[Haskell-beginners] Adding 1 to Just 9

[David McBride]

let foo = \x -> Just (x + 1)
fmap foo (Just 9)

Just (Just 10)


On Mon, May 14, 2018 at 8:15 AM, Olumide <50295 at web.de> wrote:

> Dear List,
>
> Chapter 14 of LYH appears to suggest that a Just value can be added to an
> Int. Quote from http://learnyouahaskell.com/fo
> r-a-few-monads-more#useful-monadic-functions
>
> "For instance, say we have Just 9 and the function \x -> Just (x+1). If we
> map this function over Just 9, we're left with Just (Just 10)."
>
> I've tried the following in ghci but got the error:
>
> <interactive>:12:1: error:
>     • Non type-variable argument in the constraint: Num (Maybe a)
>       (Use FlexibleContexts to permit this)
>     • When checking the inferred type
>         it :: forall a. (Num (Maybe a), Num a) => Maybe a
>
>
> Am I reading the quote wrong? Is Just (Just 10) a hypothetical?
>
> Regards,
>
> - Olumide
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