Small bugs in the haskell code snippets

[saeednj]

In the "Single Digit Math" Chapter:

1. Section "A Single Digit" (first code snippet):
   module Data.Char should be replaced with
   import Data.Char.
2. Section "Adding Types":
   The data type has a terminal type Num Int, but it is instantiated with a Char in isDigit x = Num x.
   Either change the Num Int to Num Char or construct it with the ordinal value of the character: isDigit x = Num ((ord x)-(ord '0'))

Maybe not that important in general, but for someone not proficient in haskell like me, it took a while to find why the codes are not compiling.