Operators

Operators are special symbols that perform a computation for one or more values. They are either unary, binary, or ternary.

  • Unary operators perform an operation for a single value. The unary operator symbol appears before the value.

  • Binary operators operate on two values. The binary operator symbol appears between the two values (infix).

  • Ternary operators operate on three values. The first operator symbol appears between the first and second value, the second operator symbol appears between the second and third value (infix).

Assignment Operator (=)

The binary assignment operator = can be used to assign a new value to a variable. It is only allowed in a statement and is not allowed in expressions.

1
var a = 1
2
a = 2
3
// `a` is `2`
4
5
6
var b = 3
7
var c = 4
8
9
// Invalid: The assignment operation cannot be used in an expression.
10
a = b = c
11
12
// Instead, the intended assignment must be written in multiple statements.
13
b = c
14
a = b

Assignments to constants are invalid.

1
let a = 1
2
// Invalid: Assignments are only for variables, not constants.
3
a = 2

The left-hand side of the assignment operand must be an identifier. For arrays and dictionaries, this identifier can be followed by one or more index or access expressions.

1
// Declare an array of integers.
2
let numbers = [1, 2]
3
4
// Change the first element of the array.
5
//
6
numbers[0] = 3
7
8
// `numbers` is `[3, 2]`
1
// Declare an array of arrays of integers.
2
let arrays = [[1, 2], [3, 4]]
3
4
// Change the first element in the second array
5
//
6
arrays[1][0] = 5
7
8
// `arrays` is `[[1, 2], [5, 4]]`
1
let dictionaries = {
2
true: {1: 2},
3
false: {3: 4}
4
}
5
6
dictionaries[false][3] = 0
7
8
// `dictionaries` is `{
9
// true: {1: 2},
10
// false: {3: 0}
11
//}`

Force-assignment operator (<-!)

The force-assignment operator (<-!) assigns a resource-typed value to an optional-typed variable if the variable is nil. If the variable being assigned to is non-nil, the execution of the program aborts.

The force-assignment operator is only used for resource types.

Swapping Operator (<->)

The binary swap operator <-> can be used to exchange the values of two variables. It is only allowed in a statement and is not allowed in expressions.

1
var a = 1
2
var b = 2
3
a <-> b
4
// `a` is `2`
5
// `b` is `1`
6
7
var c = 3
8
9
// Invalid: The swap operation cannot be used in an expression.
10
a <-> b <-> c
11
12
// Instead, the intended swap must be written in multiple statements.
13
b <-> c
14
a <-> b

Both sides of the swap operation must be variable, assignment to constants is invalid.

1
var a = 1
2
let b = 2
3
4
// Invalid: Swapping is only possible for variables, not constants.
5
a <-> b

Both sides of the swap operation must be an identifier, followed by one or more index or access expressions.

Arithmetic Operators

The unary pefix operator - negates an integer:

1
let a = 1
2
-a // is `-1`

There are four binary arithmetic operators:

  • Addition: +
  • Subtraction: -
  • Multiplication: *
  • Division: /
  • Remainder: %
1
let a = 1 + 2
2
// `a` is `3`

The arguments for the operators need to be of the same type. The result is always the same type as the arguments.

The division and remainder operators abort the program when the divisor is zero.

Arithmetic operations on the signed integer types Int8, Int16, Int32, Int64, Int128, Int256, and on the unsigned integer types UInt8, UInt16, UInt32, UInt64, UInt128, UInt256, do not cause values to overflow or underflow.

1
let a: UInt8 = 255
2
3
// Run-time error: The result `256` does not fit in the range of `UInt8`,
4
// thus a fatal overflow error is raised and the program aborts
5
//
6
let b = a + 1
1
let a: Int8 = 100
2
let b: Int8 = 100
3
4
// Run-time error: The result `10000` does not fit in the range of `Int8`,
5
// thus a fatal overflow error is raised and the program aborts
6
//
7
let c = a * b
1
let a: Int8 = -128
2
3
// Run-time error: The result `128` does not fit in the range of `Int8`,
4
// thus a fatal overflow error is raised and the program aborts
5
//
6
let b = -a

Arithmetic operations on the unsigned integer types Word8, Word16, Word32, Word64 may cause values to overflow or underflow.

For example, the maximum value of an unsigned 8-bit integer is 255 (binary 11111111). Adding 1 results in an overflow, truncation to 8 bits, and the value 0.

1
// 11111111 = 255
2
// + 1
3
// = 100000000 = 0
1
let a: Word8 = 255
2
a + 1 // is `0`

Similarly, for the minimum value 0, subtracting 1 wraps around and results in the maximum value 255.

