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.