Keywords are special terms that have an assigned meaning. We already came across
a few keywords in the previous chapters:
none. There are
a few others that demand extra attention.
Reflects the current block height of the Stacks blockchain as an unsigned integer. If we imagine the chain tip to be at height 5, we can read that number at any point in our code.
Reflects the current block height of the underlying burn blockchain (in this case Bitcoin) as an unsigned integer.
Contains the principal that sent the transaction. It can be used to validate the principal that is calling into a public function.
Note that it is possible for the
tx-sender to be a contract principal if the
as-contract was used to shift the sending context.
Contains the principal that called the function. It can be a standard principal
or contract principal. If the contract is called via a signed transaction
contract-caller will be equal. If the contract
calls another contract in turn, then
contract-caller will be equal to the
previous contract in the chain.