> most type theories don't enforce full parametricity at runtime
means "sometimes types can appear at run time"? If so it's true, but it's not what I understand by "parametricity".
Which is fair although necessary. It's not so much a breakage as it is a necessity, since non parametric code may rely on this too.
Or maybe it is about constrained parametricity. In any case this doesn't seem a big issue.
> most type theories don't enforce full parametricity at runtime
means "sometimes types can appear at run time"? If so it's true, but it's not what I understand by "parametricity".