Allow specifying mandatory `Option` argument

[pablosichert]

Awesome library! Had a lot of fun using it so far.

I have an edge case where one of my input argument is an Option<bool> since I want to encode the states yes/no/maybe via Some(true)/Some(false)/None. Currently, bon::builder would interpret that argument as optional, even though I want it to be mandatory.

#22 could be related in a sense that it would allow me to work around this issue.

---

For other people looking for a workaround: create a typealias type ... = Option<T>, such that bon is not aware of the top-level Option type.

A note for the community from the maintainers

Please vote on this issue by adding a 👍 reaction to help the maintainers with prioritizing it. You may add a comment describing your real use case related to this issue for us to better understand the problem domain.

[Veetaha]

Hi! Thank you for creating the issue! I've been thinking of adding smth like #[builder(must_use/explicit)] (name is not settled yet) for an Option and #[builder(default)] parameters to explicitly configure that the caller must not omit calling the setter for the optional field. Do you think this will solve it for you?

Example:

struct Foo {
     #[builder(must_use)]
     optional: Option<u32>,

     #[builder(default = 42, must_use)]
     has_default: u32,
}

let foo = Foo::builder()
    .maybe_optional(None)    // not calling this or `.optional(value)` will be a compile error
    .maybe_has_default(None) // not calling this or `.has_default(value)` will be a compile error
    .build();
    
assert_eq!(foo.optional, None);
assert_eq!(foo.has_default, 42);

Note that the workaround you suggested (hiding Option from bon behind a type alias) has a drawback in that bon won't generate a pair of methods member(value: T) and maybe_member(value: Option<T>) in this case.

[Veetaha]

I re-read the issue again. I suppose in this use case Option is used, but no None default value is not implied, right? You'd like to make it genuinely required and have the same behavior as if was just a regular field of Option<T> type without any special treatment like having a pair of member/maybe_member methods?

This would be a bit different from what I suggested higher. In this case another attribute like #[builder(transparent)] would be needed in bon (we don't have it now yet). The difference is that #[builder(transparent)] will prevent generating a pair of member/maybe_member methods, and make the argument required

[pablosichert]

You'd like to make it genuinely required and have the same behavior as if was just a regular field of Option<T> type without any special treatment like having a pair of member/maybe_member methods?

Yes, exactly. Thank you for looking into this!

The suggested #[builder(transparent)] sounds good (without having looked into which other edge cases this might open up from that).

[Veetaha]

Here are some more thoughts about this.

The proposed #[bon(transparent)] may be confusing. Users may mistakenly think that this attribute can be used to disable automatic Into conversions or some other automatic behaviours. Also, I'd want to follow the rule "add attribute only if unavoidable". Having an attribute for a niche use case (such as this) increases the complexity of the API of bon and it makes it harder for people to study it (from docs perspective the is just more attributes to learn).

Maybe it's fine to use a type alias in this case to explicitly state that bon shouldn't treat the field as Option. I guess it doesn't hurt if you need to do this very rarely.

Also another interesting workaround for this that I found is that you can prefix Option with r# to make bon stop detecting it as an Option. The r# prefix is used in Rust to prevent the compiler from treating an identifier as a keyword (special behavior).

Turns out I unintentionally coded bon such that it has a similar behavior for Option detection in this case as well. If Option is prefixed with r#, then bon stops treating the type specially. Sounds rather fun that this unintended behavior actually kinda fits the use case 🐱.

For example, this works as you'd like:

#[bon::builder]
fn foo(val: r#Option<i32>) {}

// `val` is a mandatory field that has no default and must be set
// No special `Option` handling was applied by `bon`
foo().val(Some(32)).call();

Maybe this behavior of r#Option could even be documented and guaranteed in by bon API. What do you think @pablosichert?

[pablosichert]

That's a neat little workaround – could roll with it.

However, I must say that if I was reading code that used r#Option I would immediately think that the r# is superfluous and the author forgot it there after changing away from a previously used type that clashes with a keyword.

Maybe a #[required] attribute could do?

That said, both the typealias and r# would be good enough for me to handle this case.

