Iterators provide an easy and comfortable way to provide data for a Sequences.
Iterators are special types of Methods that return a Sequence. Rather then executing all at once and returning a finished list, tough, bits of the iterator are run as needed, whenever a new ite for the sequence is requested.
Note: Since iterators are very similar to regular methods, many topics covered in the Methods topic will apply to them, as well.
Iterators are declared like normal methods, but with three important distinctions:
- Iterator methods must return a Sequence type.
- They must me marked with the
- Instead of returning a value, they pass back individual items of the sequence using
Of course, any method can return a sequence, simply by declaring a sequence type as their result type. A normal method would need to handle the work of constructing the sequence itself – which can be easy when returning data from, say an Array or a list, but becomes more complicated when the data should be generated dynamically as the sequence is enumerated.
What the iterator syntax adds to the table is that it makes it easy to write linear code that returns one item of a sequence after the other, but allow that code to be executed non-linearly, piece by piece, if and when the sequence is accessed.
While the iterator is declared to return sequence of a specific type, inside it, each use of the
yield statement will yield a single value of that type. For example
method NaturalNumbers: sequence of Integer; iterator; begin for i := 1 to Int32.Max do yield i; end; ... for each n in NaturalNumbers do writeLn(n);
Under the Hood
When an iterator is called, the body of the iterator method is not actually executed to calculate the whole sequence. Instead, a new object is instantiated, representing the sequence. Once code starts to enumerate over the sequence, for example in a
for each loop, the iterator's method body will be executed piece by piece in order to provide the sequence elements.
It will run until the first occurrence of
yield, and that value will be returned as the first value of the sequence – and then stop. As the loop continues to iterate the sequence and asks for the next item, execution resumes at that position, until the next
yield statement is encountered, and so on — until the method body reaches it's end, marking the end of the sequence.
Of course the above example is very simple (and could be easily reproduced without iterators, for example using a For Loop Expression. But iterators allow for arbitrary complexity in code, and as many
yield statements as needed.
For example, one could easily imagine a complex parser that processes data, with many nested
then Clauses or
case Statements that dive deep into a data structure and yield new items for a sequence from many places.
Oxygene;'s iterator syntax allows such complex logic to be expressed as linear flow – leaving it to the compiler to unpack the code so that for each tem of the sequence, execution runs form one yield statement to the next, and no further.
method WeirdSequenceOfNumbers(aExtraWeird: Boolean): sequence of String; iterator; begin yield 'One'; yield 'Two'; yield 'Three'; for i := 4 to 20 do yield i.ToString; yield 'Twenty-1'; if aExtraWeird then begin for i := 20 to 90 do yield (i*3).ToString; yield "Twohundred and seventyone"; end else begin for i := 20 to 90 do yield i; end; end;
Delegating Iteration to a Sequence
yield keyword also supports delegation of the iterator to a second sequence, as shown below:
var MoreValues := [3,4,5,6]; // array of Int32, can act as sequence method SomeIntegers: sequence of Int32; begin yield 1; // adds "1" to the sequence yield 2; // adds "2" to the sequence yield MoreValues; // adds "3" thru "6" to the sequence, from the array yield 7; // adds "7" to the sequence end;
When the iterator reaches the relevant
yield statement, subsequent items are retrieved from that sequence, until it ends. After that, it continues to the next local
yield statement. One can think of it as shorthand for:
yield 1; // adds "1" to the sequence yield 2; // adds "2" to the sequence for each i in MoreValues do yield i yield 7; // adds "7" to the sequence
Limitations for Code in Iterators
In general, almost all code constructs can be used in iterators, including complex loops and conditional statements.
Since yielding in an iterator returns back to the caller on every iteration, it's not allowed to
yield from within a protected block such as a
locking statement or a
finally block, as that would leave an unbalanced Stack.
In addition, while an iterator can use the
exit Statement to prematurely finish it's execution in any place, it may not return a value as part of the exit. Similarly, the
result Expression is not available.
Static Methods, Visibility and Virtuality and Other Modifiers
Since iterators are methods, they can be made static, and visibility and virtuality apply just as for regular methods.