import Duck
main = do
let mallard = makeDuck (print "(normal flying)") (print "quack") (print "I'm a mallard duck")
fly mallard -- prints "(normal flying)"
quack mallard -- prints "quack"
display mallard -- prints "I'm a mallard duck"
let mallard' = setFly (print "they cut my limbs!") mallard
fly mallard' -- prints "they cut my limbs!"
return ()
module Duck (makeDuck, fly, quack, display, setFly, setQuack) where
data Duck = Duck {
fly :: IO (),
quack :: IO (),
display :: IO ()
}
-- Duck, the ctor is not expeorted, otherwise client code could
-- pattern match on a Duck internals; instead, we expose a make
-- function equivalent to it
makeDuck :: IO () -> IO () -> IO () -> Duck
makeDuck = Duck
setFly f' (Duck f q d) = Duck f' q d
setQuack f' (Duck f q d) = Duck f' q d
-- but not setDisplay
