We begin our treatment of functions in this chapter by discussing a subset of possible functions in Motoko, namely private functions. We will explain private function arguments, argument type annotation, return type and the type of the function itself.

Private functions in Motoko may appear in Records, Objects, Classes, Modules, Actors and other places as well.

Other possible functions will be discussed in upcoming chapters:

Private functions

Lets start with most simple function in Motoko:

func myFunc() {};

The func keyword is indicating a function declaration. myFunc is an arbitrary name of the function followed by two parenthesis () and two curly brackets {}. The () are used for function arguments (inputs to the function) and the {} are used for the function body.

Note: The () in this context is not the same as the unit type!

We could explicitly tell Motoko that this is a private function by using the private keyword in front of the func keyword. This is not necessary though, because a function declaration defaults to a private function in Motoko unless declared otherwise.

Lets be more explicit about our private function, add one argument as an input and expand the body:

private func myFunc(x : Nat) {
    // function body

The function is now marked private. All arguments must be annotated. Type inference doesn't work here. In this case we take in one argument and name it x. We also type annotate it with Nat.

Lets proceed by adding a return type to this function and actually returning a value of that type:

private func myFunc(x : Nat) : Nat {
    return x;

After the function arguments we annotate the return type of this function with : Nat. If we don't annotate the return type of a private function, it defaults to the unit () return type.

Inside the body we return x, the same variable that was passed to the function. This is allowed because x also has type Nat, which is the expected return type for this function.

Lets simplify this function:

func myFunc(x : Nat) : Nat {

We removed the private keyword and simplified the return expression to just x. Even the semicolon ; is gone. But note that we can't leave out the type annotations, like we did with variables. Type inference doesn't work on function declarations except for the defaulting unit type behavior mentioned above.

Lets write a useful private function and call it:

func concat(t1 : Text, t2 : Text) : Text {
    let result = t1 # t2;

let output = concat("Hello", "world");

Our function concat takes two arguments of type Text. It also returns a Text type.

We use the text concatenation operator # to concatenate the two arguments and assign the result to a new variable. Concatenation with # only works for Text types.

The result of the concatenation t1 # t2 is another Text. We did not type annotate the variable result. Motoko automatically infers this for us.

We return result by placing it at the last line of the function without a return keyword and semicolon ;. You could be explicit by adding the return keyword and even type annotate the result variable with a : Text type, but in this case it is not necessary.

Lastly, we call this function with two text literals as the arguments and assign its result to the variable output. Again, we don't have to annotate the type of this output variable, because this is obvious from the context and Motoko will infer this information for us.

Function type

The last concept for this chapter is the type of the whole function. A function's typed arguments and return type together are used to define a type for the function as a whole. The type of the function concat above is the following:

type Concat = (Text, Text) -> Text;

let ourFunc : Concat = concat;

We used the type name Concat to define a new type (Text, Text) -> Text. This is the type of our function concat. The function type is constructed by joining three things:

  • a tuple of types for the function argument types
  • the -> keyword
  • the return type of the function

We use the Concat type to annotate the type of another variable ourFunc and assign the function name concat to it without the parenthesis and arguments like we did when we called the function. We basically renamed our function to ourFunc.