Re: [Haskell-cafe] Deprecation cycle for classes

16 Jun 2019


      On Sun, Jun 16, 2019 at 09:59:27AM +1000, Jack Kelly wrote:
...
Tom Ellis  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.
Unfortunately I don't think this scheme allows the user to write code that
is compatible with version 1 and version 2 at the same time.