Function and Lambda

Lambda

Fucntion form in nix is like <param_list>:(<return>|<func_body>)

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 workround 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 paramter count needed).

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

Set pattern paramters(Named parameters)

Another workround for multiple paramters for function is using set parttern. Paramters are separated by comma instead of semicolon.

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

INFO

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

Variadic parameter

args binds to the whole set excluding the optional attribute.

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

INFO

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: "${toJSON args}"; 
in 
  format_person { name = "john smith"; weight = 60 } # weight does not appear in parameter list but it's valid

Optional paramter

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