In addition to what others have said, there was a 24 days of ghc extensions last year which highlighted some of the more interesting extensions and more importantly, why you might use them.

https://ocharles.org.uk/blog/pages/2014-12-01-24-days-of-ghc-extensions.html

On Tue, Nov 1, 2016 at 8:41 AM, Zachary Kessin <zkessin@gmail.com> wrote:
I have noticed that almost every haskell source file I have looked at has 4-8 language extensions. It would be very nice if there was a page somewhere that listed them all and gave a short (1 paragraph) description of what that extension does and what it is normally used for. That is not a detailed description of the API but a short description of *WHY* you might want to use it. Followed by a link to the API

--
Zach Kessin
Twitter: @zkessin
Skype: zachkessin

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