CODEWITHSUNDEEP
haskell
Dario
Dario

Reputation: 995

Haskell allow only positive input

I need to define a haskell function:

func :: Int -> Int
func 1 = 1
func 2 = 2
func x = x+1

So that it allows only positive numbers. I've already had a look at a similar question: Non-negative integers

And wrote this:

newtype Positive a = Positive a

toPositive :: (Num a, Ord a) => a -> Positive a
toPositive x
    | x < 0 = error "number cannot be negative"
    | otherwise = Positive x

func :: Positive a -> a
func (Positive n) = n

Which is however already throwing errors. Thoughts?

Update:

Sample error:

*Main> func 1

<interactive>:32:6:
    No instance for (Num (Positive a0)) arising from the literal `1'
    Possible fix: add an instance declaration for (Num (Positive a0))
    In the first argument of `func', namely `1'
    In the expression: func 1
    In an equation for `it': it = func 1
*Main>

Upvotes: 0

Views: 3262

Answers (1)

Neil Forrester
Neil Forrester

Reputation: 5241

You forgot to call toPositive to convert an Int to a Positive. Call it this way:

func $ toPositive 1

Also, a quirk of Haskell is its handling of negative number literals. To avoid confusion with the subtraction operator, you must wrap them in parentheses:

func $ toPositive (-1)

Upvotes: 2

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