[Veetaha]

The term required is a bit overloaded. There are two parts of "being required".

1. The setter method must be called during building (exhaustiveness).
2. There is no default value for the member (no maybe_ method existing).

There should be some way to configure both of these independently, and I just can't find good names and syntax for that.

Maybe #[builder(required)] forces the setter not to be omitted (exhaustiveness) and removes the maybe_ method. And then #[builder(required, default)] generates the maybe_ method but still makes the setter required to call (exhaustiveness). The required, default combo looks a bit confusing, so I'm still looking for better design. I'll keep this issue open to see if I or anyone else has other bright ideas

[Veetaha]

On the second thought #[builder(required)] looks bearable when paired with #[builder(default)]. So the proposed behavior is the following:

struct Example {
    #[builder(required)]
    arg1: Option<String>,

    // A bit confusing, but it means the setter is required to call, although a default
    // value may be requested (also explicitly). That default value may be changed
    // by the implementor and the caller won't be broken if it changes
    #[builder(required, default = "foo")]
    arg2: String,

    #[builder(required, default = Some("bar".to_owned()))]
    arg3: Option<String>
}

let example = Example::builder()
    // accepts an `Option<String>`. There is no `maybe_` method.
    .arg1(Some("value".to_owned()))
    // For this member a `maybe_` method was generated (because of `#[builder(default)]`).
    // However calling the setter is still required (because of `#[builder(required)]`).
    // If you want to set the default value, pass `None` explicitly.
    // Not calling any setter for `arg2` won't compile
    .maybe_arg2(None::<String>)
    // This works the same as with `arg3`, although there is `Option<Option<String>>`
    // accepted here. If `None` is passed the default value of `Some("bar".to_owned())`
    // is used. The method `.arg3()` accept a single-level `Option<String>` though
    .maybe_arg3(None)
    .build();

assert_eq!(example.arg1, "value");
assert_eq!(example.arg2, "foo");
assert_eq!(example.arg2.as_deref(), Some("bar"));

Additionally, the macro may generate default_{member}() methods such as .default_arg2() method to explicitly request the default value for the member. This avoids the need to pass a type annotation for None literal when it's passed into a function that accepts an Option<impl Into<String>>.

[musjj]

I'm liking how the #[builder(required)] API looks.

My main use-case for bon is for enforcing immutability in certain structs (where all fields are private with builder+getter), rather for partially filling struct fields.

So something that would make it easy to make all fields mandatory during construction would be great!

[Veetaha]

Hi @musjj, I've already revisited this issue and design a bit on the background while working on #145.

To me, the feature of having exhaustiveness in the builder (where you need to fill all fields, even ones that have default values) looks quite complicated to even express and explain for the users. However, I'm not entirely giving up on this idea, and I'd be glad to have it if user feedback proves me wrong on this.

The main problem for me has been figuring out the proper naming for the different behaviors that I'd like to be configurable.

• Opt-out from the special handling for members of type Option. This means two things:
  • The setter for this field becomes required to call.
  • There is just one setter generated for the field as if it was just a regular field: {member}(Option<T>)
• Opt-out from only part of the special handling for members of type Option:
  • The setter for this field becomes required to call.
  • There still are two setters generated {member}(T) and maybe_{member}(Option<T>)

For the first behavior I'm thinking to return to the #[builder(transparent)] terminology. This tells the reader, that the field shouldn't be treated in any special way, and be a regular required field just like all fields are by default.

Then I'd like to reserve something like #[builder(explicit)] to mark fields for which the caller must invoke setters, even if these fields have default values or they are of Option<T> type.

So it would look like this:

#[derive(bon::Builder)]
struct Example {
    field1: Option<u32>,
    
    #[builder(transparent)]
    field2: Option<u32>,
    
    #[builder(explicit)]
    field3: Option<u32>
}

Example::builder()
    // `field1` has two setters generated, and it includes a `maybe_{}` setter. We could also omit
    // calling this setter.
    .maybe_field1(None)
    // `field2` has just one setter generated that accepts `Option<T>` and it's required to call
    .field2(None)
    // It works the same as `field1`, but with a difference that if you don't call any setter for
    // the `field3` the code won't compile
    .maybe_field3(None)
    .build()

I've already added support for #[builder(transparent)] in my yet-in-progress PR #145.

