---
title: Type name alias with use syntax
tags: RBS
---

# Type name alias with `use` syntax

The `use` syntax allows using type names with shorter alias names. Instead of specifying type names with qualified names, the `use` syntax defines aliases of existing type names.

It works with per-file basis. The `use` directives written in a file works only for the file.

The directive works only for RBS. There is no corresponding Ruby code.

## `use` with single type name clause

The simplest form of the directive is for single type name, without new name.

use RBS::Namespace

With the directive, the relative type name/namespace `Namespace` is resolved to `::RBS::Namespace`.

## `use` with single type name with new name clause

One *single type name* use clause can have optional `as` keyword to give a new name to the type.

use RBS::Resolver::context as resolver_context

The `resolver_context` is resolved to `::RBS::Resolver::context`.

## `use` with wildcard clause

The *wildcard* use clause allows defining same-name aliases of all types under the namespace.

use RBS::AST::*

The `RBS::AST::` namespace contains modules including `::RBS::AST::Declarations` and `::RBS::AST::Members`, and it all of the names can be accessed with the base name, as `Declarations` and `Members`.

## `use` with multiple clauses

use RBS::Namespace, RBS::TypeName as TN, RBS::AST::*

## Syntax

The `use` directive has a syntax rule as follows:

use-directive ::= `use` *use-clause* `,` ... `,` *use-clause*

use-clause ::= *type-name*
             | *type-name* `as` *simple-type-name*
             | *namespace* `::` `*`

The `*type-name*` and `*namespace*` can be either relative or absolute. Relative type names and namespaces are simply converted to absolute type name, just by adding `::` at the head of them.

## Discussion

### Using alias names to define new types

The `use` name resolution works with right-hand-side types. It cannot be used to define new types.

use RBS::TypeName

# Defines a class, ::TypeName, not ::RBS::TypeName
class TypeName
end

### Alias names resolution

The alias names works for relative type names. Fully qualified type names are not subject for `use` name resolution.

use RBS::TypeName

Foo: TypeName     # ::RBS::TypeName
Bar: ::TypeName   # ::TypeName

Note that the `use` name resolution has higher precedence than usual type name resolution.

use RBS::TypeName

class TypeName      # Defines ::TypeName
  Foo: TypeName     # ::RBS::TypeName, not ::TypeName
end

When alias names conflict, the names from later clauses wins.

use RBS::TypeName as Declarations
use RBS::AST::Declarations

Both the `use` directives at line 1 and 2 defines alias of `Declarations`, and the `::RBS::AST::Declarations` wins.