Modules¶

Chapel supports modules to manage namespaces. A program consists of one or more modules. Every symbol, including variables, functions, and types, is associated with some module.

Module definitions are described in Module Definitions. The relation between files and modules is described in Files and Implicit Modules. Nested modules are described in Nested Modules. The visibility of a module’s symbols by users of the module is described in Visibility Of A Module’s Symbols. The execution of a program and module initialization/deinitialization are described in Program Execution.

Module Definitions¶

A module is declared with the following syntax:

module-declaration-statement:
  privacy-specifier[OPT] prototype-specifier[OPT] `module' module-identifier block-statement

privacy-specifier:
  `private'
  `public'

prototype-specifier:
  `prototype'

module-identifier:
  identifier

A module’s name is specified after the module keyword. The block-statement opens the module’s scope. Symbols defined in this block statement are defined in the module’s scope and are called module-scope symbols. The visibility of a module is defined by its privacy-specifier (Visibility Of A Module).

Module declaration statements are only legal as file-scope or module-scope statements. For example, module declaration statements may not occur within block statements, functions, classes, or records.

Any module declaration that is not contained within another module creates a top-level module. Module declarations within other modules create nested modules (Nested Modules).

Prototype Modules¶

Modules that are declared with the prototype keyword use relaxed rules for error handling. These relaxed rules are appropriate for programs in the early stages of development but are not appropriate for libraries. In particular, within a prototype module errors that are not handled will terminate the program (see Prototype Mode).

Implicit modules (Files and Implicit Modules) are implicitly considered prototype modules as well.

Files and Implicit Modules¶

Multiple modules can be defined within the same file and need not bear any relation to the file in terms of their names.

Example (two-modules.chpl).

The following file contains two explicitly named modules, MX and MY.
module MX {
  var x: string = "Module MX";
  proc printX() {
    writeln(x);
  }
}

module MY {
  var y: string = "Module MY";
  proc printY() {
    writeln(y);
  }
}
Module MX defines module-scope symbols x and printX, while MY defines module-scope symbols y and printY.

For any file that contains file-scope statements other than module declarations, the file itself is treated as a module declaration. In this case, the module is implicit. Implicit modules are always prototype modules (Prototype Modules). An implicit module takes its name from the base filename. In particular, the module name is defined as the remaining string after removing the .chpl suffix and any path specification from the specified filename. If the resulting name is not a legal Chapel identifier, it cannot be referenced in a use statement.

Example (implicit.chpl).

The following file, named implicit.chpl, defines an implicitly named module called implicit.
var x: int = 0;
var y: int = 1;

proc printX() {
  writeln(x);
}
proc printY() {
  writeln(y);
}
Module implicit defines the module-scope symbols x, y, printX, and printY.

Nested Modules¶

A nested module (or sub-module) is a module that is defined within another module, known as the outer, or parent, module. An outer module can refer to the names of its sub-modules directly without a use or import statement. However, a sub-module must use or import its parent module in order to refer to its name or symbols.

An inner module’s symbols can be referenced without accessing those of its parent module by naming the inner module in a qualified manner within the use statement.

Example (nested-use.chpl).

The code
  use libsci.blas;

.. BLOCK-test-chapelpost

  } }
uses a module named blas that is nested within a module named libsci.

Files with both module declarations and file-scope statements result in nested modules.

Example (nested.chpl).

The following file, named nested.chpl, defines an implicitly named module called nested, with nested modules MX and MY.
module MX {
  var x: int = 0;
}

module MY {
  var y: int = 0;
}

use MX, MY;

proc printX() {
  writeln(x);
}

proc printY() {
  writeln(y);
}

Access of Module Contents¶

A module’s contents can be accessed by code outside of that module depending on the visibility of the module itself (Visibility Of A Module) and the visibility of each individual symbol (Visibility Of A Module’s Symbols). This can be done via the use statement (Using Modules) or qualified naming (Qualified Naming of Module Symbols).

Visibility Of A Module¶

A top-level module is available for use (Using Modules) anywhere. The visibility of a nested module is subject to the rules of Visibility Of A Module’s Symbols, where the nested module is considered a “module-scope symbol” of its outer module.

Visibility Of A Module’s Symbols¶

A symbol defined at module scope is visible from outside the module when the privacy-specifier of its definition is public or is omitted (i.e. by default). When a module-scope symbol is declared private, it is not visible outside of that module. A symbol’s visibility inside its module is controlled by normal lexical scoping and is not affected by its privacy-specifier. When a module’s symbol is visible (Visibility Of A Module), the visible symbols it contains are accessible via the use statement (Using Modules) or qualified naming (Qualified Naming of Module Symbols).

Using Modules¶

The use statement provides the primary means of accessing a module’s symbols from outside of the module. Use statements make both the module’s name and its public symbols available for reference within a given scope. For top-level modules, a use statement is required before referring to the module’s name or the symbols it contains within a given lexical scope.

Use statements can also restrict or rename the set of module symbols that are available within the scope. For further information about use statements, see The Use Statement.

