Reference immutability is a type based technique for taming mutation that has long been studied in the
context of object-oriented languages, like Java. Recently, though, languages like Scala have blurred the lines
between functional programming languages and object oriented programming languages. We explore how
reference immutability interacts with features commonly found in these hybrid languages, in particular with
higher-order functions – polymorphism – and subtyping. We construct a calculus System F<:M which encodes
a reference immutability system as a simple extension of System F<: and prove that it satisfies the standard
soundness and immutability safety properties.