Nix language

Unquoted URLs

Nix syntax supports URLs as verbatim, so one can write instead of ""

There’s was an RFC 45 accepted to deprecate verbatim URLS and provides a number of arguments how this feature does more harm than good.

rec { ... } expression

rec allows you to reference variables within an attribute set.

A simple example:

rec {
  a = 1;
  b = a + 2;

evaluating to { a = 1; b = 3; }.

b refers to a as rec makes all keys available within the attribute set.

There are a couple of pitfalls:

  • It’s possible to introduce a hard to debug error infinite recursion when shadowing a variable, the simplest example being rec { b = b; }.

  • combining with overriding logic such as overrideAttrs function in nixpkgs has a surprising behaviour of not overriding every reference.

A better way is to use simpler let .. in:

  a = 1;
in {
  a = a;
  b = a + 2;

with attrset; ... expression

It’s common to see the following expression in the wild:

with (import <nixpkgs> {});


Which brings all packages into scope of the current expression so that pkgs.git becomes git.

There are a number of problems with such approach:

  • Static analysis can’t reason about the code, because it would have to actually evaluate this file to see what variables are in scope.

  • As soon as there are two with used, it’s not clear anymore from which the variables are coming from.

  • Scoping rules around with are not intuitive, see Nix issue for details

A better way is to use a variable:

  pkgs = import <nixpkgs> {};
in ...

<...> search path

<...> is syntax, commonly <nixpkgs> is for looking up Nix expression’s path specified by shell environment variable $NIX_PATH.

Two developers on different machines are likely to have <nixpkgs> point to different revisions, which will lead to getting different results.

It’s possible to specify exact nixpkgs commit via $NIX_PATH, but that’s still problematic unless:

  1. You specify the commit at one place only and reference it else where.

  2. And you can control the environment via your source code, so that a) applies by somehow setting $NIX_PATH via nix-shell or NixOS options

See Towards reproducibility: Pinning nixpkgs for a tutorial on how to do better.

attr1 // attr2 merge operator

It merges two attribute sets:

$ nix repl
Welcome to Nix version 2.3.6. Type :? for help.

nix-repl> { a = 1; b = 2; } // { b = 3; c = 4; }
{ a = 1; b = 3; c = 4; }

However, if attribute sets are nested it doesn’t merge them:

nix-repl> :p { a = { b = 1; }; } // { a = { c = 3; }; }
{ a = { c = 3; }; }

You can see key b was removed, because whole a value was replaced.

A better way is to use pkgs.lib.recursiveUpdate function:

$ nix repl '<nixpkgs/lib>'
Welcome to Nix version 2.3.6. Type :? for help.

Loading '<nixpkgs/lib>'...
Added 364 variables.

nix-repl> :p recursiveUpdate { a = { b = 1; }; } { a = { c = 3;}; }
{ a = { b = 1; c = 3; }; }

Reproducability referencing top-level directory with ./.

Browsing GitHub source code you’re likely to see the following:

 { pkgs ? import <nixpkgs> {}

 pkgs.stdenv.mkDerivation {
   name = "foobar";

   src = ./.;

If working directory is /home/myuser/mywork/myproject, then the derivation of src will be named /nix/store/n1caswkqqp8297833y24wyg9xxhs2dc6-myproject.

The problem is that now your build is no longer reproducible, as it depends on the parent directory name that you don’t have control of in the source code.

If someone builds the project in a differently named folder, they will get a different hash of the src and everything that depends on it.

A better way is to use builtins.path:

 { pkgs ? import <nixpkgs> {}

 pkgs.stdenv.mkDerivation {
   name = "foobar";

   src = builtins.path { path = ./.; name = "myproject"; };

If you’re using git to track your code, you may also want to look at gitignoresource, which does this for you.