Using Haskell as my shell

Las Safin


Oddly, programmers use one programming language for their shell, yet another one to write programs.

When we need to run a lot of external commands, we use a shell scripting language, and when we need to write algorithms, we use a “real” programming language.

The core difference can be summarized as the lack or presence of data structures. Bash doesn’t support data structures well, something like Haskell, or any imperative language, like Python, does.

When we try to use Bash to handle structured data, it quickly goes wrong. Take a look at this stackoverflow answer for how to split a string into an array using , as a delimiter:

readarray -td '' a < <(awk '{ gsub(/, /,"\0"); print; }' <<<"$string, "); unset 'a[-1]';
declare -p a;
## declare -a a=([0]="Paris" [1]="France" [2]="Europe")

This is the answer burried underneath many other almost correct yet still incorrect answers. Quite honestly, this is horrifying. What ought to be a simple function call, has been mangled into something beyond recognition.

Yet, we still use Bash. For our shell, the most important thing isn’t how easy it is to split a string, it’s how fast and easily you can run an external command. Relying on Python for that won’t go well, as evident by this stackoverflow question; the clean answers depend on sh!

Imagine, if we could use one language for both?

There are many alternatives to Bash, but they are all fundamentally boring shells. Zsh, Fish, Oil, Elvish, Nushell, rc, es, XS, are domain specific languages, and offer no real value as a “real” programming language. Would you write your IRC client in Elvish?

Instead of making a shell language that can do more than Bash, why don’t we go the other way around, and try making an existing language usable as a shell?

For it to be a good shell, we want to make running external commands as ergonomic as possible. It shouldn’t require multiple lines of code to read a file and pipe it to a process.

There are many languages with a REPL. If we were to make the Python REPL fit for use as a shell, we could make a library to make it easier to run external commands, but due to Python’s syntax, it would likely require a great amount of overhead to execute external processes, what we’re likely to do the most in our shell. We could make a function with syntax similar to this:


While it’s not a lot of visual noise, it’s much slower to type than ls -l. The ", ,, and parentheses, take up our valuable time.

Haskell, on the other hand, has a much more lightweight syntax. Throughout this blog post I manage to make the following syntax possible:


Its REPL, GHCi, is also quite featureful, importantly supporting path completion.

While longer than ls -l, for arguments without special characters and capital letters, it grows at the same rate, 1 key stroke of overhead per argument.

To do this, I made my own library Procex.

Why did I make another library when shh, shell-conduit, Shelly.hs, etc. already exist? The reason is that these solutions are all designed around createProcess.

createProcess doesn’t support all the features you’d expect on a Unix system, notably passing arbitrary file descriptors through to the called process.

In addition, it has the issue that launching processes takes a non-trivial amount of time, around 0.5 seconds on my ODROID N2 (which is unusual hardware I admit).

On POSIX-like systems, you generally need to close all file descriptors you don’t want to pass on to the new process. This could be handles to files, pipes, etc., notably, stdin, stdout, stderr are the first file descriptors (in that order). Depending on your system, the limit is different. On my system it’s 1048576. createProcess is implemented such that it loops from stderr+1 until this limit, closing every file descriptor in this range. On my ODROID N2, this takes around a second, meaning I had to wait an extra second on every command to execute. This was not usable.

Procex doesn’t have this problem, the specifics are detailed here, though it is not of great importance to this article.

Procex basics

Let’s start off by loading Procex up in GHCi, after installing the procex package. This depends on your system, but if you’re using cabal it will generally be:

$ cabal update
$ cabal install procex --lib procex
$ cabal install pretty-simple --lib pretty-simple # Heavily recommended, gives us Text.Pretty.Simple.pPrint
$ ghci -Wall -Wno-type-defaults -XExtendedDefaultRules -XOverloadedStrings -interactive-print Text.Pretty.Simple.pPrint
> import Procex.Prelude
> import Procex.Shell
> import Procex.Shell.Labels

GHCi has a couple of problems Procex helps us work around, notably, stdin is not set to line buffering, and changing directories doesn’t affect path completion.

To fix the former, we run:


This is equivalent to hSetBuffering stdin LineBuffering.

The latter can be fixed by doing the following:

:set prompt-function promptFunction

Now we can use cd from Procex.Shell, and path completion will be different depending on your working directory, where as before, path completion would always be from the directory you started GHCi in. As a side effect the prompt will also be changed.

Procex has the concept of commands, which represent a process to execute, along with the arguments and file descriptors we want to pass to it.

To create a command, we can use the mq function. After mq you can write the arguments you want to pass, wrapping them in quotes, but without any commata, parentheses or similar.

Listing the current directory:

mq "ls" "-l"

Or if you want to use the short syntax:


The labels (prefixed with #) are interpreted as strings, where _ is replaced by -, since that character is illegal in labels.

