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Mutation Operators

Muter uses mutation operators to generate mutants in your source code. This is the list of currently available mutation operators.

Negate Conditionals

The negate conditionals operator will invert conditional operators in your code based on this table:

Original Operator Negated Operator
== !=
!= ==
>= <=
<= >=
> <
< >

The purpose of this operator is to highlight how your tests respond to changes in branching logic. A well-engineered test suite will be able to fail clearly in response to code taking a different branch than it expected.

Mutating an equality check

if myValue == 50 {
    // something happens here
}

becomes

if myValue != 50 {
    // something happens here
}

Change Logical Connector

The change logical connector operator will change conditional operators in your code based on this table:

Original Operator Changed Operator
&& ||
|| &&

The purpose of this operator is to highlight how your tests respond to changes in logic. A well-engineered test suite will be able to fail clearly in response to different logical constraints.

Mutating a Logical AND

func isValidPassword(_ text: String, _ repeatedText: String) -> Bool {
    let meetsMinimumLength = text.count >= 8
    let passwordsMatch = repeatedText == text
    return meetsMinimumLength && passwordsMatch
}

becomes

func isValidPassword(_ text: String, _ repeatedText: String) -> Bool {
    let meetsMinimumLength = text.count >= 8
    let passwordsMatch = repeatedText == text
    return meetsMinimumLength || passwordsMatch
}

Remove Side Effects

The Remove Side Effects operator will remove code it determines is causing a side effect. It will determine your code is causing a side effect based on a few rules:

  • A line contains a function call which is explicitly discarding a return result
  • A line contains a function call and doesn't save the result of the function call into a named variable or constant (i.e. a line implicitly discards a return result or doesn't produce one)
  • A line does not contain a call to print, exit, fatalError, abort, or any function listed in muter.conf.yml under the excludeCalls key.

The purpose of this operator is to highlight how your tests respond to the absence of expected side effects.

Mutating an explicitly discarded return result

func initialize() {
    _ = self.view
    view.results = self.results
}

becomes

func initialize() {
    view.results = self.results
}

Mutating a void function call

func update(email: String, for userId: String) {
    var userRecord = record(for: userId)
    userRecord.email = email
    database.persist(userRecord)
}

becomes

func update(email: String, for userId: String) {
    var userRecord = record(for: userId)
    userRecord.email = email
}

Swap Ternary

The Swap Ternary operator will swap ternary operators in your code based on this table:

Original Operator Changed Operator
<condition> ? <expression1> : <expression2> <condition> ? <expression2> : <expression1>

The purpose of this operator is to highlight how your tests respond to changes in logic. A well-engineered test suite will be able to fail clearly in response to different logical constraints.