mcc

#BabelOfCode 2024
Week 7
Language: Haskell

Confidence level: Medium low

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I was going to do Fennel this week, but then I looked at the problem and thought "this is ideal for Haskell "amb". I have been looking for an excuse to use Haskell "amb" for 25 years. So Haskell.

I have tried to learn Haskell 3 times now and failed. This "Babel of Code" thing was originally in part an excuse to do Haskell

May 1, 2025 at 4:15pmWeb
Show threadmccMay 1, 2025

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.

Show threadmccMay 1, 2025
Question: The Haskell 2010 documentation describes its basic I/O functions, somewhat ambiguously, as "character oriented". I assume this means "ASCII character oriented". Is there a way in Haskell to get equivalents of getChar, putChar, string operations etc which are *UTF-8 character* oriented? I don't need graphemes, I'm happy with codepoint resolution.
Show threadmccMay 1, 2025

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?

Haskell in 5 steps - HaskellWiki

Show threadmccMay 1, 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.

Show threadmccMay 1, 2025
Welp, after 25+ years of trying, I have written my first working Haskell program. It reads one line from stdin and then prints it back out. I have now finally used a "monad", although I still don't feel I know what one ~is~.
Show threadmccMay 1, 2025
I am trying to switch my program to Cabal-driven builds. This is one of the questions it asks you when you run `cabal init`. I think I understand what it is about the Haskell community that lead them to do it this way, but in my opinion, this is bad user experience. If the newest version of the Cabal format isn't the recommended one then why did you release it at all?
Show threadmccMay 1, 2025

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?

System.IO

Show threadmccMay 1, 2025
Answer to my previous question was I naively took the first suggestion from hls and imported GHC primitives where I should have imported System.IO. Cool. Works now
Show threadmccMay 1, 2025

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?

Show threadmccMay 1, 2025

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?

Text.Megaparsec.Char

Show threadmccMay 1, 2025

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.

Show threadmccMay 1, 2025

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?

Show threadKeith WansbroughMay 1, 2025
@mcc haven't done Haskell in a while, but I don't think it cares how many spaces, just that they're the same. Your problem is that the alignment point is the start column of the first token after the `do`.
Show threadmccMay 1, 2025
@kw217 oh no. okay, I see :(
Show threadbarsMay 1, 2025

@mcc only in the first case is the "let" keyword (and following code) aligned to "do"

It's just significant whitespace. You can either align everything on the column after "do " (that's where the 3 chars come from) or indent on a newline with an indent width of your choice.

Show threadbarsMay 1, 2025
@mcc in pic 3, it's the "if" that breaks the alignment
Show threadbarsMay 1, 2025

@mcc

This should compile, with 'do' on the prev line:

```
takeLine :: Handle -> Int -> IO Int
takeLine inHandle acc = do
inEof <- hIsEOF inHandle
if inEof
then return acc
else do
inStr <- hGetLine inHandle
let result = 1
takeLine inHandle (acc + result)
```

Show threadjleedevMay 1, 2025
@mcc Layout: https://www.haskell.org/onlinereport/haskell2010/haskellch10.html#x17-17800010.3
10 Syntax Reference

Show threadJosh HollandMay 1, 2025
@mcc in my opinion, the neatest way to get consistent indents is to have a newline straight after "do", then your preferred number of spaces the next line
Show threadShae ErissonMay 1, 2025
@mcc would you be interested in a video chat intro sometime tomorrow, east coast USA biz hours?
I got paid to write Haskell for five years, could maybe speed up your getting started process?
Show threadmccMay 1, 2025
@shapr That is a very generous offer, at the moment I don't think it's needed but thank you.
Show threadShae ErissonMay 1, 2025

@mcc If you change your mind, I'm free on Mondays and Fridays, I hope it goes well for you!

❤️ 

Show threadsamir, an unknown quantityMay 1, 2025

@mcc I am excited you’re writing Haskell. 😄

In general, it’s unlikely you want to be importing `GHC`. If ever you do need to, you’ll know.

