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A default parameter value in Swift is a predefined expression assigned to a function, method, or initializer parameter within its declaration. This allows the caller to omit the corresponding argument at the call site. When an argument is omitted, the compiler injects the default expression at compile time, and the expression is then evaluated dynamically at execution time.

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Syntax

Default values are defined by appending the assignment operator (=) and the value expression immediately after the parameter’s explicit type annotation. 
func executeTask(timeout: Double = 30.0, retryCount: Int = 3) {
    // Implementation
}

// Valid invocations:
executeTask() 
executeTask(timeout: 15.0)
executeTask(timeout: 10.0, retryCount: 5)

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Evaluation Semantics

Unlike languages where default parameters are evaluated once at declaration or compile time (e.g., Python), Swift evaluates default parameter expressions at the call site every time the function is invoked without that argument. If the default value is the result of a function call or a computed property, that execution occurs dynamically upon each invocation. 
import Foundation

func generateID() -> String {
    return UUID().uuidString
}

// generateID() is executed every time createRecord() is called without an ID
func createRecord(id: String = generateID()) {
    print(id)
}

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Parameter Ordering and Resolution

Swift does not enforce a strict rule requiring default parameters to be placed at the end of a parameter list. You can interleave parameters with and without default values. However, the compiler relies on argument labels to resolve ambiguity. When an argument is omitted, the compiler matches the remaining provided arguments to the function signature sequentially, utilizing the argument labels to map values to their corresponding parameters. 
func configure(debugMode: Bool = false, environment: String, verbose: Bool = true) {
    // Implementation
}

// The compiler uses the 'environment' label to bridge the omitted defaults
configure(environment: "Production")

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Type Annotations and Optionals

Swift does not support type inference for function or method parameters based on default values. Explicit type annotations are strictly required in the function signature. A declaration like func myFunc(value = 10) is invalid syntax and will fail to compile. Furthermore, when assigning nil as a default value, the parameter must be explicitly typed as an Optional. 
import Foundation

// Explicit type annotation is strictly required; type inference is not supported
func fetchData(completion: ((Data?) -> Void)? = nil) {
    // Implementation
}

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Caller Context via Literal Expressions

Swift provides special literal expressions such as #file, #line, #column, and #function. When these are used as default parameter values, they evaluate to the lexical context of the caller, rather than the context where the function is declared. This is a compiler-level feature heavily utilized for logging and assertion mechanisms. 
func log(message: String, file: String = #file, line: Int = #line) {
    print("[\(file):\(line)] \(message)")
}

// When invoked, #file and #line capture the location of THIS specific call site
log(message: "System initialized")
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