In Clio, functions are pure and lazy. But before we get into the details, let's define a simple function together.

fn hello person:
'hello' person -> print

In the above example, we defined a function named hello, it accepts a single argument person and says hello to that person. Let's see what happens if we run this function:

fn hello person:
'hello' person -> print
'Pouya' -> hello

If you run the above code, you will not see any output in the console. This is because all the functions you define are lazy by default. That means a call to the function will only be evaluated if the result of it is needed.

Some functions aren't lazy, for example the built-in print function is not lazy, that's why you were able to run the hello world example. Eager is opposite of lazy, when a function is eager the evaluation of the function call will be immediate.

To make a function eager we decorate our definition with the built-in eager function:

fn hello person:
'hello' person -> print
'Pouya' -> hello

Now if you run the code you will see the results in the console. Functions in Clio have implicit returns, it means the last evaluated expression will be returned from the function, for example:

fn double n:
n * 2
2 -> double -> print

You don't need to explicitly write return in your function body. Functions are pure, they have access to outside scope, but the outside scope is frozen for them, that means if you change value of a variable after you define a function, that function will still have the old value of this variable.

10 => x
fn add_to_x n:
x + n => x
20 => x
2 -> add_to_x

Functions should have at least one argument, value of a function call only depends on its arguments so a function with no arguments is going to be a constant. Since the value a function returns depends only on its arguments, it makes sense to cache the results and speed-up our function calls. This way instead of re-executing the function for same arguments, the function remembers the previous results and returns them.

We'll talk more about functions in the future.