How to write a simple Gno Library (Package)

Overview

This guide shows you how to write a simple library (Package) in Gno, which can be used by other Packages and Realms. Packages are stateless, meaning they do not hold state like regular Realms (Smart Contracts). To learn more about the intricacies of Packages, please see the Packages reference.

The Package we will be writing today will be a simple library for suggesting a random tapas dish. We will define a set list of tapas, and define a method that randomly selects a dish from the list.

Prerequisites

• Text editor

Editor support

The Gno language is based on Go, but it does not have all the bells and whistles in major text editors like Go. Advanced language features like IntelliSense are still in the works.

Currently, we officially have language support for ViM, Emacs and Visual Studio Code.

1. Setting up the work directory

We discussed Gno folder structures more in detail in the simple Smart Contract guide. For now, we will just follow some rules outlined there.

Create the main working directory for our Package:

mkdir tapas-lib

Since we are building a simple tapas Package, inside our created tapas-lib directory, we can create another directory named p, which stands for package:

cd tapas-lib
mkdir p

Additionally, we will create another subdirectory that will house our Package code, named tapas:

cd p
mkdir tapas

After setting up our work directory structure, we should have something like this:

tapas-lib/
├─ p/
│  ├─ tapas/
│  │  ├─ // source code here

2. Create tapas.gno

Now that the work directory structure is set up, we can go into the tapas sub-folder, and actually create our tapas suggestion library logic:

cd tapas
touch tapas.gno

Inside tapas.gno, we will define our library logic:

package tapas

import (
    "gno.land/p/demo/rand"
)

// List of tapas suggestions
var listOfTapas = []string{
    "Patatas Bravas",
    "Gambas al Ajillo",
    "Croquetas",
    "Tortilla Española",
    "Pimientos de Padrón",
    "Jamon Serrano",
    "Boquerones en Vinagre",
    "Calamares a la Romana",
    "Pulpo a la Gallega",
    "Tostada con Tomate",
    "Mejillones en Escabeche",
    "Chorizo a la Sidra",
    "Cazón en Adobo",
    "Banderillas",
    "Espárragos a la Parrilla",
    "Huevos Rellenos",
    "Tuna Empanada",
    "Sardinas a la Plancha",
}

// GetTapaSuggestion randomly selects and returns a tapa suggestion
func GetTapaSuggestion() string {
    // Create a new instance of the random number generator.
    // Notice that this is from an imported Gno library
    generator := rand.New()

    // Generate a random index
    randomIndex := generator.Intn(len(listOfTapas))

    // Return the random suggestion
    return listOfTapas[randomIndex]
}

There are a few things happening here, so let's dissect them:

• We defined the logic of our library into a package called tapas.
• The package imports another gno package, which is deployed at gno.land/p/demo/rand
• We use the imported package inside of GetTapaSuggestion to generate a random index value for a tapa

Conclusion

That's it 🎉

You have successfully built a simple tapas suggestion Package that is ready to be deployed on the Gno chain and imported by other Packages and Realms.