Higher Order Functions

One may define a higher order function whose context depends on the dynamic context of one or more passed in function arguments.

function has_dependent_fn_arg(
  (function()[_]: void) $f,
)[ctx $f]: void {
  /* some code */
  $f();
  /* more code */
}

The semantics of the argument-dependent-context are such that the higher order function's type is specialized per invocation.

final class Box {
  public static ?Box $global = null;
  public function __construct(public mixed $value)[] {}
}

function callee((function(Box)[_]: void) $f, Box $b)[globals, ctx $f]: void {
  /* some code */
  $f($b);
  Box::$global = $b;
  /* more code */
}

function caller()[globals]: void {
  callee(
    ($_) ==> {}, // implicitly [globals]
    new Box(null),
  ); // invoke requires context [globals]

  callee(
    ($box)[write_props] ==> {
      $box->value = 0;
    },
    new Box(null),
  ); // ERROR because invoke requires context [globals, write_props]
}

Note that this suggests and requires that all other code within callee requires only the globals context's capabilities, as the actual capabilities provided via the passed argument cannot be depended upon to be any specific capabilities.