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Propose the decision from #542, noting implementation from #563 Also integrates some of #339 into `variables.md` because that's actually how this started, looking for a proposal reference for #542 Co-authored-by: Richard Smith <richard@metafoo.co.uk> Co-authored-by: josh11b <josh11b@users.noreply.github.com>
92 lines
3.2 KiB
Markdown
92 lines
3.2 KiB
Markdown
# Name lookup
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<!--
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Part of the Carbon Language project, under the Apache License v2.0 with LLVM
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Exceptions. See /LICENSE for license information.
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SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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-->
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<!-- toc -->
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## Table of contents
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- [TODO](#todo)
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- [Overview](#overview)
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- [Unqualified name lookup](#unqualified-name-lookup)
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- [Alternatives](#alternatives)
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- [Name lookup for common, standard types](#name-lookup-for-common-standard-types)
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- [Open questions](#open-questions)
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- [Shadowing](#shadowing)
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<!-- tocstop -->
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## TODO
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This is a skeletal design, added to support [the overview](README.md). It should
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not be treated as accepted by the core team; rather, it is a placeholder until
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we have more time to examine this detail. Please feel welcome to rewrite and
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update as appropriate.
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## Overview
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Names are always introduced into some scope which defines where they can be
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referenced. Many of these scopes are themselves named. Carbon has a special
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facility for introducing a dedicated named scope just like C++, but we traverse
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nested names in a uniform way with `.`-separated names:
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```
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namespace Foo {
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namespace Bar {
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alias ??? MyInt = Int;
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}
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}
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fn F(x: Foo.Bar.MyInt);
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```
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Carbon packages are also namespaces so to get to an imported name from the
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`Abseil` package you would write `Abseil.Foo`. The "top-level" file scope is
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that of the Carbon package containing the file, meaning that there is no
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"global" scope. Dedicated namespaces can be reopened within a package, but there
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is no way to reopen a package without being a library and file _within_ that
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package.
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Note that libraries (unlike packages) do **not** introduce a scope, they share
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the scope of their package. This is based on the observation that in practice, a
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fairly coarse scoping tends to work best, with some degree of global registry to
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establish a unique package name.
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### Unqualified name lookup
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Unqualified name lookup in Carbon will always find a file-local result, other
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than the implicit "prelude" of importing and aliasing the fundamentals of the
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standard library. There will be an explicit mention of the name in the file that
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declares the name in the current or enclosing scope, which must also precede the
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reference.
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#### Alternatives
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This implies that other names within your own package but not declared within
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the file must be found by way of the package name. It isn't clear if this is the
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desirable end state. We need to consider alternatives where names from the same
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library or any library in the same package are made immediately visible within
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the package scope for unqualified name lookup.
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### Name lookup for common, standard types
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The Carbon standard library is in the `Carbon` package. A very small subset of
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this standard library is provided implicitly in every file's scope. This is
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called the "prelude" package.
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Names in the prelude package will be available without scoping names. For
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example, `Bool` will be the commonly used name in code, even though the
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underlying type may be `Carbon::Bool`. Also, no `import` will be necessary to
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use `Bool`.
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## Open questions
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### Shadowing
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We can probably disallow the use of shadowed unqualified names, but the actual
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design for such needs to be thought through.
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