Wren is deeply object oriented, so most code consists of invoking methods on objects, usually something like this:
You have a receiver expression (here
System) followed by a
., then a name
("Heyoo!")). Multiple arguments
are separated by commas:
The argument list can also be empty:
The VM executes a method call like so:
Unlike most other dynamically-typed languages, in Wren a class can have multiple methods with the same name, as long as they have different signatures. The signature includes the method’s name along with the number of arguments it takes. In technical terms, this means you can overload by arity.
For example, the Random class has two methods for getting a random integer. One takes a minimum and maximum value and returns a value in that range. The other only takes a maximum value and uses 0 as the minimum:
var random = Random.new() random.int(3, 10) random.int(4)
method, which has some kind of “optional” parameter. The body of the method
figures out how many arguments were passed and uses control flow to handle the
two different behaviors. That means first parameter represents “max unless
another parameter was passed, in which case it’s min”. Kind of gross.
In Wren, these are calls to two entirely separate methods,
int(_). This makes it easier to define “overloads” like this since you don’t
need optional parameters or any kind of control flow to handle the different
It’s also faster to execute. Since we know how many arguments are passed at compile time, we can compile this to directly call the right method and avoid any “if I got two arguments do this…” runtime work.
Some methods exist to expose a stored or computed property of an object. These are getters and have no parentheses:
"string".count 6 (1..10).min 1 1.23.sin 0.9424888019317 [1, 2, 3].isEmpty false
A getter is not the same as a method with an empty argument list. The
part of the signature, so
count() have different signatures.
Unlike Ruby’s optional parentheses, Wren wants to make sure you call a getter
like a getter and a
() method like a
() method. These don’t work:
"string".count() [1, 2, 3].clear
If you’re defining some member that doesn’t need any parameters, you need to
decide if it should be a getter or a method with an empty
() parameter list.
The general guidelines are:
If it modifies the object or has some other side effect, make it a method:
If the method supports multiple arities, make the zero-parameter case a
method to be consistent with the other versions:
Otherwise, it can probably be a getter.
A getter lets an object expose a public “property” that you can read. Likewise, a setter lets you write to a property:
person.height = 74 // Grew up!
=, this is just another syntax for a method call. From the
language’s perspective, the above line is just a call to the
person, passing in
=(_) is in the setter’s signature, an object can have both a getter
and setter with the same name without a collision. Defining both lets you
provide a read/write property.
Wren has most of the same operators you know and love with the same precedence and associativity. We have three prefix operators:
! ~ -
They are just method calls on their operand without any other arguments. An
!possible means “call the
! method on
We also have a slew of infix operators—they have operands on both sides. They are:
* / % + - .. ... << >> < <= > >= == != & ^ | is
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
- is both a prefix and an infix operator. Since they have different
-(_)), there’s no ambiguity between them.
Most of these are probably familiar already. The
... operators are
“range” operators. The number type implements those to create range
objects, but they are method calls like other operators.
is keyword is a “type test” operator. The base Object class implements
it to tell if an object is an instance of a given class. You’ll rarely need to,
but you can override
is in your own classes. That can be useful for things
like mocks or proxies where you want an object to masquerade as a certain class.
Another familiar syntax from math class is subscripting using square brackets
). It’s handy for working with collection-like objects. For example:
list // Get the first item in a list. map["key"] // Get the value associated with "key".
You know the refrain by now. In Wren, these are method calls. In the above
examples, the signature is
[_]. Subscript operators may also take multiple
arguments, which is useful for things like multi-dimensional arrays:
These examples are subscript “getters”, and there are also corresponding subscript setters:
list = "item" map["key"] = "value"
These are equivalent to method calls whose signature is
[_]=(_) and whose
arguments are both the subscript (or subscripts) and the value on the right-hand