"You can do this in almost any typed language that lets you consume things." True, but languages with support for affine types are, to my knowledge, uncommon.

"You can do this in almost any typed language that lets you consume things." True, but languages with support for affine types are, to my knowledge, uncommon. "One problem is where to put your state-specific type info when you're not using it." Can you elaborate? I have not run into this issue.