2011/8/5 Christopher Howard
But users are not the only source of ints. For example, let's say I wanted to do my own square function, with a type like so:
square :: Int -> Positive Int validatePositive :: Int -> Maybe (Positive Int)
But this means the type of square would have to be changed to square :: Int -> Maybe (Positive Int)
I would go with : validatePositive :: Int -> Maybe (Positive Int) square :: Positive Int -> Positive Int For the subtraction problem as shown by Thomas, you can subtract :: Positive Int -> Positive Int -> Maybe (Positive Int) subtract (Positive a) (Positive b) | a >= b = Just $ Positive (a-b) | otherwise = Nothing But that's what you want to avoid, so maybe it's acceptable, depending on your problem, to do: subtract :: Positive Int -> Positive Int -> Positive Int subtract (Positive a) (Positive b) | a >= b = Positive (a-b) | otherwise = Positive 0 The only other solution would be to statically guarantee that a>b, with dependent types you could probably; but since I know next to nothing about that, I have trouble seeing how could work with a simple example like that: (and I would love to learn !) 1 - read two lines from stdin 2 - convert each lines to natural numbers a and b 3 - subtract b from a , a>b should be statically guaranteed 4 - print the result David.