> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # Swift Nil-Coalescing The `??` (nil-coalescing) operator is a binary operator that unwraps an `Optional` if it contains a value, or returns a fallback default value of type `T` if the optional evaluates to `nil`. ## Syntax ```swift theme={"dark"} let result = optionalExpression ?? defaultExpression ``` Semantically, the `??` operator is a concise shorthand for the ternary conditional operator combined with forced unwrapping: ```swift theme={"dark"} // The ?? operator a ?? b // Is equivalent to: a != nil ? a! : b ``` ## Type Constraints For the compiler to resolve the `??` operator, strict type constraints apply: 1. The left-hand side (`optionalExpression`) must be of type `Optional`. 2. The right-hand side (`defaultExpression`) must evaluate to type `T` (the exact wrapped type of the left-hand side). 3. The resulting expression evaluates to a non-optional `T`. *Note: It is syntactically valid for the right-hand side to also be an `Optional`. In this scenario, the resulting expression remains an `Optional`, allowing for nil-coalescing chains (`a ?? b ?? c`).* ## Short-Circuit Evaluation The `??` operator utilizes short-circuit evaluation. The right-hand side expression is evaluated lazily. If the left-hand side optional contains a value, the right-hand side is completely ignored and never executed. This behavior is achieved in the Swift Standard Library by defining the `defaultValue` parameter as an `@autoclosure`. ## Standard Library Signature Under the hood, the operator is defined with the following function signature: ```swift theme={"dark"} public func ?? ( optional: T?, defaultValue: @autoclosure () throws -> T ) rethrows -> T ``` * `T?`: The optional value being evaluated. * `@autoclosure () throws -> T`: The default value, wrapped in an automatic closure to defer execution until it is strictly necessary (i.e., when `optional` is `.none`). * `rethrows`: Propagates any errors thrown by the `defaultValue` expression, provided the expression is actually evaluated.

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