CODEWITHSUNDEEP
listocaml
Dunno
Dunno

Reputation: 3684

I can't build a list of elements in a tree

I have a tree type defined as

type 'a tree = Nil | Node of ('a tree * 'a * 'a tree)

I need to build a list of elements in this tree. I tried (without tail-recursion):

let elements t =
  let rec walk node =
    match node with
    | Nil -> []
    | Node(lChild, x, rChild) ->
      (walk lChild) :: [x] :: (walk rChild)
  in
  walk set

But I get a type error: expression (walk lChild) has type 'a list but an expression was expected of type 'a

So I guess my question can be reduced to "how to add a list of elements to a list".

Thanks!

Upvotes: 0

Views: 84

Answers (2)

user3075773
user3075773

Reputation: 139

ivg's answer is correct. To be a bit clearer :

 let xs = [2;3;4]

1 :: xs returns [1;2;3;4], xs's type is int list, 1 is an int. So if you use an 'a list, the :: operator needs to have an 'a typed element to its left.

To concatenate lists, try 

[1] @ [2;3;4]

Upvotes: 1

ivg
ivg

Reputation: 35210

You need @ operator, that concatenates two lists.

:: is a consing operator and it can only prepend an element to the list, i.e., the left hand side of it should be an element, and the right should be the list.

Upvotes: 5

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