Running a Haskell Script without GHC

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Introduction

Given the following haskell script generate-random-samples.hs that requires mwc-random

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{-# LANGUAGE ScopedTypeVariables #-}

import System.Random.MWC
import Data.Vector.Unboxed
import Control.Monad.ST

main = do
  vs <- withSystemRandom $
        \(gen::GenST s) -> uniformVector gen 20 :: ST s (Vector Int)
  print vs

… how do you run it without having to globally install the package or having to build a whole cabal project ?

Initial approach

One of the simplest approaches using nix is the following:

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$ nix-shell --packages 'haskellPackages.ghcWithHoogle (pkgs: with pkgs; [ mwc-random ])'
nix-shell> runghc generate-random-samples.hs

In order to reuse the command so other people can run the script, you can add the following shell.nix:

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{ compiler ? "ghc881" }:
let

  pkgs = import (builtins.fetchGit {
    url = "https://github.com/NixOS/nixpkgs.git";
    rev = "890440b0273caab258b38c0873c3fe758318bb4f";
    ref = "master";
  }) {};

  ghc = pkgs.haskell.packages.${compiler}.ghcWithPackages (ps: with ps; [
          mwc-random
        ]);

in

pkgs.stdenv.mkDerivation {
  name = "ghc-env";
  buildInputs = [ ghc pkgs.cabal-install ];
  shellHook = "eval $(egrep ^export ${ghc}/bin/ghc)";
}

So now you only need to call $ nix-shell to enter into a pure shell with a specific GHC version that includes all your dependencies:

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$ nix-shell
nix-shell> runghc generate-random-samples.hs

Improving the first approach

The only issue is that you must be aware of how nix work in order to be able to run the script.

But these could be solved using bash shebangs in your haskell script:

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#!/usr/bin/env nix-shell
#!nix-shell -i runghc
{-# LANGUAGE ScopedTypeVariables #-}

import System.Random.MWC
import Data.Vector.Unboxed
import Control.Monad.ST

main = do
  vs <- withSystemRandom $
        \(gen::GenST s) -> uniformVector gen 20 :: ST s (Vector Int)
  print vs

So now, you can run your haskell script in an environment without ghc:

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./generate-random-samples.hs
[6052359640365008112,3693984866634705670,6521947999724514858,640433474764908030,-4262896110044960033,-1795671341099353119,-2220462704949887998,-248182841640258167,709016591698961687,-3622504171575206589,5987258113070378446,-159251391303273987,-8449937247808153766,6165509553180365166,-8199532339362621783,-9187765480154042269,-2389922548196927048,-4842141643835297495,-1106748185069026877,826927505518387091]

Update: 11-03-2020

After reading the following comment in reddit, I think it is worth mentioning that you can achieve better modularity in exchange of maintainability, by having everything on the haskell script, without having to depend on a shell.nix file:

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#!/usr/bin/env nix-shell
#!nix-shell -i runghc -p "haskellPackages.ghcWithPackages (pkgs: with pkgs; [ mwc-random ])"
#!nix-shell -I nixpkgs=https://github.com/NixOS/nixpkgs/archive/d373d80b1207d52621961b16aa4a3438e4f98167.tar.gz
{-# LANGUAGE ScopedTypeVariables #-}

import System.Random.MWC
import Data.Vector.Unboxed
import Control.Monad.ST

main = do
  vs <- withSystemRandom $
        \(gen::GenST s) -> uniformVector gen 20 :: ST s (Vector Int)
  print vs