Why can I use Maybe with record syntax, but not regular ADT syntax?

Question

I'm writing some data types in Haskell to represent formal English grammar.

data S = NP VP

So far so good, a sentence is just a noun phrase and a verb phrase. Marvel at the elegant beauty of algebraic data types!

I'll also define a determiner and adjective as:

data D = A | An | The
type Adj = String -- Too many adjectives for me to list, so I make it a type
                  -- synonym for String.

Now, I'm having issues defining NP, which is a noun with an optional determiner and adjective. My first natural instinct is to use Maybe:

data NP = Maybe D Maybe Adj N

which gives me the error:

Expecting one more argument to `Maybe' In the type `Maybe' In the definition of data 
constructor `Maybe' In the data type declaration for `NP'

(Note that the error doesn't change based on whether or not I have imported Data.Maybe)

The only way I ever got this to work was by using record syntax:

data NP' = NP' {determiner :: Maybe D, adjective :: Maybe Adj, noun :: N}

Why does this only work when I use record syntax?

Answer 1

Try

data NP = NP (Maybe D) (Maybe Adj) N

You need to

1. Begin with a constructor name: in this case NP
2. Apply only a single argument to Maybe which I've done by disambiguating it with parentheses
3. Denote 3 separate slots in the constructor, one for each component type

Answer 2

I presume you meant to write (with the NP data constructor):

data NP = NP Maybe D Maybe Adj N

In your example, NP is being defined as a constructor with 5 arguments, the first of which is a bare "Maybe". This doesn't work because Maybe is a type that needs one type parameter, which is what the error message is telling you.

To get your desired interpretation, you need to surround the Maybe _s with parentheses:

data NP = NP (Maybe D) (Maybe Adj) N