Scheme does the right thing here (by convention), that mutating procedures end with a bang: (string-upcase str) returns a new string, whereas (string-upcase! st

Scheme does the right thing here (by convention), that mutating procedures end with a bang: (string-upcase str) returns a new string, whereas (string-upcase! str) mutates the string in place. The details for mutation of data in scheme go beyond that, though. Sometimes procedures are "allowed but not required to mutate their argument". Most (all?) implementations do mutate, but it is still considered bad for to do something like: (define a (list 1 2 3)) (append! a (list 4)) (display a) As append! returns a list that is supposed to supercede the binding to a. Using a like that "is an error", as a valid implementation of append! may look like this (define append! append) Which would make the earlier code snippet invalid.