--------------------------------------old defs do not support "=":

data PrimaryColour = Yellow | Red | Blue

---------------------------------------------------as is shown by:
{-
Main> Yellow == Blue

ERROR: PrimaryColour is not an instance of class "Eq"
-}

-------------------------so make new type an instance of class Eq:

data PrimaryColour = Yellow | Red | Blue
instance Eq PrimaryColour
   where Yellow == Yellow = True
         Red == Red       = True
         Blue == Blue     = True
         _ == _           = True

-----------------------------------------------------which allows:
{-
Main> Yellow == Blue
False
-}


-------------------------------------this can be done more simply:

data PrimaryColour = Yellow | Red | Blue deriving Eq

---------------------------------------------which gives the same:
{-
Main> Yellow == Blue
False
Main> Yellow /= Blue
True
-}

---------but other defs of "==" could be useful for colour blinds:

data PrimaryColour = Yellow | Red | Blue
instance Eq PrimaryColour
   where _ == _ = True

-----------------------------------------------------------giving:
{-
Main> Yellow == Blue
True
-}


--------------------------------------now define your own classes:

class Mail m
   where giveAdr:: m->String
         setAdr:: String->m->m
         existsAdr:: m->Bool
         existsAdr x = (giveAdr x /= "")        -- default

data SimpleLetter a = SL String a     -- hat eine Adresse
instance Mail (SimpleLetter a)
   where giveAdr (SL s _) = s
         setAdr s (SL s' x) = (SL s x)

data BroadcastLetter a = BC [String] a   -- hat Liste von Adressen
clearAdr:: (BroadcastLetter a)->(BroadcastLetter a)
clearAdr (BC ss x) = BC [] x

instance Mail (BroadcastLetter a)
   where giveAdr (BC [] _)     = ""
         giveAdr (BC (s:ss) _) = s
         setAdr s (BC ss x) = BC (s:ss) x

---------------------------------------------------------examples:
{-
Main> existsAdr (SL "Grete" (25, "Now", 1997))
True
Main> giveAdr(setAdr "Gustav" (setAdr "Franz" 
(SL "Grete" (25, "Nov", 1997))))
"Gustav"
Main> giveAdr (clearAdr (BC ["Grete"] (25, "Nov", 1997)))
""
Main> existsAdr (clearAdr (BC ["Grete"] (25, "Nov", 1997)))
False
-}