Ability to check compatibility of two schemas

[pbcornell]

This is a feature request for a function that identifies whether two schemas are "compatible," and if not, potentially provides information about any discovered incompatibilities.

Consider use cases in which ISL defines a contract, and another ISL defines a portion of the contract actually consumed by a consumer (e.g., APIs, data publisher/consumer contracts, etc.). Before evolving a contract in a backwards incompatible way, it would be helpful to know which consumers (if any) would be affected by the breaking change.

A collapsed, canonical form of each schema with all imports inlined is likely a helpful starting point for such a compatibility check.

This is a non-trivial feature request, and any such feature would need to be precise about what "compatible" means, and clear about its limitations. The following is a non-exhaustive list of complications that come to mind:

• determining compatibility between two regex constraints is challenging at best
• logic constraints present interesting challenges, especially not
• some changes identified as incompatible may or may not actually break consumers (e.g., allowing a string to be longer), but knowing this for certain requires understanding the code that will process the data
• ...

An initial approach to this (and one that probably covers a large percentage of use cases while reducing complexity) would likely:

• focus on a structural comparison of schemas (are they of the same type? do structs have a compatible set of fields? etc.)
• avoid any comparison logic based on value constraints (e.g., regex, valid_values, length, precision)
• the not constraint is a curious one, and might be supported if the context is within a known, "closed" universe of types (in which case not can be rewritten as an any_of constraint)