1
// 00000000
2
// - 1
3
// = 11111111 = 255
1
let b: Word8 = 0
2
b - 1 // is `255`

Arithmetics on number super-types

Arithmetic operators are not supported for number supertypes (Number, SignedNumber, FixedPoint, SignedFixedPoint, Integer, SignedInteger), as they may or may not succeed at run-time.

1
let x: Integer = 3 as Int8
2
let y: Integer = 4 as Int8
3
4
let z: Integer = x + y // Static error

Values of these types need to be cast to the desired type before performing the arithmetic operation.

1
let z: Integer = (x as! Int8) + (y as! Int8)

Logical Operators

Logical operators work with the boolean values true and false.

  • Logical NOT: !a

    This unary prefix operator logically negates a boolean:

    1
    let a = true
    2
    !a // is `false`
  • Logical AND: a && b

    1
    true && true // is `true`
    2
    3
    true && false // is `false`
    4
    5
    false && true // is `false`
    6
    7
    false && false // is `false`

    If the left-hand side is false, the right-hand side is not evaluated.

  • Logical OR: a || b

    1
    true || true // is `true`
    2
    3
    true || false // is `true`
    4
    5
    false || true // is `true`
    6
    7
    false || false // is `false`

    If the left-hand side is true, the right-hand side is not evaluated.

Comparison Operators

Comparison operators work with boolean and integer values.

  • Equality: ==, for booleans and integers

    Both sides of the equality operator may be optional, even of different levels, so it is for example possible to compare a non-optional with a double-optional (??).

    1
    1 == 1 // is `true`
    2
    3
    1 == 2 // is `false`
    1
    true == true // is `true`
    2
    3
    true == false // is `false`
    1
    let x: Int? = 1
    2
    x == nil // is `false`
    1
    let x: Int = 1
    2
    x == nil // is `false`
    1
    // Comparisons of different levels of optionals are possible.
    2
    let x: Int? = 2
    3
    let y: Int?? = nil
    4
    x == y // is `false`
    1
    // Comparisons of different levels of optionals are possible.
    2
    let x: Int? = 2
    3
    let y: Int?? = 2
    4
    x == y // is `true`
  • Inequality: !=, for booleans and integers (possibly optional)

    Both sides of the inequality operator may be optional, even of different levels, so it is for example possible to compare a non-optional with a double-optional (??).

    1
    1 != 1 // is `false`
    2
    3
    1 != 2 // is `true`
    1
    true != true // is `false`
    2
    3
    true != false // is `true`
    1
    let x: Int? = 1
    2
    x != nil // is `true`
    1
    let x: Int = 1
    2
    x != nil // is `true`
    1
    // Comparisons of different levels of optionals are possible.
    2
    let x: Int? = 2
    3
    let y: Int?? = nil
    4
    x != y // is `true`
    1
    // Comparisons of different levels of optionals are possible.
    2
    let x: Int? = 2
    3
    let y: Int?? = 2
    4
    x != y // is `false`
  • Less than: <, for integers

    1
    1 < 1 // is `false`
    2
    3
    1 < 2 // is `true`
    4
    5
    2 < 1 // is `false`
  • Less or equal than: <=, for integers

    1
    1 <= 1 // is `true`
    2
    3
    1 <= 2 // is `true`
    4
    5
    2 <= 1 // is `false`
  • Greater than: >, for integers

    1
    1 > 1 // is `false`
    2
    3
    1 > 2 // is `false`
    4
    5
    2 > 1 // is `true`
  • Greater or equal than: >=, for integers

    1
    1 >= 1 // is `true`
    2
    3
    1 >= 2 // is `false`
    4
    5
    2 >= 1 // is `true`

Comparing number super-types

Similar to arithmetic operators, comparison operators are also not supported for number supertypes (Number, SignedNumber FixedPoint, SignedFixedPoint, Integer, SignedInteger), as they may or may not succeed at run-time.

1
let x: Integer = 3 as Int8
2
let y: Integer = 4 as Int8
3
4
let z: Bool = x > y // Static error

Values of these types need to be cast to the desired type before performing the arithmetic operation.

1
let z: Bool = (x as! Int8) > (y as! Int8)

Bitwise Operators

Bitwise operators enable the manipulation of individual bits of unsigned and signed integers. They're often used in low-level programming.

