It's through "gradual" typing. You check what you can as early as you wish and emit dynamic checks for the remainder. There was a claim a while ago that gradua

It's through "gradual" typing. You check what you can as early as you wish and emit dynamic checks for the remainder. There was a claim a while ago that gradual typing is inherently slower than static or dynamic, but that sounds like an implementation bug to me, not anything fundamental.