[Haskell-cafe] Deprecation cycle for classes

[Jack Kelly]

Tom Ellis <tom-lists-haskell-cafe-2017 at jaguarpaw.co.uk> writes:

> 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

What if you inserted Bar as a superclass of Foo in V2?

-- Version 1
class Foo a where
  foo :: a

-- Version 2
{-# DEPRECATED Foo "it's going away in V3" #-}
class Bar a => Foo a where
  foo :: a

class Bar a where
  bar :: a

-- Version 3
class Bar a where
  bar :: a

Users of version 1 can use Foo/foo/...

When they upgrade to version 2, they get forced to write a Bar instance,
get warned that Foo is going away, and they can use the class Bar to
implement their own instances.

Users of version 3 can only use Bar/bar/...

Does that help?

-- Jack