Okasaki’s book is generally not practical and he even expressed some surprise at its popularity due to that fact. There are some practical purely functional data structures but you’ll have to find them elsewhere.
Can you go into a bit more detail, of why it would not be practical? Are there better implementations of the data structures in the book? The book I believe is from 1995 or so, so I could well imagine, that in the meantime progress has been made. But what exactly makes the algorithms in the book impractical for usage within a functional program?
