14
Which code style to initialize structs? (lemmy.ml)
submitted 5 days ago by nutomic@lemmy.ml to c/rust@lemmy.ml
23 comments fedilink hide all child comments

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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[-] livingcoder@programming.dev 2 points 3 days ago* (last edited 3 days ago)

I prefer to encapsulate a mutable reference to the instance in a scope.

let post_form = {
    let mut post_form = PostInsertForm::new(
        // your constructor arguments
    );
    post_form.some_mutating_method(
        // mutation arguments
    );
    post_form
};

This way you're left with an immutable instance and you encapsulate all of the logic needed to setup the instance in one place.

[-] ubik@fedi.turbofish.cc 1 points 3 days ago

@livingcoder @nutomic that's a nice one. Had never thought of it. But I'd just use the builder pattern.

[-] livingcoder@programming.dev 1 points 2 days ago* (last edited 2 days ago)

Even if you were using the builder pattern, this maintains the immutable variable in the parent scope while you use the mutable variable's builder pattern methods (basically exactly as my example demonstrates) in the inner scope.

edit: Oh, I think you mean you would chain the builder pattern calls and assign it to an immutable variable. Sure, that makes sense if you own the struct.

[-] BB_C@programming.dev 20 points 5 days ago

Neither.

  • make new() give you a fully valid and usable struct value.
  • or use a builder (you can call it something else like Partial/Incomplete/whatever) struct so you can't accidentally do anything without a fully initialized value.

Maybe you should also use substructs that hold some of the info.

[-] dessalines@lemmy.ml 1 points 5 days ago* (last edited 5 days ago)

We used to have TypedBuilder (which is builder pattern), but switched to DeriveNew, as its a bit cleaner, and requires less generated code.

[-] al4s@feddit.org 21 points 5 days ago

Definitely the second one.

  1. It avoids Mut
  2. It makes clear that the initialization is over at the end of of the statement. The first option invites people to change some more properties hundreds of lines down where you won't see them.
[-] Deebster@programming.dev 12 points 5 days ago

100% the second one. It's the idiomatic way to do this in Rust, and it leaves you with an immutable object.

I personally like to move the short declarations together (i.e. body down with language_id (or both at the top)) but that's a minor quibble.

[-] AsudoxDev@programming.dev 9 points 5 days ago* (last edited 5 days ago)

Second one if a constructor or a builder is not an option. 1 is out of the question.

Why are the Lemmy devs asking for this though?

[-] nutomic@lemmy.ml 2 points 4 days ago

To decide if I should merge the linked PR or not (I did merge it).

[-] DemocratPostingSucks@lemm.ee 8 points 5 days ago

Defo the second one, the first is weird imo

[-] nutomic@lemmy.ml 5 points 5 days ago

@DemocratPostingSucks@lemm.ee @Deebster@programming.dev @al4s@feddit.org Thanks for the feedback! Personally I prefer the first option, but based on your comments I will merge the PR with the second option.

[-] al4s@feddit.org 12 points 5 days ago

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 4 days ago

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

[-] al4s@feddit.org 2 points 4 days ago

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 2 points 4 days 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 3 days ago

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

[-] nutomic@lemmy.ml 1 points 5 days ago

Thats even more verbose so the second option is better.

[-] al4s@feddit.org 6 points 5 days ago

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 4 days ago

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

[-] al4s@feddit.org 2 points 4 days ago

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 3 days ago

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

[-] al4s@feddit.org 2 points 3 days ago

Because you don't control third party libraries

[-] Nothing4You@programming.dev 3 points 5 days ago

also adding my vote for the second one

this post was submitted on 23 Sep 2024
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