Functions are merely converters of data; given arguments, they calculate results. They can be combined to create more complex ones and may also have side effects. For the reason that functions are the most fundamental part in Cloe, this page illustrates how to call and define them in detail.

Calling functions

Functions are called with arguments and compute their results depending on their actual values. Because function calls are expressions which represent their results, they are placed anywhere expressions are allowed like other functions’ arguments and let statements’ bodies.


There are 2 kinds of arguments: required and optional ones.

Required arguments

Required arguments are ones which fill required parameters of called functions. Lists can be expanded into multiple required arguments preceded by .. markers.

(someFunction x y)
(someFunction ..[1 2])

Optional arguments

Optional arguments are ones which replace default values of optional parameters defined in called functions. To be passed to functions, they must be preceded by . specifying them as optional arguments aligned as key-value pairs. Dictionaries can be expanded into optional arguments while their keys must be all strings.

(someFunction . option1 42 option2 "foo")
(someFunction . ..{"option1" 42 "option2" "foo"})

Defining functions

There are 2 ways to define functions, def statements and anonymous functions. Although the former is more expressive and common, the latter is convenient when you want to create one-off functions instantly.

Functions are characterized mainly by 2 kinds of components: signatures which determine how they receive arguments and body expressions which describe how their arguments are processed into their results.


Signatures are sets of parameters which are placeholders of values used in functions. There are 2 kinds of parameters: required and optional ones.

Required parameters

Required parameters are placeholders of required arguments. They receive and bind given names with the arguments considering their orders. Passed required arguments can also be collected into rest parameters of lists preceded by ...

Required parameters are always required. If no passed arguments can fill any required parameters on function calls, errors will be raised with appropriate messages about which parameters are missing.

(def (foo x) x)
(def (bar y rest)

Optional parameters

Optional parameters are defined with their names and default values. They must be placed after . in signatures with the default values like . someName "defaultValue". Passed optional arguments can also be collected into rest parameters of dictionaries preceded by ...

(def (foo . x 42) x)
(def (bar . rest)

def statements

They are the most basic way to create functions. Every def statement is composed of a function name, a signature, internal statements, and body expression. Internal statements are def or let statements used in other def statements defining intermediate values which are referred to in subsequent internal statements or body expressions.

(def (foo x y)
  (+ x y))

(def (sum ..args)
  (+ ..args))

(def (+all ns ms)
  (def (add ns)
    (let x (ns 1))
    (let y (ns 2))
    (+ x y))
  (let mappedFunction add)
  (map mappedFunction (zip ns ms)))

Anonymous functions

Anonymous functions are quite useful when you are creating simple functions which are used only once. The most notable trait of them is that they are not statements but expressions, which leads to less and cleaner codes in some occasions. Needless to say, they cannot be recursive by themselves because they have no names as the name suggests.

(\ (x) (+ x 42))
(\ (x ..args . y 42 ..kwargs) (+ x y))

Mutually recursive functions

Because codes in Cloe are interpreted from top to bottom, it provides a special syntax mr for creation of mutually recursive functions which refer to each other inside themselves. In mr clauses, functions are defined as usual although each one can use the others no matter where they are defined in the same clauses.

  (def (even? n)
    (if (= n 0) true (odd? (- n 1))))
  (def (odd? n)
    (if (= n 0) false (even? (- n 1)))))