Access the record fields in Ocaml

[oleg...@gmail.com]

I would like to write a function to compare if the children keys of the current node are less/greater than
their parent and I don't know how to do that.
I would really like to read more about lenses but I haven't found any material on the web.

The problem is , I can't write n.key >= n.left.key && n.key < n.right.key in the compare_children function;

Stack overflow link

type avl_tree =
    Node of avl_tree_node
  | Leaf
and avl_tree_node = { 
  key : int;
  balance : int;
  left : avl_tree;
  right : avl_tree;
}

type subtree_lens = {
  get : avl_tree_node -> avl_tree;
  set : avl_tree_node -> avl_tree -> avl_tree_node
}

let lens_right = {
  get = (fun node -> node.right);
  set = fun node t -> {node with right = t}
}

let lens_left = {
  get = (fun node -> node.left);
  set = fun node t -> {node with left = t}
}

let compare_children nd =
  match nd with
  | Leaf -> true
  | Node n -> n.key >= n.left.key && n.key < n.right.key

[Philippe Veber]

Hi Oleg,

Indeed, [n.left] is an [avl_tree], not an [avl_tree_node]. As such it is not a record type and therefore has no [key] field. You have to do pattern matching on [n.left] and [n.right] to achieve what you want, like in:

let compare_children nd =
  match nd with
  | Leaf -> true
  | Node n ->

 
      match n.left, n.right with
      | Leaf, Leaf -> ...
      | Node left, Leaf -> ...
      | Leaf, Node right -> ...
      | Node left, Node right -> n.key >= left.key && n.key < right.key

Hope this helps,

  Philippe.

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[oleg...@gmail.com]

Thank you Philippe,
Do you know where I can read about the Ocaml lenses and optionals ?

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[oleg...@gmail.com]

Thanks Philippe ,
One more question though :

This line of code from lens_right function :

set = fun node t -> {node with right = t}

it takes a node as an argument and then creates a brand new node with its right property being set to t ?

On Saturday, 3 December 2016 19:44:58 UTC+1, Philippe Veber wrote:

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[Philippe Veber]

2016-12-03 19:52 GMT+01:00 <oleg...@gmail.com>:

Thanks Philippe ,
One more question though :

This line of code from lens_right function :

set = fun node t -> {node with right = t}

it takes a node as an argument and then creates a brand new node with its right property being set to t ?

and all others properties equal to those of [node], yes.
 

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