IMO pure FP is nice because _compositionality_ is nice. If my problem has lots of simple data structures that represent mathematical objects, then usually I fin

IMO pure FP is nice because _compositionality_ is nice. If my problem has lots of simple data structures that represent mathematical objects, then usually I find that most of things I want to do with them can be succinctly modeled with pure function compositions. When my problem gets more complex, including but not limited to state, monads can restore that compositionality in a way that works with the rest of my pure code. This property is true to varying degrees outside of pure FP too. For example in Rust I find that expression-orientation makes programs fairly easy to compose, modulo thinking about ownership.