Solidity: Constant state variables are not constant

[ethernomad]

contract constantState {
    uint constant public test = block.number;
}

Every time I call test() I get the current block number. I thought the block number would be written into state during contract creation.

> constantstate.test()
31892
> constantstate.test()
31892
> constantstate.test()
31893
> constantstate.test()
31902

[ethernomad]

Removing the constant modifier makes it constant.

[ethernomad]

I suppose constant means "constantly re-evaluated". :stuck_out_tongue_winking_eye:

[chriseth]

Thanks for the find! Yeah, constant means don't store this expression in storage but rather re-evaluate it everytime it is used.

On a side note: constant for functions does not mean they are constant, it just means they should not change storage. We have to revisit these keywords at some point.

[axic]

Does it make sense to have the constant modifier on state variables? Shouldn't the memory keyword do what constant is doing for state variables?

[chriseth]

@axic you cannot combine memory with uint, it's a different thing than constant. We should check if the expression that is assigned is actually constant. We could also use a different keyword like inline because that is actually what is happening.

[axic]

@chriseth I've collected my thoughts in #992

[axic]

Based on #992, this is considered a bug.

[chriseth]

Question to think about: Should the following be valid after the change?

contract C {
  function (uint) returns (uint) constant x = f;
  function f(uint) returns (uint) {}
}

The value of the variable x is a compile-time constant, so I would say yes.

[chriseth]

... although I did not implement that for now.

[3esmit]

I found really useful to use constant expressions. Maybe we could change constant keyword to expression or volatile.

Example:

contract LockableCoin is AbstractCoin {
    //creation time, defined when contract is created
    uint public creationTime = now;
    //time constants, defines epoch size and periods
    uint public constant UNLOCKED_TIME = 25 days;
    uint public constant LOCKED_TIME = 5 days;
    uint public constant EPOCH_LENGTH = UNLOCKED_TIME + LOCKED_TIME;
    //current epoch constant formula, recalculated in any contract call
    uint public constant CURRENT_EPOCH = (now - creationTime) / EPOCH_LENGTH + 1;    
    //next lock constant formula, recalculated in any contract call
    uint public constant NEXT_LOCK = (creationTime + CURRENT_EPOCH * UNLOCKED_TIME) + (CURRENT_EPOCH - 1) * LOCKED_TIME;
(...)
}