Lambda Expressions

The main problem with regular functions is that they cannot be passed around, since polymorphism makes it really difficult. In order to have first class functions you have to use lambda expressions. They work like this:

Syntax

The syntax to create lambda expression is as follows:

[captures](arg_1: Type_1, arg_2: Type_2, ...) -> ReturnType { 
    [body]    
};

// With inferred return type
[captures](arg_1: Type_1, arg_2: Type_2, ...) { 
    [body]    
};

// Direct return
[captures](arg_1: Type_1, arg_2: Type_2, ...) expression;

Captures are optional.

Using them

In terms of functionality, they are the same as regular functions. Here is an example:

let double = (i: Int) i * 2;

print(double(4)); // Prints 8

If you want to access data outside the context of the lambda expression, you can use captures (you can only capture variables, not expressions):

let i = -1;

let count = [i]() {
    i.inc();
    return *i;
};

let c0 = count(); // 0
let c1 = count(); // 1
let c2 = count(); // 2
let c3 = count(); // 3

Of course, you can also pass lambdas as function arguments as long as you use the correct type (see function types).