---
title: "~=(_:_:)"
framework: swift
role: symbol
role_heading: Operator
path: "swift/optional/~=(_:_:)"
---

# ~=(_:_:)

Returns a Boolean value indicating whether an argument matches nil.

## Declaration

```swift
static func ~= (lhs: _OptionalNilComparisonType, rhs: borrowing Wrapped?) -> Bool
```

## Parameters

- `lhs`: A nil literal.
- `rhs`: A value to match against nil.

## Discussion

Discussion You can use the pattern-matching operator (~=) to test whether an optional instance is nil even when the wrapped value’s type does not conform to the Equatable protocol. The pattern-matching operator is used internally in case statements for pattern matching. The following example declares the stream variable as an optional instance of a hypothetical DataStream type, and then uses a switch statement to determine whether the stream is nil or has a configured value. When evaluating the nil case of the switch statement, this operator is called behind the scenes. var stream: DataStream? = nil switch stream { case nil:     print("No data stream is configured.") case let x?:     print("The data stream has \(x.availableBytes) bytes available.") } // Prints "No data stream is configured." note: To test whether an instance is nil in an if statement, use the equal-to operator (==) instead of the pattern-matching operator. The pattern-matching operator is primarily intended to enable case statement pattern matching.