I am unsure how to change the name of a class in a library whilst providing a safe deprecation cycle for the users of the library. Suppose I have version 1 of my library with a class -- Version 1 class Foo a where foo :: a and I want to migrate it to version 3 as follows, but with a deprecation cycle provided by version 2 that allows code to work with either version 1 and 2, or 2 and 3. -- Version 3 class Bar a where bar :: a Then I can try to write version 2 as -- Version 2 {-# ConstraintKinds #-} class Foo a where bar :: a bar = foo foo :: a foo = bar type Bar = Foo This satisfies the following conditions: Users of versions 1 and 2 can use * The constraint Foo * The method foo * The class Foo and method foo for implementing their own instances User of versions 2 and 3 can use * The constraint Bar * The method bar * The method bar for implementing their own instances but they cannot jointly use * The *class* Bar for implementing their own instances (since Bar is not a class, just a Constraint) Is there any way I can write a version 2 that allows users to implement instances that work with both version 2 and version 3? Thanks, Tom