In Part 3 we are focusing on building an Abstract Syntax Tree (AST) for an expression language using Java 25 features and Higher-Kinded-J.  We will create a complete expression language with parsing, type checking, optimisation, and interpretation that is type-safe, immutable, and easy to transform.

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Building an expression language interpreter using Java 25's sealed interfaces, records, and pattern matching combined with functional optics from Higher-Kinded-J. The article demonstrates how to design an immutable Abstract Syntax Tree (AST) for parsing and evaluating expressions, generate lenses and prisms for type-safe navigation, and implement optimizations like constant folding and identity simplification. Optics provide composable, bidirectional transformations that eliminate manual reconstruction boilerplate while maintaining data-oriented programming principles. The Focus DSL enables fluent navigation through nested structures, making complex tree transformations readable and maintainable.

Functional Optics for Modern Java