Derivation

The recommended way to create Derivation using shell scripts.

We used callCabal2nix function from nixpkgs to build the todo-app package above. This functio generates a Haskell package Derivation from its source, utilizing the “cabal2nix” program to convert a cabal file into a Nix derivation.

While package management is the key purpose of Nix, its derivations can also be used to produce non-package types, such as development environments (aka. “devShell”).

The directory used by Nix to store store paths (including derivations).

Normally we can achieve this by refactoring our Nix expression to be a function (see lsdFor ⤵️) that takes arguments for these variations (dir and tree ⤵️), producing the appropriate derivation as a result:

This is a simple flake that exposes a package (a writeShellApplication Derivation wrapping lsd), that can be `nix run`ed to list the contents of the root directory.

Nix expressions are evaluated to produce derivations (among other values). These derivations when realized usually produce the compiled binary packages. Sometimes, realizing a derivation can produce a Nix expression representing another derivation. This generated Nix expression too needs to be evaluated to its derivation before it can be realized. This secondary evaluation is achieved by importing from the derivation being evaluated, and is called “import from derivation” or IFD.

Note that this is the same path used by both nix build and nix shell. Each specific package is uniquely identified by their Store path; changing any part of its build recipe (including dependencies), changes that path. Hence, nix is reproducible.

The cowsay Derivation produces two output paths, the second of which is the cowsay binary package (the first one is the separate documentation path), and if you inspect that^* you will see the contents of it:

What is a Nix “package”? Technically, a Nix package is a special Store path built using instructions from a Derivation, both of which reside in the Nix Store. To see what is contained by the cowsay package, look inside its Store path. To get the store path for a package (here, cowsay), run nix build as follows:

[!info] Nix Store & Store Paths /nix/store is a special directory representing the Nix Store. The paths inside /nix/store are known as Store path. Nix fundamentally is, in large part, about manipulating these store paths in a pure and reproducible fashion; Derivation are “recipes” that does this manipulation, and they too live in the Nix Store.

[!info] nix run nix run command will run the specified package from the flake. Here nixpkgs is the flake, followed by the letter #, which is followed by the package (Derivation) name cowsay that is outputted by that flake. See Flake URL for details on the syntax.

A binary cache provides cached binaries of built Nix Derivation.