How to read token balance of another token

Question

I'm trying to write a contract which reads the token balance of an address. The token contract is already deployed. What would I need to do to read data from another contract?

Assume I'm trying to read the GNT balance of an address, this is how I'm trying to reference the Golem contract:

 GolemNetworkToken temp = GolemNetworkToken(0xa74476443119A942dE498590Fe1f2454d7D4aC0d);

However this doesn't seem to be valid Solidity syntax.

EDIT: with suggestions:

 pragma solidity ^0.4.16;


contract ERC20Basic {
  uint256 public totalSupply;
  function balanceOf(address who) constant returns (uint256);
  function transfer(address to, uint256 value) returns (bool);
  event Transfer(address indexed from, address indexed to, uint256 value);
}

contract ERC20 is ERC20Basic {
  function allowance(address owner, address spender) constant returns (uint256);
  function transferFrom(address from, address to, uint256 value) returns (bool);
  function approve(address spender, uint256 value) returns (bool);
  event Approval(address indexed owner, address indexed spender, uint256 value);
}

contract test{

  ERC20 myToken = ERC20(0xa74476443119A942dE498590Fe1f2454d7D4aC0d);

  function getTokenBalanceOf(address a) constant returns (uint balance) {
    return myToken.balanceOf(a);
  }
}

Answer 1

You need to reference the token and then the syntax that you used should work. In fact, that is what we have the ERC20 standard for: You dont need to include the specific token but just the token interface.

import "./ERC20.sol";

contract myTest {

  ERC20 myToken;

  function setToken(address tokenAddress) {
    myToken = ERC20(tokenAddress);
  }

  function getTokenBalanceOf(address h0dler) constant returns (uint balance) {
    return myToken.balanceOf(h0dler);
  }
}