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Comment by c00lio | Hacker News Reader

"Oh, you _also_ need to print something? Lets stack a few monad transformers..."

"But remember that you need the TemplateExplicative and NullUnderstanding compiler extensions!"

You can use eventlog traces, from Debug.Trace [1]. You can (traceEvent $ "look: " ++show bazinga) everywhere you need and then stare at the log to your heart content.

[1] https://hackage.haskell.org/package/base-4.18.0.0/docs/Debug...

No need for extensions, just compile and run your program slightly differently. That's the power of declarative languages.

Not everything is tracing and debugging, sometimes you really need to output intermediate results for "normal", "production" purposes. One could still abuse Debug::Trace, but that would really be ugly.

I also object to that "everywhere". It is far easier to just dump an extra 'print' line somewhere inside a for-loop than into a `foldl (*) 1 $ map (+ 3) [17, 11, 19, 23]`. And that is an easy one...

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

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.

I can't tell if you are trying to defend those languages or just piling up absurdities on the previous post in the style of "yes, and ..." improv.

I am trying to offer counterpoint to what seems to me as an unjust critique from a person who, at first sight, does not know much about Haskell.

Also, a link to a useful library is not a bad thing for anyone curious about Haskell. Thus, there's a bit of education there.

If it looks like improv, I am here every evening till Friday.

You're thinking of Haskell. F# was modelled after OCaml, which doesn't attract monad transformer stacks, and doesn't have a zoo of compiler extensions.

Well, they aren't actually compiler extensions but pre processor extensions (PPX).

And I would really like if OCaml would have had the possibility to add the needed PPXs names to the source file (like Haskell's compiler extensions). So as to not have to read the Dune (or whatever build system is used) file to get to know where `foo%bar` or `[@@foo]` is coming from and what is doing. But at least the usage of `ppxlib` nowadays should make PPXs "compose" aka. not stamping on each other's feet.

https://ocaml.org/docs/metaprogramming http://ocamlverse.net/content/metaprogramming.html

I haven’t used it for some time but OCaml certainly used to have a zoo of incompatible compiler extensions. Circa 2008 or so I once hit on the brilliant idea of using protobufs to get two mutually incompatible halves of an ocaml program to talk to one another only to find that required yet another compiler extension to work.

Are you thinking of preprocessors? Back then, it would have been via Camlp4 and Camlp5.

Aah yes I am

I'm pretty sure F# was modeled on both. There are some definite "Haskell-isms" in F#; if nothing else, monads are typically done in something more or less equivalent to the `do` notation (an `async` or `seq` block), for example.

The syntax superficially looks a lot like OCaml, but it doesn't do the cool stuff with OCaml functors and modules; you write it a lot more like Haskell most of the time.

Here is the "official" history of F#: https://fsharp.org/history/hopl-draft-1.pdf

Don Syme began with a port of Haskell to .Net, but SPJ convinced him that this is a bad idea, so he did choose OCaml. ("The Decision to Create F#", Page 9)

You have to add extra syntax to do very normal things like have more than one expression in a function.

As someone who's coded OCaml for 20 years, I have no idea what you're referring to. `let x in y`? `x; y`? `x, y`? Those are all in the base language.

>NullUnderstanding compiler extension

brilliant! lol

Is brilliant the best word? :P

"now just sprinkle some `map . sequence . map`'s here and there and you are done. who said this was difficult?"

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