Update record field value

Learning
1

Hi,

I have following record:

type node_t = Element of element_t | Text of text_t

and element_t = {
tag_name: string
children: node_t list;
mutable deleted: bool
}

and text_t = {
text: string;
line_number: int;
}

Now, I have a XML node stored in a variable called xml_node
And the data is :

xml_node = Element {tag_name = "cell"; children: []; deleted = false}

I want to update the deleted flag as “deleted=true” in the above node record.

Could you please tell me how to do it?

Thank you in advance

2

You need to access the element_t payload of the Element constructor in order to modify one of its fields. You can use pattern matching to do that, e.g

(* [set_deleted t x] sets [e.deleted] to [x] if [t = Element e]. *)
let set_deleted t x =
  match t with
  | Element e -> e.deleted <- x
  | Text _ -> ()

Best wishes,
Nicolás

3

But “e” in your example is type of node_t whereas “deleted” is a type of element_t.

4

OK, got it.
I just used your solution like below:

match xml_tree with 
        | Element (_ as e) -> 
          e.deleted <- true;
        | _ -> ()

It worked.
Thank you Nicolás.

5

Note that _ as e is equivalent to writing just e.

Best wishes,
Nicolás

6

OK Nicolás.
Thanks for the information.

7

Out of curiosity, any reason why this field is mutable? It is typical in OCaml to use immutable fields and update records with the immutable update syntax (returns a copy of the record with the given changes).

8

I am not aware of such syntax.
Could you please tell me the syntax?

9 
type example_record = {a : int; b : string};;

let one = { a = 12; b = "hello"};;

let two = { one with b = "world"};;
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