D Programming Language
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# Modules

    Module:
    	ModuleDeclaration DeclDefs
    	DeclDefs

    DeclDefs:
    	DeclDef
    	DeclDef DeclDefs

    DeclDef:
    	AttributeSpecifier
    	ImportDeclaration
    	EnumDeclaration
    	ClassDeclaration
    	InterfaceDeclaration
    	AggregateDeclaration
    	Declaration
    	Constructor
    	Destructor
    	UnitTest
    	StaticConstructor
    	StaticDestructor
     	SharedStaticConstructor
    	SharedStaticDestructor
    	ConditionalDeclaration
    	DebugSpecification
    	VersionSpecification
    	StaticAssert
    	TemplateDeclaration
    	TemplateMixinDeclaration
    	TemplateMixin
    	MixinDeclaration
    	;

Modules have a one-to-one correspondence with source files. The module name is the file name with the path and extension stripped off.

Modules automatically provide a namespace scope for their contents. Modules superficially resemble classes, but differ in that:

- There's only one instance of each module, and it is statically allocated.
- There is no virtual table.
- Modules do not inherit, they have no super modules, etc.
- Only one module per file.
- Module symbols can be imported.
- Modules are always compiled at global scope, and are unaffected by surrounding attributes or other modifiers.

Modules can be grouped together in hierarchies called packages.

Modules offer several guarantees:

- The order in which modules are imported does not affect the semantics.
- The semantics of a module are not affected by what imports it.
- If a module C imports modules A and B, any modifications to B will not silently change code in C that is dependent on A.

### Module Declaration

The ModuleDeclaration sets the name of the module and what package it belongs to. If absent, the module name is taken to be the same name (stripped of path and extension) of the source file name.

    ModuleDeclaration:
    	module ModuleFullyQualifiedName ;

    ModuleFullyQualifiedName:
    	ModuleName
    	Packages . ModuleName

    ModuleName:
    	Identifier

    Packages:
    	PackageName
    	Packages . PackageName

    PackageName:
    	Identifier

The Identifiers preceding the rightmost are the Packages that the module is in. The packages correspond to directory names in the source file path. Package names cannot be keywords, hence the corresponding directory names cannot be keywords, either.

If present, the ModuleDeclaration appears syntactically first in the source file, and there can be only one per source file.

Example:

    module c.stdio; // module stdio in the c package

By convention, package and module names are all lower case. This is because those names have a one-to-one correspondence with the operating system's directory and file names, and many file systems are not case sensitive. All lower case package and module names will minimize problems moving projects between dissimilar file systems.

If the file name of a module is an invalid module name (e.g. foo-bar.d), you may use a module declaration to set a valid module name:

    module foo_bar;

### Import Declaration

Symbols from one module are made available in another module by using the ImportDeclaration:

    ImportDeclaration:
    	import ImportList ;
    	static import ImportList ;

    ImportList:
    	Import
    	ImportBindings
    	Import , ImportList

    Import:
    	ModuleFullyQualifiedName
    	ModuleAliasIdentifier = ModuleFullyQualifiedName

    ImportBindings:
    	Import : ImportBindList

    ImportBindList:
    	ImportBind
    	ImportBind , ImportBindList

    ImportBind:
    	Identifier
    	Identifier = Identifier

    ModuleAliasIdentifier:
    	Identifier

There are several forms of the ImportDeclaration, from generalized to fine-grained importing.

The order in which ImportDeclarations occur has no significance.

ModuleFullyQualifiedNames in the ImportDeclaration must be fully qualified with whatever packages they are in. They are not considered to be relative to the module that imports them.

### Basic Imports

The simplest form of importing is to just list the modules being imported:

    import std.stdio; // import module stdio from package std
    import foo, bar;  // import modules foo and bar

    void main()
    {
        writefln("hello!\n");  // calls std.stdio.writefln
    }

How basic imports work is that first a name is searched for in the current namespace. If it is not found, then it is looked for in the imports. If it is found uniquely among the imports, then that is used. If it is in more than one import, an error occurs.

    module A;
    void foo();
    void bar();

    module B;
    void foo();
    void bar();

    module C;
    import A;
    void foo();
    void test()
    {
      foo(); // C.foo() is called, it is found before imports are searched
      bar(); // A.bar() is called, since imports are searched
    }

    module D;
    import A;
    import B;
    void test()
    {
      foo();   // error, A.foo() or B.foo() ?
      A.foo(); // ok, call A.foo()
      B.foo(); // ok, call B.foo()
    }

    module E;
    import A;
    import B;
    alias B.foo foo;
    void test()
    {
      foo();   // call B.foo()
      A.foo(); // call A.foo()
      B.foo(); // call B.foo()
    }

### Public Imports

By default, imports are private. This means that if module A imports module B, and module B imports module C, then C's names are not searched for. An import can be specifically declared public, when it will be treated as if any imports of the module with the ImportDeclaration also import the public imported modules.

