wren/doc/site/expressions.markdown

^title Expressions
^category language

Wren's syntax is based on C so if you're familiar with that (or any of the
plethora of other languages based on it) you should be right at home. Since
Wren is heavily object-oriented, you'll notice that most of the different
expression forms are just different ways of invoking methods.

## Literals

Literals produce objects of built-in types. The primitive [value](values.html)
types—numbers, strings and friends—have literal forms as do the
built in collections: [lists](lists.html) and [maps](maps.html).

[Functions](functions.html) do not have standalone a literal form. Instead,
they are created by passing a [block
argument](functions.html#block-arguments) to a method.

## Identifiers

Names in expressions come in a few flavors. A name that starts with an
underscore denotes a [field](classes.html#fields), a piece of data stored in an
instance of a [class](classes.html). All other names refer to
[variables](variables.html).

## Method calls

Wren is object-oriented, so most code consists of method calls. Most of them
look like so:

    :::dart
    System.print("hello")
    items.add("another")
    items.insert(1, "value")

You have a *receiver* expression followed by a `.`, then a name and an argument
list in parentheses. Arguments are separated by commas. Methods that do not
take any arguments can omit the `()`:

    :::dart
    text.length

These are special "getters" or "accessors" in other languages. In Wren, they're
just method calls. You can also define methods that take an empty argument list:

    :::dart
    list.clear()

An empty argument list is *not* the same as omitting the parentheses
completely. Wren lets you overload methods by their call signature. This mainly
means [*arity*](classes.html#signature)—number of parameters—but
also distinguishes between "empty parentheses" and "no parentheses at all".

You can have a class that defines both `foo` and `foo()` as separate methods.
Think of it like the parentheses and commas between arguments are part of the
method's *name*.

If the last (or only) argument to a method call is a
[function](functions.html), it may be passed as a [block
argument](functions.html#block-arguments):

    :::dart
    blondie.callMeAt(867, 5309) {
      System.print("This is the body!")
    }

Semantically, all method calls work like so:

1. Evaluate the receiver and arguments.
2. Look up the method on the receiver's class.
3. Invoke it, passing in the arguments.

## This

The special `this` keyword works sort of like a variable, but has special
behavior. It always refers to the instance whose method is currently being
executed. This lets you invoke methods on "yourself".

It's an error to refer to `this` outside of a method. However, it's perfectly
fine to use it inside a function contained in a method. When you do, `this`
still refers to the instance whose method is being called.

This is unlike Lua and JavaScript which can "forget" `this` when you create a
callback inside a method. Wren does what you want here and retains the
reference to the original object. (In technical terms, a function's closure
includes `this`.)

## Super

Sometimes you want to invoke a method on yourself, but only methods defined in
one of your [superclasses](classes.html#inheritance). You typically do this in
an overridden method when you want to access the original method being
overridden.

To do that, you can use the special `super` keyword as the receiver in a method
call:

    :::dart
    class Base {
      method {
        System.print("base method")
      }
    }

    class Derived is Base {
      method {
        super.method // Prints "base method".
      }
    }

You can also use `super` without a method name inside a constructor to invoke a
base class constructor:

    :::dart
    class Base {
      this new(arg) {
        System.print("base constructor got " + arg)
      }
    }

    class Derived is Base {
      this new() {
        super("value") // Prints "base constructor got value".
      }
    }

**TODO: constructors**

## Operators

Wren has most of the same operators you know and love with the same precedence
and associativity. Wren has three prefix operators:

    :::dart
    ! ~ -

They are just method calls on their operand without any other arguments. An
expression like `!possible` means "call the `!` method on `possible`".

We have a few other operators to play with. The remaining ones are
infix—they have operators on either side. They are:

    :::dart
    == != < > <= >= .. ... | & + - * / %

Like prefix operators, they are all funny ways of writing method calls. The
left operand is the receiver, and the right operand gets passed to it. So
`a + b` is semantically interpreted as "invoke the `+` method on `a`, passing
it `b`".

Most of these are probably familiar already. The `..` and `...` operators are
"range" operators. The number type implements those to create a
[range](values.html#ranges) object, but they are just regular operators.

Since operators are just method calls, this means Wren supports "operator
overloading" (though "operator over-*riding*" is more accurate). This can be
really useful when the operator is natural for what a class represents, but can
lead to mind-crushingly unreadable code when used recklessly. There's a reason
punctuation represents profanity in comic strips.

## Assignment

The `=` operator is used to *assign* or store a value somewhere. The right-hand
side can be any expression. If the left-hand side is an
[identifier](#identifiers), then the value of the right operand is stored in
the referenced [variable](variables.html) or [field](classes.html#fields).

The left-hand side may also be a method call, like:

    :::dart
    point.x = 123

In this case, the entire expression is a single "setter" method call. The above
example invokes the `x=` setter on the `point` object, and passing in `123`.
Sort of like `point.x=(123)`.

