[lolinder]

> In normal type coercion, the transformation is explicit and visible in the language syntax, whereas these “migrations” happen magically behind the scenes.

I was assuming that the migration would actually alter the call site in a way that is reviewable and committable, not implicitly do it every time it's needed at compile time. If that's the idea, it doesn't alter anything behind the scenes, it does so explicitly and visibly one time to change everything to meet the new requirements.

The larger problem I see is that the applications for this would be extremely limited. There's a reason why they used a simple s/a/Some a/ as an illustration—once you get beyond that the migrations will become a pain to write, a pain to validate, and likely to break tons of code in subtle ways. And since most library code changes aren't this simple kind, it's likely that it's not worth the effort of building and maintaining this migration syntax for something with so narrow an application.