[goto11]

Perhaps it is easier because Peyton Jones is a world class expert in Haskell and have 30 years experience refactoring it?

In my experience the ease of refactoring is more depending on the quality of the code you are refactoring than the language. That said, a strong type system helps avoiding stupid mistakes and Haskell have a very strong type system.

[fho]

I guess it is a bit if both. You can of course write fast and loose Haskell code, but best practices will probably prevent you from doing that.