To the first printing of the first edition, October 2019:
Due to a bug / design-choice, the numerical typeclass hierarchy in GHC differs from the Haskell Report 2010, in that Eq is not a superclass of Num. (Show is not either, but this does not affect the book). This is described in the GHC documentation.
Haskell from the Very Beginning, which previously used the Haskell Report definitions, has been updated to use the GHC ones instead.
This affects the last paragraph of Chapter 2, Question 12 of Chapter 2, and the typeclass hierarchy diagrams in Chapter 12 and its answers.
Updates have been sent to existing customers of PDF and kindle ebooks. Owners of the printed book, please contact us for an updated PDF copy containing the corrections.