CODEWITHSUNDEEP
haskell
eitan
eitan

Reputation: 59

Constructing a parametric data type

I am trying to define a data type in Haskell. This datatype will be called Node. It will be able to contain a parametric type a or Nothing. I defined it so: 

data Node a = Node (Maybe a) deriving (Show)

When I load it in GHC the following works Node Nothing

However when I type this for example Node (6)

I get the following error:

Non type-variable argument in the constraint: Num (Maybe a)
    (Use FlexibleContexts to permit this)
    When checking that ‘it’ has the inferred type
      it :: forall a. Num (Maybe a) => Node a

Why is that? what am I doing wrong?

Upvotes: 1

Views: 132

Answers (2)

Chris Stryczynski
Chris Stryczynski

Reputation: 33881

The statement data Node a = Node (Maybe a) deriving (Show) creates a data constructor (which is a function) of type Node :: Maybe a -> Node a in other words, you need to pass a value of type Maybe a to this function.

You have passed 6 which is not of type Maybe a.

Instead you can pass a Just 6 value like so: Node (Just 6).

Upvotes: 5

syntagma
syntagma

Reputation: 24314

You have defined your datatype using Maybe, so you have to wrap the value in Just:

Node (Just 6).

If you just want to have a type which can be either empty or have defined value, define it in the following way:

data Node b = Empty | Node b deriving (Show)

Upvotes: 4

Related Questions