Kotlin promises multi-paradigm support, and FP is part of that promise. But how much actually holds up? Can you write idiomatic functional Kotlin without external libraries? Let's find out.

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Kotlin supports casual functional programming well with immutability, higher-order functions, algebraic data types via sealed classes, and exhaustive pattern matching. The standard library provides rich collection operations like map, filter, and fold. However, Kotlin lacks pure FP features like higher-kinded types, type classes, built-in Either/Result for domain errors, effect systems, and automatic tail call optimization. For most practical applications, Kotlin's casual FP capabilities are sufficient, but teams requiring pure FP guarantees need external libraries like Arrow.

Functional Programming in Kotlin