What is the use of derived-mode-add-parents?

What is the use of derived-mode-add-parents?

[Pankaj Jangid]

I am working on a new -ts-mode for Move language. Rust is the closest
language so I am looking at rust-ts-mode's code. There I encountered a
call in the end,

(derived-mode-add-parents 'rust-ts-mode '(rust-mode))

I need help in understanding what this call does. The docstring says this,

--8<---------------cut here---------------start------------->8---
(defun derived-mode-add-parents (mode extra-parents)
  "Add EXTRA-PARENTS to the parents of MODE.
Declares the parents of MODE to be its main parent (as defined
in `define-derived-mode') plus EXTRA-PARENTS, which should be a list
of symbols."
  (put mode 'derived-mode-extra-parents extra-parents)
  (derived-mode--flush mode))
--8<---------------cut here---------------end--------------->8---

Please help me understand, what is the use of parent modes?

At the beginning of the file, there is

--8<---------------cut here---------------start------------->8---
(define-derived-mode rust-ts-mode prog-mode "Rust"
  "Major mode for editing Rust, powered by tree-sitter."
  :group 'rust
  :syntax-table rust-ts-mode--syntax-table
--8<---------------cut here---------------end--------------->8---

What I understand here is that we borrow certain features from parent
modes. But it is not clear to me, how we borrow these features by simply
calling defived-mode-add-parents or define-derived-mode.

Re: What is the use of derived-mode-add-parents?

[Eli Zaretskii]

> From: Pankaj Jangid <pankaj@codeisgreat.org>
> Date: Mon, 18 Mar 2024 13:04:51 +0530
> 
> I am working on a new -ts-mode for Move language. Rust is the closest
> language so I am looking at rust-ts-mode's code. There I encountered a
> call in the end,
> 
> (derived-mode-add-parents 'rust-ts-mode '(rust-mode))
> 
> I need help in understanding what this call does. The docstring says this,
> 
> --8<---------------cut here---------------start------------->8---
> (defun derived-mode-add-parents (mode extra-parents)
>   "Add EXTRA-PARENTS to the parents of MODE.
> Declares the parents of MODE to be its main parent (as defined
> in `define-derived-mode') plus EXTRA-PARENTS, which should be a list
> of symbols."
>   (put mode 'derived-mode-extra-parents extra-parents)
>   (derived-mode--flush mode))
> --8<---------------cut here---------------end--------------->8---
> 
> Please help me understand, what is the use of parent modes?

Did you read the documentation of derived-mode-add-parents in the
ELisp manual?  I think it attempts to answer your questions.

Re: What is the use of derived-mode-add-parents?

[Pankaj Jangid]

Eli Zaretskii <eliz@gnu.org> writes:

>> Please help me understand, what is the use of parent modes?
>
> Did you read the documentation of derived-mode-add-parents in the
> ELisp manual?  I think it attempts to answer your questions.

Thanks. Great reminder. 🤦