haskell : what is wrong with this piece of code

Question

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)

i meant that the function take two parameters of type MAybe and return a maybe value. any corrections? — tarak ammar
– tarak ammar, Commented Aug 7, 2017 at 18:24
yes, but in the body of the function, you can not use (Maybe 0), etc. Then you have to pick a constructor, like Just 0, not Maybe 0. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Aug 7, 2017 at 18:25
As a style thing: I recommend leaving the arguments as bare Ints rather than Maybe Ints, as in division :: Int -> Int -> Maybe Int; division _ 0 = Nothing; division a b = a `div` b. If you need the arguments to be Maybes, then you can use do notation; if ma :: Maybe Int and mb :: Maybe Int, then do a <- ma; b <- mb; division a b is well-typed. — Daniel Wagner
– Daniel Wagner, Commented Aug 7, 2017 at 18:58
@DanielWagner You're missing a Just in division a b = a `div` b. — gallais
– gallais, Commented Aug 8, 2017 at 10:15

willeM_ Van Onsem · Accepted Answer · 2017-08-07 18:42:19Z

2

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

edited Aug 7, 2017 at 18:42

answered Aug 7, 2017 at 18:24

willeM_ Van Onsem

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Comments

dfeuer · Accepted Answer · 2017-08-07 23:10:45Z

1

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.

answered Aug 7, 2017 at 23:10

dfeuer

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