.. default-domain:: chpl

.. _primers-hello2-module:

"Production-grade" hello world
==============================

`View hello2-module.chpl on GitHub <https://github.com/chapel-lang/chapel/blob/main/test/release/examples/hello2-module.chpl>`_

This program is conceptually very similar to :ref:`hello.chpl
<primers-hello>`, but it uses a more structured programming style,
explicitly defining a module, a configuration constant, and a
main() procedure.



The following statement declares a module named 'Hello'.  If a
source file contains no module declarations, the filename minus its
``.chpl`` extension serves as the module name for the code it
contains.  Thus, 'hello' would be the automatic module name for the
previous :ref:`hello.chpl <primers-hello>` example.

.. code-block:: chapel

    module Hello {

This next statement declares a `configuration constant` named
`message`.  The type is inferred to be a string since the
initializing expression is a string literal.  Users may override
the default values of configuration constants and variables on the
executable's command-line.  For example, we could change the
default message for a given run using the command line: ``./hello
--message="hiya!"``.

.. code-block:: chapel

      config const message = "Hello, world!";

Any top-level code in a module is executed as part of the module's
initialization when the program begins executing.  Thus, in the
previous one-line :ref:`hello.chpl <primers-hello>`, the presence
of a `writeln()` at the file scope formed the implicit `hello`
module's initialization and would be executed at program startup.
Since there was no explicit `main()` function or any other
top-level code, that's all that the program would do.



In this program, we define an entry point for the program by
defining a procedure named `main()`.  This will be invoked after
this module and all the modules it uses are initialized.

.. code-block:: chapel

      proc main() {
        writeln(message);
      }
    }