wren/doc/site/control-flow.markdown

^title Control Flow ^category language

Control flow is used to determine which chunks of code are executed and how many times. Branching statements deciding whether or not to execute some code and looping ones execute something more than once.

Truth

All control flow is based on deciding whether or not to do something. This decision is conditional on the value of some expression. We take the entire universe of possible values and divide them into two buckets: some we consider "true" and the rest are "false". If the expression results in a value in the true bucket, we do one thing. Otherwise, we do something else.

Obviously, the boolean true is in the "true" bucket and false is in "false", but what about values of other types? The choice is ultimately arbitrary, and different languages have different rules. Wren's rules follow Ruby:

• The boolean value false is false.
• The null value null is false.
• Everything else is true.

This means 0, empty strings, and empty collections are all considered "true" values.

If statements

The simplest branching statement, if lets you conditionally skip a chunk of code. It looks like this:

:::dart
if (ready) System.print("go!")

That evaluates the parenthesized expression after if. If it's true, then the statement after the condition is evaluated. Otherwise it is skipped. Instead of a statement, you can have a block:

:::dart
if (ready) {
  System.print("getSet")
  System.print("go!")
}

You may also provide an else branch. It will be executed if the condition is false:

:::dart
if (ready) System.print("go!") else System.print("not ready!")

And, of course, it can take a block too:

:::dart
if (ready) {
  System.print("go!")
} else {
  System.print("not ready!")
}

While statements

It's hard to write a useful program without executing some chunk of code repeatedly. To do that, you use looping statements. There are two in Wren, and they should be familiar if you've used other imperative languages.

The simplest, a while statement executes a chunk of code as long as a condition continues to hold. For example:

:::dart
// Hailstone sequence.
var n = 27
while (n != 1) {
  if (n % 2 == 0) {
    n = n / 2
  } else {
    n = 3 * n + 1
  }
}

This evaluates the expression n != 1. If it is true, then it executes the following body. After that, it loops back to the top, and evaluates the condition again. It keeps doing this as long as the condition evaluates to something true.

The condition for a while loop can be any expression, and must be surrounded by parentheses. The body of the loop is usually a curly block but can also be a single statement:

:::dart
var n = 27
while (n != 1) if (n % 2 == 0) n = n / 2 else n = 3 * n + 1

For statements

While statements are useful when you want to loop indefinitely or according to some complex condition. But in most cases, you're looping through a list, a series of numbers, or some other "sequence" object. That's what for is for. It looks like this:

:::dart
for (beatle in ["george", "john", "paul", "ringo"]) {
  System.print(beatle)
}

A for loop has three components:

1. A variable name to bind. In the example, that's beatle. Wren will create a new variable with that name whose scope is the body of the loop.

2. A sequence expression. This determines what you're looping over. It gets evaluated once before the body of the loop. In this case, it's a list literal, but it can be any expression.

3. A body. This is a curly block or a single statement. It gets executed once for each iteration of the loop.

Break statements

Sometimes, right in the middle of a loop body, you decide you want to bail out and stop. To do that, you can use a break statement. It's just the break keyword all by itself. That will immediately exit out of the nearest enclosing while or for loop.

:::dart
for (i in [1, 2, 3, 4]) {
  System.print(i)
  if (i == 3) break
}

So this program will print the numbers from 1 to 3, but will not print 4.

Numeric ranges

Lists are one common use for for loops, but sometimes you want to walk over a sequence of numbers, or loop a number of times. For that, you can create a range, like so:

:::dart
for (i in 1..100) {
  System.print(i)
}

This loops over the numbers from 1 to 100, including 100 itself. If you want to leave off the last value, use three dots instead of two:

:::dart
for (i in 1...100) {
  System.print(i)
}

This looks like some special "range" syntax in the for loop, but it's actually just a pair of operators. The .. and ... syntax are infix "range" operators. Like other operators, they are just special syntax for a regular method call. The number type implements them and returns a range object that knows how to iterate over a series of numbers.

The iterator protocol

Lists and ranges cover the two most common kinds of loops, but you should also be able to define your own sequences. To enable that, the semantics of a for are defined in terms of an "iterator protocol". The loop itself doesn't know anything about lists or ranges, it just knows how to call two particular methods on the object that resulted from evaluating the sequence expression.

When you write a loop like this:

:::dart
for (i in 1..100) {
  System.print(i)
}

Wren sees it something like this:

:::dart
var iter_ = null
var seq_ = 1..100
while (iter_ = seq_.iterate(iter_)) {
  var i = seq_.iteratorValue(iter_)
  System.print(i)
}

First, Wren evaluates the sequence expression and stores it in a hidden variable (written seq_ in the example but in reality it doesn't have a name you can use). It also creates a hidden "iterator" variable and initializes it to null.

Each iteration, it calls iterate() on the sequence, passing in the current iterator value. (In the first iteration, it passes in null.) The sequence's job is to take that iterator and advance it to the next element in the sequence. (Or, in the case where the iterator is null, to advance it to the first element). It then returns either the new iterator, or false to indicate that there are no more elements.

If false is returned, Wren exits out of the loop and we're done. If anything else is returned, that means that we have advanced to a new valid element. To get that, Wren then calls iteratorValue() on the sequence and passes in the iterator value that it just got from calling iterate(). The sequence uses that to look up and return the appropriate element.

The built-in List and Range types implement iterate() and iteratorValue() to walk over their respective sequences. You can implement the same methods in your classes to make your own types iterable.