wren/doc/site/variables.markdown

^title Variables ^category language

Variables are named slots for storing values. You can define a new variable in Wren using a var statement, like so:

:::dart
var a = 1 + 2

This creates a new variable a in the current scope and initializes it with the result of the expression following the =. Once a variable has been defined, it can be accessed by name as you would expect.

:::dart
var animal = "Slow Loris"
IO.print(animal) // Prints "Slow Loris".

Scope

Wren has true block scope: a variable exists from the point where it is defined until the end of the block where that definition appears.

:::dart
{
  IO.print(a) // ERROR! a doesn't exist yet.
  var a = 123
  IO.print(a) // "123"
}
IO.print(a) // ERROR! a doesn't exist anymore.

Variables defined at the top level of a script are top-level and are visible to the module system. All other variables are local. Declaring a variable in an inner scope with the same name as an outer one is called shadowing and is not an error (although it's not something you likely intend to do much).

:::dart
var a = "outer"
{
  var a = "inner"
  IO.print(a) // Prints "inner".
}
IO.print(a) // Prints "outer".

Declaring a variable with the same name in the same scope is an error.

:::dart
var a = "hi"
var a = "again" // ERROR!

Assignment

After a variable has been declared, you can assign to it using =:

:::dart
var a = 123
a = 234

An assignment walks up the scope stack to find where the named variable is declared. It's an error to assign to a variable that isn't defined. Wren doesn't roll with implicit variable definition.

When used in a larger expression, an assignment expression evaluates to the assigned value.

:::dart
var a = "before"
IO.print(a = "after") // Prints "after".

TODO: Top-level names.