Type Definition clingo_sys::clingo_ground_callback_t

type clingo_ground_callback_t = Option<unsafe extern "C" fn(location: *const clingo_location_t, name: *const c_char, arguments: *const clingo_symbol_t, arguments_size: usize, data: *mut c_void, symbol_callback: clingo_symbol_callback_t, symbol_callback_data: *mut c_void) -> bool>;

Callback function to implement external functions.

If an external function of form @name(parameters) occurs in a logic program, then this function is called with its location, name, parameters, and a callback to inject symbols as arguments. The callback can be called multiple times; all symbols passed are injected.

If a (non-recoverable) clingo API function fails in this callback, for example, the symbol callback, the callback must return false. In case of errors not related to clingo, this function can set error ::clingo_error_unknown and return false to stop grounding with an error.

Parameters:

• location - location from which the external function was called
• name - name of the called external function
• arguments - arguments of the called external function
• arguments_size - number of arguments
• data - user data of the callback
• symbol_callback - function to inject symbols
• symbol_callback_data - user data for the symbol callback (must be passed untouched)

Returns whether the call was successful @see clingo_control_ground()

The following example implements the external function @f() returning 42.

bool
ground_callback(clingo_location_t const *location,
                char const *name,
                clingo_symbol_t const *arguments,
                size_t arguments_size,
                void *data,
                clingo_symbol_callback_t symbol_callback,
                void *symbol_callback_data) {
  if (strcmp(name, "f") == 0 && arguments_size == 0) {
    clingo_symbol_t sym;
    clingo_symbol_create_number(42, &sym);
    return symbol_callback(&sym, 1, symbol_callback_data);
  }
  clingo_set_error(clingo_error_runtime, "function not found");
  return false;
}