Functors in nix

Functors combine functions and attribute sets

Understanding Functors

A functor is a polymorphic data type that can represent both a lambda function (x: x) and an attribute set ({ key = value; }).

This means that the Atribute set can hold data in the form of attributes.

And also be a lambda function that can operate on that data.

-> The lambda function can access the attributes of the attribute set often called self.

Examples of Functors

In the following example, build is both an attribute set and a lambda. __functor is a reserved attribute name that turns the attribute set build into a functor.

{
  build = {
    foo = 1;
    __functor = self: _arg: self.foo;
  };
}

The attribute set build can now be used as a lambda via function application.

nix-repl> build "linux"
=> 1

But at the same time build is still an Attribute set.

nix-repl> build
=> { __functor = «lambda»; foo = 1; }

Applications of Functors

Functors find practical applications including some in nixpkgs:

• lib.makeOverridable
• lib.setFunctionArgs
• lib.mirrorFunctionArgs
• lib.types and lib.options
• and many more.

When to use a functor

(Noogle opinion)

The use of functor should be avoided and is often unnecessary.

Using them can add up in complexity and makes is hard to learn and maintain a certain piece of code.