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> I develop with Haskell professionally. That foldl example of yours is pretty rare and usually dealt with the QuickCheck [1], mother of all other quickchecks. Usually, the trace will be outside of the foldl application, but you can have it there in the foldl argument, of course.

So actually not everywhere. And QuickCheck does something else entirely.

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You missed the word "usually". You really, really do not need a print within the body of a loop of any tightness. But you can have it.

The foldl example of yours should be split into a property checking and controlling for expected properties of the input. The first part is done via quickcheck and second part usually is done with assertions and/or traces.

But nothing preclude you from having your trace there, inside foldl argument. This is clearly wrong place to have it, but still you can have it there.

So I repeat, you can have your traceEvents everywhere.

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