Everyone else pretty much said it for me. To elaborate more personally: the distilled expression of pure functions as _the_ basic entity of modular engineering

Everyone else pretty much said it for me. To elaborate more personally: the distilled expression of pure functions as _the_ basic entity of modular engineering is such a huge selling point for me. Lazy evaluation is also surprisingly valuable in this vein, because I can compose components -- pass the output of a component to the input of another -- with the assurance that I'll only pay for that computation if it's actually requested. I do deeply appreciate Rust in how it supports serious functional programming idioms without requiring a GC. The value of immutable-by-default and single-ownership semantics have been infectious far beyond my direct use of Rust. But those very same features of Rust also make the pure function just a little more clunky in that environment. Oh, and being literally unable to perform I/O (or any other computational effect) outside of the right context is such a clear win for me. Very few languages, even in the functional world, have such a strict separation. The architectural and mental benefits are outstanding. Some concepts take on a surprisingly simple existence in Haskell. I recently discovered the Yoneda and codensity transformations, as so frequently discussed in Haskell... in the context of a Visitor pattern on a tree structure in Java. Visitors are literally Yonada-ified sum types, and I could only make that connection (and understand the ramifications and benefits) after seeing how it naturally arises in both languages.