parsePerson "firstName = John\nlastName = Connor\nage = 30"

parsePerson.hs

import Data.Char(isDigit)

data Error = ParsingError | IncompleteDataError | IncorrectDataError String
  deriving (Show)

data Person = Person { firstName :: String, lastName :: String, age :: Int }
  deriving (Show)

split delimiter = foldr f [[]] 
  where f c l@(x:xs) | c == delimiter = []:l
                     | otherwise = (c:x):xs

parsePerson :: String -> Either Error Person
parsePerson str 
  | notAllFields         = Left IncompleteDataError
  | incompleteDataError' = Left ParsingError
  | incompleteDataError  = Left ParsingError
  | not ageCorrect       = Left (IncorrectDataError (age''))
  | otherwise            = Right (Person firstName'' lastName'' (read age'' :: Int))
  where
    fields = (split '\n' str)
    notAllFields = length (split '\n' str) < 3
    incompleteDataError' = length (split '=' (fields !! 0)) /= 2 ||
                           length (split '=' (fields !! 1)) /= 2 ||
                           length (split '=' (fields !! 2)) /= 2
    [fnName, firstName'] = split '=' (fields !! 0)
    [lnName, lastName']  = split '=' (fields !! 1)
    [aName, age']        = split '=' (fields !! 2)
    incompleteDataError  = not (init fnName == "firstName" && 
                                init lnName == "lastName" && 
                                init aName == "age")
    age'' = tail age'
    ageCorrect = all (\x -> isDigit x) age''
    firstName'' = tail firstName'
    lastName'' = tail lastName'