Scheme was defined as an algorithmic language, not a strictly pure-functional one, though it also included some functional features and permitted other functional abstractions to be implemented.

Racket slowly added some immutable data types to Scheme, which was a nice fit for some other Scheme features that had been there from the start, and better for software engineering (e.g., enforce that a user of a module couldn't go and mutate data you were sharing with them).