When comparing Real World Haskell vs Professor Frisby's Mostly Adequate Guide to Functional Programming, the Slant community recommends Professor Frisby's Mostly Adequate Guide to Functional Programming for most people. In the question“What are the best resources to introduce programming students to Functional Programming?” Professor Frisby's Mostly Adequate Guide to Functional Programming is ranked 5th while Real World Haskell is ranked 12th.
Ranked in these QuestionsQuestion Ranking
Pros
Pro Free
Available online for free.

Pro Covers a wider range of topics than many books
Pro Free
It's available free on Github.
Pro Can submit pull requests to suggest improvements
You can submit issues to suggest improvements, or submit pull requests.
Thanks to it being open source, it's constantly being improved and updated.
Pro Uses JavaScript
JavaScript is popular, ubiquitous, and has the necessary power to teach core concepts.
Cons
Con Poorly organized and hard to follow
The book jumps around, includes confusing function names and many of the code examples don't compile.
Con Not really "real world"
The book doesn't feel very "pragmatic". The title implies a practical approach to Haskell, but it takes way too long to be able to actually create something using this book as lot of time is spent on theory.
Con Shortage of real world examples
There are very few non-trivial examples given in the book.
