Ticket #102: Tst.2.hs

data    Object = Operatortype Operator | Integertype Integer | Realtype Float   deriving (Eq,Show)
type    Stack = [Object]
type    Operator= (Stack->Stack) 
instance Eq Operator where
        _ == _ = error "typecheck"
instance Show Operator where
        show _ = error "can't show operator!"
instance Num Object where
        (Integertype a) + (Integertype b) = Integertype (a+b)
        (Realtype a) + (Realtype b) = Realtype (a + b)
        (Realtype a) + (Integertype b) = Realtype (a + fromInteger(b) )
        (Integertype b) + (Realtype a) = Realtype (a + fromInteger(b) )
        _ + _ = error "typecheck"
apply2:: (Object->Object->Object)->Stack->Stack
apply2 op (s0:(s1:s2)) = ((op s0 s1):s2)

add:: Operator
add = apply2 (+)
sub:: Operator
sub =  apply2 (-)