Scrap Distribution.Compat.Newtype

[Bodigrim]

This module was derived from https://hackage.haskell.org/package/newtype, which predates introduction of class Coercible to GHC, a more modern and better known alternative.

The class Newtype served to provide rather awkward combinators:

• ala :: (Newtype o n, Newtype o' n') => (o -> n) -> ((o -> n) -> b -> n') -> b -> o'
• alaf :: (Newtype o n, Newtype o' n') => (o -> n) -> ((a -> n) -> b -> n') -> (a -> o) -> b -> o'
• pack' :: Newtype o n => (o -> n) -> o -> n
• unpack' :: Newtype o n => (o -> n) -> n -> o

What is awkward about them? All of them completely ignore the first argument and never apply it to the value. The argument serves purely to provide a type annotation in an ancient epoch predating {-# LANGUAGE TypeApplication #-}. (Even this could have been done better by using proxy a b instead of (->) a b in type signatures, to signalize that the argument is not going to be applied to anything).

Haskell acquired class Coercible a b where coerce :: a -> b after 2011 and by now it is a standard way to deal with newtypes, reducing cognitive overhead for developers.

Please read Github PR Conventions and then fill in one of these two templates.

---

Template Α: This PR modifies behaviour or interface

• Patches conform to the coding conventions.
• Any changes that could be relevant to users have been recorded in the changelog.
  • Is the change significant? If so, remember to add significance: significant in the changelog file.
• The documentation has been updated, if necessary.
• Manual QA notes have been included.
• Tests have been added. (Ask for help if you don’t know how to write them! Ask for an exemption if tests are too complex for too little coverage!)