Approve

approve.ligo

  const emptyOps : list(operation) = list end;

  function approve (const addressSpender : address; const value : nat; var store : store) : return is
  block {
    // If sender is the spender approving is not necessary
    if sender = addressSpender then skip;
    else block {
        const senderAccount: account = getAccount(sender, store.accounts);
        var allowance: nat :=  getAllowance(addressSpender, senderAccount.allowances); 

        // Changing allowance value from non-zero value to a non-zero value is forbidden to prevent the corresponding attack vector.
        if allowance =/= 0n then failwith("UnsafeAllowanceChange");
        else block {
          senderAccount.allowances[addressSpender] := value;
          store.accounts[sender] := senderAccount;
        }
    }
  } with (emptyOps, store);