Show threadJ3RN  May 1, 2025
@samir @mcc I came here to say this 😁
Show threadbarsMay 1, 2025
@mcc any particular reason why you're importing GHC.* and not System.IO?
Show threadmccMay 1, 2025
@bars Because it's what hls recommended. Thank you
Show threadbarsMay 1, 2025
@mcc Sure. Keep us posted!
Show threadAnupam 🇮🇳《ミ》λ≡May 1, 2025
@mcc wait till you see the tiny cabal file it geberates. Who thought this was a good idea? Cabal init should ideally ask no questions at all
Show threadmccMay 1, 2025
@aj the file it generated has what appears to be hundreds of lines of comments actually
Show threadAnupam 🇮🇳《ミ》λ≡May 1, 2025
@mcc oh god I had forgotten about the comments
Show threadAaron Sawdey, Ph.D.May 1, 2025
@mcc Maybe they liked the python2/python3 schism so much they would like to see if they can get that for their community?
Show threadMarcel(le)May 1, 2025
@mcc LTS and supporting multiple versions of GHC in the wild... 😮‍💨
Debian stable is at GHC 9.0.2, Debian unstable at GHC 9.6.6 and upstream GHC is maintained at versions 9.6 up to 9.12. I have seen some good work from Cabal devs, but feature sets may be more practical to have packages available over a broad range of versions. I totally agree and would want new packages to take up the new features, though!
Show threadmccMay 1, 2025
@mu Oh, is that the idea? That 3.0 is LTS?
Show threadMarcel(le)May 1, 2025
@mcc I think it's a bit conservative for that. Cabal 3.0 is relatively old, and I suspect the choices in cabal init might not be up to date.
Show threadOwen NelsonMay 1, 2025
@mcc I heard they are like burritos
Show threadmccMay 1, 2025
@onelson I heard that too but it doesn't help me
Show threadOwen NelsonMay 1, 2025
@mcc yeah
Show threadOwen NelsonMay 1, 2025
@mcc monads: very carb heavy
Show threadMegMay 1, 2025
@onelson @mcc bananurrito
Show threadMegMay 1, 2025
@onelson @mcc (I hate these metaphors so much because if you take them to their logical conclusion you have to start thinking about things like "now you unwrap this banana and put these bananas inside and then wrap it all up into one new banana")
Show threadOwen NelsonMay 1, 2025

@megmac @mcc idk if this is anything but I have long held that monads are just like null propagation in sql, e.g. how `null or false` is `null` not `false`.

I'm not even sure this is accurate, tbh.

Show threadMegMay 1, 2025

@onelson @mcc I think analogies just don't really work at all, whether to abstractions like food or to other different sorts of languages.

Imo the useful definition of a monad is just that it is a particular kind of nested data structure that happens to be useful for describing computation constructively. A math nerd will tell you that's not all it is, but from a programmer's perspective that's why it matters.

Show threadRobin AdamsMay 1, 2025
@onelson @megmac @mcc Null propogation is basically the Maybe monad. So yes that's a monad, but just one example of a monad: there are lots of other different things monads can do.
Show threadRobin AdamsMay 1, 2025
@onelson @megmac @mcc Actually it's a good way to understand them: a monad represents something that can be propogated.
Show threadRobin AdamsMay 1, 2025
@onelson @megmac @mcc Wait, maybe that's just a functor.
Show threadrk: it’s hyphen-minus actuallyMay 1, 2025

@mcc

Monad monoid endofuctors etc

Show threadGlyphMay 1, 2025
@mcc at one point I understood Monads by implementing Deferred (which everyone kept telling me was actually a Monad) in Haskell. As I recall, I started with about 700 lines of code and eventually got it down to 0 as I finally, fully realized the idea that it was a monad. But the experience of learning Haskell this way felt like ascending in a roguelike; I accomplished something, but I could not take it with me. Today, I do not know what a Monad is and I cannot write Haskell.
Show threadMirrors And StuffMay 1, 2025
@glyph @mcc this is also my experience of Haskell. At one point in time, my language of choice. Now, don't ask me to explain anything about it.
Show threadSteve HoldenMay 1, 2025
@glyph @mcc we live in a mysterious universe.
Show threadJordanMay 1, 2025
@mcc It’s Rust’s ? being implicitly added on every line.
Show threadaseMay 1, 2025
@mcc -odir and -hidir for where to put the files. i think --make and -threaded are there by default today
Show threadaseMay 1, 2025
@mcc i guess the "normal" way to use haskell is exclusively via cabal which puts all the output files elsewhere
Show threadmccMay 1, 2025

@annanannanse Wait, what? I can use cabal for normal builds?

Are these the directions to use ?

https://cabal.readthedocs.io/en/stable/nix-local-build.html

3.1. Quickstart — Cabal 3.14.2.0 User's Guide

Show threadaseMay 1, 2025
@mcc i'm afraid i'm not up to speed on howto cabal best (i only use haskell via a custom build system that also just calls ghc...), but you need a .cabal file defining your exe and the `cabal build/run` should work.
Show threadرہگزرMay 1, 2025

@mcc @annanannanse You need a cabal project to be able to use `cabal build` or `cabal run`. There is `cabal init` to produce a skeleton. Not so simple unfortunately.

I think for simple tasks it is better to run stuff from ghci and just forego compiling entirely.

Show threadNot a Spring OnionMay 1, 2025

@mcc

Feels a bit like programming in C.

"Just put rm *.o in your makefile."

"That's how it has always been."

"It's not the job of the compiler to not leave a mess."

Show threadStagsMay 1, 2025

@mcc While researching this online, I found a build tool called cabal.

cabal build --builddir=dist/build-artifacts

Show threadMarcel(le)May 1, 2025
@mcc Isn't that like rustc/cargo, most people using cargo instead of rustc directly? 🤔
Show threadJonathan DowlandMay 1, 2025
@mcc I *think* Char is actually (a) Unicode
Show threadJonathan DowlandMay 1, 2025
@mcc *checks* yes https://hackage.haskell.org/package/base-4.21.0.0/docs/Data-Char.html
Data.Char