The helpers you’ll likely be interested in are all in Procex.Quick.

diff-ing two strings, then capturing the output:

diff :: ByteString -> ByteString -> IO ByteString
diff x y = captureLazyNoThrow $ mq "diff" (pipeArgStrIn x) (pipeArgStrIn y)

cat-ing a string:

mq "cat" <<< "Hello World!\n"

Piping curl to kak:

mq "kak" <| mq "curl" "-sL" "" -- The reverse will wait for curl to end instead of kak

stat-ing all the entries in your directory:

import System.Directory

listDirectory "." >>= mq "stat"

Piping curl to a file:

captureLazy (mq "curl" "-sL" "") >>= B.writeFile "./myip.json"

Piping stdout and stderr to different places:

import qualified Data.ByteString.Lazy as B

  (pipeHOut 1 $ \_ stdout -> B.hGetContents stdout >>= B.putStr)
  (pipeHOut 2 $ \_ stderr -> B.hGetContents stderr >>= B.writeFile "./log")

pipeHOut gives us the raw handle, allowing us to handle the data in Haskell, allowing us to use all the usual Haskell libraries we’d use.

In general, it is a better idea to rely on Haskell alternatives to the tools in coreutils, as they are fit for Bash and traditional shells:

Setting up your shell with Nix

You need to copy this directory, fix shellrcSrcPath, then refer to the derivation built by default.nix in your environment.systemPackages, or whatever you prefer. The derivation produces a single file bin/s that launches your shell.

The equivalent of your .bashrc will be in the ShellRC.hs file. GHCi commands will have to be put directly into default.nix.

All the imports in your ShellRC.hs file will in addition be available in the shell.

The :li command will reload the ShellRC.hs file from source instead of using the pre-compiled version from the nix store.

Setting up your shell without Nix

Let’s make a $HOME/.ghci-shell.hs file, with the same purpose as the .bashrc file.

Let’s for now put this inside:

:set -Wall -Wno-type-defaults -XExtendedDefaultRules -XOverloadedStrings -interactive-print Text.Pretty.Simple.pPrint
import Procex.Prelude
import Procex.Shell
import Procex.Shell.Labels

:set prompt-function promptFunction

You can then launch your shell with:

env GHCRTS="-c" ghci -ignore-dot-ghci -ghci-script "$HOME/.ghci-shell.hs"

This should work fine, but your init script won’t be compiled, whereas it will with Nix.

Speeding up typing

While the number of characters isn’t very different compared to Bash, there are some tricks to make it faster to type.

I’m using a Japanese keyboard with the UK layout. I don’t use the extra Japanese keys, so I have rebound the Hiragana_Katakana key (2 keys right of space) to ", a valuable trick that is applicable to Bash too and has also saved my fingers from unnecessary pain holding down shift.

I’ve also renamed mq to ξ as such:

ξ :: (QuickCmd a, ToByteString b) => b -> a
ξ = mq

I’ve bound my unused Muhenkan key (1 key left of space) to that to save another key stroke.

I recommend omitting extraneous spaces whenever possible, since the code in your shell is write-once-read-never:


Since I need to hold down shift to type _, I’ve mapped my unused Henkan key (1 key right of space) to it to save one more key stroke.

My .XCompose:

<Henkan> : "_"
<Hiragana_Katakana> : "\""
<Muhenkan> : "ξ"

You’re likely better off doing this by modifying your XKB layout, but I didn’t want to delve into that mess.

With this we’re down to 34 key strokes on my keyboard.

The equivalent command in Bash:

nix build nixpkgs#hello -o out

This took me 31 key strokes, surprisingly quite close!

You could further save key strokes by renaming functions in Procex to shorter names, however, I am of the belief that the user should choose the names, not just for functions from Procex, but also for other common functions they use. I’ve myself made aliases to Data.ByteString.Lazy.UTF8.toString, Data.ByteString.Lazy.UTF8.fromString, and some other common functions I use a lot.

Internal design of Procex

The first step was making my own glue code in C for interfacing with the vfork and execve for creating processes, as detailed here. You could do this in Haskell if you’re careful, but file descriptors in the child, which would effectively be another Haskell thread, would point to different things than the parent. This is problematic since handles from the environment will now suddenly point to different things, but only in the child. Because of this the code that runs in the child before execve is in C.

If you didn’t bother reading the above article, the gist is that the glue code provides functions that combine the forking and execution, in addition to allowing file descriptors to be set up for the child. This is then bound to inside Procex.Execve.

We interface to it from Procex.Core, which defines the core Cmd type.

Cmd is internally Args -> IO (Async ProcessStatus), where Args is a record of the raw arguments to pass as ByteStrings, the file descriptors to pass (and how to map them), and what “executor” to use (used to allow exec-ing without fork-ing).

This design was chosen as it is easy to compose. The exported functions are:

Notably, Procex.Core does not expose any overlapping functionality, since it’s only meant to expose the core interface.

These all internally wrap the original function passed, resulting in a new function that takes Args.

When we run a command, we simply pass it an empty Args, then each “layer” will add what it needs to it, then finally reaching the root function defined in makeCmd', that calls the functions defined in the glue code (bound in Procex.Execve).

Procex.Process provides functionality that is commonly needed when executing processes, and wraps over Procex.Core. It defines a family of pipe* functions, which make pipes, then pass one end of the pipe (as a file descriptor) to the process, and the other end to something else.

