this post was submitted on 23 Sep 2024
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Rust Programming

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Which of these code styles do you find preferable?

First option using mut with constructor in the beginning:

  let mut post_form = PostInsertForm::new(
    data.name.trim().to_string(),
    local_user_view.person.id,
    data.community_id,
  );
  post_form.url = url.map(Into::into);
  post_form.body = body;
  post_form.alt_text = data.alt_text.clone();
  post_form.nsfw = data.nsfw;
  post_form.language_id = language_id;

Second option without mut and constructor at the end:

  let post_form = PostInsertForm {
    url: url.map(Into::into),
    body,
    alt_text: data.alt_text.clone(),
    nsfw: data.nsfw,
    language_id,
    ..PostInsertForm::new(
      data.name.trim().to_string(),
      local_user_view.person.id,
      data.community_id,
    )
  };

You can see the full PR here: https://github.com/LemmyNet/lemmy/pull/5037/files

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[–] al4s 13 points 2 months ago (2 children)

If you're ever forced to do something the second way, you can also wrap it in braces, that way you end up with an immutable value again:

let app = {
  let mut app = ...
  ...
  app
};
[–] taladar@sh.itjust.works 1 points 2 months ago (1 children)

Why not just a let app = app; line after the let mut app = ...; one?

[–] al4s 2 points 2 months ago (2 children)

A scope groups the initialization visually together, while adding the let app = app; feels like it just adds clutter - I'd probably just leave it mut in that case.

[–] BB_C@programming.dev 3 points 2 months ago

Rebinding with and without mut is a known and encouraged pattern in rust. Leaving things as mut longer than necessary is not.

[–] taladar@sh.itjust.works 1 points 2 months ago

But a scope adds a nesting level which adds a lot more visual clutter.

[–] nutomic@lemmy.ml 1 points 2 months ago (1 children)

Thats even more verbose so the second option is better.

[–] al4s 6 points 2 months ago (1 children)

Yeah if you have the second option, use it, but if the struct has private fields it won't work.

[–] taladar@sh.itjust.works 2 points 2 months ago (1 children)

The first one won't work either for private fields.

[–] al4s 2 points 2 months ago (1 children)

You can have setters that set private fields, there are also sometimes structs with mixed private and public fields

[–] taladar@sh.itjust.works 1 points 2 months ago (1 children)

But why not use a proper builder pattern in that case?

[–] al4s 2 points 2 months ago

Because you don't control third party libraries