nix-language

name	nix-language
description	Use when working with Nix expression language fundamentals including syntax, expressions, functions, builtins, let-in bindings, with expressions, inherit, rec, attrsets, attribute sets, string interpolation, derivations, import, fetchurl, fixed-point patterns, nixpkgs lib functions, lambdas, operators, or lazy evaluation.
user-invocable	false

Nix Language

Core Data Types

Type	Example	Notes
String	"hello ${name}"	Interpolation with ${}, multiline with ''
Integer	42
Float	1.0, 3.14	Rarely used in nixpkgs
Boolean	true / false
Null	null
Path	./foo.nix, <nixpkgs>	Paths are resolved relative to the file
List	[ 1 2 3 ]	Space-separated, heterogeneous
Attribute set	{ a = 1; b = 2; }	Key-value map, semicolons required
Function	x: x + 1	Single-argument, curried

Functions

Nix functions take exactly one argument. Multi-argument is done via currying or attrset patterns:

# Curried
add = a: b: a + b;

# Attrset pattern (most common in nixpkgs)
mkDerivation = { pname, version, src, ... }: { /* ... */ };

# With defaults
mkDerivation = { pname, version, src, buildInputs ? [], ... }: { /* ... */ };

The @ pattern captures the full attrset: { pname, ... } @ args:.

Key Constructs

let-in

Introduces local bindings. Bindings can reference each other (order doesn't matter due to laziness):

let
  x = 1;
  y = x + 1;
in x + y

with

Brings attrset attributes into scope. Doesn't shadow existing bindings:

with pkgs; [ git curl wget ]
# equivalent to: [ pkgs.git pkgs.curl pkgs.wget ]

inherit

Shorthand for x = x; in attrsets:

{ inherit src version; }
# equivalent to: { src = src; version = version; }

{ inherit (pkgs) git curl; }
# equivalent to: { git = pkgs.git; curl = pkgs.curl; }

rec (recursive attrsets)

Allows self-reference within attrsets. Avoid when possible — prefer let-in:

rec {
  x = 1;
  y = x + 1;  # works because of rec
}

Conditionals

if condition then valueA else valueB

No if without else — it's an expression, not a statement.

String Patterns

# Multiline (strips leading whitespace)
''
  line 1
  line 2
''

# Escape interpolation in multiline
''
  literal ''${not-interpolated}
''

# String interpolation
"Hello ${name}, you have ${toString count} items"

Derivations

A derivation is a build instruction. builtins.derivation is low-level; use stdenv.mkDerivation in practice:

stdenv.mkDerivation {
  pname = "hello";
  version = "1.0";
  src = ./src;
  buildInputs = [ pkg-config openssl ];
  buildPhase = ''
    make
  '';
  installPhase = ''
    mkdir -p $out/bin
    cp hello $out/bin/
  '';
}

Key concept: $out is the Nix store path where the build output goes.

Important Builtins

Builtin	Purpose
builtins.map	Transform list elements
builtins.filter	Filter list by predicate
builtins.attrNames	Get attrset keys as list
builtins.attrValues	Get attrset values as list
builtins.hasAttr	Check if key exists
builtins.elem	Check if element in list
builtins.toString	Convert to string
builtins.toJSON	Serialize to JSON
builtins.fromJSON	Parse JSON string
builtins.readFile	Read file contents at eval time
builtins.fetchurl	Fetch URL at eval time
builtins.trace	Debug print during evaluation
import	Load and evaluate a .nix file

nixpkgs lib Essentials

lib.mkIf condition value        # Conditional config (NixOS modules)
lib.mkMerge [ a b ]             # Merge multiple configs
lib.optionals bool list         # Conditional list items
lib.optional bool value         # Conditional single item as list
lib.filterAttrs pred set        # Filter attrset
lib.mapAttrs f set              # Transform attrset values
lib.genAttrs names f            # Generate attrset from names
lib.concatMapStringsSep sep f l # Map + join strings
lib.recursiveUpdate a b         # Deep merge attrsets

Laziness

Nix is lazy — values are only computed when needed. This allows:

• Infinite data structures (nixpkgs is a giant attrset, only needed packages are evaluated)
• Self-referencing configurations (NixOS modules reference each other)
• throw in unused branches doesn't error

Anti-Patterns

• Avoid rec when let-in works — rec makes the whole attrset depend on itself
• Avoid with pkgs; in large scopes — makes it unclear where names come from, doesn't shadow let-bindings
• Don't use builtins.toPath — it's deprecated
• Don't read secrets at eval time — builtins.readFile embeds content in the store (world-readable)