Leveraging Context Variables in ErgoScript

ErgoScript provides the capability to create intricate programs with predicates that are context-dependent. Let's take a look at an example:

HEIGHT < 4000000            // address 2fp75qcgMrTNR2vuLhiJYQt

This script uses the context variable HEIGHT, which signifies the height of the block in which the transaction is mined. If the blockchain height is less than 4,000,000, the box associated with this address becomes "anyone-can-spend". Conversely, if the blockchain height equals or exceeds 4,000,000, the box becomes "no-one-can-spend".

Apart from HEIGHT, ErgoScript supports other context variables such as OUTPUTS, INPUTS, and minerPubKey. These variables offer additional tools for writing more specific and flexible scripts. For more details on context variables and their usage, please refer to the ErgoScript language specification.

Context extension values attached to inputs can also carry reusable script or proof data. Since Sigma 6.0, a transaction can provide script bytes through one input's context extension and let other inputs read the same value, which can be cleaner than duplicating the same script/proof material on every input. Keep variable IDs explicit and document which input supplies each value; otherwise multi-input contracts become hard to audit.