CODEWITHSUNDEEP
haskell
tarak ammar
tarak ammar

Reputation: 15

haskell : what is wrong with this piece of code

I'm trying to compile this piece of code but it doesn't work and I don't get it:

division :: Maybe Int -> Maybe Int -> Maybe Int
division _ (Maybe 0) = Nothing
division (Maybe a) (Maybe b) = Just (a `div` b)

Upvotes: 1

Views: 95

Answers (2)

dfeuer
dfeuer

Reputation: 48631

Based on Daniel Wagner's comment and Willem Van Onsem's point about Integral, this is how I would write the function:

division :: Integral a => a -> a -> Maybe a
division _ 0 = Nothing
division p q = Just (div p q)

To use it with Maybe arguments, you can simply write

division <$> m <*> n

or, equivalently,

liftA2 division m n

where liftA2 is in Control.Applicative.

Upvotes: 1

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 477684

Maybe a is defined as:

data Maybe a = Just a | Nothing

so you can not put Maybe a in the function defintion, only in the signature:

division :: Maybe Int -> Maybe Int -> Maybe Int
division _ (Just 0) = Nothing
division (Just a) (Just b) = Just (a `div` b)

Furthermore the function is not total: not every possible case of input is handled by the function, so this could result in an exception. So you better return something in those cases (usually one returns Nothing):

division :: Maybe Int -> Maybe Int -> Maybe Int
division _ (Just 0) = Nothing
division (Just a) (Just b) = Just (a `div` b)
division _ _ = Nothing

Finally we can use Integral i as type, instead of Int and make the function more generic:

division :: Integral i => Maybe i -> Maybe i -> Maybe i
division _ (Just 0) = Nothing
division (Just a) (Just b) = Just (a `div` b)
division _ _ = Nothing

Upvotes: 2

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