CODEWITHSUNDEEP
listhaskellnested
The Unfun Cat
The Unfun Cat

Reputation: 32018

How do I write a nested list of lists with my custom list implementation?

I am wondering whether my definition of a list is less capable than the regular implementation. I am having problems creating a list of lists with the below definition:

data List a =
  Nil
  | Cons a (List a)
  deriving (Eq, Ord, Show)

I do not see how to create a list like [[2],[3]] with my definition of list.

The simplest instance of a nested list is just [[]] which I think I can recreate with Cons Nil Nil. The second simplest instance of a nested list is [[1]]. This is Cons (Cons 1 Nil) Nil afaics.

However, like I said do not see how to create the list [[2],[3]] with my definition above. How do I do it? (It should be possible, since an exercise asks me to create a monad instance for the list definition above).

Upvotes: 0

Views: 151

Answers (1)

kennytm
kennytm

Reputation: 523764

Let's first consider, how would you express [x,y]?

Cons x (Cons y Nil)

Now put in x = [2]:

Cons (Cons 2 Nil) (Cons y Nil)

and y = [3]:

Cons (Cons 2 Nil) (Cons (Cons 3 Nil) Nil)

Upvotes: 3

Related Questions