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](syntax.html#blocks):

    :::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](lists.html), 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](values.html#ranges), 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](expressions.html#operators), they are just
special syntax for a regular method call. The number type implements them and
returns a [range object](values.html#ranges) 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](lists.html) and [Range](values.html#ranges) 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.