Good idea, I like that it's minimalist and straight-to-the point (as opposed to e.g. exercism with its million features).
Nit: I think that when an exercise was just attempted, it should not come up immediately again (soon, yes, but w/ some enforced delay, maybe) — otherwise it feels a bit tedious (even though it's probably decent for internalizing it)
Nit: I think that when an exercise was just attempted, it should not come up immediately again (soon, yes, but w/ some enforced delay, maybe) — otherwise it feels a bit tedious (even though it's probably decent for internalizing it)