Scala

Use when scala collections including immutable/mutable variants, List, Vector, Set, Map operations, collection transformations, lazy evaluation with views, parallel collections, and custom collection builders for efficient data processing.

Use when scala's advanced type system including generics, variance, type bounds, implicit conversions, type classes, higher-kinded types, path-dependent types, and abstract type members for building type-safe, flexible APIs.

Use when functional programming patterns in Scala including higher-order functions, immutability, pattern matching, algebraic data types, monads, for-comprehensions, and functional composition for building robust, type-safe applications.