Modules

C3 groups functions, types, variables and macros into namespaces called modules. When doing builds, any C3 file must start with the module keyword, specifying the module. When compiling single files, the module is not needed and the module name is assumed to be the file name, converted to lower case, with any invalid characters replaced by underscore (_).

A module can consist of multiple files, e.g.

file_a.c3

module foo;

/* ... */

file_b.c3

module foo;

/* ... */

file_c.c3

module bar;

/* ... */

Here file_a.c3 and file_b.c3 belong to the same module, foo while file_c.c3 belongs to to bar.

Details

Some details about the C3 module system:

  • Modules can be arbitrarily nested, e.g. module foo::bar::baz; to create the sub module baz in the sub module bar of the module foo.
  • Module names must be alphanumeric lower case letters plus the underscore character: _.
  • Module names are limited to 31 characters.
  • Modules may be spread across multiple files.
  • A single file may have multiple module declarations.
  • Each declaration of a distinct module is called a module section.

Importing modules

Modules are imported using the import statement. Imports always recursively import sub-modules. Any module will automatically import all other modules with the same parent module.

foo.c3

module some::foo;
fn void test() {}

bar.c3

module bar;
import some;
// import some::foo; <- not needed, as it is a sub module to "some"
fn void test()
{
    foo::test();
    // some::foo::test() also works.
}

In some cases there may be ambiguities, in which case the full path can be used to resolve the ambiguity:

abc.c3

module abc;
struct Context
{
    int a;
}

def.c3

module def;
struct Context
{
    void* ptr;
}

test.c3

module test1;
import def, abc;
// Context c = {} <- ambiguous
abc::Context c = {};

Implicit imports

The module system will also implicitly import:

  1. The std::core module (and sub modules).
  2. Any other module sharing the same top module. E.g. the module foo::abc will implicitly also import modules foo and foo::cde if they exist.

Visibility

All files in the same module share the same global declaration namespace. By default a symbol is visible to all other modules. To make a symbol only visible inside the module, use the @private attribute.

module foo;

fn void init() { .. }

fn void open() @private { .. }

In this example, the other modules can use the init() function after importing foo, but only files in the foo module can use open(), as it is specified as private.

It's possible to further restrict visibility: @local works like @private except it's only visible in the local context.

// File foo.c3
module foo;
fn void abc() @private { ... }
fn void def() @local { ... }

// File foo2.c3
module foo;
fn void test()
{
    abc(); // Access of private in the same module is ok
    // def(); <- Error: function is local to foo.c3
}

Overriding symbol visibility rules

By using import <module> @public, it's possible to access another moduleĀ“s private symbols. Many other module systems have hierarchal visibility rules, but the import @public feature allows visibility to be manipulated in a more ad-hoc manner without imposing hard rules.

For example, you may provide a library with two modules: "mylib::net" and "mylib::file" - which both use functions and types from a common "mylib::internals" module. The two libraries use import mylib::internals @public to access this module's private functions and type. To an external user of the library, the "mylib::internals" does not seem to exist, but inside of your library you use it as a shared dependency.

A simple example:

// File a.c3
module a;

fn void a_function() @private { ... }

// File b.c3
module b;

fn void b_function() @private { ... }

// File c.c3
module c;
import a;
import b @public;

fn void test() 
{
  a::a_function(); // <-- error, a_function is private
  b::b_function(); // Allowed since import converted it to "public" in this context.
}

Note: @local visibility cannot be overridden using a "@public" import.

Changing the default visibility

In a normal module, global declarations will be public by default. If some other visibility is desired, it's possible to declare @private or @local after the module name. It will affect all declaration in the same section.

module foo @private;

fn void ab_private() { ... } // Private

module foo;

fn void ab_public() { ... } // Public

module bar;
import foo;

fn void test()
{
    foo::ab_public(); // Works
    // foo::ab_private(); <- Error, private method
}

If the default visibility is @private or @local, using @public sets the visibility to public:

module foo @private;

fn void ab_private() { ... }        // Private
fn void ab_public() @public { ... } // Public

Linker visibility and exports

A function or global prefixed extern will be assumed to be linked in later. An "extern" function may not have a body, and global variables are prohibited from having an init expression.

The attribute @export explicitly marks a function as being exported when creating a (static or dynamic) library. It has no particular effect when compiling as an executable.

