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

The Maybe type has two constructors: Nothing and Just a, where a is a type variable. Nothing itself does not determine (constrain) the type, but in any reasonable context you'll find something like (I do not claim it to be syntactically correct Haskell but something I would have written 10 years ago):

if (foo == 5)
then Nothing
else Just 5

And then type inference will tell you that (assume 5 was an argument to this code snippet 'foo' of type Int) that foo has type:

foo :: Int -> Maybe Int

But as the previous poster pointed out, Maybe a is a perfectly good type. You'll find the same non-issue with [] ie the list type with [] (the constructor for an empty list).