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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

<<__EntryPoint>>
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​

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
<?hh

<<__EntryPoint>>
function main(): void {
  print("Hello, World!\n");
  exit(0);
}