Attributes and C Interop

Attributes

#name lines before a declaration change how it behaves. Stack them, or group them in one #[...] line.

import @json

#[doc("A person with a name and age"), json]
const Person struct {
    name string
    age i64
}

do main() {
    mut person = Person{name: "Ann", age: 41}
    println(json.stringify(person))
}

Attribute	Applies to	Effect
#doc("...")	functions, structs, enums	Documentation for gray doc
#json	structs	Enables JSON encoding and decoding
#flags	enums	Each variant is one bit
#error_code	enums	Adds variants to ErrorCode
#strict	when	Requires every variant to be handled
#discard	functions	Callers may ignore the return value
#deprecated("...")	functions, structs, enums	Warns at every use
#test	functions	Run by gray test

#discard marks a function whose result is safe to ignore.

#discard
do log_and_count(message string) -> i64 {
    println(message)
    return 1
}

do main() {
    log_and_count("hello")
}

Calling C

Import a header with extern import, then call any function through the extern. prefix.

extern import "stdio.h"

do main() {
    extern.puts("hello from C")
}

Give a C call’s result a type by assigning it to an annotated variable, and use c_string() to turn a char* result into a string. Match C’s argument widths with i32, u32 and f32.

extern import "stdlib.h"

do main() {
    mut home string = c_string(extern.getenv("HOME"))
    println(len(home) > 0)
}

Memory safety does not cross the C boundary: the pointer checker cannot follow a pointer once C holds it.