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78 lines
2.2 KiB
Markdown
78 lines
2.2 KiB
Markdown
^title Variables
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Variables are named slots for storing values. You define a new variable in Wren
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using a `var` statement, like so:
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:::wren
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var a = 1 + 2
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This creates a new variable `a` in the current scope and initializes it with
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the result of the expression following the `=`. Once a variable has been
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defined, it can be accessed by name as you would expect.
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:::wren
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var animal = "Slow Loris"
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System.print(animal) //> Slow Loris
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## Scope
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Wren has true block scope: a variable exists from the point where it is defined
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until the end of the [block](syntax.html#blocks) where that definition appears.
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:::wren
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{
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System.print(a) //! "a" doesn't exist yet.
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var a = 123
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System.print(a) //> 123
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}
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System.print(a) //! "a" doesn't exist anymore.
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Variables defined at the top level of a script are *top-level* and are visible
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to the [module](modules.html) system. All other variables are *local*.
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Declaring a variable in an inner scope with the same name as an outer one is
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called *shadowing* and is not an error (although it's not something you likely
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intend to do much).
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:::wren
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var a = "outer"
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{
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var a = "inner"
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System.print(a) //> inner
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}
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System.print(a) //> outer
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Declaring a variable with the same name in the *same* scope *is* an error.
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:::wren
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var a = "hi"
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var a = "again" //! "a" is already declared.
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## Assignment
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After a variable has been declared, you can assign to it using `=`:
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:::wren
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var a = 123
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a = 234
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An assignment walks up the scope stack to find where the named variable is
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declared. It's an error to assign to a variable that isn't defined. Wren
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doesn't roll with implicit variable definition.
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When used in a larger expression, an assignment expression evaluates to the
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assigned value.
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:::wren
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var a = "before"
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System.print(a = "after") //> after
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If the left-hand side is some more complex expression than a bare variable name,
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then it isn't an assignment. Instead, it's calling a [setter method][].
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[setter method]: method-calls.html#setters
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**TODO: Top-level names.**
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<a class="right" href="functions.html">Functions →</a>
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<a href="control-flow.html">← Control Flow</a>
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