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

Question

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).

kennytm · Accepted Answer · 2016-05-20 17:04:50Z

3

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)

answered May 20, 2016 at 17:04

kennytm

526k111 gold badges1.1k silver badges1k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

The Unfun Cat Over a year ago

I am amazed that I was able to understand how to make the monad instance but not see this. Must have forgotten a Nil somewhere and had the REPL choke. Oh, well.

Collectives™ on Stack Overflow

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

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related