Effective Haskell: Omitted essential code lines (ch. 5, p. 198, pdf, P1.0)


On page 198, when phantom types are introduced, when listing other changes to the code example at hand, also this necessary change (adding the type parameter a) should be mentioned:

users :: [User a]

Without it the code won’t compile. The change is visible a couple pages later in the full final listing, but it’s confusing to the reader (me) that it wasn’t mentioned earlier.