Function

n: n % 2 != 0 # tells whether a number is odd

Function call

Function call by parameters after function or function variable

let is_odd = n: n % 2 != 0; in {
   is_odd 3 # false 
}

Curried Function

Multiple parameters for nix function? Sadly no such thing exists in nix. But one workaround is currying. Currying simply stores state into next function returned from previous, to perform the chaining. It's recursive-like, but with absolute recursion count(which is the parameter count needed).

let add = x: y: x + y; in add 2 3 # returns 5

Named Parameters

Another workaround for multiple paramters for function is using set parttern.

let format_person = { name, age }: { name = name; age = age }; 
in 
  format_person { name = "jane"; age =  18 }

Optional Parameter

let
  format_person =
    {
      name ? "john smith",
      age ? null,
    }:
    "${name} aged ${age}";
in
format_person { name = "jane"; }

NOTE

Set parttern is similar to named paramters, they're all named, can be sepecified by any order.

Parameter List

args binds to the whole set excluding the optional attribute.

let
  format_person =
    {
      name,
      age,
      height ? 0,
      ...
    }@args:
    "${builtins.toJSON args}";
in
format_person {
  name = "john smith";
  age = 18;
} # `height` is not included in json

NOTE

If no other paramters are expected, do omit ... to prevent messing things up.

There's no signiture checking for argument count, so adding attributes that are not defined in function definition is ok.

let
  format_person =
    {
      name,
      age,
      height ? 0,
      ...
    }@args:
    "${builtins.toJSON args}";
in
format_person {
  name = "john smith";
  weight = 60;
} # weight does not appear in parameter list but it's valid