Metaprogramming

std::meta is the entry point for Noir's metaprogramming API. This consists of comptime functions and types used for inspecting and modifying Noir programs.

Functions

type_of

type_of

pub comptime fn type_of<T>(x: T) -> Type {}

^{_{Source code: noir_stdlib/src/meta/mod.nr#L27-L29}}

Returns the type of a variable at compile-time.

Example:

comptime {
    let x: i32 = 1;
    let x_type: Type = std::meta::type_of(x);

    assert_eq(x_type, quote { i32 }.as_type());
}

unquote

unquote

pub comptime fn unquote(code: Quoted) -> Quoted {

^{_{Source code: noir_stdlib/src/meta/mod.nr#L19-L21}}

Unquotes the passed-in token stream where this function was called.

Example:

comptime {
    let code = quote { 1 + 2 };

    // let x = 1 + 2;
    let x = unquote!(code);
}

derive

derive

#[varargs]
pub comptime fn derive(s: StructDefinition, traits: [TraitDefinition]) -> Quoted {

^{_{Source code: noir_stdlib/src/meta/mod.nr#L48-L51}}

Attribute placed on struct definitions.

Creates a trait impl for each trait passed in as an argument. To do this, the trait must have a derive handler registered with derive_via beforehand. The traits in the stdlib that can be derived this way are Eq, Ord, Default, and Hash.

Example:

#[derive(Eq, Default)]
struct Foo<T> {
    x: i32,
    y: T,
}

fn main() {
    let foo1 = Foo::default();
    let foo2 = Foo { x: 0, y: &[0] };
    assert_eq(foo1, foo2);
}

derive_via

derive_via_signature

pub comptime fn derive_via(t: TraitDefinition, f: DeriveFunction) {

^{_{Source code: noir_stdlib/src/meta/mod.nr#L68-L70}}

Attribute placed on trait definitions.

Registers a function to create impls for the given trait when the trait is used in a derive call. Users may use this to register their own functions to enable their traits to be derived by derive.

Because this function requires a function as an argument which should produce a trait impl for any given struct, users may find it helpful to use a function like std::meta::make_trait_impl to help creating these impls.

Example:

#[derive_via(derive_do_nothing)]
trait DoNothing {
    fn do_nothing(self);
}

comptime fn derive_do_nothing(s: StructDefinition) -> Quoted {
    let typ = s.as_type();
    quote {
        impl DoNothing for $typ {
            fn do_nothing(self) {
                println("Nothing");
            }
        }
    }
}

As another example, derive_eq in the stdlib is used to derive the Eq trait for any struct. It makes use of make_trait_impl to do this:

derive_eq

comptime fn derive_eq(s: StructDefinition) -> Quoted {
    let signature = quote { fn eq(_self: Self, _other: Self) -> bool };
    let for_each_field = |name| quote { (_self.$name == _other.$name) };
    let body = |fields| {
        if s.fields().len() == 0 {
            quote { true }
        } else {
            fields
        }
    };
    crate::meta::make_trait_impl(
        s,
        quote { Eq },
        signature,
        for_each_field,
        quote { & },
        body,
    )
}

^{_{Source code: noir_stdlib/src/cmp.nr#L10-L30}}

make_trait_impl

make_trait_impl

pub comptime fn make_trait_impl<Env1, Env2>(
    s: StructDefinition,
    trait_name: Quoted,
    function_signature: Quoted,
    for_each_field: fn[Env1](Quoted) -> Quoted,
    join_fields_with: Quoted,
    body: fn[Env2](Quoted) -> Quoted,
) -> Quoted {

^{_{Source code: noir_stdlib/src/meta/mod.nr#L87-L96}}

A helper function to more easily create trait impls while deriving traits.

Note that this function only works for traits which:

1. Have only one method
2. Have no generics on the trait itself.

• E.g. Using this on a trait such as trait Foo<T> { ... } will result in the generated impl incorrectly missing the T generic.

If your trait fits these criteria then make_trait_impl is likely the easiest way to write your derive handler. The arguments are as follows:

• s: The struct to make the impl for
• trait_name: The name of the trait to derive. E.g. quote { Eq }.
• function_signature: The signature of the trait method to derive. E.g. fn eq(self, other: Self) -> bool.
• for_each_field: An operation to be performed on each field. E.g. |name| quote { (self.$name == other.$name) }.
• join_fields_with: A separator to join each result of for_each_field with. E.g. quote { & }. You can also use an empty quote {} for no separator.
• body: The result of the field operations is passed into this function for any final processing. This is the place to insert any setup/teardown code the trait requires. If the trait doesn't require any such code, you can return the body as-is: |body| body.

Example deriving Hash:

derive_hash

comptime fn derive_hash(s: StructDefinition) -> Quoted {
    let name = quote { Hash };
    let signature = quote { fn hash<H>(_self: Self, _state: &mut H) where H: std::hash::Hasher };
    let for_each_field = |name| quote { _self.$name.hash(_state); };
    crate::meta::make_trait_impl(
        s,
        name,
        signature,
        for_each_field,
        quote {},
        |fields| fields,
    )
}

^{_{Source code: noir_stdlib/src/hash/mod.nr#L137-L151}}

Example deriving Ord:

derive_ord

comptime fn derive_ord(s: StructDefinition) -> Quoted {
    let signature = quote { fn cmp(_self: Self, _other: Self) -> std::cmp::Ordering };
    let for_each_field = |name| quote {
        if result == std::cmp::Ordering::equal() {
            result = _self.$name.cmp(_other.$name);
        }
    };
    let body = |fields| quote {
        let mut result = std::cmp::Ordering::equal();
        $fields
        result
    };
    crate::meta::make_trait_impl(s, quote { Ord }, signature, for_each_field, quote {}, body)
}

^{_{Source code: noir_stdlib/src/cmp.nr#L216-L231}}