Since these are just regular method calls, you can define whatever setters you
like in your classes. However, you cannot change the behavior of *simple*
assignment. If the left-hand side is a variable name or field, an assignment
expression will always just store the value there.

## Subscript operators

Most languages use square brackets (`[]`) for working with collection-like
objects. For example:

    :::dart
    list[0]    // Gets the first item in a list.
    map["key"] // Gets the value associated with "key".

You know the refrain by now. In Wren, these are just method calls that a class
may define. Subscript operators may take multiple arguments, which is useful
for things like multi-dimensional arrays:

    :::dart
    matrix[3, 5]

Subscripts may also be used on the left-hand side of an assignment:

    :::dart
    list[0] = "item"
    map["key"] = "value"

Again, these are just method calls. The last example is equivalent to invoking
the `[]=` method on `map`, passing in `"key"` and `"value"`.

## Logical operators

The `&&` and `||` operators are not like the other infix operators. They work
more like [control flow](control-flow.html) structures than operators because
they conditionally execute some code&mdash;they short-circuit. Depending on the
value of the left-hand side, the right-hand operand expression may or may not
be evaluated. Because of this, they cannot be overloaded and their behavior is
fixed.

A `&&` ("logical and") expression evaluates the left-hand argument. If it's
[false](control-flow.html#truth), it returns that value. Otherwise it evaluates
and returns the right-hand argument.

    :::dart
    System.print(false && 1)  // false
    System.print(1 && 2)      // 2

An `||` ("logical or") expression is reversed. If the left-hand argument is
[true](control-flow.html#truth), it's returned, otherwise the right-hand
argument is evaluated and returned:

    :::dart
    System.print(false || 1)  // 1
    System.print(1 || 2)      // 1

## The conditional operator `?:`

Also known as the "ternary" operator since it takes three arguments, Wren has
the little "if statement in the form of an expression" you know and love from C
and its brethren.

    :::dart
    System.print(1 != 2 ? "math is sane" : "math is not sane!")

It takes a condition expression, followed by `?`, followed by a then
expression, a `:`, then an else expression. Just like `if`, it evaluates the
condition. If true, it evaluates (and returns) the then expression. Otherwise
it does the else expression.

## The `is` operator

Wren has one last expression form. You can use the `is` keyword like an infix
operator. It performs a type test. The left operand is an object and the right
operand is a class. It evaluates to `true` if the object is an instance of the
class (or one of its subclasses).

    :::dart
    123 is Num     // true
    "s" is Num     // false
    null is String // false
    [] is List     // true
    [] is Sequence // true

## Precedence

When you mix these all together, you need to worry about
*precedence*&mdash;which operators bind more tightly than others&mdash;and
*associativity*&mdash;how a series of the same operator is ordered. Wren mostly
follows C, except that it fixes the bitwise operator mistake. The full
precedence table, from highest to lowest, is:

<table class="precedence">
  <tbody>
    <tr>
      <th>Prec</th>
      <th>Operator</th>
      <th>Description</th>
      <th>Assoc</th>
    </tr>
    <tr>
      <td>1</td>
      <td><code>()</code> <code>[]</code> <code>.</code></td>
      <td>Grouping, Subscript, Method call</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>2</td>
      <td><code>-</code> <code>!</code> <code>~</code></td>
      <td>Negate, Not, Complement</td>
      <td>Right</td>
    </tr>
    <tr>
      <td>3</td>
      <td><code>*</code> <code>/</code> <code>%</code></td>
      <td>Multiply, Divide, Modulo</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>4</td>
      <td><code>+</code> <code>-</code></td>
      <td>Add, Subtract</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>5</td>
      <td><code>..</code> <code>...</code></td>
      <td>Inclusive range, Exclusive range</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>6</td>
      <td><code>&lt;&lt;</code> <code>&gt;&gt;</code></td>
      <td>Left shift, Right shift</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>7</td>
      <td><code>&lt;</code> <code>&lt;=</code> <code>&gt;</code> <code>&gt;=</code></td>
      <td>Comparison</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>8</td>
      <td><code>==</code></td>
      <td>Equals</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>8</td>
      <td><code>!=</code></td>
      <td>Not equal</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>9</td>
      <td><code>&amp;</code></td>
      <td>Bitwise and</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>10</td>
      <td><code>^</code></td>
      <td>Bitwise xor</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>11</td>
      <td><code>|</code></td>
      <td>Bitwise or</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>12</td>
      <td><code>is</code></td>
      <td>Type test</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>13</td>
      <td><code>&amp;&amp;</code></td>
      <td>Logical and</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>14</td>
      <td><code>||</code></td>
      <td>Logical or</td>
      <td>Left</td>
    </tr>
    <tr>
      <td>15</td>
      <td><code>?:</code></td>
      <td>Conditional</td>
      <td>Right</td>
    </tr>
    <tr>
      <td>16</td>
      <td><code>=</code></td>
      <td>Assign</td>
      <td>Right</td>
    </tr>
  </tbody>
</table>