Rust Programming @lemmy.ml nutomic @lemmy.ml 3 mo. ago

Which code style to initialize structs?

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

23 comments

Definitely the second one.

It avoids Mut

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.
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.
- We used to have TypedBuilder (which is builder pattern), but switched to DeriveNew, as its a bit cleaner, and requires less generated code.
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.
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?
- To decide if I should merge the linked PR or not (I did merge it).
Defo the second one, the first is weird imo
@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.
- 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 };
  
  Why not just a let app = app; line after the let mut app = ...; one?
  
  Thats even more verbose so the second option is better.
also adding my vote for the second one
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.
- @livingcoder @nutomic that's a nice one. Had never thought of it. But I'd just use the builder pattern.
  
  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.

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