hack — consolidated language guide — DedSec Project

Hack

What is different from PHP

Hack is a typed language in the HHVM ecosystem. A file starting with <?hh is not a PHP CLI program; executing it with php does not validate Hack semantics. The type checker can reject incompatible parameter types, while business constraints such as positivity still require explicit validation.

Small executable lab for a configured HHVM project

Save as lesson.hack. The <<__EntryPoint>> attribute marks the entry function. Check your installed HHVM release and project configuration before choosing a launch command.

<?hh
function double_positive(int $n): int {
  if ($n < 0) {
    throw new \InvalidArgumentException('negative');
  }
  return $n * 2;
}
<<__EntryPoint>>
function main(): void {
  echo double_positive(4), "\n";
  try {
    double_positive(-1);
  } catch (\InvalidArgumentException $_) {
    echo "rejected\n";
  }
}

Expected output, not executed here: 8 followed by rejected. The int type alone does not guarantee non-negative values; the explicit check establishes that contract.

Diagnosis

• PHP parse error: the wrong runtime is being used. Confirm that HHVM executes the Hack file.
• Type error for a string: an arbitrary string cannot satisfy an int argument. Parse and validate at the input boundary instead of hiding the mismatch with a cast.
• Unhandled InvalidArgumentException: decide whether callers should catch, translate or propagate the error. Avoid broad catches that conceal unrelated failures.
• Entry point or project failure: check .hhconfig, installed HHVM version and launch command. Available tooling varies by release.
• Dependency failure: Hack/HHVM is not automatically compatible with every current PHP package.

Mastery pathway

1. With actual Hack tooling, record HHVM and type-checker versions, run the example and save both output lines.
2. Change the first call to double_positive("4"), predict the type error, then restore the integer.
3. Add tests for zero, one and a negative number. Explain why positivity is a runtime contract rather than a property of int.
4. Compare exceptions with an explicit result representation for a caller-facing API, and justify how the caller recovers.

Verification: static review only in this environment; HHVM is not installed. Do not report runtime success without executing the code. Reference: https://docs.hhvm.com/hack/

Hack — Expanded practice workbook

Read the baseline source

<?hh
<<__EntryPoint>>
function main(): void {
  echo "Hello, Ada!\n";
}

Relevant environment: Run with HHVM in a configured Hack project.. Verify that the matching compiler, host or simulator is available before treating output as authoritative. For scripts that can interact with personal files or systems, use a scratch workspace and read-only fixtures by default.

Detailed working objective

Use Hack's typed collections and a shape for validated JSON-like input, then compare nullable types and runtime enforcement in HHVM.

The failure that must be understood

Hack syntax and HHVM runtime are not interchangeable with PHP 8; check typed mode and installed HHVM support.