Contexts, Words & Macros

Macros work. Contexts and words do not yet.

Macros

Declare a macro with macro name(param: meta_type) { body } and call it with the @ prefix. Parameter meta-types are identifier, expr, condition, block, and body.

macro twice(v: expr) {
    v + v
}

fn use_macro(v: i32): i32 {
    @twice(v)
}

A normal macro is hygienic: bindings introduced by the template are renamed, so they cannot capture or be captured by names at the call site. Every other name in the template resolves through the call-site scope, with globals and imports visible when they are not shadowed. A macro body is not analysed as code before expansion; resolution is keyed by node identity, not by source text.

raw macro opts out of hygiene. Its body splices literally into the call site, and names resolve there first and only then fall back to the module and global scope. Use it when the macro is meant to touch the caller's bindings.

Tag macros

A tag macro is invoked with an HTML-like statement form. It takes a body parameter and, optionally, the special attributes parameter, which exposes named attributes as attributes.name.

tag macro RunTwice(attributes, body: body) {
    let title = attributes.title;
    body;
    body;
}

fn main(): i32 {
    var total = 0;
    <RunTwice title: 1>
        total = total + 1;
    </RunTwice>
    total
}

Tag macros are statement-position only. Misuse has dedicated diagnostics: E2017 for a tag macro used as a value, E2018 for a mismatched closing tag, E2019 for an unknown attribute, and E2020 for attributes passed to a macro that does not accept them. The general macro codes E2011-E2016 cover unknown macros, arity, argument kind, recursion, duplicates, and raw macro used as a value.

Contexts and words

context blocks, use statements, and the prefix, suffix, infix, and nop word declarations are accepted as declarations and then skipped entirely: the body is consumed without semantics. Word call and word sequence expressions have no parser support.

Read the Contexts, Words, and Macros reference chapters for the intended design, and Implementation Status for the boundary.