple guard throwing an exception if the second argument is less than the first argument. Immediately after, a while loop is created, where the first argument is incremented until the first argument is less or equal to the second one.

The yield statement adds an object to the result of a generator function.

-- Dart Language Specification, Chapter 18, Section 16, page 213

In this loop, the keyword yield followed by variable (or any valid terms) is used. This specific expression is important, because it will adds the object into the result of the generator. In other words, if another function is calling the previous function inside a loop, the sequence of integer will be printed to the screen.

Let execute that from the console.

$ dart run bin/int_generator.dart
0
1
2
3
4
5
6
7
8
9
10

yield is not the only unique term here able to put an object inside a generator, yield* (yield-each) can also be used to push a series of objects, in fact a subtype of Iterable class.

The yield-each statement adds a series of objects to the result of a generator function.

-- Dart Language Specification, Chapter 18, Section 17, page 214

Iterable<int> int_generator(int x, int y) sync* {
  if (y<x) throw "y<x"; 
  yield* [x,y];
}

int main() {
  print(int_generator(0, 10));
  for (var i in int_generator(0, 10)) {
    print(i);
  }
  return 0;
}

Let execute this code.

$ dart run bin/int_each_generator.dart 
(0, 10)
0
10

To be honest, I don't currently understand why it returns a record on the first part of the program. I was reading the specification but I was unable to find and answer. I will let this problem for future-me. A quick note though: a generator cannot return a value.

Case ⟨Generator functions⟩. It is a compile-time error if a return statement of the form return e; appears in a generator function.

-- Dart Language Specification, Chapter 18, Section 12, page 211

Based on the documentation, it also seems yield* can be used to improve recursive function performance. Here the one of the previous example implemented using recursive function call.

Iterable<int> int_rec_generator(int x, int y) sync* {
  if (x<=y) {
    yield x;
    yield* int_rec_generator(x+1, y);
  }
}

int main() {
  print(int_rec_generator(0, 10));
  return 0;
}

Let run it now.

$ dart run bin/int_recursive_generator.dart 
(0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10)

Same result than before, but it was using a recursive function. If I remember correctly, calling recursively a function in Dart with tail queue optimization and trampoline is a very bad idea (it will consume a lot of stack and memory). Anyway, let switch to another topic: Iterable class.

Iterable class

We know how to create generators, and then Iterable from scratch, but what we can do with this kind of objects? Let read the definition from the API documentation.

A collection of values, or "elements", that can be accessed sequentially.

-- Dart Language Documentation, Iterable class

Not all features will be illustrated there, only the one looking interesting and useful for my own use cases.

• Iterate.generate() constructor creates a new Iterable object using Integer by default (like a Range/Sequence). The second argument offers a way to change the type returned based on the integer sequence produced (a bit like a map()).

int main() {
  var x = Iterable.generate(10);
  print("generate(10): $x");

  var y = Iterable.generate(26, (int x) => String.fromCharCode(x+65));
  print("generate(10, _): $y");

  return 0;
}

$ dart run bin/t.dart 
generate(10): (0, 1, 2, 3, 4, 5, 6, 7, 8, 9)
generate(10, _): (A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z)

• Iterable.contains() method returns true if the object passed in the first argument is present in the collection of objects managed by the Iterable.

$ cat bin/contains.dart 
void main() {
  var a = Iterable.generate(26, (int x) => String.fromCharCode(x+65));
  print('"A"? ${a.contains("A")}');
  print('65? ${a.contains(65)}');
}

$ dart bin/contains.dart 
"A"? true
65? false

• Iterable.elementAt() method returns the element present at the position defined by the first argument. If the position does not exist, it throws a RangeError exception.

$ cat bin/elementAt.dart 
void main() {
  var x = Iterable.generate(10);
  print(x.elementAt(0));
  print(x.elementAt(9));
  print(x.elementAt(1024));
}

$ dart bin/elementAt.dart 
0
9
Unhandled exception:
RangeError (index): Index out of range: index should be less than 10: 1024
#0      IndexError.check (dart:core/errors.dart:520:7)
#1      _GeneratorIterable.elementAt (dart:core/iterable.dart:928:16)
#2      main (file:///dart/t/bin/elementAt.dart:5:11)
#3      _delayEntrypointInvocation.<anonymous closure> (dart:isolate-patch/isolate_patch.dart:314:19)
#4      _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:193:12)

• Iterable.every() method returns true if the collection of items satisfies the anonymous function passed in argument.

$ cat bin/every.dart 
void main() {
  var x = Iterable.generate(10);
  print(x.every((y) => y >= 0 ));
  print(x.every((y) => y == String ));
}

$ dart bin/every.dart 
true
false

• Iterable.fold() method reduces the collection of items using a combine function present in second argument and an initial value defined in the first argument.

void main() {
  Iterable<int> i = Iterable.generate(10);
  print(i.fold<int>(1, (v, e) => v + e));

  var s = Iterable.generate(26, (int x) => String.fromCharCode(x+65));
  print(s.fold<String>("string: ", (v, e) => "${v} ${e}"));
}

$ dart bin/fold.dart 
46
string:  A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

• Iterable.forEach() method iterates of the collection of item. This method returns nothing.

void main() {
  var list = [1,2,3,4, "end"];
  list.forEach((x) => print(x));
}

$ dart bin/forEach.dart 
1
2
3
4
end

• Iterable.join() method joins the collection of item with a String and converts all object in the collection t String.

void main() {
  var list = ["start", 1, 2, 3, "end"];
  print(list.join(", "));
}

$ dart bin/join.dart 
start, 1, 2, 3, end

• Iterable.map() method modifies each element of the collection by calling a function defined in the first argument.

void main() {
  var m = [
    {'a': 1},
    {'b': 2}
  ];
  print(
    m.map(
      (x) => {x.values.first: x.keys.first}
    )
  );
}

$ dart bin/map.dart 
({1: a}, {2: b})

•  Iterable.reduce() methods is similar to Iterable.fold() excepts it does not have an initial value defined.

void main() {
  Iterable<int> list = Iterable.generate(20);
  print(list.reduce((a, e) {
    print("$a $e");
    if (e%2==0) {
      return a + e;
    }
    else {
      return a;
    }
  }));
}

$ dart bin/reduce.dart 
0 1
0 2
2 3
2 4
6 5
6 6
12 7
12 8
20 9
20 10
30 11
30 12
42 13
42 14
56 15
56 16
72 17
72 18
90 19
90

Way more methods are available, but it was not expected to go so deep in this post. If you want to go even deeper, you will need to read the test cases and the Dart implementation:

• Iterable test cases can be found in the files tests/corelib/iterable*.dart from Dart SDK.

• Iterable implementation source code can be found in sdk/lib/core/iterable.dart.

With these 2 links, you will have more than enough to understand how things are working.

Cover Image by Will H McMahan on Unsplash