Allow invariant checks in `.build()`

[Banyc]

I want something like this:

pub struct S {
    x: i32,
    y: i32,
}

impl S {
    fn check_rep(&self) {
        assert!(self.x < self.y);
    }
}

When .build() is called, I also want check_rep to be called.

[benluelo]

Just stumbled upon this issue while looking through the repo, but you could do something like this:

pub struct S {
    xy: Xy
}

pub struct Xy {
    x: i32,
    y: i32,
}

impl Xy {
    pub fn new(x: i32, y: i32) -> Result<Self, Whatever> {
        if x < y {
            Ok(Self { x, y })
        } else {
            Err(/* whatever */)
        }
    }

    // also add some getters for x and y
}

i.e. just encode the invariants into the type system

[Banyc]

@benluelo its like a builder pattern and I have another idea:

pub struct S {
    x: i32,
    y: i32,
}

pub struct SBuilder {
    pub x: i32,
    pub y: i32,
}

impl SBuilder {
    pub fn build(self) -> Result<S, Whatever> {
        if self.x < self.y {
            Ok(S { x, y })
        } else {
            Err(/* whatever */)
        }
    }
}

[mo8it]

I would implement it like this:

fn build(self) -> Result<S, Whatever> {
    …
    validate(S {x, y})
}

Where validate is a function pointer fn(S) -> Result<S, Whatever> that is specified as a struct builder attribute #[builder(prebuild = "validate")].

I would call it something like prebuild instead of validator because it can be used for post-processing and not only validation.

@idanarye Does this look fine to you?

[Techassi]

I can tackle this if @idanarye approves.

[idanarye]

What's the signature of validate? How does the TypedBuidler macro know what the error type is? Also, why prebuild when it happens after the struct is built? Shouldn't it be postbuild?

[idanarye]

Since the crate is split now and we can bring in regular structs, who about something like this:

#[derive(TypedBuilder)]
#[builder(postbuild)]
struct S {
    x: i32,
    y: i32,
}

impl PostBuild for S {
    type Output = Result<Self, Whatever>;

    fn process(self) -> Self::Output {
        validate(&self)?; // or just do the validation here
        Ok(self)
    }
}

#[builder(postbuild)] will tell the macro to use PostBuild::Output and PostBuild::process. Alternatively, the macro could always use the PostBuild trait, but without #[builder(postbuild)] it would generate a simple implementation of it (and when Rust finally stabilizes specialization we could have a blanket implementation and remove this setting)

[mo8it]

Nice approach! Using PostBuild only when the attribute is specified sounds better to me.

[idanarye]

I wonder about that. I think always using PostBuild and generating one when the setting is not passed will result in simpler code because we won't have to conditionally change the generated code of the build method.

[Techassi]

Yeah I agree, we should always use the PostBuild::Output. This simplifies the code generation for the .build() method. I will open a PR and will start working on this.

[Techassi]

I created an initial draft PR over at #95 which implements this feature. It should be noted that it currently is a WIP PR.