Question

I have recently started diving into Haskell. Its quite interesting, but the definition of Nothing baffles me and is not sinking in. On GHCI

Prelude> :t Nothing
Nothing :: Maybe a

Shouldn’t Nothing be Just Nothing, why Maybe a?

Answer 1

Here is the definition of the Maybe type:

data Maybe a = Nothing | Just a

This states that for some type a, the type Maybe a has two constructors: Nothing and Just a (a being the same type variable that Maybe is applied to on the LHS).

Hopefully it’s clear how Just "foo" has type Maybe String and Just 'c' has type Maybe Char. But what type is Nothing? Since Nothing doesn’t contain any value, its constructor doesn’t use the a type variable, and therefore there’s nothing that constrains its type to a particular form of Maybe.

That makes it a “polymorphic constant”. Nothing can be used in a context in which any Maybe type is expected, because it works equally well in all of them. That’s why GHCi reports its type as simply Maybe a: the a is a variable which could represent any type, and which type it refers to will be determined by inference based on the context in which the Nothing value is used. But when you give Nothing by itself, there are no additional clues about its specific type, so you just see the variable.

> :t Nothing
Nothing :: Maybe a
> :t (Nothing :: Maybe Int)
(Nothing :: Maybe Int) :: Maybe Int
> :t (Nothing :: Maybe Char)
(Nothing :: Maybe Char) :: Maybe Char

And by the way, Just Nothing :: Maybe (Maybe a); it’s a Maybe wrapped in another Maybe.