  • Bitwise AND: a & b

    Returns a new integer whose bits are 1 only if the bits were 1 in both input integers:

    1
    let firstFiveBits = 0b11111000
    2
    let lastFiveBits = 0b00011111
    3
    let middleTwoBits = firstFiveBits & lastFiveBits // is 0b00011000
  • Bitwise OR: a | b

    Returns a new integer whose bits are 1 only if the bits were 1 in either input integers:

    1
    let someBits = 0b10110010
    2
    let moreBits = 0b01011110
    3
    let combinedbits = someBits | moreBits // is 0b11111110
  • Bitwise XOR: a ^ b

    Returns a new integer whose bits are 1 where the input bits are different, and are 0 where the input bits are the same:

    1
    let firstBits = 0b00010100
    2
    let otherBits = 0b00000101
    3
    let outputBits = firstBits ^ otherBits // is 0b00010001

Bitwise Shifting Operators

  • Bitwise LEFT SHIFT: a << b

    Returns a new integer with all bits moved to the left by a certain number of places.

    1
    let someBits = 4 // is 0b00000100
    2
    let shiftedBits = someBits << 2 // is 0b00010000
  • Bitwise RIGHT SHIFT: a >> b

    Returns a new integer with all bits moved to the right by a certain number of places.

    1
    let someBits = 8 // is 0b00001000
    2
    let shiftedBits = someBits >> 2 // is 0b00000010

For unsigned integers, the bitwise shifting operators perform logical shifting, for signed integers, they perform arithmetic shifting.

Ternary Conditional Operator

There is only one ternary conditional operator, the ternary conditional operator (a ? b : c).

It behaves like an if-statement, but is an expression: If the first operator value is true, the second operator value is returned. If the first operator value is false, the third value is returned.

The first value must be a boolean (must have the type Bool). The second value and third value can be of any type. The result type is the least common supertype of the second and third value.

1
let x = 1 > 2 ? 3 : 4
2
// `x` is `4` and has type `Int`
3
4
let y = 1 > 2 ? nil : 3
5
// `y` is `3` and has type `Int?`

Casting Operators

Static Casting Operator (as)

The static casting operator as can be used to statically type cast a value to a type.

If the static type of the value is a subtype of the given type (the "target" type), the operator returns the value as the given type.

The cast is performed statically, i.e. when the program is type-checked. Only the static type of the value is considered, not the run-time type of the value.

This means it is not possible to downcast using this operator. Consider using the conditional downcasting operator as? instead.

1
// Declare a constant named `integer` which has type `Int`.
2
//
3
let integer: Int = 1
4
5
// Statically cast the value of `integer` to the supertype `Number`.
6
// The cast succeeds, because the type of the variable `integer`,
7
// the type `Int`, is a subtype of type `Number`.
8
// This is an upcast.
9
//
10
let number = integer as Number
11
// `number` is `1` and has type `Number`
12
13
// Declare a constant named `something` which has type `AnyStruct`,
14
// with an initial value which has type `Int`.
15
//
16
let something: AnyStruct = 1
17
18
// Statically cast the value of `something` to `Int`.
19
// This is invalid, the cast fails, because the static type of the value is type `AnyStruct`,
20
// which is not a subtype of type `Int`.
21
//
22
let result = something as Int

Conditional Downcasting Operator (as?)

The conditional downcasting operator as? can be used to dynamically type cast a value to a type. The operator returns an optional. If the value has a run-time type that is a subtype of the target type the operator returns the value as the target type, otherwise the result is nil.

The cast is performed at run-time, i.e. when the program is executed, not statically, i.e. when the program is checked.

1
// Declare a constant named `something` which has type `AnyStruct`,
2
// with an initial value which has type `Int`.
3
//
4
let something: AnyStruct = 1
5
6
// Conditionally downcast the value of `something` to `Int`.
7
// The cast succeeds, because the value has type `Int`.
8
//
9
let number = something as? Int
10
// `number` is `1` and has type `Int?`
11
12
// Conditionally downcast the value of `something` to `Bool`.
13
// The cast fails, because the value has type `Int`,
14
// and `Bool` is not a subtype of `Int`.
15
//
16
let boolean = something as? Bool
17
// `boolean` is `nil` and has type `Bool?`

Downcasting works for concrete types, but also works e.g. for nested types (e.g. arrays), interfaces, optionals, etc.

1
// Declare a constant named `values` which has type `[AnyStruct]`,
2
// i.e. an array of arbitrarily typed values.
3
//
4
let values: [AnyStruct] = [1, true]
5
6
let first = values[0] as? Int
7
// `first` is `1` and has type `Int?`
8
9
let second = values[1] as? Bool
10
// `second` is `true` and has type `Bool?`

Force-downcasting Operator (as!)

