This time we examine how the Focus DSL lets you express complex tree traversals and optimisations in fluent, composable chains; turning verbose recursion into declarative paths like .departments().each().employees().each().salary().

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Functional optics provide a composable way to traverse and transform tree structures in Java. The Focus DSL's TraversalPath enables fluent navigation through nested data with methods like `.departments().each().employees().each().salary()`, eliminating manual recursion. The article demonstrates building traversals for an expression AST, implementing compiler optimizations (constant folding, identity simplification, dead branch elimination) as composable passes, and collecting information through folds. Bottom-up and top-down traversal strategies handle different transformation needs. The approach separates navigation logic from transformation logic, making tree operations declarative and type-safe while bridging to effect-polymorphic operations for validation and error accumulation.

Functional Optics for Modern Java

Building a Universal Expression Traversal

Filtered Traversals and Conditional Updates