Really, as far as I am aware from asking people, there is no de facto standard, people have strong opinions about which they prefer but with little specific evidence other than “it works for me in most cases”.
The haskell-indentation package and haskell-indent packages are the most popular:
- both have a tab cycle
- both are complicated code-bases
- both have plenty of edge cases where they break down
Each will Do The Right Thing in different cases, but again, there are no unit tests, no real parser, etc. just a bunch of special cases that the author(s) happened to think of, so the documentation and understanding on how they behave in a rigorous sense is sparse. haskell-indentation understands more Haskell than haskell-indent, but plenty of people will tell you, including me, that you spend a lot of time cycling through indentations to get to the right one.
The haskell-simple-indent package has no knowledge of Haskell, it only knows about column alignment and it does not have a tab cycle, it has indentation and de-indentation. This is the version I use because of the aforementioned edge cases in the previous modes, which can be rather distracting at times.
I think with such libraries it will be difficult to describe their differences other than anecdotically at this point. I would welcome some rigorous comparison (though time may well be better spent writing a better mode).
simple-indent is maintained by me. I don’t know whether the authors of the others are actively interested in maintenance.