The book on immutable and persistent data structures is named "Purely Functional Data Structures" so I don't think there's anything wrong here. Functional means

The book on immutable and persistent data structures is named "Purely Functional Data Structures" so I don't think there's anything wrong here. Functional means a lot more than programming with functions nowadays. If you can't look past the letters of the word to see its meaning in a larger context idk what to say.