The force-downcasting operator as! behaves like the conditional downcasting operator as?. However, if the cast succeeds, it returns a value of the given type instead of an optional, and if the cast fails, it aborts the program instead of returning nil,

1
// Declare a constant named `something` which has type `AnyStruct`,
2
// with an initial value which has type `Int`.
3
//
4
let something: AnyStruct = 1
5
6
// Force-downcast the value of `something` to `Int`.
7
// The cast succeeds, because the value has type `Int`.
8
//
9
let number = something as! Int
10
// `number` is `1` and has type `Int`
11
12
// Force-downcast the value of `something` to `Bool`.
13
// The cast fails, because the value has type `Int`,
14
// and `Bool` is not a subtype of `Int`.
15
//
16
let boolean = something as! Bool
17
// Run-time error

Optional Operators

Nil-Coalescing Operator (??)

The nil-coalescing operator ?? returns the value inside an optional if it contains a value, or returns an alternative value if the optional has no value, i.e., the optional value is nil.

If the left-hand side is non-nil, the right-hand side is not evaluated.

1
// Declare a constant which has an optional integer type
2
//
3
let a: Int? = nil
4
5
// Declare a constant with a non-optional integer type,
6
// which is initialized to `a` if it is non-nil, or 42 otherwise.
7
//
8
let b: Int = a ?? 42
9
// `b` is 42, as `a` is nil

The nil-coalescing operator can only be applied to values which have an optional type.

1
// Declare a constant with a non-optional integer type.
2
//
3
let a = 1
4
5
// Invalid: nil-coalescing operator is applied to a value which has a non-optional type
6
// (a has the non-optional type `Int`).
7
//
8
let b = a ?? 2
1
// Invalid: nil-coalescing operator is applied to a value which has a non-optional type
2
// (the integer literal is of type `Int`).
3
//
4
let c = 1 ?? 2

The type of the right-hand side of the operator (the alternative value) must be a subtype of the type of left-hand side, i.e. the right-hand side of the operator must be the non-optional or optional type matching the type of the left-hand side.

1
// Declare a constant with an optional integer type.
2
//
3
let a: Int? = nil
4
let b: Int? = 1
5
let c = a ?? b
6
// `c` is `1` and has type `Int?`
7
8
// Invalid: nil-coalescing operator is applied to a value of type `Int?`,
9
// but the alternative has type `Bool`.
10
//
11
let d = a ?? false

Force Unwrap Operator (!)

The force-unwrap operator (!) returns the value inside an optional if it contains a value, or panics and aborts the execution if the optional has no value, i.e., the optional value is nil.

1
// Declare a constant which has an optional integer type
2
//
3
let a: Int? = nil
4
5
// Declare a constant with a non-optional integer type,
6
// which is initialized to `a` if `a` is non-nil.
7
// If `a` is nil, the program aborts.
8
//
9
let b: Int = a!
10
// The program aborts because `a` is nil.
11
12
// Declare another optional integer constant
13
let c: Int? = 3
14
15
// Declare a non-optional integer
16
// which is initialized to `c` if `c` is non-nil.
17
// If `c` is nil, the program aborts.
18
let d: Int = c!
19
// `d` is initialized to 3 because c isn't nil.

The force-unwrap operator can only be applied to values which have an optional type.

1
// Declare a constant with a non-optional integer type.
2
//
3
let a = 1
4
5
// Invalid: force-unwrap operator is applied to a value which has a
6
// non-optional type (`a` has the non-optional type `Int`).
7
//
8
let b = a!
1
// Invalid: The force-unwrap operator is applied
2
// to a value which has a non-optional type
3
// (the integer literal is of type `Int`).
4
//
5
let c = 1!

Precedence and Associativity

Operators have the following precedences, highest to lowest:

  • Unary precedence: -, !, <-
  • Cast precedence: as, as?, as!
  • Multiplication precedence: *, /, %
  • Addition precedence: +, -
  • Bitwise shift precedence: <<, >>
  • Bitwise conjunction precedence: &
  • Bitwise exclusive disjunction precedence: ^
  • Bitwise disjunction precedence: |
  • Nil-Coalescing precedence: ??
  • Relational precedence: <, <=, >, >=
  • Equality precedence: ==, !=
  • Logical conjunction precedence: &&
  • Logical disjunction precedence: ||
  • Ternary precedence: ? :

All operators are left-associative, except for the following operators which are right-associative:

  • Ternary operator
  • Nil-coalescing operator

Expressions can be wrapped in parentheses to override precedence conventions, i.e. an alternate order should be indicated, or when the default order should be emphasized e.g. to avoid confusion. For example, (2 + 3) * 4 forces addition to precede multiplication, and 5 + (6 * 7) reinforces the default order.