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The @discardableResult attribute is a compiler directive applied to function, method, or initializer declarations to suppress the default compiler warning emitted when a caller invokes the function without capturing or utilizing its return value. By default, Swift enforces strict return value utilization through static analysis. If a function returns a value, the compiler expects the caller to assign it to a variable, evaluate it in an expression, or explicitly discard it using the wildcard pattern (_ =). Applying @discardableResult alters this behavior for the decorated declaration, signaling to the compiler that ignoring the return value is an intended operation, thereby silencing the unused-result warning.

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Syntax

The attribute is placed immediately preceding the func or init keyword. 
@discardableResult
func computeAndCache() -> Int {
    let calculatedValue = 42
    // Internal state mutation or caching logic
    return calculatedValue
}

// Caller behavior
computeAndCache()               // Valid: No compiler warning generated
let result = computeAndCache()  // Valid: Return value remains fully accessible

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Technical Characteristics

  • Compile-Time Only: The attribute strictly affects compile-time static analysis. It does not alter the runtime behavior, memory allocation, or the actual return type of the function.
  • Caller Flexibility: It provides the caller with the option to either capture the returned data or safely ignore it without requiring the explicit _ = functionCall() syntax.
  • Redundancy: Applying @discardableResult to a function that returns Void (or ()) is redundant, as the Swift compiler inherently permits Void return values to be implicitly discarded.
  • Protocol Conformance: When applied to a method requirement within a protocol, the attribute dictates the compiler behavior for any type calling the method through the protocol interface. Conforming types can also independently apply the attribute to their specific implementations.
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