Code Product

This crate provides two things:

1. A library to generate code by repetively expanding macros. This is the primary objective of this crate as it enables code generation in a convenient way from other proc macros. The Syntax is there as well.
2. The standalone product!{} and product_items!{} macros to generate code using the library as it is useful on its own.

Use-Cases

This macro system is useful to generate boilerplate code that repeats in similar ways.

Product expansion example

The name product is because it expands to the product (each by each) of all defined sets. For example given are the two sets of defintions 'Foo and Bar' and 'This and That', showing different syntactic variants:

# use code_product::product;
# trait Trait<T>{}
# struct This<T>(T); struct That<T>(T);
# struct Foo; struct Bar;
product!{
    // Rather elaborate form with named definitions:
    // define `Type` to expand to `This` and `That`
    $(Type: (This) (That))
    // and inline define `T` to expand to `Foo` and `Bar`
    impl Trait<$($T: (Foo)(Bar))> for $Type<$T> {}
}

or

# use code_product::product;
# trait Trait<T>{}
# struct This<T>(T); struct That<T>(T);
# struct Foo; struct Bar;
product!{
    // Alternative form inlining definition and reference by index:
    impl Trait<$((Foo)(Bar))> for $((This)(That))<$0> {}
}

either of the above will expand four times to:

# trait Trait<T>{}
# struct This<T>(T); struct That<T>(T);
# struct Foo; struct Bar;
impl Trait<Foo> for This<Foo> {}
impl Trait<Foo> for That<Foo> {}
impl Trait<Bar> for This<Bar> {}
impl Trait<Bar> for That<Bar> {}

Linear expansion example

In linear expansion scopes in square brackets each definition has to define the same number of itens. These are then iterated together. This gives more control over the expansions as each possible combination has to be defined manually.

Exmaple: Pair Substr with &str and with CowStr and its reversed forms.

product! {
    $[
        $(Lhs: (SubStr)(&str)(SubStr)(CowStr))
        $(Rhs: (&str)(SubStr)(CowStr)(SubStr))

        impl PartialOrd<$Rhs> for $Lhs {
            fn partial_cmp(&self, other: &$Rhs) -> Option<std::cmp::Ordering> {
                (**self).partial_cmp(other)
            }
        }
    ]
}