In principle, we need nothing more, but this is not very ergonomic to use as a shell. Each argument we want to pass to a process needs a cmd & passArg "myarg", and passArg doesn’t even work when you’re in a shell:

Often, in our shell, we’ll pass paths as arguments, but if you pass in non-ASCII paths to passArg as a literal, they will get mangled. The top bit of each byte in the string will simply be unset by the IsString implementation of ByteString, since it’s not UTF-8 aware, so it doesn’t know how to encode such bytes into the ByteString.

To avoid this problem, we need a helper function that takes a String instead of a ByteString, so that we don’t use ByteString’s IsString instance.

In Procex.Quick we define a ToByteString class, that has a single toByteString member. It has an instance for [a] where a ~ Char (defined this way to aid type defaulting), such that we can define functions that take any a where ToByteString a.

To attain a Bash-like syntax that is more concise, a QuickCmd class is defined, with quickCmd :: QuickCmd a => Cmd -> a.

It has three instances:

QuickCmdArg has all the instances you can guess, String, ByteString, etc. We actually can’t use ToByteString for our instances for QuickCmdArg, as that would 1) require UndecidableInstances and 2) make type inference not work in a lot of cases.

Wrapping it all up, we have the mq function that wraps makeCmd and quickCmd, as shown in the basic examples.

There are also various operators that wrap over Procex.Process and call Data.ByteString.Lazy.hGetContents for you, e.g. <<<, |>, <!|, and the capture* family of functions.

The capture* functions all wrap captureFdsAsHandles, which simply runs a command and provides the handles to the specified file descriptors. They all output a ByteString, which can be read lazily or strictly.

An important part of running commands is also checking for failures. In Bash, we have set -e. In Procex, run runs commands synchronously and waits for them to exit, throwing if the command failed. This obviously works fine for capture* functions that wait for the command to finish, but what about when we’re using lazy IO?

The answer is more lazy IO. We attach a “finalizer” to the ByteString (GitHub):

attachFinalizer :: IO () -> ByteString -> IO ByteString
attachFinalizer finalizer str = B.fromChunks <$> go (B.toChunks str)
    go' :: [BS.ByteString] -> IO [BS.ByteString]
    go' [] = finalizer >> pure []
    go' (x : xs) = (x :) <$> go xs

    go :: [BS.ByteString] -> IO [BS.ByteString]
    go = unsafeInterleaveIO . go'

A Data.ByteString.Lazy.ByteString is internally isomorphic to a list of Data.ByteString.ByteString. By converting it to and then from a list of such chunks, we can insert lazy IO into it, executing the finalizer when we reach the nil case using unsafeInterleaveIO.

In practice this works quite well, but some times we don’t want it to err, for example when we’re using diff. diff returns a non-zero exit code when the inputs differ, but we want to ignore that, so for each lazy capture* function there is a -NoThrow version. This could be extended to allow filtering what exit codes you want to ignore, but this would complicate the “quick” module, and if you want more advanced behavior, you’d likely be better off using Procex.Core and Procex.Process directly, then passing the resulting Cmd -> Cmd to mq.

The label trick

Procex.Shell.Labels contains this:

{-# OPTIONS_GHC -Wno-orphans #-}

module Procex.Shell.Labels where

import Data.Functor
import Data.Proxy (Proxy (..))
import GHC.OverloadedLabels (IsLabel (..))
import GHC.TypeLits (KnownSymbol, symbolVal)

instance (a ~ String, KnownSymbol l) => IsLabel l a where
  fromLabel =
    symbolVal (Proxy :: Proxy l) <&> \case
      '_' -> '-'
      x -> x

Labels like #label when -XOverloadedLabels is enabled are translated into something like fromLabel @"label".

The reason, it’s IsLabel l a where a ~ String instead of IsLabel l String, is that with the latter, type inference wouldn’t work properly, meaning something like mq #echo wouldn’t type check.

With this instance, fromLabel @"label" will be inferred to be of the type String, causing it to be evaluated as "label".

This will likely conflict with other uses of labels, so you might not want it if you use other libraries that use labels.

Conclusion after 6 months of using Haskell as my shell

In the beginning it was certainly painful, it was as if I had to relearn talking. Thankfully GHCi provides an escape hatch: :! allows you to shell out to sh easily.

In the process of switching my shell to Haskell, I also got a lot faster at writing Haskell. Haskell is now the primary interface through which I use my computers, and it has been very pleasant. I no longer have to deal with regexes, since I can whip out a full parser combinator library any time. You could likely also include a PostgreSQL library in the shell to access databases without going through the psql REPL.

I’ve also been removing my scripts one by one completely, replacing them with simple Haskell functions in my ShellRC.hs, where they can interface with structured data rather than raw bytes.

Future work

Advanced completion like in Fish would be quite nice, but unfortunately GHCi is a bit hard to customize due to its integration into the GHC source code. Perhaps a GHCi alternative external to GHC could be implemented, or the Idris REPL could be modified instead, since it seems more amenable to customisation.

Related articles

About me

I like dependent types.


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