Swift Operators
What would a program be without operators? A mishmash of classes, namespaces, conditionals, loops, and namespaces signifying nothing.
Operators are what do the work of a program. They are the very execution of an executable; the teleological driver of every process. Operators are a topic of great importance for developers and the focus of this week’s NSHipster article.
Operator Precedence and Associativity
If we were to dissect an expression —
say 1 + 2
—
and decompose it into constituent parts,
we would find one operator and two operands:
1  +  2 

left operand  operator  right operand 
Expressions are expressed in a single, flat line of code, from which the compiler constructs an AST, or abstract syntax tree:
For compound expressions,
like 1 + 2 * 3
or 5  2 + 3
,
the compiler uses rules for
operator precedence and associativity
to resolve the expression into a single value.
Operator precedence rules,
similar to the ones you learned in primary school,
determine the order in which different kinds of operators are evaluated.
In this case, multiplication has a higher precedence than addition,
so 2 * 3
evaluates first.
1  +  (2 * 3) 

left operand  operator  right operand 
Associativity determines the order in which
operators with the same precedence are resolved.
If an operator is leftassociative,
then the operand on the lefthand side is evaluated first: ((5  2) + 3
);
if rightassociative,
then the righthand side operator is evaluated first: 5  (2 + 3)
.
Arithmetic operators are leftassociative,
so 5  2 + 3
evaluates to 6
.
(5  2)  +  3 

left operand  operator  right operand 
Swift Operators
The Swift Standard Library includes most of the operators
that a programmer might expect coming from another language in the C family,
as well as a few convenient additions like
the nilcoalescing operator (??
)
and pattern match operator (~=
),
as well as operators for
type checking (is
),
type casting (as
, as?
, as!
)
and forming open or closed ranges (...
, ..<
).
Infix Operators
Swift uses infix notation for binary operators (as opposed to, say Reverse Polish Notation). The Infix operators are grouped below according to their associativity and precedence level, in descending order:
BitwiseShiftPrecedence
<<
 Bitwise left shift
>>
 Bitwise right shift
MultiplicationPrecedence
*
 Multiply
/
 Divide
%
 Remainder
&*
 Multiply, ignoring overflow
&/
 Divide, ignoring overflow
&%
 Remainder, ignoring overflow
&
 Bitwise AND
AdditionPrecedence
+
 Add

 Subtract
&+
 Add with overflow
&
 Subtract with overflow

 Bitwise OR
^
 Bitwise XOR
RangeFormationPrecedence
..<
 Halfopen range
...
 Closed range
CastingPrecedence
is
 Type check
as
 Type cast
NilCoalescingPrecedence
??

nil
Coalescing
ComparisonPrecedence
<
 Less than
<=
 Less than or equal
>
 Greater than
>=
 Greater than or equal
==
 Equal
!=
 Not equal
===
 Identical
!==
 Not identical
~=
 Pattern match
LogicalConjunctionPrecedence
&&
 Logical AND
LogicalDisjunctionPrecedence

 Logical OR
DefaultPrecedence
(None)
AssignmentPrecedence
=
 Assign
*=
 Multiply and assign
/=
 Divide and assign
%=
 Remainder and assign
+=
 Add and assign
=
 Subtract and assign
<<=
 Left bit shift and assign
>>=
 Right bit shift and assign
&=
 Bitwise AND and assign
^=
 Bitwise XOR and assign
=
 Bitwise OR and assign
Unary Operators
In addition to binary operators that take two operands, there are also unary operators, which take a single operand.
Prefix Operators
Prefix operators come before the expression they operate on. Swift defines a handful of these by default:

