I am not sure whether the reason I previously failed Haskell is

1. Because it's actually hard
2. Because of a mental block caused by failing at it more than once already
3. Because Haskell users are really bad at explaining things

I think it's a little 2 and mostly 3. I *love* ML, I know two MLs (3 if you count Rust) plus have in the past written my own ML. I understand the parts of Haskell that are just ML and get lost whenever I hit "do"— the point of divergence from ML; the dreaded Monad.

In the Haskell docs

https://wiki.haskell.org/Haskell_in_5_steps

It states this is how you build a Haskell program to run it.

Assuming I realize I can drop -threaded, is actually the easiest/correct way to build a Haskell program to run in the year 2025?

I run the given ghc --make command. It leaves some crap in src/. Say I do not want intermediate files in my source tree. I would like them to be moved to bin/ or obj/ or something, or simply not retained. Is this possible, or is Haskell in 2025 simply a "leaves crap in src/" kind of language in 2025?

I found -no-keep-hi-files and -no-keep-o-files (despite them technically not being documented) but say I want to retain them, just in a place of my choosing.

I am attempting to call "openFile" on the first command line argument in Haskell¹. It doesn't like it.

I'm not sure I'm looking at the right docs. I searched Google for "haskell system.io" and got https://hackage.haskell.org/package/base-4.21.0.0/docs/System-IO.html#v:openFile . I don't know if this is the newest Haskell2010 or if $GOOG is confused.

The doc (every doc I find) claims the type of openFile is FilePath -> IOMode -> IO Handle. But hls on my computer seems to think it's FilePath ->IOMode -> Bool -> IO Handle. Am I missing something?

Haskell people, please help me.
There are 3 image attachments to this post, showing the same code block but with different amounts of indentation.

The first code block works,
the second block does not work,
the third one REALLY does not work.

According to my editor, none of these blocks of code contains tabs.

Haskell appears (?) to treat three spaces, four spaces, and eight spaces radically differently.

I dislike multiple-of-3 indents.

What do I need to read to understand what I am missing?

Another cursed question.

See attachment 1. This code compiles.

Reading the documentation ( https://hackage.haskell.org/package/megaparsec-9.7.0/docs/Text-Megaparsec-Char.html#v:space ), I realize I do not want space but "space1" (see attachment 2).

I change the symbol "L.space" to "L.space1". No!! Says GHC. L does *not* export space1!! only space!!

But the documentation says it exports space1?

My problem can be explained if when I put dependency "megaparsec ^>=9.7.0" in my cabal file it picked like version 5 or 6 or something.

Is there a way to get cabal to print out for me what version it actually chose of each solved dependency? In npm or Rust for example I would consult the lock file.

I never solved the space1 problem but worked around it with a solution from @dysfun . I now have three new questions.

1. In attachment 1, why is "return" not required on L.decimal? I originally wrote "return" and it gave me a hint saying I could remove it, and removing it works. But return *is* required on (lsum, nums)?

2. In attachment 2: If att. 1 is allowed, why is this not allowed? It gives "parse error (possibly incorrect indentation or mismatched brackets)" on the _. Wrong type syntax?

One more question (I think this question might be outright goofy).

Is there a specific syntax for "calling a monad of a different type" from the current monad?

I have constructed a megaparsec-monad combinator that parses my pattern. I've made an IO-monad function that reads lines one at a time. If I call the megaparsec combinator I made inside my IO monad, I get a confusing error.

The megaparsec tutorial implies a combinator can act like a function that takes strings: https://markkarpov.com/tutorial/megaparsec.html#forcing-consumption-of-input-with-eof

So the answer to my last question was to use "parse"/"runParser" (aliases for 1 function) from megaparsec. Great.

It's not working and I think the problem is I don't understand destructuring. I want the equivalent of Rust

let Some(x) = f() else { panic!("Not found!"); }

I *think* I'm getting an Either, and I need to match its cases. But the way I know how to do that is "case…of". And the arrows in that "point the wrong way"?? Compare this similar attempt to unpack a list into its 1 item:

Alright. Thanks for the explanations y'all. I am now correctly parsing my input file. Current source:

https://github.com/mcclure/aoc2024/blob/d23ee9139e6645022504fce2dc26f39601e66933/07-01-operations/app/Puzzle.hs

I STILL can't figure out how to make the match on line 43 give a more human-readable error (on the array destructure on the command line argument processing) than "user error (Pattern match failure in 'do' block at app/Puzzle.hs:43:5-12)", but since I'm the only one running this I guess this only matters for "this hurts my sense of professionalism!!" reasons.

I will say. I could have done *this entire 50 line Haskell program* in the following two lines of perl:

perl -e 'open(FH,"$ARGV[0]"); while (<FH>) { /^(\d+):\s+(\d+(?:\s+\d+)*)$/ or die "Invalid input"; my $sum=$1; my @‍ops = split(/\s+/, $2); }'

…and I suspect writing that not only required far, far less thought for me, but would have required far less thought for someone who was already versed in both Haskell and Megaparsec.

(EDIT: Note to run this snippet you must remove the 0-width space.)