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In the Nix language

In the Nix language

Unquoted URLs

Nix syntax supports URLs as verbatim, so one can write https://example.com instead of "https://example.com"

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:

let
  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

Here are some better alternatives:

# instead of:
with (import <nixpkgs> {});

# try this instead:
let
  pkgs = import <nixpkgs> {};
  inherit (pkgs) curl jq;
in ...

# instead of:
buildInputs = with pkgs; [ curl jq ];

# try this instead:
buildInputs = builtins.attrValues {
  inherit (pkgs) curl jq;
};

# or this:
buildInputs = lib.attrVals ["curl" "jq"] pkgs

<...> search path

You will often see Nix language code samples that refer to <nixpkgs>.

<...> is special syntax that was introduced in 2011 to conveniently access values from the shell environment variable $NIX_PATH.

This means, the value of a search path depends on external system state. When using search paths, the same Nix expression can produce different results.

In most cases, $NIX_PATH is set to the latest channel when Nix is installed, and is therefore likely to differ from machine to machine.

Note

Channels are a way of distributing Nix software. They are being phased out, but still used by default.

For example, two developers on different machines are likely to have <nixpkgs> point to different revisions of nixpkgs. Builds may work for one and fail for the other, causing confusion.

It’s possible to specify an exact nixpkgs revision via $NIX_PATH. But that is still problematic, unless:

  1. you specify the commit at one place only,

    and

  2. you control the shell environment via your source code, setting $NIX_PATH via nix-shell or NixOS options.

Note

We recommend to avoid using search paths and to disable channels by permanently setting NIX_PATH= to be empty.

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; }; }

Reproducibility 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 with the name attribute set to something fixed. This will derive the human readable portion of the store path from name instead of the working directory:

 { 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.

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