ch02/parsing/exercise_4.hs has a bug.

[wind2412]

If we use

main = do
print (parse parseExpr "err..." "#t")

, the bug will occur:

Left "err..." (line 1, column 2):
unexpected "t"
expecting "d", "b", "o" or "x"

Because parseNumber conflicts with parseBool. It will consume a # symbol.
Changing code at line 97 to be <|> try parseNumber will fix the bug;)