What is the type of Nothing in Haskell?

Question

I\'m at page 118 of the book \"Learn You a Haskell for Great Good!\"

It is written there:

ghci> :t Nothing 
Nothing :: Maybe a

H

Answer 1

Is this not in contradiction with the rule the only concrete types can have values?

Since functions are first-class values in Haskell, this purported rule would mean that polymorphic functions such as map and foldr would be impossible to implement.

There are, in fact, a lot of polymorphic non-function values in Haskell, such as

1 :: Num a => a
Nothing :: Maybe a
[] :: [a]
Left 1 :: Num a => Either a b

etc. These values exist for every instantiation of a (and b).