I wrote (to the Haskell Mailing List)
> newtype Fruit = Int
> newtype Apples = Apples Fruit
> newtype Bananas = Bananas Fruit
>
> noBananas :: Bananas -> Bool
> noBananas (Bananas bananas)
> = bananas == 0
>
> yesWeHaveNoBananas
> = noBananas (Bananas 0)
>
> yesWeHaveNoApples
> = noBananas (Apples 0)
> (Hope this is correct, I don't have a Haskell 1.3 compiler handy)
It isn't. Second try:
newtype Fruit = Fruit Int
newtype Apples = Apples Fruit
newtype Bananas = Bananas Fruit
noBananas :: Bananas -> Bool
noBananas (Bananas (Fruit bananas))
= bananas == 0
yesWeHaveNoBananas
= noBananas (Bananas (Fruit 0))
yesWeHaveNoApples
= noBananas (Apples (Fruit 0))
Even more constructors!
Cheers,
Ronny Wichers Schreur
