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Name lookup

Table of contents

TODO

This is a skeletal design, added to support the overview. It should not be treated as accepted by the core team; rather, it is a placeholder until we have more time to examine this detail. Please feel welcome to rewrite and update as appropriate.

Overview

Names are always introduced into some scope which defines where they can be referenced. Many of these scopes are themselves named. Carbon has a special facility for introducing a dedicated named scope just like C++, but we traverse nested names in a uniform way with .-separated names:

namespace Foo {
  namespace Bar {
    alias ??? MyInt = Int;
  }
}

fn F(x: Foo.Bar.MyInt);

Carbon packages are also namespaces so to get to an imported name from the Abseil package you would write Abseil.Foo. The "top-level" file scope is that of the Carbon package containing the file, meaning that there is no "global" scope. Dedicated namespaces can be reopened within a package, but there is no way to reopen a package without being a library and file within that package.

Note that libraries (unlike packages) do not introduce a scope, they share the scope of their package. This is based on the observation that in practice, a fairly coarse scoping tends to work best, with some degree of global registry to establish a unique package name.

Unqualified name lookup

Unqualified name lookup in Carbon will always find a file-local result, other than the implicit "prelude" of importing and aliasing the fundamentals of the standard library. There will be an explicit mention of the name in the file that declares the name in the current or enclosing scope, which must also precede the reference.

Alternatives

This implies that other names within your own package but not declared within the file must be found by way of the package name. It isn't clear if this is the desirable end state. We need to consider alternatives where names from the same library or any library in the same package are made immediately visible within the package scope for unqualified name lookup.

Name lookup for common, standard types

The Carbon standard library is in the Carbon package. A very small subset of this standard library is provided implicitly in every file's scope. This is called the "prelude".

Names in the prelude will be available without a package qualifier. For example, the name Type can be directly used in code without a Carbon. qualifier, even though it belongs to the Carbon package, and no import is necessary to use the name Type.

Open questions

Shadowing

We can probably disallow the use of shadowed unqualified names, but the actual design for such needs to be thought through.