This module contains implementation of Extensible values.

Extensible values are an alternative representation of sum-types for Michelson. Instead of representing them as nested options, we treat them as (Natural, ByteString) pair, where the first element of the pair represents the constructor index, while the second is a packed argument.

With such a representation sum types can be easily upgraded: it is possible to add new elements to the sum type, and the representation would not change.

However, such representation essentially limits the applicability of the values. This module does not provide Michelson-level function to unwrap the value because it would require traversing all the possible options in the contract code. While this is possible, it is very inefficient. Up to this moment, we have not come up with a decent reason to allow such behavior, so Extensible types are write-only in Michelson code. They can be unwrapped off-chain with fromExtVal.

In order to preserve previous values during migrations, users should ONLY APPEND items to the underlying sum type. Changing, reordering and deleting items is not allowed and would lead to compatibility breakage. Currently, this restriction in not enforced. Only no-argument and one-argument constructors are supported.

GOOD: -- `Extensible GoodSumTypeV1` is backwards compatible -- with `Extensible GoodSumTypeV2` data GoodSumTypeV1 = A Natural | B data GoodSumTypeV2 = A Natural | B | C MText

BAD: -- `Extensible BadSumTypeV1` is NOT backwards compatible -- with `Extensible BadSumTypeV2` data BadSumTypeV1 = A | B data BadSumTypeV2 = A Natural | B | C MText