How can I handle an error from another function in Haskell?

Question

I'm still a beginner in Haskell.

My code

secureDivide :: Int -> Int -> Maybe Int
secureDivide _ 0 = Nothing
secureDivide 0 _ = Nothing
secureDivide x y = Just (x `div` y)

addOne:: Maybe Int -> Maybe Int
addOne (Just n) = Just (n + 1)

Actually I don't have any problem and get the result I want with 'secureDivide'.

Example : secureDivide 10 0 -> Nothing

But when I try something like this :

mySecureNext (mySecureDiv 10 0) -> I have an 'Exception' and not 'Nothing'

Is there a way to handle an error to a message without import something with the if-else statement like if error = Nothing else Just ... (or other option) ?

you need to handle the Nothing case in your addOne function. A better pattern is to simply define your addOne function as Int -> Maybe Int. Then you can utilize the fact that Maybe is a monad and do secureDivide >>= addOne — Chase
– Chase, Commented Sep 8, 2022 at 14:58
addOne = fmap (+1). The definition of fmap takes care of both Just arguments and Nothing arguments. Your current definition duplicates the Just logic while ignoring the Nothing logic. — chepner
– chepner, Commented Sep 8, 2022 at 15:12
Also, you can drop secureDivide 0 _ = Nothing. There is no problem dividing 0 by a nonzero value, so it doesn't need to be caught and handled separately. (If you do handle it separately, the result should be Just 0, not Nothing.) — chepner
– chepner, Commented Sep 8, 2022 at 15:13

N0lim · Accepted Answer · 2022-09-12 20:09:40Z

3

In Data.Maybe there is function

maybe :: b -> (a -> b) -> Maybe a -> b

Examples of use

maybe 0 (+1) Nothing
> 0
maybe 0 (+1) (Just 10)
> 11

About addOne

a) You can add alternative variant of execution

addOne :: Maybe Int -> Maybe Int
addOne (Just n) = Just (n + 1)
addOne Nothing = Nothing

addOne (mySecureDiv 10 0)

b) You can change function to monadic function

addOne :: Int -> Maybe Int
addOne n = Just (n + 1)

And compose functions by

-- (>>=) :: m a -> (a -> m b) -> m b
-- or specific version
-- (>>=) :: Maybe a -> (a -> Maybe b) -> Maybe b

(mySecureDiv 10 0) >>= addOne

c) You can make pure function

addOne :: Int -> Int
addOne n = n + 1

And compose functions by

-- fmap :: (a -> b) -> f a -> f b
-- or specific version
-- fmap :: (a -> b) -> Maybe a -> Maybe b
-- or infix version <$>

fmap addOne (mySecureDiv 10 0)
addOne <$> (mySecureDiv 10 0)

edited Sep 12, 2022 at 20:09

answered Sep 11, 2022 at 14:27

N0lim

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1 Comment

dsagman Over a year ago

The multiple ways to solve with a monad or with compose is super helpful. This really "clicked" in my brain. Thank you.

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