You ask about how to learn about GHC extensions to the Haskell language.
You’ll get a better answer to that if asked as a separate Topic (not least, you can supply the context in which it arises) - checking that any existing Topic has not met your needs, for example (from August 2023):
In short:
- on reference material, Chapter 6 Language Extensions of the GHC User’s Guide is definitive;
- on tutorial-like material, various bloggers have written on the subject. One that comes to my mind is, around Christmas 2014, @ocharles wrote a series of posts 24 Days of GHC Extensions. (My browser warns that site is not secure; I ignored that warning.); and
- although this will not be for most beginners, when you have more Haskell under your belt, you can learn from the proposals to collect groups of language extensions as ‘editions’ and the discussions of those proposals. This is the proposal for the
GHC2021edition: ghc-proposals/proposals/0380-ghc2021.rst at master · ghc-proposals/ghc-proposals · GitHub and this for theGHC2024edition: ghc-proposals/proposals/0613-ghc2024.rst at master · ghc-proposals/ghc-proposals · GitHub