问在数据验证中使用“任一”

EN

module Student where

data Student = Student {firstName::FirstName, lastName::LastName, age::Age}
  deriving Show

newtype FirstName = FirstName String
  deriving Show

newtype LastName = LastName String
  deriving Show

newtype Age = Age Int
  deriving Show

mkStudent :: String -> String -> String -> Either String Student
mkStudent fn ln a = 
   Student <$> validate fn
           <*> validate ln
           <*> validate a

aceptableLetters = ['a'..'z']++['A'..'Z']

validateFn :: String -> Either String FirstName
validateFn fn 
   | length fn < 2 = Left "First name has to at least 2 letters"
   | length fn > 100 = Left "First name is limited to 100 characters"
   | not . null $ filter (\c -> not . elem c $ aceptableLetters) fn = Left "First name contains unacceptable chars"
   | otherwise = Right . FirstName $ fn

validateLn :: String -> Either String LastName
validateLn lastName 
   | length lastName < 2 = Left "Last name has to at least 2 letters"
   | length lastName > 100 = Left "Last name is limited to 100 characters"
   | not . null $ filter (\c -> not . elem c $ aceptableLetters) lastName = Left "Last name contains unacceptable chars"
   | otherwise = Right . LastName $ lastName

validateA :: String -> Either String Age
validateA a
   | age <= 18 = Left "Student has to be at least 18"
   | age > 100 = Left "Student has more than 100 years. Probably it is an error."
   | otherwise = Right . Age $ age
   where
    age = read a

class Validate v where
    validate :: String -> Either String v

instance Validate FirstName where
    validate=validateFn

instance Validate LastName where
    validate=validateLn

instance Validate Age where
    validate=validateA