Having trouble with returning fixed-size arrays

Question

So I'm having two problems with returning arrays from external functions:

Case One: arrays in getters

contract A {
  mapping(address => uint8[10]) public gameBoards;
}

// test.sol
function testGetGameBoard() {
  address myAddress;
  uint8[10] memory gameBoard = a.boards(address);
  Assert.equal(gameBoard[0], 0, 'uint8 arrays are initialized with 0');
  // TypeError: Member "equal" not unique after argument-dependent lookup in type(library Assert)

From this answer it looks like the compiler can't infer gameBoard[0] is a uint8? I'm not sure what's going on.

Case Two: arrays in structs

contract A {
  mapping(address => Game) public games;
  struct Game {
    address player;
    uint8[10] board;
  }
}

// test.sol
function testGetGame() {
  address myAddress;
  var(player, board) = a.games(myAddress);
  // TypeError: Not enough components (1) in value to assign all variables (2).

From what I understand, return values need to be known in size, which prevents you from returning dynamic arrays and strings. But a fixed array uint8[10] should be known in size, right? It's not clear to me why I'm getting both of these errors. Any explanation as to what's going on would be much appreciated.

Answer 1

Yes, the size of uint8[10] is known. The problem is, there are two types of uint8[10]:

uint8[10] storage var1;
uint8[10] memory var2;

If you create a uint8[10] memory, the memory required for the array is dynamically allocated when the contract code starts running. It will be destroyed when the contract execution finishes.

A uint8[10] storage is more like a pointer to a uint8[10] which is stored in the blockchain as a contract member variable. A uint8[10] storage is not an array itself.

It is not possible to directly assign a uint8[10] memory to a uint8[10] storage pointer.

You should probably read the Solidity manual on this:

http://solidity.readthedocs.io/en/develop/types.html#data-location

I hope this helps.