Hell: A Haskell Shell | Hacker News Reader

Hell: A Haskell Shell

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117 pointstikhonj37 comments

It is not a real shell, since it misses basic pipes. It is just a library for renaming system functions to their Bash-like names...

Move on, and look at these:

* http://hackage.haskell.org/package/Shellac

* http://hackage.haskell.org/package/pipes-shell

* http://hackage.haskell.org/package/HSH

* http://hackage.haskell.org/package/shellish

I'm not sure of the types of the functions, but Haskell's arrow typeclass could do arrows in theory.

> since it misses basic pipes

To be fair, you don't actually need them

  lelf:~/srcs/hell$ reverse <$> pwd
  "lleh/scrs/flel/sresU/"

But yes, right now it's just a bunch of renames.

Might note this was "whipped this up in a few hours". I suspect this is just an experiment. But I think this might evolve into something serious, lots of interest was expressed in /r/haskell.

On a serious side note, it doesn't need pipes since the shell utilities are wrapped as function which are composable in Haskell.

The last 3 links are to shell-scripting libraries, which have a different goal than creating an interactive shell. I would recommend shelly, the library I created: https://hackage.haskell.org/package/shelly

Wouldn't pipes be implemented as monads (arrows)? I think it'd be defeating the point to bolt on a Bourne shell convention for Haskell shell, when there's a native capability to be used...

How does something this bad end up the top comment? Three out of four links you provided are completely irrelevant. It is not missing pipes. Pipes are a simple composition operator. Haskell already has that, no need to make a special version for this.

While it's true that you can't really pipe programs conveniently yet it sure supports composition with all functions as you suggest. Just wanted to note in case that was what the comment above was thinking about. Except that I pretty much agree with you, current top comment is crap.

Otherwise the syntax would live up to the shell's name.

See: http://www.haskell.org/haskellwiki/Simple_Unix_tools & http://www.haskell.org/ghc/docs/7.6.2/html/libraries/base-4....

ps: Haskell is such a Gloriously Happy Land, why make it "hell"?

Read the page?

"Irony" is a side effect. Haskell is pure .. :P

I feel like "hell" is a perfect description for trying to use Haskell. Had to suffer through it for my first quarter of intro to CS and was only too glad to move on.

There's also scsh, which uses Scheme:

http://www.scsh.net/

Unfortunately, on FreeBSD:

    ===>  scsh-0.6.7 is marked as broken: fails to install on amd64.

And I really wanted to play with it. Seems like I will have to help in Scsh reimplementation in Racket before I can do this.

It's not much of an interactive shell, although it's perfectly cromulent for writing shell scripts.

Hell? This could be heaven! For everyone.

    >> ls *.*

Vs.

    >> fmap (filter (isInfixOf ".")) ls'

No sale!

Actually that was meant to be a joke. Is that really a reason to downvote?

Yes, this isn't reddit.

Truly clever, relevant humor is perfectly acceptable. It's the cheap stuff that deserves to be downvoted. The post in question is, AFAICT, only guilty of making a relevant point in a slightly humorous manner.

Completely fair criticism. Obviously, I don't mean to continue writing such lengthy code on the command-line, but in order to feel out what's good enough, I need the shell. The brevity can come later. E.g. there's no reason

    ls "*.*"

couldn't be defined, e.g.

    ls = run . ("ls " ++)

I'd argue that even the double-quotes in `ls "."` are too much for normal usage. There have been many attempts at creating non POSIX-y shells based on various languages, but AFAICT it's a non-starter to require special syntax when running basic command lines. I'm mostly not writing shell scripts, I'm using a command line to interact with the system. As such, I want no barrier to running programs yet a lightweight way to leverage a "real" programming language. Redefining tools like `ls` seems like a red herring as well. I'm not against built-ins (they're necessary), but the Unix toolchain isn't going anywhere anytime soon. You'll still need to pull data out of git, logfiles, munch that data with external tools, etc. As such it's worth thinking about how to best bridge these worlds. If the shell environment can provide a better ls, that's fine, but I suspect it's a design bug if it feels like it needs the built-in for day-to-day usage.

