a classy little scripting language


Values are the built-in atomic object types that all other objects are composed of. They can be created through literals, expressions that evaluate to a value. All values are immutable—once created, they do not change. The number 3 is always the number 3. The string "frozen" can never have its character array modified in place.

Booleans #

A boolean value represents truth or falsehood. There are two boolean literals, true and false. Their class is Bool.

Numbers #

Like other scripting languages, Wren has a single numeric type: double-precision floating point. Number literals look like you expect coming from other languages:


Numbers are instances of the Num class.

Strings #

A string is an array of bytes. Typically, they store characters encoded in UTF-8, but you can put any byte values in there, even zero or invalid UTF-8 sequences. (You might have some trouble printing the latter to your terminal, though.)

String literals are surrounded in double quotes:

"hi there"

A handful of escape characters are supported:

"\0" // The NUL byte: 0.
"\"" // A double quote character.
"\\" // A backslash.
"\%" // A percent sign.
"\a" // Alarm beep. (Who uses this?)
"\b" // Backspace.
"\f" // Formfeed.
"\n" // Newline.
"\r" // Carriage return.
"\t" // Tab.
"\v" // Vertical tab.

A \u followed by four hex digits can be used to specify a Unicode code point:

System.print("\u0041\u0b83\u00DE") AஃÞ

A capital \U followed by eight hex digits allows Unicode code points outside of the basic multilingual plane, like all-important emoji:

System.print("\U0001F64A\U0001F680") 🙊🚀

A \x followed by two hex digits specifies a single unencoded byte:

System.print("\x48\x69\x2e") Hi.

Strings are instances of class String.

Interpolation #

String literals also allow interpolation. If you have a percent sign (%) followed by a parenthesized expression, the expression is evaluated. The resulting object’s toString method is called and the result is inserted in the string:

System.print("Math %(3 + 4 * 5) is fun!") Math 23 is fun!

Arbitrarily complex expressions are allowed inside the parentheses:

System.print("wow %((1..3).map {|n| n * n}.join())") wow 149

An interpolated expression can even contain a string literal which in turn has its own nested interpolations, but doing that gets unreadable pretty quickly.

Ranges #

A range is a little object that represents a consecutive range of numbers. They don’t have their own dedicated literal syntax. Instead, the number class implements the .. and ... operators to create them:


This creates a range from three to eight, including eight itself. If you want a half-inclusive range, use ...:


This creates a range from four to six not including six itself. Ranges are commonly used for iterating over a sequences of numbers, but are useful in other places too. You can pass them to a list‘s subscript operator to return a subset of the list, for example:

var list = ["a", "b", "c", "d", "e"] 
var slice = list[1..3] 
System.print(slice) [b, c, d]

Their class is Range.

Null #

Wren has a special value null, which is the only instance of the class Null. (Note the difference in case.) It functions a bit like void in some languages: it indicates the absence of a value. If you call a method that doesn’t return anything and get its returned value, you get null back.

Lists → ← Syntax