1-ValidEmail.hs

module ValidEmail (mkEmail, emailToString, EmailAddress) where

newtype EmailAddress = ValidAddress String

validate :: String -> Boolean 
validate email = ...

mkEmail :: String -> Maybe ValidEmail
mkEmail = case validate email of
  True -> Just $ ValidAddress email
  False -> Nothing

emailToString :: EmailAddress -> String
emailToString (ValidAddress email) = email

-- Note: ValidAddress is not exported, so its impossible
-- to make a valid email without using mkEmail

2-MyFunctions.hs

module MyFunctions (sendMsg)_ where

import ValidEmail (EmailAddress)


-- this function can now specify its demands
sendMsg :: EmailAddress -> IO ()
sendMsg = ... -- no validation required. We can use emailToString if we need 
              -- to convert the address to form the email headers 

3-OtherModuleTypeError.hs

module OtherModule where

import MyFunctions (sendMsg)


main = do
  email <- getLine
  -- I dont need to remember that sendMsg demands a valid email, the compiler will tell me.
  -- I don't have to remember it anywhere I use this function. If I use this function at least 2 times,
  -- writing the type already paid off, after that, everything is a gain.
  -- This is a type error
  sendMsg email
  
  

4-OtherModuleOk.hs

module OtherModule where

import MyFunctions (sendMsg)
import ValidEmail (mkEmail)

main :: IO ()
main = do
  unknownString <- getLine
  maybeEmail <- mkEmail unknownString
  -- I only need to check this once. after that, all functions that use the EmailAddress
  -- type can pass it around freely without any checks whatsoever.
  
  -- Furthermore, I don't have to keep track if I'm inside the "checked" subset of functions or I am not
  -- The compiler keeps track of that for me - if it takes a String, i don't need to pre-check. If it 
  -- takes an EmailAddress, I must hand off a valid one either by making one from a string, or passing
  -- a valid one.
  
  case maybeEmail of
    Just emailAddr -> sendMsg emailAddr
    -- warning if we don't handle other case
    Nothing -> putStrLn "Invalid email"