DIP37

Title:	Importing Packages as if They Were Modules
DIP:	37
Version:	1
Status:	Verbally approved by Walter and Andrei at dconf 2013, but some details may need to be ironed out if anyone comes up with issues with it.
Created:	2013-05-05
Last Modified:	2013-05-05
Author:	Jonathan M Davis
Related DIPs:	DIP15 and DIP16
Links:	Enhancement Request in Bugzilla — Pull Request with the necessary changes to the compiler

Abstract

Provide a way to import a package. This is a variation on DIP15.

Rationale

Currently, it's impossible to split up a module into a package in place without breaking code, and we'd like to be able to do that. There are also people who want the ability import packages as a whole (as evidenced by the all.d idiom which some people have been using).

Description

If a package has a package.d file in it, then an import statement which imports the package will import that file.

So, if there's a foo/bar/package.d and code imports foo.bar, then the compiler will import foo.bar.package, and all aspects of importing will function as they normally do except that instead of typing foo.bar.package, foo.bar is typed by itself. And by using the file package.d, we completely avoid the risk of breaking existing code as package is a keyword and therefore is normally illegal as a module name.

If there is no package.d file in a package, then importing the package will be an error as it has been (though the error message will probably indicate something about package.d). Also, having a package and module with the same name will result in an ambiguity error when you try and import them (e.g. foo/bar/package.d and foo/bar.d).

Usage

This will allow us to do something like take std/datetime.d and split into something like

   std/datetime/common.d
   std/datetime/interval.d
   std/datetime/package.d
   std/datetime/timepoint.d
   std/datetime/timezone.d

and std/datetime/package.d could then look something like

   /++ Package documentation here +/
   module std.datetime.package;

   public import std.datetime.common;
   public import std.datetime.interval;
   public import std.datetime.timepoint;
   public import std.datetime.timezone;

Code which imports std.datetime would then be unaffected by the change, and new code could choose either to import std.datetime or to directly import the new sub-modules.

This is identical to what some projects have been doing with all.d, where they have a foo/bar/all.d which publicly imports all of the bar package, except that this provides additional syntactic sugar for it.

Another benefit is that this then gives us a way to document an entire package. By putting a ddoc comment on the module declaration at the top of the package.d file, a ddoc page for the package as a whole can be created.

Also, because package.d simply takes advantage of what the module and import system can already do (the only major new thing being that importing the package would then import its package.d file instead of getting an error), this change is incredibly straightforward and allows us to have full control over what gets imported when importing the package (e.g. only publicly importing modules which are intended to be part of its public API and not importing modules which are intended for internal use).

Copyright

This document has been placed in the Public Domain.