That's the converse of my point. The article claimed immutable data is important for a good REPL and I said that the claim was an overstatement. I didn't say that immutable data makes a REPL impossible.
I quite like immutable datastructures, as it happens.
Common Lisp has some immutable data structure such as, oh, numbers. If you arithmetically encode a record of data into a bignum integer, that is immutable.
I quite like immutable datastructures, as it happens.