partially

Provides a configurable derive macro that generates another struct with the same fields, but wrapped in Option<T>, and implements Partial to allow conditionally applying the generated struct fields to the base struct fields.

partially provides the Partial trait, which allows applying another structs values to self, with the intent that the other struct mirrors the fields of self, but wrapped in Option<T>.

Further, partially_derive (or partially with the derive feature enabled) supports automatically generating a mirrored struct with each field wrapped in Option<T>, and generates a Partial implementation that allows applying the Some fields of the mirrored struct to the base struct. I expect most folks will be most interested in using the derive macro.

Usage

With derive:

// `partially` installed with feature `derive`
use partially::Partial;

// define a base structure, with the `Partial` derive macro
#[derive(partially_derive::Partial)]
// further, instruct the macro to derive `Default` on the generated structure
#[partially(derive(Default))]
struct Data {
    // since no field options are specified, this field will be mapped
    // to an `Option<String>` in the generated structure
    value: String,
}

// example usage
fn main() {
    // since we derived default for the generated struct, we can use that
    // to obtain a partial struct filled with `None`.
    let empty_partial = PartialData::default();

    // we can, of course, also specify values ourself
    let full_partial = PartialData {
        value: Some("modified".to_string()),
    };

    // define a "base" that we'll operate against
    let mut full = Data {
        value: "initial".to_string(),
    };

    // apply the empty partial
    full.apply_some(empty_partial);

    // note that applying the empty partial had no effect
    assert_eq!(full.value, "initial".to_string());

    // apply the full partial
    full.apply_some(full_partial);

    // note that applying the full partial modified the value
    assert_eq!(full.value, "modified".to_string());
}

Without derive:

use partially::Partial;

struct Base {
    value: String,
}

#[derive(Default)]
struct PartialBase {
    value: Option<String>,
}

impl partially::Partial for Base {
    type Item = PartialBase;

    #[allow(clippy::useless_conversion)]
    fn apply_some(&mut self, partial: Self::Item) {
        if let Some(value) = partial.value {
            self.value = value.into();
        }
    }
}

fn main() {
    let empty_partial = PartialBase::default();
    let full_partial = PartialBase {
        value: Some("modified".to_string()),
    };

    let mut data = Base {
        value: "initial".to_string(),
    };

    data.apply_some(empty_partial);

    assert_eq!(data.value, "initial".to_string());

    data.apply_some(full_partial);

    assert_eq!(data.value, "modified".to_string())
}

Struct Options

derive

Usage example: #[partially(derive(Debug, Default))].

Instructs the macro to generate a #[derive(...)] attribute on the generated struct.

rename

Usage example: #[partially(rename = "MyGeneratedStruct")].

Instructs the macro to use a given identifier for the generated struct. By default, PartialBaseStructName is used, where BaseStructName is the name of the original struct.

crate

Usage example: #[partially(crate = "my_partially_crate")].

Instructs the macro to use a different base path for the Partial trait implementation. By default, partially is used. This can be useful if you've forked the partially crate.

Field Options

rename

Usage example: #[partially(rename = "new_field_name")].

Instructs the macro to use a given identifier for the generated field. By default, the same name as the base struct is used.

omit

Usage example: #[partially(omit)].

Instructs the macro to omit the field from the generated struct. By default, no fields are omitted.

transparent

Usage example: #[partially(transparent)].

Instructs the macro to skip wrapping the generated field in Option<T>, instead transparently mirroring the field type into the generated struct.

as_type

Usage example: #[partially(as_type = "Option<f32>")].

Instructs the macro to use the provided type instead of Option<T> when generating the field. Note that the provided type will be used verbatim, so if you expect an Option<T> value, you'll need to manually specify that.

Note: When using as_type, the given type must Into<BaseType> where BaseType is the original field type. This is required for Partial trait implementation.