+
: Unary plus 

: Unary minus 
!
: Logical NOT 
~
: Bitwise NOT 
...
: Openended partial range 
..<
: Closed partial range
For example,
the !
prefix operator
negates a logical value of its operand
and the 
prefix operator
negates the numeric value of its operand.
!true // false
(1.0 + 2.0) // 3.0
Postfix Operators
Unary operators can also come after their operand,
as is the case for the postfix variety.
These are less common;
the Swift Standard Library declares only the
openended range postfix operator, ...
.
let fruits = ["🍎", "🍌", "🍐", "🍊", "🍋"]
fruits[3...] // ["🍊", "🍋"]
Ternary Operators
The ternary ?:
operator is special.
It takes three operands
and functions like a singleline ifelse
statement:
evaluate the logical condition on the left side of the ?
and produces the expression on the left or righthand side of the :
depending on the result:
true ? "Yes" : "No" // "Yes"
In Swift,
Ternary
is defined lower than Default
and higher than Assignment
.
But, in general, it’s better to keep ternary operator usage simple
(or avoid them altogether).
Operator Overloading
Once an operator is declared, it can be associated with a type method or toplevel function. When an operator can resolve different functions depending on the types of operands, then we say that the operator is overloaded.
The most prominent examples of overloading can be found with the +
operator.
In many languages, +
can be used to perform
arithmetic addition (1 + 2 => 3
)
or concatenation for arrays and other collections ([1] + [2] => [1, 2]
).
Developers have the ability to overload standard operators by declaring a new function for the operator symbol with the appropriate number and type of arguments.
For example, to overload the *
operator
to repeat a String
a specified number of times,
you’d declare the following toplevel function:
func * (lhs: String, rhs: Int) > String {
guard rhs > 0 else {
return ""
}
return String(repeating: lhs, count: rhs)
}
"hello" * 3 // hellohellohello
This kind of language use is, however, controversial. (Any C++ developer would be all too eager to regale you with horror stories of the nondeterministic havoc this can wreak)
Consider the following statement:
[1, 2] + [3, 4] // [1, 2, 3, 4]
By default, the +
operator concatenates the elements of both arrays,
and is implemented using a generic function definition.
If you were to declare a specialized function
that overloads the +
for arrays of Double
values
to perform memberwise addition,
it would override the previous concatenating behavior:
// 👿
func + (lhs: [Double], rhs: [Double]) > [Double] {
return zip(lhs, rhs).map(+)
}
[1.0, 3.0, 5.0] + [2.0, 4.0, 6.0] // [3.0, 7.0, 11.0]
Herein lies the original sin of operator overloading: ambiguous semantics.
It makes sense that +
would work on numbers — that’s maths.
But if you really think about it,
why should adding two strings together concatenate them?
1 + 2
isn’t 12
(except in Javascript).
Is this really intuitive? …or is it just familiar.
Something to keep in mind when deciding whether to overload an existing operator.
Defining Custom Operators
One of the most exciting features of Swift (though also controversial) is the ability to define custom operators.
Consider the exponentiation operator, **
,
found in many programming languages,
but missing from Swift.
It raises the lefthand number to the power of the righthand number.
(The ^
symbol, commonly used for superscripts,
is already used by the
bitwise XOR operator).
Exponentiation has a higher operator precedence than multiplication, and since Swift doesn’t have a builtin precedence group that we can use, we first need to declare one ourselves:
precedencegroup ExponentiationPrecedence {
associativity: right
higherThan : MultiplicationPrecedence
}
Now we can declare the operator itself:
infix operator ** : ExponentiationPrecedence
Finally, we implement a toplevel function using our new operator:
import Darwin
func ** (lhs: Double, rhs: Double) > Double {
return pow(lhs, rhs)
}
2 ** 3 // 8
When you create a custom operator, consider providing a mutating variant as well:
infix operator **= : AssignmentPrecedence
func **= (lhs: inout Double, rhs: Double) {
lhs = pow(lhs, rhs)
}
var n: Double = 10
n **= 1 + 2 // n = 1000
Use of Mathematical Symbols
A custom operator can use combinations of the characters
/
, =
, 
, +
, !
, *
, %
, <
, >
, &
, 
, ^
, or ~
,
and any characters found in the
Mathematical Operators
Unicode block, among others.
This makes it possible to take the square root of a number
with a single √
prefix operator:
import Darwin
prefix operator √
prefix func √ (_ value: Double) > Double {
return sqrt(value)
}
√4 // 2
Or consider the ±
operator,
which can be used either as an infix or prefix operator
to return a tuple with the sum and difference:
infix operator ± : AdditionPrecedence
func ± <T: Numeric>(lhs: T, rhs: T) > (T, T) {
return (lhs + rhs, lhs  rhs)
}
prefix operator ±
prefix func ± <T: Numeric>(_ value: T) > (T, T) {
return 0 ± value
}
2 ± 3 // (5, 1)
±4 // (4, 4)
Custom operators are hard to type, and therefore hard to use, so exercise restraint with exotic symbols. Code should be typed, not be copypasted.
When overriding or defining new operators in your own code, make sure to follow these guidelines:
 Don’t create an operator unless its meaning is obvious and undisputed. Seek out any potential conflicts to ensure semantic consistency.
 Pay attention to the precedence and associativity of custom operators, and only define new operator groups as necessary.
 If it makes sense, consider implementing assigning variants
for your custom operator (e.g.
+=
for+
).