All symbols from a publicly imported module are also aliased in the importing module. This means that if module D imports module C, and module C publicly imports module B which has the symbol bar, in module D you can access the symbol via bar, B.bar, and C.bar.

    module A;
    void foo() { }

    module B;
    void bar() { }

    module C;
    import A;
    public import B;
    ...
    foo();	// call  A.foo()
    bar();	// calls B.bar()

    module D;
    import C;
    ...
    foo();	 // error, foo() is undefined
    bar();	 // ok, calls B.bar()
    B.bar(); // ditto
    C.bar(); // ok, calls C.bar() which is an alias to B.bar()

### Static Imports

Basic imports work well for programs with relatively few modules and imports. If there are a lot of imports, name collisions can start occurring between the names in the various imported modules. One way to stop this is by using static imports. A static import requires one to use a fully qualified name to reference the module's names:

    static import std.stdio;

    void main()
    {
      writefln("hello!");           // error, writefln is undefined
      std.stdio.writefln("hello!"); // ok, writefln is fully qualified
    }

### Renamed Imports

A local name for an import can be given, through which all references to the module's symbols must be qualified with:

    import io = std.stdio;

    void main()
    {
      io.writefln("hello!");        // ok, calls std.stdio.writefln
      std.stdio.writefln("hello!"); // error, std is undefined
      writefln("hello!");           // error, writefln is undefined
    }

Renamed imports are handy when dealing with very long import names.

### Selective Imports

Specific symbols can be exclusively imported from a module and bound into the current namespace:

    import std.stdio : writefln, foo = writef;

    void main()
    {
      std.stdio.writefln("hello!"); // error, std is undefined
      writefln("hello!");           // ok, writefln bound into current namespace
      writef("world");              // error, writef is undefined
      foo("world");                 // ok, calls std.stdio.writef()
      fwritefln(stdout, "abc");     // error, fwritefln undefined
    }

static cannot be used with selective imports.

### Renamed and Selective Imports

When renaming and selective importing are combined:

    import io = std.stdio : foo = writefln;

    void main()
    {
      writefln("bar");           // error, writefln is undefined
      std.stdio.foo("bar");      // error, foo is bound into current namespace
      std.stdio.writefln("bar"); // error, std is undefined
      foo("bar");                // ok, foo is bound into current namespace,
                                 // FQN not required
      io.writefln("bar");        // ok, io=std.stdio bound the name io in
                                 // the current namespace to refer to the entire module
      io.foo("bar");             // error, foo is bound into current namespace,
                                 // foo is not a member of io

### Scoped Imports

Import declarations may be used at any scope. For example:

    void main() {
      import std.stdio;
      writeln("bar");
    }

The imports are looked up to satisfy any unresolved symbols at that scope. Imported symbols may hide symbols from outer scopes.

In function scopes, imported symbols only become visible after the import declaration lexically appears in the function body. In other words, imported symbols at function scope cannot be forward referenced.

    void main() {
      void writeln(string) {}
      void foo() {
        writeln("bar"); // calls main.writeln
        import std.stdio;
        writeln("bar"); // calls std.stdio.writeln
        void writeln(string) {}
        writeln("bar"); // calls main.foo.writeln
      }
      writeln("bar"); // calls main.writeln
      std.stdio.writeln("bar");  // error, std is undefined
    }

### Module Scope Operator

Sometimes, it's necessary to override the usual lexical scoping rules to access a name hidden by a local name. This is done with the global scope operator, which is a leading ‘.’:

    int x;

    int foo(int x)
    {
      if (y)
        return x;  // returns foo.x, not global x
      else
        return .x; // returns global x
    }

The leading ‘.’ means look up the name at the module scope level.

### Static Construction and Destruction

Static constructors are code that gets executed to initialize a module or a class before the main() function gets called. Static destructors are code that gets executed after the main() function returns, and are normally used for releasing system resources.

There can be multiple static constructors and static destructors within one module. The static constructors are run in lexical order, the static destructors are run in reverse lexical order.

Static constructors and static destructors run on thread local data, and are run whenever threads are created or destroyed.

Shared static constructors and shared static destructors are run on global shared data, and constructors are run once on program startup and destructors are run once on program termination.

### Order of Static Construction

Shared static constructors on all modules are run before any static constructors.

The order of static initialization is implicitly determined by the import declarations in each module. Each module is assumed to depend on any imported modules being statically constructed first. Other than following that rule, there is no imposed order on executing the module static constructors.

Cycles (circular dependencies) in the import declarations are allowed as long as not both of the modules contain static constructors or static destructors. Violation of this rule will result in a runtime exception.

### Order of Static Construction within a Module

Within a module, the static construction occurs in the lexical order in which they appear.

### Order of Static Destruction

It is defined to be exactly the reverse order that static construction was performed in. Static destructors for individual modules will only be run if the corresponding static constructor successfully completed.

Shared static destructors are executed after static destructors.

### Order of Unit tests

Unit tests are run in the lexical order in which they appear within a module.

### Mixin Declaration

    MixinDeclaration:
        mixin ( AssignExpression ) ;

The AssignExpression must evaluate at compile time to a constant string. The text contents of the string must be compilable as a valid DeclDefs, and is compiled as such.

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