Nix Language Reference

{}

👆 That's the simplest bit of nix! Nix is really simple.

# example.nix
{
  whatCanNixDo = "It can store data";
  whatAreTheCurlyBrackets = "They define a set of key value pairs known as attributes";
  amIAnAttribute = "Yes, you are the third attribute in this attribute set";
  whichAttributeAmI = 4;
  soYouCanStoreNumbersAlso = true;
  andBooleanValuesToo = {
    yes = "You can store boolean values";
    andAlso = "You can store atrribute sets within attribute sets!";
  };
  "can key names have spaces" = "Sure";
  whatAboutLists = [
    "lists can contain anything separated by a space, such as:"
    20 21 22
    false
  ];
}

The nix command can pull from this data:

nix eval --file example.nix whatCanNixDo                # "It can store data"
nix eval --file example.nix whichAttributeAmI           # 4
nix eval --file example.nix andbooleanValuesToo.yes     # "You can store boolean values"
nix eval --file example.nix "can key names have spaces" # Sure
nix eval --file example.nix whatAboutLists.2            # 20

Referring to yourself

You can use the attributes of a set as variables for use within that set, by using the rec keyword:

rec {
  ten = 10;
  nine = ten - 1;
  five = nine - 4;
  zero = ten - 10;
}

Variables

When you want to resuse data that you don't want to expose as an attribute, you can "let" a variable exist "in" the scope of a particular attribute set during the evaluation which won't itself become an attribute of the set.

let
  activity = "writing Nix!";
in {
  onMonday = "On Monday I shall be " + activity;
  onTuesday = "And On Tuesday I'll likely try " + activity;
  onWednesday = "Ooo I don't know, maybe more " + activity;
}

Functions

Nix comes with many builtin functions.

rec {
  twentyThreePointThreeFour = 23.34;
  twentyThree = builtins.ceil twentyThreePointThreeFour;
  twentyFour = builtins.floor twentyThreePointThreeFour;
}

And you can declare your own functions too.

let
  subtractOne = number: number - 1;
  multiplyThenSubtractOne = firstNumber: secondNumber: firstNumber * secondNumber - 1;
in {
  ninetyNine = subtractOne 100;
  five = subtractOne 6;
  eleven = multiplyThenSubtractOne 2 6;
}