nix-script

Brian Hicks, February 23, 2021

I like writing quick little scripts to avoid having to remember how to do things. Most of the time I start in bash, thinking the task won't be too complicated... but then before I know it I'm having to reread the bash man pages to figure out how arrays work for the thousandth time.

So really, I'd rather write scripts in a language that offers some more safety and better data structures. We're trying to learn Haskell at work, so maybe that?

Let's see a hello world:

#!/usr/bin/env runghc

main :: IO ()
main = putStrLn "Hello, World!"

That seems pretty reasonable, and only takes like 300ms to compile and run. Not awful, but it's not as fast as a bash script, and it means I have to use Haskell's standard prelude instead of something safer like relude or nri-prelude.

It's reasonable to solve this, though: I can just use nix-shell to get a ghc with packages.

#!/usr/bin/env nix-shell
#!nix-shell -p "(pkgs.haskellPackages.ghcWithPackages (ps: [ ps.text ]))"
#!nix-shell -i runghc

{-# LANGUAGE OverloadedStrings #-}

import Data.Text.IO

main :: IO ()
main = Data.Text.IO.putStrLn "Hello, World!"

Well, that works, but I've traded speed for flexibility: run time has grown to over 2 seconds! Eep!

I don't think I should have to make this trade, so I wrote nix-script. It transparently manages a compilation cache for these kinds of scripts, and lets you specify dependencies and build commands inside your source file!

That means the nix-shell example above can be rewritten like so:

#!/usr/bin/env nix-script
#!buildInputs (pkgs.haskellPackages.ghcWithPackages (ps: [ ps.text ]))
#!build ghc -O -o $OUT_FILE $SCRIPT_FILE

{-# LANGUAGE OverloadedStrings #-}

import Data.Text.IO

main :: IO ()
main = Data.Text.IO.putStrLn "Hello, World!"

The first time you run that, it'll compile the script to a binary, then run it. That takes about two seconds on my machine. The second time, and going forward until you change the script, it detects that it already compiled to a binary, so it just runs the compiled version. That takes 30ms or so for me! Big improvement!

But it's really not that fun to have to figure out that #!build line every time, and I always forget how to call pkgs.haskellPackages.ghcWithPackages correctly... so there's also a wrapper script called nix-script-haskell that makes this nicer:

#!/usr/bin/env nix-script-haskell
#!haskellPackages text

{-# LANGUAGE OverloadedStrings #-}

import Data.Text.IO

main :: IO ()
main = Data.Text.IO.putStrLn "Hello, World!"

And, in addition to the speed boost, we can depend on any package in the Nix ecosystem! For example, here's how you'd add and call jq:

#!/usr/bin/env nix-script-haskell
#!runtimeInputs jq

import System.Process

main :: IO ()
main = do
  formatted <-
    readProcess
      "jq"
      ["--color-output", "."]
      "{\"name\": \"Atlas\", \"species\": \"kitty cat\"}"
  putStr formatted

It's also pretty easy to create more wrapping interpreters, so we also ship one for bash. Even though it's not a compiled language, we can cache the nix environment with the exact dependencies the script needs!

You can get this at github.com/BrianHicks/nix-script. There are installation instructions in the README, both for standalone use and use within a larger Nix project.

Enjoy, and let me know if—and how—you use this!