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Comment by thesz | Hacker News Reader

With eventlog you have lightweight profiling and logging tool for "normal", "production" purposes. You can correlate different metrics of your program with your messages. This is not an abuse of Debug.Trace (notice the dot), it is normal state of affairs, regularly used and RTS is optimized for that use case.

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.

[1] https://hackage.haskell.org/package/QuickCheck

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Eventlog is just `unsafePerfemIO*`, so you could just use that instead and not "hide" it and feel better about that.

The "correct" answer to `foldl` would be `scanl` and printing the result of it.

https://hackage.haskell.org/package/base-4.18.0.0/docs/Prelu...

Eventlog traces are RTS calls wrapped into unsafePerformIO, you are right. The trace part of eventlog is optimized for, well, tracing and is very, very lightweight. It is also safe from races, whereas simple unsafePerformIO (putStrLn $ "did you meant that? " ++ show (a,b,c)) is not.

In my opinion, eventlog traces make much better logging than almost anything I've seen.

Right now, developing with C++, I miss the power of Haskell's RTS.

The point I was trying to make was, that if all you want/need is a `putStr`, just use `unsafePerformIO`.

Haskell's (GHC's) Eventlog is nice, but a binary format and not comparable to text output.

> 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.

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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