Variables

A variable is a name that holds a value or object. All variables in Mojo are mutable—their value can be changed. (If you want to define a constant value that can't change at runtime, see the alias keyword.)

note

Mojo formerly supported the let keyword for declaring immutable variables. This has been removed to simplify the language, and for other reasons discussed elsewhere. To simplify the migration of older code, let declarations are currently supported, but function the same as var declarations.

Undeclared variables

Within a def function or a REPL environment, you can create a variable with just a name and a value. For example:

name = "Sam"

A variable declared without var follows s.

note

Undeclared variables are not allowed in an fn function or as a struct field.

Declared variables

You can declare a variable with the var keyword. For example:

var name = "Sam"
var user_id: Int

The name variable is initialized to the string "Sam". The user_id variable is uninitialized, but it has a declared type, Int for an integer value. All declared values are typed—either explicitly with a type annotation or implicitly when they're initialized with a value.

Since declared variables are strongly typed, you can't assign a variable a value of a different type, unless those types can be implicitly converted. For example, this code will not compile:

var user_id: Int = "Sam"

In addition to typing, declared variables also follow lexical scoping, unlike undeclared variables.

Finally, using var helps prevent runtime errors caused by typos. For example, if you misspell the name of an undeclared variable, Mojo simply instantiates a new variable using the misspelled name. But when all mutable variables must be first declared with var (which is the case inside an fn function), then misspellings such as the following are caught by the compiler:

var name = "Sam"
# Somewhere later...
nane = "Sammy"  # This is not allowed in an `fn` function

Although you can use var in a def function, this benefit is realized only when used inside an fn function, where the Mojo compiler will flag undeclared variables (such as the above nane) as unknown declarations.

note

When using Mojo in a REPL environment, top-level variables (variables outside a function or struct) do not require var declarations.

Type annotations

Although Mojo supports dynamic variable types (it can infer a value type at runtime), it also supports static type annotations on variables. This enables strong compile-time type checking for variables, which can make your code more predictable, manageable, and secure (especially when combined with type checking in fn functions).

To specify the type for a variable, add a colon followed by the type name:

var name: String = "Sam"

This way, name can never be assigned a value that's not a string (or that cannot be implicitly converted to a string).

note

You must declare a variable with var to use type annotations.

If a type has a constructor with just one argument, you can initialize it in two ways:

var name1: String = "Sam"
var name2 = String("Sam")

Both of these lines invoke the same constructor to create a String from a StringLiteral.

Late initialization

Using type annotations allows for late initialization. For example, notice here that the z variable is first declared with just a type, and the value is assigned later:

fn my_function(x: Int):
    var z: Float32
    if x != 0:
        z = 1.0
    else:
        z = foo()
    print(z)

fn foo() -> Float32:
    return 3.14

note

Late initialization works only if the variable is declared with a type.

Implicit type conversion

Some types include built-in type conversion (type casting) from one type into its own type. For example, if you assign a number to a String, it creates the string "1" instead of a compiler error:

var number: String = 1

As shown above, value assignment can be converted into a constructor call if the target type has a constructor that takes a single argument that matches the value being assigned. So, this code uses the String constructor that takes an integer: __init__(inout self, num: Int).

Implicit conversion follows the logic of overloaded functions, because that's exactly what's happening here: assigning a number to a String variable is exactly the same as this:

var number = String(1)

Thus, if you call a function that requires an argument of a certain type (such as String), you can pass in any value as long as that value type can implicitly convert to the required type (using one of the type's overloaded constructors).

For example, you can pass an Int to a function that expects a String, because String includes a constructor that takes an Int:

fn take_string(version: String):
    print(version)

fn pass_integer():
    var version: Int = 1
    take_string(version)

For more details on implicit conversion, see Constructors and implicit conversion.

Variable scopes

Variables declared with var are bound by lexical scoping. This means that nested code blocks can read and modify variables defined in an outer scope. But an outer scope cannot read variables defined in an inner scope at all.

For example, the if code block shown here creates an inner scope where outer variables are accessible to read/write, but any new variables do not live beyond the scope of the if block:

def lexical_scopes():
    var num = 10
    var dig = 1
    if True:
        print("num:", num)  # Reads the outer-scope "num"
        var num = 20        # Creates new inner-scope "num"
        print("num:", num)  # Reads the inner-scope "num"
        dig = 2             # Edits the outer-scope "dig"
    print("num:", num)      # Reads the outer-scope "num"
    print("dig:", dig)      # Reads the outer-scope "dig"

lexical_scopes()

num: 10 num: 20 num: 10 dig: 2

Note that the var statement inside the if creates a new variable with the same name as the outer variable. This prevents the inner loop from accessing the outer num variable. (This is called "variable shadowing," where the inner scope variable hides or "shadows" a variable from an outer scope.)

The lifetime of the inner num ends exactly where the if code block ends, because that's the scope in which the variable was defined.

This is in contrast to undeclared variables (those without the var keyword), which use function-level scoping (consistent with Python variable behavior). That means, when you change the value of an undeclared variable inside the if block, it actually changes the value for the entire function.

For example, here's the same code but without the var declarations:

def function_scopes():
    num = 1
    if num == 1:
        print(num)   # Reads the function-scope "num"
        num = 2      # Updates the function-scope variable
        print(num)   # Reads the function-scope "num"
    print(num)       # Reads the function-scope "num"

function_scopes()

1 2 2

Now, the last print() function sees the updated num value from the inner scope, because undeclared variables (Python-style variables) use function-level scope (instead of lexical scope).