It isn’t just RWH - every piece of documentation about Haskell is out of date in some ways. In a way, it reminds me of the time before Haskell 98 arrived, when academics and educators were often caught out by changes between versions of the language.
Unfortunately, the efforts put into establishing a new language standard (Haskell 2020) seems to have gone the way of the Mary Celeste. In the absence of the so-called “dead hand” of standards, we currently have this deluge of extension upon extension which erodes the accuracy of documentation.
Perhaps the best that can be achieved right now is the approach used by another evolving language: Rust - using RFCs and a measured release system (and centralised documentation) to steady the pace of progress…
Maybe it’s time to contemplate a stable educational variant/subset of Haskell (e.g. like the old Helium project) which the authors of textbooks and static online content can focus on with confidence - otherwise we may lose “mindshare” to e.g. a reinvigorated Miranda®. (The various Simple Haskell articles could be useful in such an endeavour.)
Otherwise (and given the unfortunate presence of “unsafe…” primitives in Haskell 2010), a renewed focus on making Haskell 98 the introductory path to the full “bells-whistles-and-claxons” Haskell, complete with separate implementations and documentation (https://www.haskell98.org) may be the simplest option, for now.
