[trealira]

Yes, it does imply that, except since Haskell is lazy, you'll be holding onto a thunk until the IO function is evaluated, so you won't have a list of 10 million messages in memory up until you're printing, and even then, lists are lazy, too, so you won't ever have all entries of the list in memory at once, either, because list entries are also thunks, and once you're done printing it, you'll throw it away and evaluate a thunk to create the next cons cell in the list, and then you evaluate another thunk to get the item that the next cell points to and print it. Everything is implicitly interleaved.

In the case above, where I constructed a really long string, it depends on the type of string you use. I used lazy Text, which is internally a lazy list of strict chunks of text, so that won't ever have to be in memory all at once to print it, but if I had used the strict version of Text, then it would have just been a really long string that had to be evaluated and loaded into memory all at once before being printed.

[tpmoney]

Sorry, I lack a lot of context for Haskell and its terms (my experience with FP is limited largely to forays into Lisp / Clojure), but if I'm understanding right, you're saying because the collection is being lazily evaluated, the whole process up to the point of re-combining the items back into their final collection will be happening in a parallel manner, so as long as the IO is ordered to occur before that final collection, it will occur while other items are still being processed? So if the program were running and the system crashed half way through, we'd still have logs for everything that was processed up to the point it crashed (modulo anything that was inflight at the time of the crash)?

What happens if there are multiple steps with logging at each point? Say perhaps a program where we want to:

1) Read records from a file

2) Apply some transformations and log

3) Use the resulting transformations as keys to look up data from a database and log that interaction

4) Use the result from the database to transform the data further if the lookup returned a result, or drop the result otherwise (and log)

5) Write the result of the final transform to a different file

and do all of the above while reporting progress information to the user.

And to be very clear, I'm genuinely curious and looking to learn so if I'm asking too much from your personal time, or your own understanding, or the answer is "that's a task that FP just isn't well suited for" those answers are acceptable to me.

[trealira]

> And to be very clear, I'm genuinely curious and looking to learn so if I'm asking too much from your personal time, or your own understanding, or the answer is "that's a task that FP just isn't well suited for" those answers are acceptable to me.

No, that's okay, just be aware that I'm not an expert in Haskell and so I'm not going to be 100% sure about answering questions about Haskell's evaluation system.

IO in Haskell is also lazy, unless you use a library for it. So it delays the action of reading in a file as a string until you're actually using it, and in this case that would be when you do some lazy transformations that are also delayed until you use them, and that would be when you're writing them to a file. When you log the transformations, only then do you start actually doing the transformations on the text you read from the file, and only then do you open the file and read a chunk of text from it, like I said.

As for adding a progress bar for the user, there's a question on StackOverflow that asks exactly how to do this, since IO being lazy in Haskell is kind of unintuitive.

https://stackoverflow.com/questions/6668716/haskell-lazy-byt...

The answers include making your own versions of the standard library IO functions that have a progress bar, using a library that handles the progress bar part for you, and reading the file and writing the file in some predefined number of bytes so you can calculate the progress yourself.

But, like the other commenter said, you can also just do things in IO functions directly.

[mrkeen]

It's entirely up to you. You can just write Haskell with IO everywhere, and you'll basically be working in a typical modern language but with a better type system. Main is IO, after all.

> if the program were running and the system crashed half way through, we'd still have logs for everything that was processed up to the point it crashed

Design choice. This one is all IO and would export logs after every step:

  forM_ entries $ \entry -> do
      (result, logs) <- process entry
      export logs
      handle result

Remember, if you can do things, you can log things. So you're not going to encounter a situation where you were able to fire off an action, but could not log it 'because purity'.