# PredicateExpression A component expression that makes up part of a predicate. ``` protocol PredicateExpression

``` ## Overview To transform a predicate, define a protocol for the result and add conformance on each predicate expression type that you support. For example: ```swift protocol ProsePredicateExpression { func proseQuery() -> String } // Repeated for each supported operator. extension PredicateExpressions.Equal: ProsePredicateExpression where LHS: ProsePredicateExpression, RHS: ProsePredicateExpression { func proseQuery() -> String { return lhs.proseQuery() + " is equal to " + rhs.proseQuery() } } extension Predicate { func proseQuery() -> String? { guard let expression = expression as? ProsePredicateExpression else { return nil } return expression.proseQuery() } } ``` ## Topics ### Evaluating a predicate expression [`Output`](/documentation/Foundation/PredicateExpression/Output) [`evaluate(_:)`](/documentation/Foundation/PredicateExpression/evaluate(_:)) --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)