Qualified Naming of Module Symbols¶

When a module’s symbol is visible—via a use statement, or lexically for nested modules—its public symbols can be referred to via qualified naming with the following syntax:

module-access-expression:
  module-identifier-list . identifier

module-identifier-list:
  module-identifier
  module-identifier . module-identifier-list

This allows two symbols that have the same name to be distinguished based on the name of their module. Using qualified naming in a function call restricts the set of candidate functions to those in the specified module.

If code refers to symbols that are defined by multiple modules, the compiler will issue an error. Qualified naming can be used to disambiguate the symbols in this case.

Example (ambiguity.chpl).

In the following example,

module M1 {
  var x: int = 1;
  var y: int = -1;
  proc printX() {
    writeln("M1's x is: ", x);
  }
  proc printY() {
    writeln("M1's y is: ", y);
  }
}

module M2 {
  use M3;
  use M1;

  var x: int = 2;

  proc printX() {
    writeln("M2's x is: ", x);
  }

  proc main() {
    M1.x = 4;
    M1.printX();
    writeln(x);
    printX(); // This is not ambiguous
    printY(); // ERROR: This is ambiguous
  }
}

module M3 {
  var x: int = 3;
  var y: int = -3;
  proc printY() {
    writeln("M3's y is: ", y);
  }
}

The call to printX() is not ambiguous because M2’s definition shadows that of M1. On the other hand, the call to printY() is ambiguous because it is defined in both M1 and M3. This will result in a compiler error. The call could be qualified via M1.printY() or M3.printY() to resolve this ambiguity.

Module Initialization¶

Module initialization occurs at program start-up. All module-scope statements within a module other than function and type declarations are executed during module initialization. Modules that are not referred to, including both top-level modules and sub-modules, will not be initialized.

Example (init.chpl).

In the code,
var x = foo();       // executed at module initialization
writeln("Hi!");      // executed at module initialization
proc sayGoodbye {
  writeln("Bye!");   // not executed at module initialization
}
The function foo() will be invoked and its result assigned to x. Then “Hi!” will be printed.

Module initialization order is discussed in Module Initialization Order.

Module Deinitialization¶

Module deinitialization occurs at program tear-down. During module deinitialization:

If the module contains a deinitializer, which is a module-scope function named deinit(), it is executed first.
If the module declares module-scope variables, they are deinitialized in the reverse order of their declaration.

Module deinitialization order is discussed in Module Deinitialization Order.

Program Execution¶

Chapel programs start by initializing all modules and then executing the main function (The main Function).

The main Function¶

The main function must be called main and must have zero arguments. It can be specified with or without parentheses. In any Chapel program, there is a single main function that defines the program’s entry point. If a program defines multiple potential entry points, the implementation may provide a compiler flag that disambiguates between main functions in multiple modules.

Implementation Notes.

In the current Chapel compiler implementation, the – –main-module flag can be used to specify the module from which the main function definition will be used.

Example (main-module.chpl).

Because it defines two main functions, the following code will yield an error unless a main module is specified on the command line.
module M1 {
  const x = 1;
  proc main() {
    writeln("M", x, "'s main");
  }
}

module M2 {
  use M1;

  const x = 2;
  proc main() {
    M1.main();
    writeln("M", x, "'s main");
  }
}
If M1 is specified as the main module, the program will output:
M1's main
If M2 is specified as the main module the program will output:
M1's main
M2's main
Notice that main is treated like just another function if it is not in the main module and can be called as such.

To aid in exploratory programming, a default main function is created if the program does not contain a user-defined main function. The default main function is equivalent to

proc main() {}

Example (no-main.chpl).

The code
writeln("hello, world");
is a legal and complete Chapel program. The startup code for a Chapel program first calls the module initialization code for the main module and then calls main(). This program’s initialization function is the file-scope writeln() statement. The module declaration is taken to be the entire file, as described in Files and Implicit Modules.

Module Initialization Order¶

Module initialization is performed using the following algorithm.

Starting from the module that defines the main function, the modules named in its use statements are visited depth-first and initialized in post-order. If a use statement names a module that has already been visited, it is not visited a second time. Thus, infinite recursion is avoided.

Modules used by a given module are visited in the order in which they appear in the program text. For nested modules, the parent module and its uses are initialized before the nested module and its uses.

Example (init-order.chpl).

The code
module M1 {
  use M2.M3;
  use M2;
  writeln("In M1's initializer");
  proc main() {
    writeln("In main");
  }
}

module M2 {
  use M4;
  writeln("In M2's initializer");
  module M3 {
    writeln("In M3's initializer");
  }
}

module M4 {
  writeln("In M4's initializer");
}
prints the following
In M4's initializer
In M2's initializer
In M3's initializer
In M1's initializer
In main
M1, the main module, uses M2.M3 and then M2, thus M2.M3 must be initialized. Because M2.M3 is a nested module, M4 (which is used by M2) must be initialized first. M2 itself is initialized, followed by M2.M3. Finally M1 is initialized, and the main function is run.

Module Deinitialization Order¶

Module deinitialization is performed in the reverse order of module initialization, as specified in Module Initialization Order.