On "feel", it also feels like there should be a seamless syntactic protocol for post-POSIX shells, but I've yet to nail down how it would work. Straw-man requirements:

    - Absolutely no special syntax to run programs
    - Low-drama way to write "real code", i.e. native 
      scripting language syntax.
    - Great integration via syntax, shell built-ins for 
      working in Unix-y pipeline.
    - Replace powerful yet bizarre bash/zsh parameter 
      constructs with better native language tools.

By low-drama, think of the success of Markdown vs. HTML, TeX, etc. for markup. Not as powerful, but much easier to use for its use cases. IMO, this is the most powerful thing about the POSIX shells -- there's zero ceremony around running programs and dealing with I/O redirection. That's also the compromise, as things like quoting semantics get hairy. One way a post-POSIX shell could go is to retain this seamlessness while adding a touch more modality so that the worst problems of POSIX shells are eliminated. For example, multiple-quoting or re-quoting should never be needed, real data structures are both available and the default, etc.

Anyhow, Hell looks like a fun experiment and sandbox. Enjoy!

I agree completely. I've been thinking about this a lot, and non-quoted strings strike me as an absolute requirement for anything that aims to replace the bourne shell and its descendants (as opposed to complementing them in the way that Perl/Python/Ruby do it). But obviously it's hard to have non quoted strings in a programming language.

I need to play more with TCL (the one non sh-like language I know that does this) but I think even it has some very strange features that grow out of treating ordinary text as strings.

A shell made in a pure functional language... It does not look apealing (the main use of a shell is to create side effects), but I'll sure try it.

The main use of Haskell is to create side effects. It just does so in a principled, constrained manner.

There's a misunderstanding at work here about how Haskell actually regards side effects. In a few words: It provides an API for declaring side effects, and forces you to use this API when chaining them together, but it does not refuse to acknowledge their existence.

Haskell's reputation exaggerates how this API was less powerful, or missing altogether, in early versions of the language. But its reputation is not what you write programs in.

No misunderstanding. I know Haskell, and I do like it. But monads (each one) are better constrained to a small part of your code, otherwise you negate all the power you gain by declaring the side effects. And the entire point of a shell is to do IO all over the place.

sigh

Functional languages are all about keeping side effects under control so that you can tell what will and what will not change the state of the system. Think of it as incorporating awareness of program state into the type-system of the language.

As others have replied to you, I think you misunderstand what side effects & IO are.

Imagine a "normal" shell type chain:

    ps -ef | grep 'elephant' | sort -nk2 >> logfile.txt

neither `grep`, nor `sort` should have any side effects, and if they do, then you have potential for weird bugs later on. (imagine your / is mounted read only, and sort has a side effect of writing everything to a temp file. now suddenly this otherwise sane looking chain stops working).

In the command, you have a single input, and a single output (which has a specific mode, append). Those are the only IO functions which you've specified, but you have no guarantee that anything else will not make a mess of things.

The dream for "pure functional" shell scripting would be where the side effects are more obvious, and controllable, not to eliminate them.

Sort does have the side effect of writing to a temp file, when the input to be sorted is large. :)

Except that usually grep and sort can read files, or entire directories, which is considered a "side effect" in Haskell.

(Not to mention checking locales to decide how to sort, checking if the output is terminal so that we can add colors, but only if the environment variable says so, and on, and on...)

You can write C in Haskell (I do sometimes, for SPOJ or something) if you really want. However, Haskell lets you say that something has no side effects, which is very useful for reasoning about programs.

And almost all Haskell programs do perform side-effects, they just demarcate the sections of their logic that do and don't have effects.

The meme that Haskell can't do side-effects or that's its hard just won't die...

I think most Unix commands have a function type which is not representable in haskell, if you don't want to fall back to strings everywhere. So you end up with many functions like ls and ls'. For a shell I prefer a dynamically typed language.

Typeclasses to the rescue..

    ls :: LsOptions -> IO Listing

    instance ToString Listing where ...

    instance Default LsOptions where def = LsOptions []

Now a single `ls` function can yield regular textual output, or the more specific Listing type can be used if you want to say, map over the files listed.

    toString <$> ls def

    map toUpper . filesListed <$> ls def

The repl can automate the application of toString of course, so the user can just enter `ls def`. Someone better at this than I could probably find a way to make the def parameter be optional, too.

It looks more usable than scsh, and I like the name.

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