So I think "easy to mentally model" gets harder and harder to achieve as the type system becomes more powerful. If your conclusion is that we keep the type system limited in power, that's valid! but not what I'm exploring.
Another point that I failed to make in the post (thanks for the pushback!) is that type inference algorithms leave you high & dry in the presence of a type error. Then you're left with lots of hidden state (the internal steps the algorithm took before arriving at the error) and the final conclusion, which, if you're lucky, points you to two places in the code where the inferred types are in disagreement.
With my proposed system, all types are annotated all the time (to be shown or hidden via editor flag, or on hover), and the annotations are updated in response to actions taken within the editor. The "algorithm" becomes extremely simple, with almost no intermediate steps.
Of course, proof will be in the pudding, if I can actually achieve a pleasant editing experience :)
Another point that I failed to make in the post (thanks for the pushback!) is that type inference algorithms leave you high & dry in the presence of a type error. Then you're left with lots of hidden state (the internal steps the algorithm took before arriving at the error) and the final conclusion, which, if you're lucky, points you to two places in the code where the inferred types are in disagreement.
With my proposed system, all types are annotated all the time (to be shown or hidden via editor flag, or on hover), and the annotations are updated in response to actions taken within the editor. The "algorithm" becomes extremely simple, with almost no intermediate steps.
Of course, proof will be in the pudding, if I can actually achieve a pleasant editing experience :)