Functions

A function is a piece of behaviour with a name, a list of parameters, and a result. You have been calling them since Chapter 1; this chapter is about writing them.

Functions in Zuri are values. You can put one in a list, hand it to another function, return one from a function, store one in a dictionary, and call whatever comes back:

def double(n) {
  return n * 2
}

var operations = { twice: double, thrice: @(n) => n * 3 }

var chosen = operations.twice

echo chosen(5)
echo operations['thrice'](5)
echo [1, 2, 3].map(double)

10
15
[2, 4, 6]

That one property is what makes map, filter, reduce, every callback in the standard library, and every route handler in Chapter 15 possible.

Note the shape of those two calls. operations.twice reads the function out of the dictionary, and then you call what you read. Writing operations.twice(5) in one step does not work: a dot followed by a call looks for a method on the dictionary, and a dictionary has no method called twice. Either read it into a variable first, as above, or index with brackets and call the result: operations['twice'](5).

This chapter covers declaring a function, the spellings an anonymous one can take, how a closure captures the variables around it, and the optional type annotations that make the runtime check arguments for you.