let foo = \x -> Just (x + 1) fmap foo (Just 9) Just (Just 10) On Mon, May 14, 2018 at 8:15 AM, Olumide <50295@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 _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners