Haskell beginner

Question

I don't understand why I get the following response from GHCi. Isn't Maybe a constructor function?

Prelude> :t Maybe

<interactive>:1:1: Not in scope: data constructor `Maybe'
Prelude> let e = Maybe 5

<interactive>:1:9: Not in scope: data constructor `Maybe'

Answer 1

Maybe is a type constructor, and its two possible data constructors are Nothing and Just. So you have to say Just 5 instead of Maybe 5.

> let x = Just 5
> x
Just 5
> let y = Nothing
> y
Nothing
> :type x
x :: Maybe Integer
> :type y
y :: Maybe a
> :info Maybe
data Maybe a = Nothing | Just a     -- Defined in Data.Maybe
instance Eq a => Eq (Maybe a) -- Defined in Data.Maybe
instance Monad Maybe -- Defined in Data.Maybe
instance Functor Maybe -- Defined in Data.Maybe
instance Ord a => Ord (Maybe a) -- Defined in Data.Maybe
instance Read a => Read (Maybe a) -- Defined in GHC.Read
instance Show a => Show (Maybe a) -- Defined in GHC.Show

Maybe is a type constructor because it is used to construct new types (the result type depends on the type of a in Maybe a), where such a type might be Maybe Int (notice, there's no type param a anymore, i.e. all type parameters are bound). Just a and Nothing are data constructors because they're used to construct instances of a certain Maybe type, for example Just Int creates instances of Maybe Int.

Another major difference is that you can only use data constructors when pattern matching. You can't say:

case foo of
     Maybe a -> ...

You'll have to say:

case foo of
     Just a  -> ...
     Nothing -> ...