People often say that before they realize how complex and messy real-world Haskell looks like. The indentation-based rules aren't even defined anywhere except via imperative code in the implementation.
That's because real-world Haskell tends to not be idiomatic, and it involves a lot of directly telling GHC what to do. Mathematicians don't have to (and rarely, if ever) make efficient code. GHC is a really advanced and impressive compiler, but that really doesn't amount to much. At some point the language just devolves into understanding what arcane compiler internals you need to invoke, and it's more like a bunch of C macros with a fancy runtime.
Nothing forces it to be messy. A well-tended haskell codebase is great. On the oposite, I would say that a poor haskell codebase is painful, but far from being as painful as a poor java/js codebase.
Anecdotally, in my years of haskell at separate places, I had to debug/repl maybe had to repl once or twice. I wish I could say that of mycurrent java gig.
My experience is similar, and in addition I would say that I have come across badly written, difficult to understand Haskell code, but it was always straightforward to refactor it into something clearly written.
I've written Haskell code in production and I still find the syntax the most eloquent. And you don't have to know the indentation rules, you just pick them up from usage. Just like you don't have to know the grammar rules.
Maybe it's because I code Python professionally, or because I use an autoformatter (ormolu) for Haskell, but I have literally never had an issue with the indentation rules.
If that's true, that's such a disappointment. I always thought of Haskell as the pinnacle of mathematical discipline in programming (though too hard for me to wrap my head around!).
