A first approximative intuition is to think of Functors as containers (or more like a context) and of fmap as a way to apply a transformation function of your choice on the contained value, respecting the signification of that context. For example, Maybe represents the possibility of having or not having a value, therefore, fmap will apply your transformation on that value if it exists, otherwise you're still left with nothing. This example might seem straight forward but it had to be defined somewhere, thus, made possible by the Functor instance implementation for the type Maybe. Let's have a look at it: fmap :: Functor f => (a -> b) -> f a -> f b Specialized: fmap ~ (a -> b) -> Maybe a -> Maybe b And concretize further: fmap ~ Num n => (n -> n) -> Maybe n -> Maybe n As you can see, given a transformation function and maybe some numeral, you'll get maybe another numeral. The implementation lools like this: fmap f (Just n) = Just (f n) fmap f Nothing = Nothing Thus, starting with Nothing, we cannot apply our tranformation so you stay with Nothing. Similarly, with Just n, we're able to pattern match to obtain that n, apply our transformation f on that n, and then rewrap everything back into Just. You can see how the value cannot escape its container / context. Of course there are more complex such containers / context. Either represents a choice between two values. [] contains multiple values, IO contains (side-)effects, and so on. Hope this helps. nitrix On Mon, May 14, 2018, 08:18 David McBride wrote:

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/for-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

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