Launching emacs with configured packages

Question

Let's say I want to use elisp functions in a script . I rely in my script on some library, dash which I require.

#!/usr/local/bin/emacs --script
(require 'dash)

(message (describe-variable 'load-path))

As is, I will get

$ ./loader.sh
Cannot open load file: No such file or directory, dash

What is the simplest way to make a set of packages accessible to an elisp environment used in a script ?

PS : The equivalent for haskell, using nix, would be

#! /usr/bin/env nix-shell
#! nix-shell -i runghc -p "haskellPackages.ghcWithPackages(p: with p; [type-level-sets])"
#! nix-shell -I nixpkgs=channel:nixos-18.03


{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE TypeApplications #-}

module Main where

import Data.Type.Set (Set(..), Proxy(..))

class Get a s where
  get :: Set s -> a

instance {-# OVERLAPS #-} Get a (a ': s) where
  get (Ext a _) = a

instance {-# OVERLAPPABLE #-} Get a s => Get a (b ': s) where
  get (Ext _ xs) = get xs

main :: IO ()
main = do
  let lst = Ext "hello" $ Ext 10 $ Empty
  putStrLn $ show $ get @String lst

Answer 1

I have one answer with Nix, but I am curious as to how it is possible to do so without Nix

#! /usr/bin/env nix-shell
#! nix-shell -i "emacs --script"  -p "pkgs.emacsWithPackages(epkgs: (with epkgs.melpaPackages; [ dash ]))"
#! nix-shell -I nixpkgs=channel:nixos-18.03
(package-initialize)
(require 'dash)

(message "Hi")

Answer 2

Just put the directory where dash.el[c] resides in your load-path value, before you require that library:

(add-to-list 'load-path "/YOUR/DIRECTORY/CONTAINING/DASH/")
(require 'dash)

Answer 3

Add

(package-initialize)

or for more modern Emacsen you can use

(package-activate-all)

In either case, it needs to come before the require.