“A man is rich in proportion to the number of things which he can afford to let alone.”
― Henry David Thoreau, Walden or, Life in the Woods
If I have to do the proof, then as you say, it's probably harder and (in many cases) more error prone than if I didn't use a proof. If the language can do it for me, and I get the lack of errors without having to do the work (and without the chance of me making the mistakes)? Yeah, now we're talking. (Yes, I am aware that I still have to design the types to fit what the program is doing.)
If a tool introduces complexity, it has to make up for it by eliminating at least that much complexity somewhere else. If it doesn't, the tool isn't worth using.
If I have to do the proof, then as you say, it's probably harder and (in many cases) more error prone than if I didn't use a proof. If the language can do it for me, and I get the lack of errors without having to do the work (and without the chance of me making the mistakes)? Yeah, now we're talking. (Yes, I am aware that I still have to design the types to fit what the program is doing.)
If a tool introduces complexity, it has to make up for it by eliminating at least that much complexity somewhere else. If it doesn't, the tool isn't worth using.