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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[–] [email protected] 12 points 3 weeks 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
};
[–] [email protected] 1 points 3 weeks ago (1 children)

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

[–] [email protected] 2 points 3 weeks 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 3 points 3 weeks ago

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

[–] [email protected] 1 points 3 weeks ago

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

[–] [email protected] 1 points 3 weeks ago (1 children)

Thats even more verbose so the second option is better.

[–] [email protected] 6 points 3 weeks ago (1 children)

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

[–] [email protected] 2 points 3 weeks ago (1 children)

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

[–] [email protected] 2 points 3 weeks ago (1 children)

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

[–] [email protected] 1 points 3 weeks ago (1 children)

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

[–] [email protected] 2 points 3 weeks ago

Because you don't control third party libraries