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haskellghci

fmap a 2-arity function on Nothing should return Nothing?

From the source of Maybe in ghc:

instance Functor Maybe where
    fmap _ Nothing  = Nothing
    fmap f (Just a) = Just (f a)

When fmap is applied to Nothing, it should return Nothing.

For example, run this in ghci (v8.2.2):

Prelude> fmap (+1) Nothing
Nothing

However, when I apply a function with an arity of 2:

Prelude> fmap (++) Nothing
<interactive>:11:1: error:
    • No instance for (Show ([a0] -> [a0]))
        arising from a use of ‘print’
        (maybe you haven't applied a function to enough arguments?)
    • In a stmt of an interactive GHCi command: print it

In fact, the result seems to be Nothing:

Prelude> import Data.Maybe
Prelude Data.Maybe> isNothing $ fmap (++) Nothing
True

My question is, does fmap (++) Nothing really return Nothing?

Solution

  • Yes, fmap f Nothing = Nothing no matter what f is or what type it has. The Show instance for Maybe looks like

    instance Show a => Show (Maybe a) where
  ...

    So you can't show or print a value of type Maybe (A -> B) because functions don't have a Show instance.