WDYT about this design @musjj?

So something that would make it easy to make all fields mandatory during construction would be great!

Is your use case for a #[builder(transparent)] or rather #[builder(explicit)]? I.e. would you still like to have two setters generated for the optional field?

Note that #[builder(explicit)] can also be combined with #[builder(default)]:

struct Example {
    #[builder(explicit, default = 42)]
    field1: u32
} 

Example::builder()
    // We can't omit calling a setter for `field1`. This requires the caller to 
    // explicitly state "yes, I acknowledge there is a `field1`" and I want to
    // use a default value for it
    .maybe_field1(None)
    .build()

Right now I'm not adding #[builder(explicit)] attribute yet (as I said, it's quite hard to explain and understand for the readers I think). However, if it better fits your use case, I can still add it. The problem is that this attribute would introduce a dimension into the previously two-dimensional optional/required behavior, where it would allow for optional-implicit and optional-explicit members.

With this complex behaviors having #[builder(required)] becomes confusing (the reader doesn't know if it means whether there still are a pair of {member}/maybe_{member} setters generated. It also becomes easy to confuse it with #[builder(explicit)], so I'm thinking of using #[builder(transparent)] naming instead to future-proof for a potential #[builder(explicit)] attribute in the future.

[musjj]

For my personal use-case, I'm heavily preferring the #[builder(transparent)] method. Maybe you can wait for feedback from other users to see if there are demands for a #[builder(explicit)].

Btw, any chance of allowing the attribute to be used on the top level? So something like:

#[derive(bon::Builder)]
#[builder(transparent)] // All fields have to be explicitly constructed
struct Example {
    field1: Option<u32>,
    field2: Option<u32>,
    field3: Option<u32>
}

It would make it harder to forget to mark a field.

[Veetaha]

I see, thank you for the feedback! I'll just postpone the #[builder(explicit)] for now, and wait for someone with a use case for it.

Regarding the top-level config, it does make sense! I can make an extension of the #[builder(on(...))] syntax for this. So it would look like this if you want to blanket-apply transparent:

#[builder(on(_, transparent))]

With this you can also select members of specific types. For example, say you want to apply transparency only for members of type Option<u32>, but not touch members of Option<String> or Option<bool>:

#[builder(on(u32, transparent))]

Note that I plan to extend the syntax of the selector part of the on(selector, attrs...) attribute, and allow selecting members not only by their types, but for example on(prefix(foo_), ...) to select all members with a specific prefix, or other properties in the future.

With the bare #[builder(transparent)] syntax at the top-level the users may confuse this attribute to apply to the builder type itself (instead of individual fields), so on(...) wrapper makes it more clear that this config is inherited at field-level.

[adrian-budau]

Hello, I found myself in need of this feature with no workarounds, though I only really require the #[builder(explicit)] part.

I use protobuf for some between-service communication, and, at the moment I use prost with proto3 files, which for the sake of my explanation here is harder to change than adding a feature to bon.
All of my protobuf messages live in a separate crate so they can be used both between the server and the client(s).
To not break semver constantly, these protobuff rust structs are marked #[non_exhaustive] and use the builder pattern though bon.

Now proto3 has the unfortunate side-effect of having all fields optional, even if you mark them required, so there is no way to force every depender of a crate which exports proto3 messages to fill a field, and even though None is a valid value for a field, I'd rather the consumer of this type think before supplying None

proto3 file:

message Foo {
 // multiple fields
 // ...
 required Bar bar = 10;
}

in rust would be

[derive(Clone, Debug, PartialEq, Message, bon::Builder)]
#[non_exhaustive]
pub struct Foo {
  // the fields added here
  // ..
  bar: Option<Bar>, // notice this is not mandatory in bon, but it would be if I could use #[builder(explicit)]
}

I hope this counts as a valid use-case since this would greatly help me, thanks!