Hello world

Start by installing GreyCat on your machine instruction can be found here.

Create a folder called hello and open it in your editor (preferably VSCode with plugin).

Create a file named project.gcl.

Add the following content to it and update the version string with the latest one from here:

@library("std", "version");

fn main() {
    println("Hello world");
}

First install all dependencies in our case only the std library

greycat install

In a terminal execute the following command to get the first Hello World message:

greycat run
# Hello world

Stateful (Database)

GreyCat is also a database,so how do i store my data ?

All you need is to declare a variable at the root of your file

var count: node<int?>;

fn main() {
    // The first time we run our app count is null
    if(*count == null){
        count.set(0);
    }
    count.set(*count + 1);
    println(*count);
}

The above code can also be written simpler as

var count: node<int?>;

fn main() {
    count.set((count.resolve() ?? 0) + 1);
    println(*count);
}

All variables in GreyCat are declared with the var keyword, but only variables declared at the root of your file ,meaning not inside functions or types are accessible, think about it as the entry point to your graph.

We call them module variables, they have to be a node or one of the node indexes like nodeList, in our case count is an integer, the ? after the type makes it nullable, since you can not directly set the value inline we need to initialize it to null at runtime.

Now every time we run our app count will be incremented.

greycat run
# 1

greycat run
# 2

greycat run
# 3

Server

Starting a server and exposing endpoints, is also very simple and straightforward.

All you need is to add @expose to any function, this kind of works like a decorator adding extra functionality to your function.

In the following example we are exposing a getCounter function that returns the stored count value.

@expose
fn getCounter(): int {
    return *count;
}

To start GreyCat in server mode. All you need is to call greycat with the serve argument.

greycat serve --user=1
# 4 // GreyCat always executes code inside main before starting the server
# GreyCat is serving on port: 8080

GreyCat allows you to pass additional options when starting a server, in our case –user=1 will treat all request to the server as user with the id 1(admin user) more about this can be found here

Making a POST request to greycat where we specify our endpoint module::function

curl -X POST http://localhost:8080/project::getCounter
# 5

With arguments and modifying the database.

@expose
fn setCounter(val: int): int {
    count.set(val);
    return *count;
}

curl -X POST -d '[0]' http://localhost:8080/project::getCounter
# 0

This is but a simple example and introduction to the language, the real advantage and magic comes from the notion of node nodeTime nodeGeo and others that make GreyCat a powerful tool to scale temporal, geographical, and complex relational data.