4 import Text.ParserCombinators.ReadP hiding ((+++))
10 instance Read Expr where
11 readPrec = parseNum +++ parseAdd
12 where parseNum = Num <$> readPrec
13 -- use 'prec 1' and 'step' so that parsing 'a'
14 -- can only go one step deep, to prevent ininfite
16 parseAdd = prec 1 $ do