Semantic versioning for PIG type universes

[dlurton]

PIG domains could be given semantic versioning to help with identifying their persisted instances.

Example:

// universe_version is the proposed addition
(universe_version  { major: 1, minor: 2 ))

// calcluator_ast is just an example domain for clarity.
(define calculator_ast
    (domain
        (sum operator (plus) (minus) (times) (divide) (modulo))
        (sum expr 
            (lit value::int)
            (binary op::operator left::expr right::expr)
        )
    )
)

When converted to s-expressions, they might look like this:

(tree
  { domain: calculator_ast, major: 1, minor: 2 } 
  (lit 42)
)

universe_version and the tree s-expression wrapper being the new things here. The version would apply to all domains in the type domain since changing one domain can impact any domain that permutes from it.

This helps us to deal with some of the challenges that arise when loading data from an older type universe into a newer generated domain.

When the s-expressions are created by the generated code, they must be wrapped in a tree node with the same version numbers as the type universe file it was generated from. When converting back from s-expressions, the generated code must ensure that the major version is the same and that the minor version is equal or less than the same in the type universe file. The only caveat is that the major and minor versions must be manually maintained--and we'll have to consider carefully if the changes we make should be a major or minor version bump.

[lziq]

For the version number, I would recommend defining it at the top of the PIG type universe file to indicate that it belongs to all the permuted PIG domains.