Using functions and types from other modules

As a rule, functions, macros, constants, variables and types in the same module do not need any namespace prefix. For imported modules the following rules hold:

  1. Functions, macros, constants and variables require at least the (sub-) module name.
  2. Types do not require the module name unless the name is ambiguous.
  3. In case of ambiguity, only so many levels of module names are needed as to make the symbol unambiguous.
// File a.c3

module a;

struct Foo { ... }
struct Bar { ... }
struct TheAStruct { ... }

fn void anAFunction() { ... }

// File b.c3

module b;

struct Foo { ... }
struct Bar { ... }
struct TheBStruct { ... }

fn void aBFunction() { ... }

// File c.c3
module c;
import a, b;

struct TheCStruct { ... }
struct Bar { ... }

fn void aCFunction() { ... }

fn void test()
{
    TheAStruct stA;
    TheBStruct stB;
    TheCStruct stC;
    // Name required to avoid ambiguity;
    b::Foo stBFoo;
    // Will always pick the current module's 
    // name.
    Bar bar;
    // Namespace required:
    a::aAFunction();
    b::aBFunction();
    // A local symbol does not require it:
    aCFunction(); 
}

This means that the rule for the common case can be summarized as

Types are used without prefix; functions, variables, macros and constants are prefixed with the sub module name.

Module sections

A single file may have multiple module declarations, even for the same module. This allows us to write for example:

// File foo.c3
module foo;
fn int hello_world()
{
    return my_hello_world();
}

module foo @private;
import std::io;         // The import is only visible in this section.
fn int my_hello_world() // @private by default
{
    io::printn("Hello, world\n");
    return 0;
}

module foo @test;
fn void test_hello() // @test by default
{
    assert(hello_world() == 0);
}

Versioning and dynamic inclusion

NOTE: This feature may significantly change.

When including dynamic libraries, it is possible to use optional functions and globals. This is done using the @dynamic attribute.

An example library could have this:

dynlib.c3i

module dynlib;
fn void do_something() @dynamic(4.0)
fn void do_something_else() @dynamic(0, 5.0)
fn void do_another_thing() @dynamic(0, 2.5)

Importing the dynamic library and setting the base version to 4.5 and minimum version to 3.0, we get the following:

test.c3

import dynlib;
fn void test()
{
    if (@available(dynlib::do_something))
    {
        dynlib::do_something();
    }
    else
    {
        dynlib::do_someting_else();
    }  
}

In this example the code would run do_something if available (that is, when the dynamic library is 4.0 or higher), or fallback to do_something_else otherwise.

If we tried to conditionally add something not available in the compilation itself, that is a compile time error:

if (@available(dynlib::do_another_thing))  
{
    dynlib::do_another_thing(); // Error: This function is not available with 3.0
}

Versionless dynamic loading is also possible:

maybe_dynlib.c3i

module maybe_dynlib;
fn void testme() @dynamic;

test2.c3

import maybe_dynlib;
fn void testme2()
{
    if (@available(maybe_dynlib::testme))
    {
        dynlib::testme();
    }
}

This allows things like optionally loading dynamic libraries on the platforms where this is available.

Textual includes

$include

It's sometimes useful to include an entire file, doing so employs the $include function. Includes are only valid at the top level.

File Foo.c3

module foo;

$include("Foo.x");

fn void test() 
{
    io::printf("%d", testX(2));
}    

File Foo.x

fn testX(int i) 
{ 
    return i + 1; 
}

The result is as if Foo.c3 contained the following:

module foo;

fn testX(int i) 
{ 
    return i + 1; 
}

fn void test() 
{
    io::printf("%d", testX(2));
}    

The include may use an absolute or relative path, the relative path is always relative to the source file in which the include appears.

Note that to use it, the trust level of the compiler must be set to at least 2 with the -t option (i.e. use -t2 or -t3 from the command line).

$exec

An alternative to $include is $exec which is similar to include, but instead includes the output of an external program as the included text.

An example:

import std::io;

// On Linux or MacOS this will insert 'String a = "Hello world!";'
$exec("echo", "String a = \\\"Hello world!\\\"\\;");

fn void main()
{
    io::printn(a);
}

Using $exec requires trust level 3, which is enabled with -t3 from the command line.