Comment by eru

Python's Hypothesis has some very clever features to deal with shrinking past a monadic bind.

If I remember right, it basically uses a binary 'tape' of random decisions. Shrinking is expressed as manipulations of that tape. Your generators (implicitly) define a projection from that tape to your desired types. Shrinking an early part of the tape, leave the later sub-generators to try and re-use the later parts of the tape.

That's not guaranteed to work. But it doesn't have to work reliably for every shrink operation the library tries! It's sufficient, if you merely have a good-enough-chance to recover enough of the previous structure to trigger the bug again.