ApplyIf: Seamless Conditionals for Builder Chains

This crate provides the functions apply_if and apply_if_some for consuming builders, as well as apply_if_mut and apply_if_some_mut for mutable builder chains. They allow us to conditionally invoke builder functions while keeping the nice chaining syntax.

Examples

These examples use the derive_builder crate to create a builder and show how to use the apply_if... functions to conditionally invoke builder functions without breaking the chain.

Mutable Builders

This example uses a mutable builder pattern, which is why apply_if_mut and apply_if_some_mut are used.

extern crate derive_builder;
use derive_builder::Builder;
use apply_if::ApplyIf;

#[derive(Debug,PartialEq,Builder)]
struct Value{
  #[builder(default = 123)]
  first: i32,
  #[builder(default = 42.)]
  second: f32,
}

fn main () {
    // apply_if_mut only applies the function
    // if the condition is true
    let value = ValueBuilder::default()
        .apply_if_mut(true, |builder| builder.first(100))
        .second(1.337)
        .build()
        .unwrap();
    assert_eq!(value, Value{first: 100, second: 1.337});

    let value = ValueBuilder::default()
        .apply_if_mut(false, |builder| builder.first(100))
        .second(1.337)
        .build()
        .unwrap();
    assert_eq!(value, Value{first: 123, second: 1.337});

    // apply_if_some_mut only applies the function
    // if the optional contains a value
    
    let value = ValueBuilder::default()
        .apply_if_some_mut(Some(100), |builder,val| builder.first(val))
        .second(1.337)
        .build()
        .unwrap();
    assert_eq!(value, Value{first: 100, second: 1.337});

    let value = ValueBuilder::default()
        .apply_if_some_mut(None, |builder,val| builder.first(val))
        .second(1.337)
        .build()
        .unwrap();
    assert_eq!(value, Value{first: 123, second: 1.337});
}

Immutable Builders

This example uses a consuming builder pattern, which is why apply_if and apply_if_some are used.

extern crate derive_builder;
use derive_builder::Builder;
use apply_if::ApplyIf;

#[derive(Debug,PartialEq,Builder)]
#[builder(pattern = "immutable")]
struct Value{
  #[builder(default = 123)]
  first: i32,
  #[builder(default = 42.)]
  second: f32,
}

fn main () {
    // apply_if only applies the function
    // if the condition is true
    let value = ValueBuilder::default()
        .apply_if(true, |builder| builder.first(100))
        .second(1.337)
        .build()
        .unwrap();
    assert_eq!(value, Value{first: 100, second: 1.337});

    let value = ValueBuilder::default()
        .apply_if(false, |builder| builder.first(100))
        .second(1.337)
        .build()
        .unwrap();
    assert_eq!(value, Value{first: 123, second: 1.337});

    // apply_if_some only applies the function
    // if the optional contains a value
    let value = ValueBuilder::default()
        .apply_if_some(Some(100), |builder,val| builder.first(val))
        .second(1.337)
        .build()
        .unwrap();
    assert_eq!(value, Value{first: 100, second: 1.337});

    let value = ValueBuilder::default()
        .apply_if_some(None, |builder,val| builder.first(val))
        .second(1.337)
        .build()
        .unwrap();
    assert_eq!(value, Value{first: 123, second: 1.337});
}