I've dreamed / complained about the lack of such a thing for ages. Glad somebody with brains and motivation has started to do something about it: ML-TID: A Type Inference Debugger for ML in Education.
Student programmers can struggle to understand how their code is wrong, while expert programmers become frustrated when the safety mechanisms of a language render programming more cumbersome. Across the spectrum of languages, those with type inference encourage the user to elide information that would otherwise catch errors at their source, making them more difficult to resolve.
In this paper, we present a novel debugging tool for ML that should lower the barrier to entry. ML-TID1 exposes a standard debugging UI that allows the user to step-through the ML type inference algorithm in a controlled environment. We present details of the concept and implementation and results from a study of 20 undergraduates.