Object and Collection

Nix has only two kinds of collection:

• Attribute Set: a similar concept to object literal.
• List: generic bucket for elements.

Attribute Set

Set is similar to object literal in some languages like javascript and lua. Properties of a set are called attributes in nix

{ username = "john smith"; email = "example@example.com" }

Attribute name can be any string including interpolated string.

let foo = "bar";
baz = {
    ${foo} = 0
} in { }

Recursive Set

Attributes can reuse value from other attributes in scope, the set should annotate with leading rec

TIP

The attribute order doesn't matter.

rec {
    area = height * width;
    height = 10;
    width = 5;
}

Expanding Attributes

let
    foo = 123;
in
{
    inherit foo;
}

# equvalent to

let
    foo = 123;
in
{
    foo = foo;
}

Can expand multiple attributes from another attribute set:

let
    foo = { bar = 123; baz = "123"; };
in
{
    inherit (foo) bar baz;
}

# equvalent to

let
    foo = { bar = 123; baz = "123"; };
in
{
    bar = foo.bar;
    baz = foo.baz;
}

Access Attribute

Use . to access attribute of a set with either identifier or key string.

let languages = {
    "C#" = "OOP";
    Rust = "Multi-Paradigm";
    Lua = "dynamic scripting";
}; lua = "Lua"; in {
    language."C#";
    language.Rust;
    language.${lua}
}

Searching Path Validation

Use ? to validate whether the search path exists in an attribute set.

let
  foo = {
    bar = null;
    baz = {
      qux = 123;
    };
  };
in
[ 
  (foo ? bar)
  (foo.baz ? qux)
  (foo ? baz.qux) 
]

Merging

// operator will try to merge left and right operand, if attribute with same name has different values, the value from right operand will be taken.

{ foo = null; bar = null } // { foo = 1; baz = null }
# retuns { foo = 1; bar = null; baz = null; }

Callable Set

Specifying __functor meta attribute to make the attribute set to be a callable function as well. The functor can have a parameter self that targets to the attribute set itself. The nested function takes the actual paramter can be applied to the functor.

let
foo = {
    __functor = self: x: builtins.toString x;
};
in
    foo 123 # x is 123 here

Equality Checking

List and set are compared by their structure, not their references.

let 
    foo = { bar = { baz = {} }; }; 
    qux = { bar = { baz = {} }; };
in
    assert foo == qux; # true

List

List elements are separated by spaces

["a" 1 { foo = "bar" } (func 1)]

NOTE

To evaluate value returned from a function as a element of list, enclose it by ().

Concatenation

["a", 1 { foo = "bar" }] ++ [1 2 3]