Source Code Fundamentals: Program Structure

A Hack program consists of one or more source files; these source files should have the extension .hack, and contain one or more declarations of the following kind, in any order:

  • alias declaration
  • class
  • enumerated type
  • function
  • inclusion directive
  • interface
  • namespace
  • trait
  • use

Consider the following file:

#!/usr/bin/env hhvm

function main(): void {
  print("Hello, World!\n");

In this example, the start-up function is called main; however, that was an arbitrary choice of name; it could just as easily have been run, do_it, or make_magic. (What makes main the start-up function is the presence of the attribute __EntryPoint.)

Including Other Files

A file can import another file via inclusion.

Legacy Files

Meta Engineer?

All Hack source code at Meta should use the .php file extension and Hack strict mode (plain <?hh header line).

The .php extension is still in use in older Hack code, though not recommended for new code. Hack code using this extension must start with an optional shebang line (e.g. #!/usr/bin/env hhvm), followed by a header line. The header line can be one of the following:

  • <?hh // strict: this is currently the recommended header, and makes Hack work in the documented ways
  • <?hh // partial: this loosens several restrictions to ease migration from other languages. It is strongly discouraged for new code.
  • <?hh: depending on the version of HHVM/Hack, this may be equivalent to strict, partial, or throw an error. It is strongly discouraged for both new and existing code.

The following file is equivalent to the earlier example:

#!/usr/bin/env hhvm

function main(): void {
  print("Hello, World!\n");
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