enso/docs/syntax/imports.md

layout	title	category	tags	order
developer-doc	Imports and Exports	syntax	syntax imports modules	4

Imports and Exports

In order to properly modularise and work with Enso code, the language provides a robust mechanism for importing code from modules, and also re-exporting that code from modules.

• Qualified Names
• Import Syntax
  • Qualified Imports
  • Unqualified Imports
• Imports with multiple targets
• Export Syntax
  • Qualified Exports
  • Unqualified Exports
  • Visibility of Export Bindings
  • Implicit exports
    • Defined entities in a module
    • Synthetic module
• Exports with multiple targets

Qualified Names

In the following text, entity shall denote a module, a method (instance, static, extension, conversion or foreign), type, or a type constructor.

Both imports and exports require the use of qualified entity names. A qualified name consists of the library namespace (usually organization under which its published) and the library name, followed by module names mirroring the source tree of the library, followed by an entity name within that module. For example the file src/Stuff/Things/Util.enso inside the library My_Lib published by the user wdanilo would have the following qualified name: wdanilo.My_Lib.Stuff.Things.Util and the type My_Type within that module would have the qualified name wdanilo.My_Lib.Stuff.Things.Util.My_Type. To facilitate library renaming (or deciding on the publishing organization later in the development cycle, or working on a project that won't be published) it is possible to use the keyword project instead of namespace and project name, to import a file in the same project. Therefore, the file src/Varia/Tools/Manager.enso in My_Lib published (or not) by wdanilo may use project.Stuff.Things.Util to refer to the previously mentioned file.

Currently, the project keyword works only in import and export statements. Note that it is possible to export a symbol from the current project with the project keyword without first importing it.

Import Syntax

There are two main ways of importing a module into the current scope.

Qualified Imports

These imports consist of the word import followed by a qualified name of an entity. This can be optionally followed by the as word, and a referent name of the entity as it should be visible in the importing scope.

The only name brought into scope by such an import is the name of the entity (or the name provided after the as keyword, if provided).

Unqualified Imports

Unqualified imports are broken up into three main categories:

1. Unrestricted Imports: These import all symbols (entities) from the module (or from a type) into the current scope. They consist of the keyword from, followed by a qualified module name, followed by an optional rename part (using the as keyword), then the keywords import all. For example:

   from Standard.Base.Data.List as Builtin_List import all
   

2. Restricted Imports: These import a specified set of names for use as automatically resolved referent names. They consist of the keyword from, followed by a qualified module name (with optional as-rename), then the word import followed by a coma-separated list of referent names to be imported. For example:

   from Standard.Base.Data.List import Cons, Nil
   

3. Hiding Imports: These are the inverse of restricted imports, and import all symbosl other than the named ones. They consist of the from keyword, followed by a qualified module name (with optional as-rename), then the words import all hiding, followed by a coma-separated list of referent names to be excluded from the import. For example:

   from Standard.Base.Data.List import all hiding Cons, Nil
   

Imports in Enso may introduce ambiguous symbols, which is treated as a compilation error. Ideally, the error should be delayed until one of the ambiguous symbols is used in Enso code.

Imports with multiple targets

Import of one symbol can resolve to multiple targets in case of extension or conversion methods. For example, the following import in Main.enso: A_Module.enso:

type My_Type
type Other_Type
My_Type.method = 42
Other_Type.method = 42

Main.enso:

import project.A_Module.method

imports both My_Type.method and Other_Type.method methods.

Note that import project.A_Module.My_Type.method would lead to a compilation error, as it is only possible to import constructors from a type, not methods.

Export Syntax

In order to allow for easy composition and aggregation of code, Enso provides its users with a mechanism to export imported elements from modules. They appear in Enso as follows:

Qualified Exports

These exports consist of the word export followed by a qualified name of an entity. This can be optionally followed by the as word, and a referent name of the entity as it should be visible in the exporting scope.

The only name brought into scope by such an export is the name of the entity (or the name provided after the as keyword, if provided).

Unqualified Exports

Unlike imports, exports cannot be used with the all and hiding keywords. So the only supported syntax is to export a list of names with restricted exports.

Restricted Exports: These export a specified set of names, behaving as though they were redefined in the current scope. They consist of the keyword from, followed by a qualified module or type name (with optional as-rename), then the word export followed by a coma-separated list of names to be exported. For example:

from Standard.Base.Data.List export Cons, Nil, from_vector

In essence, an export allows the user to "paste" the contents of the module or type being exported into the module declaring the export. This means that exports that create name clashes must be resolved at the export site.

Visibility of Export Bindings

Bindings exported from a module X are available in an identical fashion to bindings that are defined in the module X.

Implicit exports

The compiler inserts implicit exports for entities defined in a module and for submodules of a synthetic module. A synthetic module is basically a directory in the source structure.

Defined entities in a module

Entities defined in a module are automatically exported from the module. This means that the following modules are semantically identical:

type My_Type
method x = x

export project.Module.My_Type
export project.Module.method
type My_Type
method x = x

Synthetic module

Consider a project named Proj with the following source structure:

Proj/src/Synthetic_Mod/Module.enso:

type My_Type

Proj/src/Main.enso:

import project.Synthetic_Mod.Module.My_Type

We can import submodules of Synthetic_Mod, because the compiler automatically inserts exports for them. Internally, Synthetic_Mod is represented as a module with single export:

export project.Synthetic_Mod.Module

Exports with multiple targets

Export of a single symbol can be resolved to multiple targets (entities) in case of extension or conversion methods. Similarly to imports with multiple targets, the following export in A_Module.enso:

export project.A_Module.export

exports both My_Type.method and Other_Type.method methods.